The Kidney and Osmoregulation
The Kidney: Structure and Function
The kidneys are the primary organs of excretion (removal of metabolic waste) and osmoregulation (control of water potential of body fluids). Each kidney contains approximately one million nephrons — the functional units responsible for filtering blood and producing urine.
Gross Structure of the Kidney
A longitudinal section through the kidney reveals three regions:
- Cortex — the outer region; contains the Bowman's capsules, proximal and distal convoluted tubules, and the upper parts of the loops of Henle and collecting ducts
- Medulla — the inner region; contains the loops of Henle and collecting ducts; divided into pyramids
- Pelvis — a funnel-shaped cavity where urine collects before passing to the ureter
Blood enters via the renal artery and leaves via the renal vein. Urine drains through the ureter to the bladder.
The Nephron
Each nephron consists of:
1. Bowman's capsule (renal capsule) — a cup-shaped structure surrounding a knot of capillaries called the glomerulus
2. Proximal convoluted tubule (PCT) — the first coiled section
3. Loop of Henle — a hairpin loop that descends into the medulla and returns to the cortex
4. Distal convoluted tubule (DCT) — the second coiled section
5. Collecting duct — carries urine from several nephrons through the medulla to the pelvis
Ultrafiltration
Ultrafiltration occurs in the Bowman's capsule. Blood enters the glomerulus via the afferent arteriole (wider) and leaves via the efferent arteriole (narrower). The difference in diameter creates high hydrostatic pressure in the glomerulus, forcing small molecules out of the blood and into the capsule.
What is filtered (forms the glomerular filtrate):
- Water, glucose, amino acids, urea, inorganic ions, vitamins
What is NOT filtered (too large to pass through):
- Blood cells, platelets, plasma proteins (e.g. albumin)
The filtration barrier consists of three layers:
1. Capillary endothelium — has fenestrations (pores) that allow plasma through but retain blood cells
2. Basement membrane — a fine mesh that acts as the main molecular filter, retaining proteins
3. Podocytes — specialised cells of the Bowman's capsule with finger-like pedicels that wrap around capillaries, leaving filtration slits between them
The glomerular filtration rate (GFR) in humans is approximately 125 mL/min (~180 litres/day). Clearly, most of this must be reabsorbed — otherwise we would lose our entire blood volume in minutes.
Selective Reabsorption
Most of the filtrate is reabsorbed in the proximal convoluted tubule (PCT):
| Substance | % Reabsorbed in PCT | Mechanism |
|---|---|---|
| Glucose | ~100% | Na⁺-glucose cotransport (secondary active transport): Na⁺/K⁺ ATPase on the basal membrane pumps Na⁺ out, creating a gradient; Na⁺ re-enters the cell from the lumen via cotransporters, dragging glucose with it. Glucose exits the cell into blood by facilitated diffusion |
| Amino acids | ~100% | Similar cotransport mechanisms to glucose |
| Water | ~65% | Osmosis — follows the reabsorption of solutes; the PCT is very permeable to water |
| Na⁺ | ~65% | Active transport (Na⁺/K⁺ ATPase on basal membrane) |
| Urea | ~50% | Passive diffusion (concentration rises in tubule as water is reabsorbed) |
Adaptations of PCT cells for reabsorption:
- Microvilli (brush border) — increase surface area for absorption
- Many mitochondria — provide ATP for active transport
- Basal infoldings — increase surface area of the basal membrane for Na⁺/K⁺ ATPase
- Tight junctions between cells — prevent uncontrolled leakage of substances back into the tubule
The Loop of Henle: Creating the Medullary Gradient
The loop of Henle creates a gradient of increasing solute concentration (decreasing water potential) in the medulla through a countercurrent multiplier mechanism. This gradient is essential for the production of concentrated urine in the collecting duct.
The Countercurrent Multiplier
Descending limb:
- Permeable to water (aquaporins in the membrane)
- Impermeable to ions
- As filtrate descends into the increasingly concentrated medulla, water leaves by osmosis
- The filtrate becomes progressively more concentrated
Ascending limb:
- Impermeable to water
- Actively pumps Na⁺ and Cl⁻ out of the filtrate into the medullary tissue (thick ascending limb)
- The thin ascending limb allows some passive diffusion of Na⁺ out
- The filtrate becomes progressively more dilute as it ascends
The result is a solute concentration gradient in the medulla — lowest near the cortex, highest deep in the medulla. This gradient is maintained by the countercurrent flow (descending and ascending limbs running in opposite directions) and by the vasa recta (straight capillaries that loop alongside the loop of Henle), which remove reabsorbed water and solutes without dissipating the gradient.
Urea recycling also contributes to the medullary gradient — urea diffuses out of the collecting duct (in the presence of ADH) into the deep medulla, increasing the solute concentration.
The Collecting Duct and Water Reabsorption
As filtrate passes through the collecting duct, which descends through the hypertonic medulla, water can be reabsorbed by osmosis — but only if the duct wall is permeable to water. The permeability of the collecting duct is controlled by antidiuretic hormone (ADH).
Osmoregulation: The Role of ADH
Osmoregulation is the control of the water potential of body fluids. It is regulated by a negative feedback mechanism involving ADH.
When Water Potential of Blood Decreases (e.g. Dehydration, Salty Food)
1. Osmoreceptors in the hypothalamus detect the decrease in water potential (the cells shrink as water leaves by osmosis)
2. The hypothalamus stimulates the posterior pituitary gland to release more ADH into the blood
3. ADH travels in the blood to the collecting ducts and DCT of the kidneys
4. ADH binds to receptors on the collecting duct cells, triggering the insertion of aquaporin water channel proteins into the cell membranes (via exocytosis of vesicles containing aquaporins)
5. The collecting duct becomes more permeable to water
6. More water is reabsorbed by osmosis from the collecting duct into the hypertonic medullary tissue, and then into the blood via the vasa recta
7. A small volume of concentrated urine is produced
8. Blood water potential returns to normal → negative feedback reduces ADH release
When Water Potential of Blood Increases (e.g. Drinking Lots of Water)
1. Osmoreceptors detect the increase in water potential
2. Less ADH is released from the posterior pituitary
3. Fewer aquaporins are inserted into collecting duct membranes (existing aquaporins are removed by endocytosis)
4. The collecting duct becomes less permeable to water
5. Less water is reabsorbed
6. A large volume of dilute urine is produced
7. Blood water potential returns to normal
Kidney Failure and Treatment
When kidneys fail, waste products accumulate and water/ion balance is disrupted. Treatments include:
Dialysis:
- Haemodialysis — blood is passed through an artificial kidney machine containing a partially permeable membrane. Dialysis fluid on the other side has normal plasma concentrations of glucose and ions but no urea, so urea diffuses out of the blood. Typically 3 sessions/week, 4-5 hours each.
- Peritoneal dialysis — dialysis fluid is introduced into the abdominal cavity; the peritoneum acts as the dialysis membrane. Can be performed at home.
Kidney transplant:
- A donor kidney (from a living or deceased donor) is surgically implanted
- Advantages: restores normal function; no dietary restrictions; better quality of life
- Challenges: requires immunosuppressant drugs (risk of infection); rejection risk; organ shortage
Exam Tips
- AQA commonly asks you to explain how the loop of Henle creates the medullary gradient — describe the properties of each limb and the countercurrent multiplier clearly
- For ADH, always state the full pathway: stimulus → osmoreceptors → hypothalamus → posterior pituitary → ADH → aquaporins → effect on water reabsorption
- Remember that ADH does NOT make the medullary gradient — it controls how much water is reabsorbed from the collecting duct using the gradient that already exists
- When describing ultrafiltration, mention all three layers of the filtration barrier and explain why proteins stay in the blood