Urine Formation

Urine Formation Urine Formation occurs as a series of three steps: Filtration Reabsorption Secretion

Filtration Filtration: the movement of fluids from the blood into the Bowman’s capsule of the nephron. Remembering that the glomerulus acts as a high-powered filter and has 1.5X the pressure of a normal capillary bed.

Filtration Dissolved solutes pass from the blood into the Bowman’s capsule along the pressure gradient, but not all solutes make it through.

Filtration Things that pass through the glomerulus include: ammonia, urea, water, sodium (Na+) and other cations, chloride (Cl-) and other anions, glucose, amino acids and protons.

Filtration Things that DO NOT pass through the glomerulus include: blood plasma proteins, Red Blood Cells, White Blood Cells and platelets.

Reabsorption Reabsorption is the transfer of essential solutes and most of the water back into the blood stream (peritubular capillaries). For every 120 ml of fluid, the amount of fluid filtered by your kidneys each minute, 1 ml of urine is formed and 119 ml of fluids and solutes are reabsorbed back into the bloodstream.

Reabsorption It occurs by both passive and active transport. Numerous mitochondria are in the kidney to help provide energy, but the energy only lasts for so long. Threshold level: when the maximum amount of substances have been reabsorbed and the energy is spent. Any solute leftover in the nephron when the energy is spent will get excreted in the urine.

Reabsorption

Reabsorption Glucose and amino acids are actively reabsorbed from the proximal tubule. Water is passively reabsorbed from the proximal tubule.

Reabsorption The cells surrounding the Loop of Henle have a high concentration of dissolved solutes (salt) which creates an osmotic gradient.

Reabsorption Water will then move out of the Loop of Henle and into the surrounding cells/bloodstream to try to balance the solute concentration. Therefore more water is reabsorbed passively in the descending Loop of Henle.

Counter-Current Multiplier System This refers to the process of concentrating the urine in the mammalian kidney. It is all based on the fact that the Loop of Henle consists of two parallel tubes and descends into the hypertonic medulla region of the kidney.

Counter-Current Multiplier System Water flows out (reabsorbed) from the descending limb of the Loop of Henle into the medulla. The ascending limb is impermeable to water, but here NaCl (salt) is actively reabsorbed into the medulla, effectively diluting the fluid left in the nephron.

Counter-Current Multiplier System This reabsorption of salt in the ascending limb, dilutes the urine and also allows for more water to be reabsorbed in the descending limb. This countercurrent flow multiplies the osmotic gradient between the fluid in the nephron and the medulla, thus also increasing the amount of water being reabsorbed.

Reabsorption As water gets reabsorbed, the solutes in the nephron become quite concentrated and some, like urea, will diffuse back into the bloodstream… but this is so small an amount that it is not a worry. Most will be left in the nephron and leave the body in the urine.

Reabsorption Water being absorbed in the proximal tubule (65%) and the descending Loop of Henle (25%) is said to be obligatory because it is due to osmosis. Water being reabsorbed in the collecting duct (10%) is said to be facultative because it is under the control of the anti-diuretic hormone (ADH).

Secretion Secretion is the release of the substances into the filtrate (urine) from the cells surrounding the nephron. Occurs in the proximal and distal tubules as well as the collecting ducts.

Secretion It is done to: Release any toxins/drugs that have not been filtered Maintain the electrolyte (Ion) balance of the body If certain cations are reabsorbed then others must be secreted in order to keep the balance Balance the acidity/alkalinity of the blood. It is usually an acid being secreted (proton containing compound) therefore urine is usually slightly acidic. Bicarbonate ion (HCO3-) is never secreted as it is integral in maintenance of blood pH. Acidic juices like cranberry juice cause our urine to increase in acidity which helps protect us from UTIs and prevent kidney stones.

The rest is Urine The remaining filtrate left in the collecting duct after filtration, reabsorption and secretion has occurred will travel to the renal pelvis, out the ureters and into the bladder for storage until the next urination event.