Kidney function
Ultrafiltration occurs in the glomerulus, where the efferent arteriole is narrower than
the afferent arteriole, creating high hydrostatic pressure. This pressure forces water
and small solutes out of the blood and through the capillary wall, the basement
membrane, and the epithelium of the Bowman's capsule, where podocytes are
found. This liquid, now called glomerular filtrate, contains water, ions, glucose,
amino acids, and urea but excludes large molecules like proteins and blood cells
due to the filtering role of the basement membrane.
The filtrate then moves into the proximal convoluted tubule (PCT), where selective
reabsorption occurs. Glucose and amino acids are co-transported with sodium ions,
while water is reabsorbed by osmosis. PCT has microvilli and folds to increase
surface area. Sodium ions are actively pumped out of the PCT cells into the blood by
the Na⁺/K⁺ pump, creating a sodium ion gradient. Sodium ions then move back
into the epithelial cells of the PCT via co-transporter proteins, bringing glucose
and amino acids along with them, against their concentration gradients.
(Selective reabsorption broadly occurs throughout the nephron, AQA tends to
emphasize osmoregulation and ADH for Loop of Henle, DCT and collecting duct)
In the ascending limb, sodium and chloride ions are actively transported out into
the medulla, reducing the water potential of the medulla. The AL is impermeable to
water, so no water leaves the filtrate. As filtrate reaches the Loop of Henle, water is
reabsorbed via osmosis into the surrounding medulla, which has a low water
potential created by the ascending limb (AL). The descending limb (DL) is
permeable to water but impermeable to ions. Because of water lost in the DL
caused by low water potential in the medulla caused by AL, urine becomes more
concentrated, as it reaches the bottom of the loop, the ions diffuse out of the AL.
The ion transport in the ascending limb ensures that the interstitial fluid in the
medulla is highly concentrated. This concentration gradient is maintained and
increases deeper into the medulla. The gradient allows water to be reabsorbed from
the filtrate in the collecting duct via osmosis.
Osmoregulation
When blood water potential is low (e.g., dehydration), osmoreceptors in the
hypothalamus detect this as they lose water and shrink/shrivel via osmosis. This
stimulates the posterior pituitary gland to release antidiuretic hormone (ADH)
into blood. ADH binds to receptors on the distal convoluted tubule (DCT) and
collecting duct, triggering vesicles containing aquaporins to fuse with the cell
membrane. This increases the permeability of the DCT and collecting duct to water,
so more water is reabsorbed into the blood by osmosis. This produces a smaller
volume of concentrated urine.
Ultrafiltration occurs in the glomerulus, where the efferent arteriole is narrower than
the afferent arteriole, creating high hydrostatic pressure. This pressure forces water
and small solutes out of the blood and through the capillary wall, the basement
membrane, and the epithelium of the Bowman's capsule, where podocytes are
found. This liquid, now called glomerular filtrate, contains water, ions, glucose,
amino acids, and urea but excludes large molecules like proteins and blood cells
due to the filtering role of the basement membrane.
The filtrate then moves into the proximal convoluted tubule (PCT), where selective
reabsorption occurs. Glucose and amino acids are co-transported with sodium ions,
while water is reabsorbed by osmosis. PCT has microvilli and folds to increase
surface area. Sodium ions are actively pumped out of the PCT cells into the blood by
the Na⁺/K⁺ pump, creating a sodium ion gradient. Sodium ions then move back
into the epithelial cells of the PCT via co-transporter proteins, bringing glucose
and amino acids along with them, against their concentration gradients.
(Selective reabsorption broadly occurs throughout the nephron, AQA tends to
emphasize osmoregulation and ADH for Loop of Henle, DCT and collecting duct)
In the ascending limb, sodium and chloride ions are actively transported out into
the medulla, reducing the water potential of the medulla. The AL is impermeable to
water, so no water leaves the filtrate. As filtrate reaches the Loop of Henle, water is
reabsorbed via osmosis into the surrounding medulla, which has a low water
potential created by the ascending limb (AL). The descending limb (DL) is
permeable to water but impermeable to ions. Because of water lost in the DL
caused by low water potential in the medulla caused by AL, urine becomes more
concentrated, as it reaches the bottom of the loop, the ions diffuse out of the AL.
The ion transport in the ascending limb ensures that the interstitial fluid in the
medulla is highly concentrated. This concentration gradient is maintained and
increases deeper into the medulla. The gradient allows water to be reabsorbed from
the filtrate in the collecting duct via osmosis.
Osmoregulation
When blood water potential is low (e.g., dehydration), osmoreceptors in the
hypothalamus detect this as they lose water and shrink/shrivel via osmosis. This
stimulates the posterior pituitary gland to release antidiuretic hormone (ADH)
into blood. ADH binds to receptors on the distal convoluted tubule (DCT) and
collecting duct, triggering vesicles containing aquaporins to fuse with the cell
membrane. This increases the permeability of the DCT and collecting duct to water,
so more water is reabsorbed into the blood by osmosis. This produces a smaller
volume of concentrated urine.
