Urine Formation (essay)

hydrostatic pressure pushes fluid through the fenestrations of the glomerulus and filtration slits of the podocytes of the glomerular capsule. Only blood cells and proteins are held back by the basement membrane and produces the filtrate, 180 liters a day

Reabsorption starts at the proximal convoluted tubule (PCT). 65% of Na+ and water of the filtrate are reabsorbed here along with other needed substqance such as glucose, amino acids, K+. The microvilli of the PCT facilitate this. Na+ enters the cell at its luminal surface by facilitate diffusion. The gradient favoring diffusion is maintained by active transport of Na+ out of the basolateral side of the cell. The Na+ then diffuse into the peritubular capillaries. Cl- follows Na+. Water follows also and enters the peritubular capillaries

Reabsorption in the Loop of Henle is 10% of the water of the filtrate is reabsorbed here by the countercurrent multiplier system. Na+ is actively pumped out of the ascending limb and results in the medulla having a high osmolality. This high concentration of Na+ draws water out of the descending limb which is permeable to water but not salt. The more concentrated salt solution rounding the bend now results in more active transport of Na+ out of the filtrate. This Na+ pulls more water out from the descending limb. This is the multiplier effect.

under the influence of aldosterone Na+ is reabsorbed here in exchange for K+. As a result, just about all the filtered sodium is reclaimed which is very important for maintaining adequate blood pressure.

25% of the filtered water remains. This is reabsorbed in variable amounts related to the body's state of hydration due to ADH. ADH, released from the posterior pituitary gland, stimulates the addition of water pores in the collecting duct cells. water leaves the filtrate by osmosis through these pores due to the high concentration of Na+ in the interstitial fluid of the medulla, set up by the loop of Henle. some substances are added to the filtrate by secretion. K+, H+, and some drugs are removed from the blood this way.

the opposite of reabsorption and usually involves active transport. The substances are removed form the peritubular capillaries, go through the tubule cells and into the filtrate. Mostly secretion occurs in the PCT.