Homeostasis and the Kidney: MDCAT Biology notes

Osmoregulation

Osmoregulation is the regulation of solutes and of the gain and loss of water between an organism and its environment. In humans the kidneys do this, along with excretion of nitrogenous waste. The kidneys weigh less than 1% of body mass yet receive about 20% of the cardiac output.

The nephron

Each kidney has about a million nephrons. Blood enters the glomerulus (a knot of capillaries inside the cup-shaped Bowman's capsule) by the wide afferent arteriole and leaves by the narrower efferent arteriole. The efferent arteriole then forms peritubular capillaries around the tubule; in juxtamedullary nephrons it also forms the vasa recta, capillary loops alongside the loop of Henle.

Filtrate path: Bowman's capsule → proximal convoluted tubule (PCT) → loop of Henle (descending and ascending limbs) → distal convoluted tubule (DCT) → collecting duct → renal pelvis → ureter.

Nephron type	Position	Loop of Henle
Cortical	Mostly in cortex	Short
Juxtamedullary	At the cortex-medulla border	Long, deep into medulla; builds the osmotic gradient and conserves water when water is short

Steps of urine formation

1. Pressure filtration in the glomerulus: water, glucose, amino acids, salts and urea pass into Bowman's capsule. Blood cells, platelets and large proteins such as albumin stay in the blood.
2. Selective reabsorption: most useful substances, all glucose and amino acids, and most water and $Na^+$ are reabsorbed in the PCT. Further water and $Na^+$ are reabsorbed in the collecting duct under hormonal control.
3. Tubular secretion: the DCT actively secretes $H^+$ (regulating blood pH), $K^+$ and some drugs into the filtrate.

Loop of Henle: countercurrent multiplier

The descending limb is permeable to water; the ascending limb is impermeable to water and actively pumps out $Na^+$ and $Cl^-$. Flow in opposite directions multiplies the salt concentration in the medulla. Urea diffusing out of the lower collecting duct further raises the gradient of the inner medulla. This gradient allows water to be drawn out of the collecting duct, so the loop of Henle is what makes hypertonic urine possible.

Hormonal control

• ADH (antidiuretic hormone, vasopressin) is made in the hypothalamus and released from the posterior pituitary. When water is lost (e.g. dehydration), blood osmotic pressure rises; osmoreceptors in the hypothalamus detect this and ADH is released. ADH makes the collecting duct permeable, so water is reabsorbed and a small volume of hypertonic urine is produced. After drinking excess water, ADH falls and large volumes of dilute urine are passed. ADH deficiency causes diabetes insipidus.
• Aldosterone from the adrenal cortex activates the sodium-potassium pumps of the distal tubule, increasing $Na^+$ reabsorption into the plasma.
• The kidney also helps control blood calcium by reabsorbing $Ca^{2+}$ and activating vitamin D.

Kidney disorders

• Kidney stones: calcium oxalate stones are linked with oxalate-rich foods such as leafy vegetables; hyperparathyroidism raises blood calcium (hypercalcaemia) and leads to calcium phosphate stones. Treatment includes lithotripsy.
• Kidney failure: end-stage renal failure leads to uraemia, accumulation of urea in blood; treated by dialysis or transplant.

Common MDCAT traps

• ADH gives hypertonic, low-volume urine; lack of ADH gives dilute, high-volume urine.
• ADH is released by the posterior pituitary, but osmoreceptors are in the hypothalamus.
• The ascending limb lets salt out but not water.
• Glomerular capillaries rejoin into the efferent arteriole, not the collecting duct.
• Juxtamedullary, not cortical, nephrons build the medullary gradient.

Quick revision

• Glucose is filtered; albumin and RBCs are not.
• PCT reabsorbs most water and $Na^+$.
• DCT secretes $H^+$ to control pH.
• Aldosterone: $Na^+$ reabsorption.
• Countercurrent multiplier is in the loop of Henle.