O-Level Excretion in Humans

The two processes are ultrafiltration and selective reabsorption. In ultrafiltration, blood enters the glomerulus under high pressure (because the afferent arteriole is wider than the efferent arteriole). This forces small molecules, water, glucose, salts and urea out through the capillary walls into the Bowman's capsule, while large molecules such as plasma proteins and blood cells stay in the blood. In selective reabsorption, as the filtrate flows along the kidney tubule, useful substances are reabsorbed back into the blood: all the glucose and some salts are reabsorbed (glucose by active transport) and water is reabsorbed by osmosis. The remaining liquid, containing water, urea and excess salts, forms urine.

During the marathon much water is lost as sweat, so the blood becomes more concentrated (lower water potential). (1) Osmoreceptors in the hypothalamus detect the fall in blood water potential. (2) The pituitary gland releases more ADH into the blood. (3) ADH makes the walls of the collecting ducts (and distal tubules) more permeable to water. (4) So more water is reabsorbed from the filtrate back into the blood. (5) A smaller volume of more concentrated urine is produced, conserving water and raising the blood water potential back to normal.

After heavy sweating, water is lost from the blood so the blood water potential falls (becomes more negative/concentrated). This is detected by osmoreceptors in the hypothalamus. The pituitary gland releases more ADH (antidiuretic hormone) into the blood. ADH travels to the kidneys and makes the walls of the collecting ducts (and distal tubules) more permeable to water. More water is therefore reabsorbed from the filtrate back into the blood by osmosis, raising the blood water potential back to normal. Because more water is reabsorbed, the urine produced is smaller in volume and more concentrated (contains a higher concentration of solutes such as urea/salts, but the same amount of solute in less water).