Control of blood water potential - AL only (3.6.4.3)
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Mammals have two kidneys that are found next to the spinal cord in the back of the abdominal cavity.
The kidney can be broken down into three regions:
- Outer region: cortex.
- Inner region: medulla.
- Centre: pelvis, leading to the ureter.
The nephron is responsible for filtering blood and producing urine. Several processes occur at different regions of the nephron.
There are several blood vessels that are key to nephron function:
- The renal artery carries oxygenated blood containing urea and other substances into the kidney.
- This branches into the afferent arteriole, which carries blood into the glomerulus at high pressure.
- The glomerulus is a network of capillaries that are surrounded by the Bowman’s capsule, where ultrafiltration occurs.
- Blood leaves the glomerulus through the efferent arteriole and is carried to other capillaries around the tubule.
- Finally, blood drains into the renal vein, which carries filtered blood away from the kidney.
Selective reabsorption occurs in the proximal convoluted tubule.
Sodium ions are actively transported from epithelial cells into the blood, lowering the sodium ion concentration inside the cells. Sodium ions then diffuse from the filtrate into the epithelial cells via co-transporter proteins carrying glucose with them.
Glucose leaves the cells and moves into the blood by facilitated diffusion. Under normal conditions, all glucose is reabsorbed in the proximal convoluted tubule.
Water is reabsorbed by osmosis due to the lower water potential of the blood.
Cells lining the proximal convoluted tubule are specialised and adapted for selective reabsorption:
- Their cytoplasm contains many mitochondria to provide ATP for active transport.
- They possess microvilli to increase the surface area for reabsorption.
- Their membranes contain numerous co-transporter proteins for the uptake of glucose, amino acids and sodium ions from the filtrate.
Following selective reabsorption in the proximal convoluted tubule, the filtrate is mainly water, urea and excess ions.
The loop of Henle maintains a gradient of sodium ions in the medulla, creating a low water potential. The descending limb is permeable to water, so water leaves the filtrate by osmosis. The ascending limb is impermeable to water, and sodium and chloride ions leave the tubule, lowering the water potential of the medulla.
Water is reabsorbed in the distal convoluted tubule and collecting duct by osmosis. ADH increases the permeability of the walls of the distal convoluted tubule and collecting duct, allowing more water to be reabsorbed into the blood. This produces more concentrated urine.
In the Bowman’s capsule, ultrafiltration forms the glomerular filtrate.
Small molecules such as water, glucose, ions, and urea are forced out of the blood, while proteins and blood cells remain in the capillaries.
Blood flows through the glomerulus and due to the afferent arteriole being wider than the efferent arteriole, hydrostatic pressure in the glomerulus is higher than in the Bowman’s capsule.
This pressure forces water and small molecules, such as glucose, ions, urea and amino acids, out of the blood and into the Bowman’s capsule. Large proteins and blood cells are too large to pass through the filtration barrier and remain in the blood.
The filtration barrier between capillaries and the Bowman’s capsule are adapted to enable ultrafiltration to occur:
- Capillary endothelium is fenestrated (has pores) for blood plasma and dissolved substances to pass through.
- Basement membrane is a fine mesh, acting like a sieve, to ensure large molecules do not pass through.
- Podocytes (specialised cells) in the Bowman’s capsule epithelium have filtration slits between them that allow small molecules to pass into the Bowman’s capsule lumen and prevent large molecules from passing through.
Osmoregulation is the control of the water potential of the blood.
It maintains a stable balance of water and dissolved substances, preventing cells from gaining or losing excessive water by osmosis. This helps prevent cells from bursting or shrinking and maintains normal blood volume, blood pressure and cell function.
A high water potential (high concentration of water) means water is more likely to move out of the blood down its concentration gradient by osmosis. A low water potential means water is less likely to move into the blood by osmosis.
The hypothalamus contains osmoreceptors that detect changes in the water potential of the blood. It controls the release of ADH (antidiuretic hormone) from the posterior pituitary gland.
If blood water potential is too low, more ADH is released, increasing the permeability of the distal convoluted tubule and collecting duct to water. More water is reabsorbed into the blood, producing a smaller volume of concentrated urine.
If blood water potential is too high, less ADH is released, reducing water reabsorption and producing a larger volume of dilute urine.