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Urine formation is taught in the note for Regulation in Mammals. Read it below and look for the section on this topic.

Regulation in Mammals

14 min read

Feedback mechanisms

Homeostasis in mammals is maintained by negative feedback: a receptor detects a change from the set point and a control centre sets off a response which reverses the change. Positive feedback makes a change bigger, as in the contractions of childbirth.

Negative feedbackA loop that keeps a body condition near a set point. A receptor detects a change, a control centre such as the brain compares it with the set point, and an effector acts to bring the condition back. When the condition returns to normal, the correcting action switches off. Body temperature, blood sugar and water balance are controlled this way.Set point (e.g. 37 °C)Receptor detects the changeControl centre (brain)Effector actsCondition returns to normaltoo hot or too cold?sweat or shiver

The kidney and urine formation

Urine is made in the nephron in three steps. In ultrafiltration the high pressure in the glomerulus forces small molecules (water, glucose, urea, salts) into Bowman’s capsule, but not blood cells or large proteins. In selective reabsorption in the proximal tubule all the glucose, amino acids and most of the water and salts are returned to the blood. In the loop of Henle and the distal tubule more water and salts are adjusted. The final fluid is urine.

A nephronThe filtering unit of the kidney. Blood enters the glomerulus, a knot of capillaries inside Bowman’s capsule, where water, glucose, salts and urea are filtered out under pressure. In the proximal tubule all the glucose and much water are reabsorbed. The loop of Henle and distal tubule adjust water and salts, and the collecting duct carries urine to the ureter.Bowman's capsuleglomerulusproximal tubuleloop of Henledistal tubulecollecting duct → urineblood inFiltered: water, glucose,salts, ureaReabsorbed: all glucose,most water and salts

Osmoregulation

Osmoregulation is the control of the water potential of the blood. If the blood is too concentrated (after sweating or too little water), the hypothalamus detects it and the pituitary gland releases ADH (antidiuretic hormone). ADH makes the collecting ducts more permeable to water, so more water is reabsorbed and a small amount of concentrated urine is made. If the blood is too dilute, less ADH is released and a large amount of dilute urine is made.

ConditionADHCollecting ductUrine
DehydratedHighMore permeableSmall volume, concentrated
Plenty of waterLowLess permeableLarge volume, dilute
As ADH rises, less urine is made.
Try itOsmoregulation

Choose how much water the body has lost. The blood concentration rises, ADH is released, and urine volume falls.

Check yourself Which structure filters the blood in the nephron?
Check yourself What does ADH do?
Check yourself Which substance is not found in urine of a healthy person?

Key terms

Ultrafiltration
Filtration under pressure in the glomerulus.
Selective reabsorption
Taking useful substances back into the blood.
ADH
A hormone that controls water reabsorption.
Osmoregulation
Control of the water potential of the blood.

Summary

  • Negative feedback keeps conditions near a set point.
  • Urine is formed by ultrafiltration, reabsorption and adjustment in the tubule.
  • ADH controls how much water is reabsorbed.
  • Dehydration gives little concentrated urine; plenty of water gives much dilute urine.
Feedback mechanisms Osmoregulation