Homeostasis

• Excess amino acids in the diet are deaminated (in the liver), producing urea, which enters the blood circulatory system to be transported to the kidneys.

• The kidney is composed of about a million small tubules, called nephrons, organised within distinct layers - the outer cortex and the inner medulla.

• Each nephron is supplied with blood vessels, which form a knot of capillaries, the glomerulus, enclosed by the cup-shaped Bowman's capsule, and which surround all other parts of the nephron.

• Kidneys regulate the internal environment by consantly adjusting the composition of the blood. They are organs of excretion and osmoregulation.

• Ultrafiltration takes place from the glomerulus into the Bowman's capsule.

• Small molecules are filtered through, by the basement membrane of the glomerulus endothelium, under high hydrostatic pressure.

• The glomerular filtrate is hypotonic to the blood as it lacks plasma proteins

Blood entering the glomerulus (from the afferent arteriole) has a high hydrostatic pressure because:

• the renal arteries are wide, short and close to the heart
• the afferent arteriole is wider than the efferent, creates a bottleneck effect
• coiling of capillaries in the glomerulus further restricts blood flow, increasing pressure

The filter - 3 layers that separate plasma from filtrate:

• Capillary endothelium - single layer of squamous cells with pores
• Basement membrane - EFFECTIVE FILTER, determines which components of blood enter the Bowman's capsule
• Inner layer of Bowman's capsule - podocytes - with foot-like processes which surround capillaries but have spacious gaps - filtration slits

• In the proximal convoluted tubule, glucose, amino acids and some ions are actively transported back into the blood during selective reabsorption.

• Water follows by osmosis and the filtrate becomes isotonic to the blood.

• The loop of Henle creates hypertonic conditions in the medulla, giving a decreasing water potential gradient from the boundary with the cortex to the inner medulla. This enables water to be withdrawn from the collecting ducts if they are permeable to water.

• Before the filtrate passes down the collecting duct, it travels through the distal convoluted tubule where further ions are actively reabsorbed.

• Antidiuretic hormone (ADH) makes the wall of the distal convoluted tubule and collecting ducts permeable to water by causing water channel proteins, aquaporins, to open.

• ADH is produced in the hypothalamus and stored in the pituitary gland from where it is secreted into the blood when hypothalamic osmoreceptors detect that the blood water has fallen below the norm.

• ADH enables water to be reabsorbed through the tubule walls so that hypertonic urine is produced.

When blood is too dilute....

The blood develops a higher solute potential (less concentrated)

Detected by osmoreceptors in the hypothalamus

Inhibits ADH release

Walls of DCT and collecting ducts become less permeable

Less water reabsorbed into blood

Large quantities of dilute (hypotonic) urine produced

When blood is too concentrated...

Solute potential of the blood becomes more negative

Detected by osmoreceptors in hypothalamus

Walls of DCT and collecting duct become more permeable

More water is reabsorbed into bloodstream

Solute potential of blood returns to normal

Smaller volume of more concentrated urine produced