User:Chela5808/My Sandbox 3

= Unit 5 =

Introduction
In this unit we will look at how the human body works. The study of processes which take place in the human body is called physiology. The study of internal structures of human systems is called anatomy. We will also look at systems that make the body function.

Upon completion of this unit you will be able to:

Respiratory system
Introduction

Gas exchange or respiration takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane, but for large organisms it usually is carried out through respiratory systems.

Features of gaseous exchange surfaces

Large animals have developed respiratory surfaces that increase the surface area for gaseous exchange, thus making respiration possible for larger bodies. The place where exchange of gases occurs, is called the respiratory surface. This surface is found in organisms e.g., lungs in animals such as mammals, gills in fish and the skin an earthworm. These surfaces as found in various organisms must allow efficient exchange of gases in order to maintain life. All gaseous exchange surfaces must have the following features:


 * large surface area;
 * moisture;
 * contact with blood capillaries and
 * extreme thinness.

Why do you think the above requirements are important for a large or big organism/animal e.g., an elephant? To start with, organisms like an elephant are multicellular, meaning they consisting of many cells. Let us examine how significant these features are to ensure efficient exchange of gases.

Large surface area

Millions of alveoli increase the surface area and thus maximise efficiency for gas exchange. It allows more (a lot of) gas to diffuse across it at the same time.

Moisture

Molecules must dissolve in water/moisture before diffusion will occur. It prevents cells from drying out and dying.

Transport network

The surface area must be in contact with the blood capillaries as they take the gases to and from the cells that need them. The transport network provides a rich blood supply which maintains a steep diffusion gradient and favours the rapid diffusion of gases.

Extremely thin and permeable

Gases diffuse quickly across the cell membrane due to a short diffusion distance between the two cell layers.

The maintenance of the life of an organism depends on how effective gas exchange is taking place at the gas exchange surface. This will partly depend on the existing gradient difference across the cell membrane.

The diaphragm

It is a sheet of muscular tissue which separates the thorax from the abdomen. Together with intercostal muscles and the ribs, it helps with the breathing in and breathing out mechanisms/ changes the volume of the lungs during breathing.

The air passage

When we breathe in air, it has a route that it follows. This route is called an air passage. We will now discuss the parts that form this air passage.

The nose and the mouth

Air passes through the nose into your respiratory system. You take in air by breathing through your nose. When you have influenza (flu) your nose becomes blocked and it is difficult to breathe through it. You then have to breathe through your mouth. The lining of the nose has ciliated hairs which clean the air that we inhale.

The nasal cavities

The nasal cavities connect the nose with the air passage which leads to the lungs. Cells lining this passage secrete sticky mucus which moistens dry air. Germs and dust present in the air are trapped and prevented from entering the lungs. Blood vessels in this passage warm cold air.

The trachea, bronchi and bronchioli

The trachea is also called the windpipe. The air from the mouth and the nose passes through the trachea into the lungs. The trachea contains incomplete rings of cartilage. Do you still remember what cartilage is? The cartilage keeps the trachea permanently open. . You find a lining in the wall of the trachea. This lining is called the mucous membrane.

The inner surface of the trachea also has cilia which move all the time. The cilia drive the mucous which traps dust particles upwards so that it can be swallowed or coughed out. The mucous membrane traps dust particles and bacteria in the air so that we can inhale clean air. The trachea divides into two bronchi. One bronchus goes to the left lung and the other to the right lung. Within the lungs each bronchus branches into smaller tubes called bronchioles.

The lungs

The lungs are situated in the thoracic cavity. The lungs are protected by the ribs and the chest bone. The lungs have a spongy texture. They are elastic and can expand. The surface of the lungs is covered by the pleural membrane. The size of the lungs depends on age, body size and gender. The surface of the lungs is covered by the pleural membrane. The alveoli are well supplied with a network of tiny blood capillaries. You now know the parts that form the air passage. Let us see how gases are exchanged during breathing.

How do we breathe?

Have you noticed how your lungs move when you breathe in and out? Look at the two diagrams shown below.

breathing in(inspiration)                breathing out (expiration)

Can you see any difference? Write your answers in your notebook. Did you notice the following? In diagram A the size of the lungs is larger and in diagram B the size of the lungs is smaller. What do we learn from this? We learn that if we breathe in and out, the chest changes in shape. This happens because the ribs and diaphragm move.

In diagram A, when we breathe in, the intercostal muscles contract. These pull the rib cage upwards and outwards so that the chest cavity can increase in volume. The diaphragm muscles contract and the diaphragm flattens. The chest gets even larger. Air moves in from high to low air pressure through the windpipe to fill up extra space due to inspiration. This is an active process.

Diagram B shows what happens when we breathe out.

The intercostal muscles relax and the ribs are lowered. The diaphragm muscles relax and move upwards. This decreases the volume of the chest cavity and lungs. Air is forced out of the lungs due to expiration. This is a passive process, but becomes active during coughing etc. Are you coping with this new information? Try the practical activity to help you understand expiration and inspiration better.

The amount of air that we breathe in differs from the air that we breathe out.

The table below shows the difference between the amounts of air you breathe in and out.

You now realise the importance of our lungs. We must take good care of them. In this last part we are going to look at what will happen when we do not take good care of our lungs.

Lung disorders

Your respiratory system is very important. As we said, we need clean air to be healthy. As you may recall, we said that gaseous exchange takes place in the alveoli in the lungs. The alveoli are very delicate. (Something which is delicate is very soft and sensitive.) They are easily damaged. Dust, dirt, bacteria, viruses and polluted gases enter your alveoli during gaseous exchange. Since our lungs are warm and moist they are an ideal breeding place for all these organisms. Your breathing system is designed to protect your lungs from harmful substances in the air that you may breathe in. The epithelial ciliated cells lining the trachea are covered by mucous. Dust and bacteria get trapped in the mucous. Special cells called goblet cells secrete the mucous.

The other cells have cilia that move back and forth and push the mucous up and out of the lungs. When this does not happen, normal functioning of the lungs will be disturbed and lung disorders may occur.

How does smoking affect the respiratory system?

Have you noticed the warnings on cigarette packets and in cigarette advertisements?

Here is one such statement: “Smoking causes lung cancer, heart diseases, emphysema and may complicate pregnancy.” Some of the most common lung disorders affecting Namibians are due to tobacco smoking. Tobacco smoke contains many chemicals which are harmful to the body. Some of the chemicals in tobacco are irritants which damage the lungs. Nicotine is a stimulant in tobacco. It damages the heart, blood vessels and nerves. Nicotine is an addictive substance. This is why smokers find it hard to give up smoking. Tar is another substance which is found in cigarette smoke. It can cause cancer. Carbon monoxide is a gas that is produced in cigarette smoke. This is a poisonous gas which stops blood carrying oxygen round the body. Smoking does not only cause lung disorders, but many other disorders like heart diseases, emphysema, cancer and bronchitis to mention but a few.

We will discuss each in turn in the next session.

Diseases caused by smoking

Lung cancer

There is scientific evidence that smoking cigarettes can increase lung cancer.

Lung cancer is one of the major killer diseases in the world. Let us look at how lung cancer develops.

Lung cancer may develop as the lungs produce new tissue to defend themselves against the constant irritation of nicotine and tar. Carcinogenic substances like tar in tobacco cause lung cancer and form a tumour. The tumour spreads through the lungs and destroys the normal lung tissue.

If you are not a smoker at present, please do not try it. If you are a smoker, how about giving it up? I know it is difficult, but it is worth trying.

Heart diseases

It is evident that heart diseases are more common among smokers than among non-smokers. The nicotine and carbon monoxide from cigarette smoke increase the tendency for blood to clot and so, block the coronary arteries. Carbon monoxide is also absorbed by haemoglobin so that less oxygen is carried to the blood.

Emphysema

Emphysema occurs when many of the delicate walls of the alveoli become torn and broken. Why? Because phagocytic cells become more and more to get rid of dust particles in cigarette smoke (phagocytosis leads to emphysema). Other parts of the alveoli become thicker so there is less space for gaseous exchange. Breathing becomes very difficult.

Bronchitis

Smoking irritates the air passage to the lungs. The smoking stops the cilia in the air passage from moving, because carbon monoxide and nicotine paralyse the cilia. The bronchi become swollen and clogged with mucous. A person coughs to push the mucous upwards. This is how bronchitis develops.

Other respiratory disorders and their causes

Air pollution also contributes to some diseases. Air is polluted with smoke, dust and harmful gases.

Do you still remember the causes of air pollution that we discussed in Grade 9? I hope you do. Most of these substances come from power stations, factories and vehicles. Some of the pollutants affect the air passage in the respiratory system. Sulphur dioxide is one of the harmful gases produced when fossil fuel is burnt. Sulphur dioxide enters the air passage and makes the bronchiole contract. This makes it difficult for air to pass into the lungs. During rainy seasons or when flowers bloom, pollen is carried into the air and causes hayfever. The lining of the nasal cavity becomes sensitive and inflamed and produces a large amount of mucous. The nose runs and a person sneezes a lot.

The polluted gas in air also causes asthma. The muscles in the walls of the bronchioles contract, so the tubes get narrow. This makes it difficult to breathe and a person wheezes.

How did you find this section? Try the last self-mark activity in this section to see how well you understand the topic.

Blood circulation
In Section 1, we looked at how the respiratory system works. In this section we will look at the heart and blood vessels

The circulatory system

The circulatory system is made up of the heart, blood vessels and blood. A blood vessel is a tube through which blood flows. The blood flows from the heart to the lungs and then from the lungs back to the heart. This completes one circulation. The blood flows from the heart to the rest of the body and then from the body back to the heart. In other words, the blood flows twice through the heart for every complete circuit of the body. This is called double circulation.

Study this diagram and make sure you know all the labelled parts.

What is blood?

Blood is mainly composed of a fluid called plasma which contains red blood cells, white blood cells and platelets. Why is blood red? It is because it contains more red blood cells than white blood cells and because red blood cells contain the pigment haemoglobin. An adult person has approximately 5

litres of blood in his or her body. Let us discuss each component of blood in turn.

Red blood cells

There are several things about RBCs that make them unusual.

·        A RBC has a strange shape -- small biconcave disc that is round and flat, sort of like a shallow bowl.

·        A RBC has no nucleus. The nucleus is extruded from the cell as it matures.

·        A RBC can change shape to an amazing extent, without breaking, as it squeezes single file through the capillaries

·        A RBC contains hemoglobin, a molecule specially designed to hold oxygen and carry it to cells that need it.

The primary function of red blood cells is to transport oxygen from the lungs to the cells of the body. RBCs contain a protein called hemoglobin that actually carries the oxygen. Hemoglobin combines loosely with oxygen in the lungs where the oxygen level is high and then easily releases it in the capillaries where the oxygen level is low. Each molecule of hemoglobin contains four iron atoms. Each iron atom can bind with one molecule of oxygen (which contains two oxygen atoms, called O2) for a total of four oxygen molecules or eight atoms of oxygen for each molecule of hemoglobin. The iron in hemoglobin gives blood its red color.

White blood cells

Our blood also contains white blood cells. There are several types of white blood cells. White blood cells are larger than red blood cells and have nuclei. They have different shapes and sizes. White blood cells are located in the lymph nodes and the spleen. Their main function is to produce antibodies that destroy foreign chemicals and pathogenic microorganisms in the blood. They are the “soldiers” of the body that protect it against disease(s).

Platelets

Platelets are small fragments of cells formed in the bone marrow. Their function is to clot blood. For example, if you cut your finger accidentally with a knife, blood flows from your finger. The platelets trap red blood cells. The blood changes into a thick substance which is called a blood clot. This stops the bleeding. The clot hardens to form a scab.

Plasma

Plasma is the liquid component of blood. It is a pale yellow fluid that contains mainly water and salt. Plasma transports hormones, dissolved substances like amino acids, glucose, vitamins, minerals, and waste products like carbon dioxide and other waste products. Plasma contains blood protein. Such protein is called fibrinogen. This protein helps with the clotting of blood.

Exchange of substances between blood and tissues

Between our cells there are narrow spaces filled with a watery fluid. This is called tissue fluid. Lymph fluid forms a link between blood and cells. Substances moving between them must pass through tissue fluid. It can only enter and leave the circulatory system through capillaries. Tissue fluid is formed at the arterial end of the capillaries where high blood pressure forces fluid out through the thin walls of the capillaries into spaces between the cells. The tissue fluid contains oxygen, glucose and amino acids. It does not contain protein molecules, because they are too large to pass through the capillary walls. Cells absorb oxygen, glucose and amino acids by diffusion and active transport from the tissue fluid. Carbon dioxide and nitrogenous waste diffuse from cells into the tissue fluid at the venous end of the capillaries. Not all fluid which leaves the capillaries is returned to the bloodstream. Some filters into small lymph capillaries and become lymph. This is again a lot of new information. If you found it difficult, read it again until you understand it properly. Let us continue with the lymphatic system and its functions.

Lymphatic system

The lymphatic system is a system of tiny tubes which end blindly among tissues. The lymph vessels form a network throughout the body. Unlike the blood, the lymph system is a one-way street draining lymph from tissue and returning it to the blood. These lymph capillaries are very permeable. Because they are not pressurized, the lymph fluid can drain easily from the tissue into the lymph capillaries. Lymph means clear water and it is basically the fluid and protein that have been squeezed out of the blood (i.e. blood plasma). The lymph is drained from the tissue in microscopic blind-ended vessels called lymph capillaries.

Functions of the lymphatic system

The lymphatic system works in close cooperation with other body systems to perform the following important functions.

·        It works with the circulatory system to deliver nutrients, oxygen and hormones to the cells that make up the tissues of the body.

·        It removes excess fluid, waste, debris, dead blood cells, pathogens, cancer cells and toxins from these cells and the spaces between them.

·        It collects protein molecules created within the cells and return these proteins to the bloodstream. Because the molecules are too large to pass through the capillaries of the circulatory system, they must be transported through the lymphatic system until they return to the bloodstream.

·        It aids the immune system in destroying pathogens and filtering waste so that the lymph can be safely returned to the circulatory system.

• It transfers substances between tissues and blood.

• It returns plasma to the blood.

• It absorbs fat to discharge it to the blood.

• Destruction of bacteria by phagocytes.

• Antibodies are produced by the  lymphocytes.

Now let us look at the functions of blood.

Functions of blood

Blood has some important functions. Let us discuss each in turn.

Transport

Blood transports oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. It transports dissolved food substances (glucose, fats and amino acids) from the small intestine (ileum) to various parts of the body. Blood carries waste products from the liver to the kidneys and skin. Blood carries hormones from the endocrine glands to the tissues on which they act.

Temperature regulation

Blood distributes heat from active organs (liver, muscles) to the rest of the body. By variation in the amount of blood flowing to the skin, the amount of heat lost from the body can be changed according to the body temperature and external conditions.

Maintaining a constant internal environment

Blood temperature, osmotic pressure, acidity and amounts of substances such as sugar, hormones and salts are kept at constant levels. The maintenance of steady conditions is called homeostasis. Thus, the body tissue is supplied with a fluid of constant composition. You will learn more about homeostasis in Section 3.

Healing and prevention of infection

By clotting, blood prevents fluid from being lost from cuts and wounds. It prevents infection by healing wounds and destroying invading pathogenic microbes.

How blood flows in the body

Have you ever noticed bluish “tubes” underneath your skin on your arms? These are tubes that carry blood. These tubes are called blood vessels.

Do you know how these vessels carry blood around the body? Blood cannot flow through the body by itself. It is pumped through the blood vessels by the heart. Blood leaving the heart travels through blood vessels called arteries. Blood returning to the heart goes through blood vessels called '''veins. Very thin blood vessels, called capillaries, '''connect the arteries to the veins. It is the capillaries which supply cells with glucose, amino acids, oxygen and other substances.

Blood vessels 

Arteries

These are thick-walled structures which carry blood away from the heart.

The diagram below shows a cross-section of an artery.

Veins

Blood from capillaries run into minute venules (a small vein) and then into the veins. Veins always carry blood to the heart.

The diagram below shows a cross-section of a vein

The table below shows the differences between arteries and veins

Capillaries

Capillaries are small, thin-walled blood vessels. Blood capillaries connect arteries to veins. When an artery enters an organ of the body, it branches into small arteries called arterioles, which in turn branch into many capillaries.

The capillaries join to form venules. The thin walls of capillaries are permeable, to allow water and dissolved substances such as oxygen, carbon dioxide and dissolved food to pass in and out.

The diagram below shows a capillary circulation with an organ.

The heart

Have you seen the heart of any animal? If you have not seen one yet, I suggest you visit an abattoir and ask the staff there to show you the heart of any animal. The heart of an adult person weighs about 300 g. The heart is made up of cardiac muscle. Glucose and oxygen are supplied to the heart muscles by coronary arteries. Coronary veins take away CO2 and other waste products.

The diagram below shows the heart of a human as seen from the outside.

You must constantly refer to this diagram when you read the content that follows.

Your heart is situated in the thoracic cavity. The main organ of the circulatory system is the heart. It never stops working. Even when you are at rest or asleep, your heart still beats. The heart beat of a normal person is 70 beats a minute.

The diagram below shows a longitudinal section of a heart.

You have to read this section more than once, as it contains a lot of new information.

The heart consists of four chambers. The two upper chambers are called the atriums and the two chambers below are ventricles. Both atriums and ventricles have muscular walls which contract and pump blood. The left ventricle which pumps oxygenated blood all round the body is the most muscular. The right and left sides of the heart are separated by a septum.

I hope that you are coping with all these new terms. It is important that you understand them all. Please read the information more than once if you find it difficult. Let us continue with the working of the heart.

How the heart pumps blood

Deoxygenated blood (blood without oxygen) from the body enters the right atrium of the heart from the vena cavae. Look again at the diagram and make sure you see the vena cavae.

• The left and right atriums are connected with the left and right ventricles through an opening.

• The opening is controlled by a valve which allows blood to flow only from the atrium to the ventricle.

• The valve between the left atrium and left ventricle is the mitral or bicuspid valve.

• The valve between the right atrium and right ventricle is the tricuspid valve.

• The right ventricle pumps blood to the lungs through the pulmonary artery.

• The pulmonary vein brings oxygenated blood (blood that contains oxygen) from the lungs into the left atrium.

• Oxygenated blood passes from the left atrium through the bicuspid valve to the left ventricle.

• The left ventricle pumps oxygenated blood out into the aorta.

Do you see the aorta on the diagram? The aorta carries oxygenated blood to the different parts of the body. In the pulmonary artery and aorta, semilunar valves are found which, together with the bi- and tricuspid valves, prevent the blood from flowing backwards. The muscle of the atrium and the ventricle is supplied with food and oxygen by coronary arteries. The heart pumps blood by contracting. The heart becomes smaller so blood gets squeezed out into the arteries. Then, the heart relaxes again and fills up with blood from the veins.

When the heart is relaxed, we speak of the diastole phase. During this phase the atriums and ventricles relax. Deoxygenated blood from the body enters the right atrium through the vena cavae. Oxygenated blood from the lungs enters the left atrium through the pulmonary veins. As a result, the heart fills up with blood.

When the heart contracts, we speak of the systole phase. During this phase, the atriums contract forcing blood into the ventricles through the bi- and tricuspid valves.

In the systole phase, the ventricles contract and pump blood into the arteries
Ventricular systole

The ventricles contract and the valves between the atriums and the ventricles close. As a result, the blood from the left ventricle is pumped along the aorta to the organs of the body. Blood from the right ventricle is pumped along the pulmonary artery to the lungs and forced out of the heart. One complete relaxation and contraction is called a heartbeat.

The pulse

The heart pumps blood round the body.

The sudden expansion of the arteries as blood enters from the heart is transmitted as a pressure wave. This can be felt as a pulse at a pressure point. The pressure point is where an artery lies near the skin. The artery can be pressed against a bone to feel the pulse.

Diseases of the circulatory system

The heart needs a good supply of food and oxygen. It gets these through its own blood supply carried in the coronary arteries. When the arteries become blocked, the heart will not get sufficient blood supply. This blockage is called a heart attack. When a small part of the coronary arteries gets blocked, the heart attack is mild.

When the blockage is large, the heart does not get enough oxygen and the heart attack is severe. In this case, a person may die. How do arteries get blocked? A fatty substance called cholesterol, sometimes makes the arteries stiff and even blocks them. As a result, blood stops flowing through the arteries. This is called artherosclorosis. If this happens in an artery that takes blood to the heart muscles, a heart attack known as thrombosis can occur. This is because the thickened walls have reduced the diameter of the artery and it has become blocked by a blood clot. The heart muscle receives less blood which means less food and oxygen. It eventually stops working. Stress and tension of modern life, over-eating and drinking too much alcohol may cause high blood pressure. This puts an extra strain on the heart and may lead to heart failure.

''Can you think of other factors that can increase your chances of suffering from heart disorders? Write your answer in your notebook.''

They include:

•      lack of exercise;

•      overweight or obesity;

•      smoking cigarettes;

•      eating too much fatty food stuffs like ice-cream, butter;


 * eggs, fried food, fatty meat, etc

•      stress;

•      sex and age;


 * heredity and
 * other diseases like diabetes mellitis, etc.

Can you think of anything we can do to decrease the chances of getting a heart attack?

You know that blood is supplied to all parts of the body through the circulatory system. This blood is pumped from the heart. The heart provides the power to drive the blood through blood vessels. It is vital for your health that you keep the circulatory system in good working order.

How can you achieve this? This can be done by leading a healthy lifestyle. To keep pumping, your heart needs food and oxygen. Your heart gets these substances from the coronary arteries that supply food and oxygen to the heart.

When these arteries become blocked, the heart cannot function properly and this may result in heart diseases.

You can avoid this by:

• using less fat in your diet;

• eating food which has less cholesterol;

• eating less red meat which contains fat;

• eating less dairy products, like butter, cream, full cream milk, etc.;

• eating less eggs, especially egg yolks;

• eating more fresh fruit and vegetables;

• not smoking;

• exercising regularly; and

• taking time to relax.

Excretion and water balance
The removal of waste products is important. This is done by an organ system called the excretory system. The process of regulating and maintaining water balance is another function in our body. You will learn about the functions of water and the regulation of the water content in the body in this section.

You know that excess amino acids cannot be stored in the body. Your cells respire and produce waste materials such as carbon dioxide and water.

''Think carefully and explain how these waste products are removed from your body. Write your ideas in your notebook.''

We can say that excretion is the removal of chemical waste products produced during metabolic reactions.

Excretory products in human beings

Your body excretes water, carbon dioxide and nitrogenous compounds.

Carbon dioxide and water are formed as a result of tissue respiration in your body. The word equation is as follows:

glucose + oxygen carbon dioxide + water + energy.

Sources of nitrogenous waste

Nitrogenous compounds come from excess or unusable protein in a diet and from the breakdown of damaged and dead cells. This protein is broken down into amino acids and deaminated by the liver. The amino group is removed from the amino acids and converted into glucose and urea. Most waste products containing nitrogen are toxic (poisonous) and must therefore be removed quickly and efficiently.

The human excretory system

Study the diagram and make sure you know all labelled parts.

The kidneys

Kidneys play an important role in the excretory system. There are two kidneys in your body. You know that unused proteins are changed into amino acids which are converted into urea. Urea produced in the liver, is carried by the blood. As blood enters the kidneys through the renal artery, it carries urea and oxygenated blood.

The kidneys produce a solution of urea in water called urine. The renal vein takes deoxygenated blood away from the kidneys. The kidneys are connected to the bladder by means of the ureter. The ureter carries urine to the bladder. The bladder is drained by the urethra, which opens directly outside in females and through the penis in males. On a hot day or after doing exercise, the volume of urine decreases and it has a yellowish colour.

Composition of urine - normal person

• proteins 0%

• urea 2%

• uric acid 0.05%

• ammonia 0.05%

• water 95%

• chloride 0.6%

• other substances 1.8%

•      Did you know?

•      Only a small portion of kidney tissue is used at a given   time. As a result, a person can live with just one kidney.

•      More than a litre of blood is filtered through each   kidney every minute of the day.

Let us look at two other excretory organs.

Excretion by the skin and other excretory substances (e.g. mineral salt) that leave the body as sweat.

Excretion by the lungs When you exercise, you sweat a lot. Sweat contains waste products. Sweat is produced in the sweat glands. A sweat gland is a coiled tube and lies in the dermis but opens on the skin’s surface into a sweat pore. The sweat gland is well supplied with blood capillaries and water

We already talked about the breathing system in section 1. You may remember that during expiration the breathing system removes carbon dioxide and water vapour. The lungs are excretory organs, because they help your body get rid of carbon dioxide. Carbon dioxide and water vapour are waste products of cell respiration. It diffuses out of the blood into the air in the alveoli of the lungs and is breathed out with expired air. Carbon dioxide in the lungs is poisonous and must therefore be removed from the lungs.

Maintaining water balance

Water is constantly lost from and gained by the body. Do you agree?

Water is lost from the body through urine, sweat, expired breathed air and faeces.

When do you feel thirsty? You feel thirsty when you do hard work or a lot of exercise. Thirst is an indication that your body needs water. Your blood becomes too concentrated when you do not drink enough water. This is detected by special cells in your brain that secrete a water retention hormone. This hormone passes through the blood stream to the kidneys and alerts them to reabsorb more water into the blood stream. More concentrated urine is formed. In this way the concentration of blood is maintained.

You take in water by drinking tea, water, coffee, juices, soft drinks and milk. You also take in water by eating watery foodstuffs like fruit and vegetables. Your body also makes a little water as a by-product of tissue respiration.

Your blood becomes diluted when you drink water. Some of the water moves through the kidneys to the bladder. Less water retention hormones are secreted and diluted urine is formed. Water is removed as urine. Your bladder is full and you feel the need to urinate. In this way your body regulates its water balance. You keep many things constant inside your body. Examples are body temperature and the level of glucose in the blood. The process of keeping the internal environment (body fluid) constant is called homeostasis.

Feedback control of water retention hormone

The flow diagram above explains how the water retention hormone works.

Why do you need a water balance?

Your body needs a water balance, because the amount of water in body fluids must be kept steady. When the amount of water taken in by the body is not sufficient, the body will soon become dehydrated. To carry out vital functions, there must be enough water in the body. When the amount of water is too little, dehydration takes place in cells.

Nervous system and hormonal control
When you run say a 100 metre race, many things happen in your body. While you are running, your muscles contract and relax so that the running action is smooth and efficient. During the race you breathe rapidly and your heart beats faster. After the race your heartbeat and breathing rate return to normal. All the activities need to be controlled and co-ordinated. This is done by two systems, the nervous system and the hormone system. In this section we will discuss the nervous system and hormonal control.

The nervous system

In this section, we will look at how the nervous system receives and interprets stimuli and coordinates activities in your body. Your body is able to detect stimuli around it and react to any changes. This ability is called sensitivity.

Sensitivity allows you to get food and avoid danger. This is the job of the nervous system. The nervous system is composed of the central nervous system (brain and spinal cord), the peripheral nervous system which has nerves connected to receptors (sense cells) and effectors (muscles and glands). The nervous system allows coordination between stimulus and response.

The parts of the nervous system

The diagram shows the main parts of the central nervous system

A. The central nervous system

The central nervous system is composed of the brain and the spinal cord.

The central nervous system contains grey and white matter. The grey matter contains cell bodies and blood vessels while the white matter consists of nerve fibres. The brain and the spinal cord need to be protected. The brain is found inside the skull and the spinal cord in the backbone. The delicate tissues of the brain and spinal cord are enclosed within tough protective membranes.

These membranes secrete cerebrospinal fluid, which helps to protect the brain and spinal cord by acting as a shock absorber.

The brain is a large delicate organ, made up of three parts: the cerebrum (made up of two cerebral hemispheres), the cerebellum and the medulla oblongata.

The functions of the parts of the brain are as follows.

•      The cerebrum is the centre of intelligence and consciousness.

•      The cerebellum co-ordinates the working of the muscles for balance, for posture and movement.

•      The medulla oblongata controls breathing, heart rate, blood flow and temperature.

•      The spinal cord is an extension of the brain. It is protected by the vertebral column. It contains spinal nerves which go to all parts of the body.

Functions of the spinal cord

The spinal cord is an extension of the brain. It is protected by the vertebral column. It contains spinal nerves which go to all parts of the body.

B. The peripheral nervous system

The peripheral nervous system consists of nerves that connect the central nervous system with all parts of the body. The nerves are made up of thousands of long thin nerve fibres. The nerves provide the central nervous system with information about what happens inside and outside the body.

Now, let us study the nerve cells.

Nerve cells

The nervous system is made up of nerve cells called neurons. Each neuron consists of a cell body which gives rise to numerous branching processes. Processes that conduct impulses away from the cell body are called axons. Processes which conduct impulses towards the cell body are called dendrites.

Three types of neurons which differ in function and in shape are described and shown below.

Sensory neurons

Sensory neurons conduct impulses from sensory cells(receptors) to the brain or spinal cord. Their dendrites are connected to sensory cells. The cell body is some distance away from the sensory cells.

Motor neurons

Motor neurons conduct impulses from the brain or spinal cord to the muscles and glands (effectors). The cell body of the motor neuron is in the brain or spinal cord and it has a long axon.

Intermediate neurons

Intermediate neurons connect the sensory neurons to the motor neurons. The intermediate neurons are found in the brain or spinal cord. Intermediate neurons have many short dendrites which collect messages from other neurons in the brain or spinal cord.

The junction between two neurones is called a synapse. There are millions of these synapses in the nervous system. They act like miniature switching stations, where the arrival of one impulse causes the start of another impulse in the next neurone. The synapses form the basis of the mechanisms of the nervous system.

This again, is a lot of new information. Did you find it easy to understand? If not, please read the information again.

The functions of the peripheral nervous system

Here is a list of the functions of the peripheral nervous system.

•      It conducts impulses rapidly from sensory cells to the brain or spinal cord and from there to the organs that respond.

•      It coordinates the activities of various parts of the body into a common action e.g. playing a guitar or sewing a dress.

•      It controls or directs the activities of different parts of the body.

Reflex arc and reflex action (function of spinal cord)

The simplest form of reaction in the nervous system is reflex action. This is a rapid involuntary response to a stimulus and is not under conscious control of the brain. A response that we cannot control, is called involuntary action.

The basic path of a reflex arc is shown below in the order in which the action is produced.

Some other examples of involuntary responses are:

•      when a hungry person smells food, the salivary glands secrete saliva;

•      the  heart beating and

•      breathing and blinking.

Voluntary action

Voluntary actions involve conscious thought. Voluntary actions are controlled by the cerebral cortex in the brain and not by the spinal cord. Can you think of and write down some examples of voluntary and involuntary responses in your notebook.

Does your answer include the following?

Some examples of voluntary responses are:

• smiling and waving your hand when seeing your friend;

• jumping over a thorny bush;

• running away from a running bull;

• holding a falling toddler and

• catching a book falling from a bookshelf.

Think of other examples of voluntary responses.

Many things influence the normal functioning of the nervous system negatively. Two examples are alcohol and drugs. Let us study these influences.

Alcohol and drug abuse

People of all walks of life drink alcohol. The way we use alcohol affects our lives. In most communities in Namibia a variety of alcoholic drinks are available in shebeens and bottle stores. The type of alcohol contained in many drinks is ethanol. It is a drug which is absorbed through the stomach.

Alcohol can affect the nervous system if it is misused. Drinking large quantities of alcohol is sedative.

Excessive consumption of alcohol has many long-term and short-term effects e.g.,

•      inhibition of nerve impulse conduction;

•      slow reflexes which lead to dangerous driving and sometimes death;

•      depression of the visual centre of the brain, leads to poor coordination and as a result accidents may happen;

•      depression of the breathing centre of the brain may cause death;

•      lack of appetite causes malnutrition;

•      irritation of the lining of the stomach causes vomiting;

•      conversion products to the liver causes hardening of the lining of the liver passages and

•      destruction of sperm leads to infertility.

The word drug is used to describe any chemical substance that changes the way the body works. Medicines are useful drugs that improve the health of people or that help fight diseases. Medical drugs kill microbes. Drugs are sometimes not used correctly or are used for other purposes and may be harmful. We say that drugs are abused. Many drugs are illegal, because of their risk to our health. These include hard drugs like cocaine and heroin and soft drugs like cannabis and ecstasy.

When a person takes a drug regularly, he or she becomes dependent on it. Then, he or she becomes addicted. Hard drugs such as heroin and cocaine are particularly addictive. Once the body has become used to it, trying to go without it causes painful withdrawal symptoms. Addiction may alter a person’s lifestyle and relationships with others. Continued abuse of drugs can irreversibly damage the body.

The diagram below shows how people abuse alcohol.

The effects of drugs on the body

Some people in Namibia use drugs like dagga and mandrax. As we have mentioned, dagga affects the nervous system. Mandrax is a sleeping pill that has a calming effect. When mandrax is taken for a long time, the nervous system may be damaged. It can even cause death.

Drugs cause many social problems. People who use drugs regularly may experience behavioural problems. Society may not accept them. They may become anti-social, irresponsible and use abusive language. They sometimes cannot cope with their work or education.

When a person takes drugs regularly, he/she may end up taking an overdose of the drug.

This will lead to death. Mixing alcohol and other drugs is very dangerous. The combined effect damages the heart and a person may even die.

Hormonal control

Imagine that you have hit the jackpot and won 1 million Namibia dollars. You will definitely be excited and very happy. Do you know what makes you so excited?

This is due to hormonal activities in your body. In this section we will look at what hormones are and what they do in our bodies. Hormones are chemicals produced in glands which control the activities of the body.

You know how the nervous system works. The hormone system and the nervous system coordinate and help different parts of the body to work together. Hormones produced in glands, are released into the bloodstream. The blood carries the hormones through the body to specific cells or organs. Hormones take longer to move through the body than an impulse from the nervous system.

The diagram shows the position of glands that produce hormones.

The growth hormone

The growth hormone is secreted by the pituitary gland during childhood. If the pituitary gland does not work normally, growth is affected. A person with too little growth hormones becomes a dwarf. A person with too many of the hormone may suffer from gigantism.

Some important applications of hormones.

Let us study the hormones secreted by the different glands in the body.

Thyroxine

The thyroxine hormone is secreted by the thyroid glands. Thyroxine stimulates the metabolic rate. In children it is essential for growth. A common cause of thyroxine deficiency is lack of iodine in the diet, because iodine is an important constituent of thyroxine. When the body lacks iodine, the person suffers from goitre.

Insulin

The pancreas produces a hormone called insulin. Insulin reduces the level of glucose in the blood. Insulin promotes the absorption of glucose from the blood by the cells of the body. Diabetes mellitus is an illness caused by insufficient insulin produced in the pancreas. Diabetes is treated by giving patient insulin injections daily. In some patients it can be controlled by a special diet or tablets.

Adrenaline

The adrenaline glands produce adrenaline. This hormone is called the fight or flight hormone, because it prepares the body for action. It speeds up heartbeat and breathing and raises the blood pressure. It allows more glucose to be absorbed into the blood to give you energy.

Finally, we will look at the difference between nerve and hormonal control since both are responsible for coordination.

Sense organs

Why do you quickly pull your hand away when you accidentally touch a hot plate? It is because your sense organs tell you to do so. You receive information about changes as stimuli around you through your sense organs.

Can you think of some sense organs? Have you mentioned the eyes, ears, tongue, skin and the nose? You use these organs to detect stimuli from the environment. The picture below shows some of the sense organs.

When a sense organ detects a stimulus such as sound or light, it sends messages along nerve cells to the brain. A nerve is a special cell and the brain is an organ.

The sense organs have nerve cells which are able to detect stimuli called receptors. The stimuli are changed into nerve impulses. The skin is the largest sense organ that responds to a range of stimuli such as touch, pressure, heat, pain or cold. The eye is a sense organ that responds to the stimulus of light. The ear responds to sound, the nose to smell and the tongue to the stimulus of taste.

Let us look at the importance of the sense organs.

The eye

Close your eyes for a few minutes and think of problems faced by those who are blind. You will realise how important your eyes are. They are located in sockets in the skull. The skull helps to protect them from damage. You will learn more about the skull in the next section. The eyes are held in place by muscles. These let you move your eyes from side to side and up and down and also to rotate the eye.

The diagram below shows a cross-section of an eye.

How you see things

When you look at something (an object like a person, a pencil, etc.), light goes from the object to your eye like this:

An upside-down picture of the object is formed on a layer called the retina at the back of your eye.

Look at the diagram of the eye again and try to locate the retina to help you understand the process better. The retina contains light-sensitive nerve cells. It sends nerve impulses (messages) along the optic nerve to your brain. The brain interprets the message as vision and you see the object the right way up.

The ear

The ear is a sense organ sensitive to sound waves, the orientation and movement of the head. The ears give you a sense of hearing and balance.

The sense of hearing is important to humans and all living organisms. It warns us against danger and helps us to communicate. You have one ear on each side of the head. This helps you tell where sound comes from. Ears change vibrations in the air into nerve impulses which travel to the brain where they are interpreted as sound.

Are you still enjoying studying this part?

I would like you to do the following self-mark activity to check your progress so far.

Assignment
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Feedback ===

{| class="prettytable" {| class="prettytable"
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&lt;center&gt;
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Assessment 2&lt;/center&gt; &lt;/center&gt; 1. oxygen, glucose, amino acids, carbon dioxide, urea and hormones

2. The white blood cells engulf and digest pathogenic microorganisms.

3.

a) bone marrow

b) red, white

c) bicon cave

d) nucleus, haemoglobin

e) oxygen, haemoglobin, oxyhaemoglobin

f) tissues

g) carbon dioxide

4. a)

i)

1 – left atrium

2 – biscupid valve

3 – tendon

4 – left ventricle

ii)

A – aorta

B – pulmonary artery

C – pulmonary vein

D – vena cava vein


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{| class="prettytable" {| class="prettytable"
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&lt;center&gt;
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Assessment 4&lt;/center&gt; &lt;/center&gt; 1. The central nervous system is made up of the brain and spinal cord

2. The body parts must work together, therefore coordination is necessary

3. A nerve is a collection of neurons.

4.

a) – (ii)

b) – (iii)

c) – (i)

d) – (iv)

5. When a person takes alcohol and drugs regularly, the person becomes dependent on it.

6. causes the brain to shrink, dizziness, unnecessary mistakes

7.

·        Sound waves

·        Light

·        Chemicals (Smell/odour)

·        Chemicals (taste)

·        Temperature, pain, touch


 * }

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Optional Multimedia Resources
Internet Resources

Diagram of the respiratory system:



Animation of breathing:

http://www.biotopics.co.uk/humans/inhexh.html

http://www.smm.org/heart/lungs/breathing.htm

Read more about the importance of breathing:

http://www.drstandley.com/bodysystems_respiratory.shtml

http://serendip.brynmawr.edu/sci_edu/waldron/pdf/BreathingLabProtocol.doc

Click the following links to do an enrichment activity on breathing:

http://www.lessonplanspage.com/SciencePELungs-TheBreathingMachine25.htm

http://www.galaxy.net/%7Ek12/body/lungs.shtml

Additional information on the effect of pollution on breathing:

http://www.ehow.com/how-does_4564272_smoking-affect-lungs.html

Different illustrations of the circulatory system:





View the types of white blood cells:

http://www.unomaha.edu/hpa/blood.html

Learn more about the plasma:

http://www.purchon.com/biology/plasma.htm

Learn more about the exchange of substances between blood and tissue:

http://www.msss.sd8.bc.ca/HOMEWORK/Knapik/capillary%20tissue%20exchange.doc

View images of the lymph system:



http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph3.html

View the animation of a heartbeat:

http://www.ahealthyme.com/Imagebank/heart.swf

For more information on how to take your pulse:

http://www.cchs.net/health/health-info/docs/0900/0984.asp?index=5508

CD-ROM Resources

Audio - the dangers of smoking on lungs

Video - human body system

Interactive activities - activities on hyperlinks

Animation - body processes