Kidney structure, function and adaptation

In a nutshell

The kidneys control the removal of waste products from the blood utilising specialised structures which enable filtration. The kidneys also control the blood water potential and adaptations in these systems can enable mammals to exist in contrasting environments.

Kidneys

The important jobs of the kidneys is to remove waste products like urea and to regulate the water potential of the blood. Regulation of water potential is called osmoregulation.

The kidneys are involved in the urinary system which is responsible for osmoregulation and removal of water products.

Biology; Excretion; KS5 Year 12; Kidney structure, function and adaptation

1.	Renal veins	Transports clean blood from the kidneys back into the bloodstream.
2.	Renal arteries	Transports blood from the bloodstream to the kidneys.
3.	Kidneys	Removes substances like urea from the blood through excretion and balances the water potential.
4.	Ureters	Transports urine from the kidneys to the bladder.
5.	Bladder	Store urine before excretion.
6.	Urethra	Transports urine from the bladder to outside the body.

Nephrons

Waste is removed from the bloodstream by long tubules called nephrons. This process is called filtration.

Biology; Excretion; KS5 Year 12; Kidney structure, function and adaptation — A nephron: A shows the cortex and B shows the medulla. 1. Blood capillaries; 2. Bowman's capsule; 3. Proximal convoluted tubule (PCT), 4. Distal convoluted tubule (DCT), 5. The collecting duct, 6. The loop of Henle.

Filtration

Filtration of the bloodstream takes place in the nephrons.

1.	Blood from the renal artery moves to afferent arterioles and then to capillaries in the cortex of the kidney. A bundle of capillaries in the Bowman's capsule is called the glomerulus and is where filtration takes place. The efferent arterioles that take filtered blood away from the glomerulus are smaller in diameter than the afferent arterioles meaning blood pressure in the glomerulus is high.
2.	The high blood pressure in the glomerulus forces small molecules and water molecules out of the blood and into the Bowman's capsule. The small molecules move through three cell membranes. These molecules are called the glomerular filtrate. The glomerular filtrate moves through the capillary wall, the basement membrane and finally through the epithelium of the Bowman's capsule.
3.	The glomerular filtrate now moves through the PCT, loop of Henle and DCT where selective reabsorption takes place. The wall epithelial of the PCT has microvilli that have a large surface area for reabsorption. Water and glucose are reabsorbed back into the blood. Glucose is actively transported back into the bloodstream using ATP.
4.	Water absorption takes place in the PCT, loop of Henle, DCT and collecting duct. The filtrate that remains after selective reabsorption is urine and usually contains water, dissolved salts, hormones and excess vitamins.

Osmoregulation

Water potential needs to be regulated for the body and cells to function properly. Water is lost constantly through excretion so the kidneys must keep the water potential at the same level.

The loop of Henle in the nephrons is responsible for osmoregulation as well as filtration. The two "limbs" of the loop of Henle control sodium ion potential to control the amount of water reabsorption. The filtrate moves down the descending limb first and then up the ascending limb.

1.	At the top of the ascending limb, sodium ions are actively transported into the medulla. This creates a low water potential in the medulla. The ascending limb is impermeable to water.
2.	This low water potential means water molecules move out of the descending limb and into the medulla. The descending limb is permeable to water. The water that moves into the medulla is reabsorbed into the bloodstream through the nephron capillary network.
3.	Water that moves up the ascending limb is reabsorbed into the blood by osmosis in the DCT and collecting duct.

Kidney adaptations

The kangaroo rat is a desert mammal. Conservation of water is critical to the survival of kangaroo rats and only a very small amount can be lost during excretion. Therefore, they has adaptations in its kidneys preventing excess water loss.

The kangaroo rat produces urine that is $20$ times more concentrated when compared to human urine. The kangaroo rat has very long loops of Henle. This multiplies the counter-current system.

The counter current system in the loop of Henle is the constant movement of ions out of the ascending limb, this maintains the water potential out of the descending limb along its whole length. The long loops of Henle also allow more water to be reabsorbed into the blood.