23.2: Filtration and Urine Formation

The function of the kidneys is to filter, reabsorb, secrete, and excrete. Every day the kidneys filter nearly 180 liters of blood, initially removing water and solutes but ultimately returning nearly all filtrates into circulation with the help of osmoregulatory hormones. This process removes wastes and toxins but is also crucial to maintain water and electrolyte levels. Most of these functions are performed by the tiny but numerous nephrons contained within the kidneys.

Blood enters the renal corpuscle of the nephron through a glomerulus of capillaries. The capillaries are surrounded by a structure called the Bowman’s capsule which absorbs water and most solutes from the blood. The blood pressure from capillaries pushes these into the capsules. If the blood pressure is too high, as seen in hypertension, the capillaries can weaken and harden, reducing the ability of the kidney to filter the blood.

The filtrate from the corpuscles empty into the proximal convoluted tubules and the descending portions of the Loop of Henle. Here nearly 70% of solutes—salt, glucose, amino acids, and bicarbonates—are reabsorbed into the surrounding capillaries. Circulating blood hormones involved in osmoregulation induce reabsorption of sodium, calcium, or more water if needed to increase or decrease blood pressure and regulate electrolytes.

Secretions from the vessels remove any remaining waste products from the blood into the distal convoluted tubules of nephrons. Nitrogenous waste like creatinine and urea is secreted as well as potassium and ammonium ions. To adjust blood pH, hydrogen and bicarbonate ions can also be removed into the distal tubules. From here the remaining filtrate, or urine, is collected by the renal pelvis and excreted from the kidneys through the ureter.

In hibernating animals, such as bears and ground squirrels, urine production is reduced or stopped altogether to conserve water during a period when no food or water is ingested. In their hypothermic state, renal vessels constrict and prevent the flow of blood into the glomerulus. This halts kidney function until the animal emerges from hibernation.

The kidneys filter the blood several times a day via a multi step process performed mostly in the nephron, the main functional unit.

Starting in the renal corpuscle water and most solutes are filtered from the capillaries in the glomerulus into the surrounding Bowman's capsule. The pressure in blood vessels forces the filtrate into the capsule.

In the proximal convoluted tubule, or PCT some solutes are reabsorbed into the capillaries based on blood chemistry such as glucose, bicarbonates, and amino acids.

As the filtrate descends the loop of Henle more water is reabsorbed into the blood via aquaporin channels. This increases blood volume and pressure. On its ascending journey sodium and chloride ions are also reabsorbed.

When the filtrate arrives in the distal convoluted tubule, or DCT more waste products like creatinine and urea from the blood are secreted. Potassium, ammonium, and hydrogen ions are also removed to help adjust blood pH and electrolyte content.

Final solute and water adjustments are made in the collecting tubule which then moves the filtrate, purified as urine for pooling in the renal pelvis. Upon leaving the kidneys urine moves to the bladder and is excreted.