Urinary System

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This chapter first discusses homeostasis in the body and the mechanisms that maintain water balance. It then describes the organs of the urinary system and the formation of urine by the kidneys.


After completing this section, you should know:

  • the parts of the urinary system
  • the structure and function of a kidney
  • the structure and function of a kidney tubule or nephron
  • the processes of filtration, reabsorption, secretion and concentration that convert blood to urine in the kidney tubule
  • the function of antidiuretic hormone in producing concentrated urine
  • the composition, storage and voiding of normal urine
  • abnormal constituents of urine and their significance
  • the functions of the kidney in excreting nitrogenous waste, controlling water levels and regulating salt concentrations and acid-base balance
  • that birds do not have a bladder


  • The excretory system consists of paired kidneys and associated blood supply. Ureters transport urine from the kidneys to the bladder and the urethra with associated sphincter muscles controls the release of urine.
  • The kidneys have an important role in maintaining homeostasis in the body. They excrete the waste product urea, control the concentrations of water and salt in the body fluids, and regulate the acidity of the blood.
  • A kidney consists of an outer region or cortex, inner medulla and a cavity called the pelvis that collects the urine and carries it to the ureter.
  • The tissue of a kidney is composed of masses of tiny tubes called kidney tubules or nephrons. These are the structures that make the urine.
  • High-pressure blood is supplied to the nephron via a tuft of capillaries called the glomerulus. Most of the contents of the blood except the cells and large protein molecules filter from the glomerulus into the (Bowmans) capsule. This fluid flows down a coiled part of the tubule (proximal convoluted tubule) where useful substances like glucose, amino acids and various ions are reabsorbed. The fluid flows to a looped portion of the tubule called the Loop of Henle where water is reabsorbed and then to another coiled part of the tubule (distal convoluted tubule) where more reabsorbtion and secretion takes place. Finally the fluid passes down the collecting duct where water is reabsorbed to form concentrated urine.

Learning Activities

Library of Resources

Presentations and Blackboard quizzes can be accessed by students of Otago Polytechnic.