Control of blood water potential

In a nutshell

Control of the water potential of the blood is overseen by the kidneys. Kidneys contain specialised structures called nephrons which enable transfer of molecules from the bloodstream into urine. Hormones are released into the bloodstream to keep the water potential of the blood constant at times of hydration and dehydration.

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; Homeostasis; KS5 Year 12; Control of blood water potential

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	Stores 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; Homeostasis; KS5 Year 12; Control of blood water potential — 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.

Hormone control

Water potential is regulated by the hormone antidiuretic hormone (ADH). ADH is released at the posterior pituitary gland. The hypothalamus has osmoreceptors which detect changes in blood water potential.

Dehydrated

When blood water potential drops, the hypothalamus detects this and the posterior pituitary gland releases ADH. ADH induces changes in the DCT and collecting duct that make their membranes more permeable to water.

This means more water is reabsorbed into the blood through osmosis. At a dehydrated state the kidneys release a small amount of highly concentrated urine.

Hydrated

When blood water potential is low, the hypothalamus instructs the posterior pituitary gland to stop ADH release. Less ADH in the bloodstream means the DCT and collecting duct are less permeable to water.

This means less water is reabsorbed into the blood through osmosis. In a hydrated state, the kidneys release a large amount of dilute urine.