Blood enters the kidneys through the renal artery and then passes through capillaries in the cortex (outer layer) of the kidneys. As blood passes through capillaries in the cortex, substances are filtered out of the blood into long tubules that surround the capillaries in a process called ultrafiltration. Useful substances, like glucose and water, are reabsorbed back into the blood, while unwanted substances, like urea, pass along to the bladder where they are excreted as urine in a process called selective reabsorption.
The steps to ultrafiltration are:
1) Blood from the renal artery enters into the smaller afferent arterial in the cortex of the kidney. This arterial splits into a glomerulus, which is a bundle of capillaries looped inside a structure called a Bowman’s capsule. This is where ultrafiltration takes place.
2) The efferent arterial takes blood away from the glomerulus, and has a smaller radius than the afferent arterial, so the glomerulus is put under high pressure.
3) The high hydrostatic pressure forces out ions, H2O, glucose, and urea from the capillaries through fenestrations. This filtered liquid is called glomerular filtrate and leaves the glomerulus via the efferent arteriole.
The steps the selective reabsorption are:
1) The epithelium of the wall of the PCT has microvilli to help the reabsorption of the useful substances back into the blood from the glomerular filtrate, via active transport and diffusion.
2) Glucose is reabsorbed back into the blood initially by diffusion, but when the concentration of glucose is higher in the blood than the glomerular filtrate, glucose is actively transported into the blood against its concentration gradient.
3) 50% of urea is reabsorbed unintentionally because the molecule is very small. As a result, urea can diffuse directly through cell surface membranes into the blood.
4) The volume of water that is reabsorbed into the blood depends on the water potential value in the blood.
The water potential value will be detected by osmoreceptors in the hypothalamus in the brain. These receptors will then send a signal to the pituitary gland. The pituitary gland then releases more of the hormone ADH (if there isn’t much water in the blood), which binds to receptors in the plasma membrane of the DCT and collecting duct, so there are more aquaporins. As a result, the plasma membrane will become more permeable to H2O, so more H2O is absorbed back into the blood by osmosis.
If there’s a lot of water in the blood, the high water potential value will be detected by osmoreceptors in the brain which would then send a signal to the pituitary gland. Therefore the pituitary gland will release less ADH. As a result, the plasma membrane of the DCT and collecting duct will be less permeable to H2O, and thus less H2O will be absorbed back into the blood by osmosis.