- a fibrous capsule - an outer membrane that protects the kidney
- a cortex - a lighter coloured outer region made up of Bowman's capsules convoluted tubules. and blood vessels
- medulla - a darker coloured inner region made up of loops of Henle, collecting ducts, and blood vessels
- renal pelvis - a funnel shaped cavity that collects urine into the ureter
- ureter - a tube that carries urine to the bladder
- renal artery - supplies the kidney with blood from the heart via the aorta
- renal vein - returns blood to the heart via the vena cava
The nephron is the functional unit of the kidney. It is a narrow tube with two twisted ends separated by a central hairpin loop. Each one is made up of a:
- Bowman's capsule - cup shaped and surrounds a mass of blood capillaries known as the glomerulus. The inner layer is made up of podocytes
- proximal convoluted tubule - a series of loops surrounded by blood capillaries. Made of thin epithelial cells which have microvilli
- Loop of Henle - a long hairpin loop that extends from the cortex into the medulla of the kidney and back again surrounded by blood capillaries
- distal convoluted tubule - a series of loops surrounded by blood capillaries. Made up of epithelial cells but is surrounded by fewer capillaries than the proximal convoluted tubule
- collecting duct - a tube which a number of distal convoluted tubules empty into. Lined by epithelial cells and widens as it empties into the pelvis of the kidney.
Blood vessels:
- afferent arteriole arises from the renal artery and supplies the nephron with blood
- glomerulus is a many branched knot of capillaries from which fluid is forced out the blood
- efferent arteriole is a tiny vessel that leaves the glomerulus
- blood capillaries is a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule from where they reabsorb mineral salts glucose and water. They merge together to form the renal vein.
The nephron works in 4 stages:
- formation of the glomerular filtrate by ultrafiltration
- the walls of the glomerular capillaries are made up of endothelial cells with pores between them. There is a build up of hydrostatic pressure as the diameter of the afferent arteriole is greater than of the efferent arteriole. As a result water/glucose/mineral ions are squeezed out of the capillary to form the glomerular filtrate. Large proteins/blood cells cannot pass across.
- The inner layer of the renal capsule is made up of highly specialised cells canned podocytes. These have gaps between their branches and the filtrate can pass between these cells.
- the endothelium of the glomerular capillaries have spaces up to 100nm wide
- reabsorption of glucose and water by the proximal convoluted tubule
- in the proximal convoluted tubule almost 85% of the filtrate is reabsorbed back into the blood. Due to ultrafiltration most small molecules are removed but most of these are useful so they get reabsorbed
- the proximal convoluted tubules are adapted to reabsorb substances by having epithelial cells that have:
- microvilli
- infoldings to give a large surface area
- a high density of mitochondria to provide ATP for active transport
- The process is as follows:
- sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away so the sodium concentration of these cells is lowered
- sodium ions diffuse down a concentration gradient from the lumen into the epithelial lining cells by carrier proteins
- these carrier proteins carry another molecule (glucose/amino acids/chloride ions) with them (co-transport)
- the molecule which have been co-transported into the cells of the proximal convoluted tubule diffuse into the blood
- maintenance of a gradient of sodium ions in the medulla by the loop of Henle
- The loop of Henle is responsible for water being reabsorbed from the collecting duct by concentrating the urine so that it has a lower water potential than the blood. The concentration of urine produced is directly related to the length of the loop of Henle. It has two regions: The descending limb (highly permeable to water), and the ascending limb (impermeable to water)
- The loop of Henle acts as a countercurrent multiplier:
- sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by mitochondria in the cells of its wall
- This creates a low water potential/high ion concentration in the region of the medulla between the two limbs (the intestinal region). The thick walls are impermeable to water so very little escapes the ascending limb.
- The walls of the descending limb are permeable to water so it passes out by osmosis into the interstitial space and enters blood capillaries
- the filtrate progressively loses water as it moves down the descending limb lowering its water potential
- at the base sodium ions diffuse out the filtrate and as it moves up the ascending limb these ions are also actively pumped out so the filtrate develops a progressively high water potential
- in the space between the ascending limb and the collecting duct there is a gradient of water potential with the highest in the cortex sand the lowest in the the further down into the medulla
- the collecting duct is permeable to water so as the filtrate moves down it water passes out by osmosis. This passes into blood vessels that occupy this space and is carried away
- As water passes out of the filtrate its water potential is lowered but the water potential is also lowered in the interstitial space so water continues to move out by osmosis down the whole length of the collecting duct. The countercurrent multiplier ensures that there is always a water potential gradient drawing water out of the tubule
- reabsorption of water by the distal convoluted tubule and collecting ducts
- the main role of the distal convoluted tubule is to make final adjustments to the water and salts that are reabsorbed and to control pH of the blood by selecting which ions to reabsorb. The homeostatic control of osmoregulation in the blood is achieved by a hormone that acts on the distal convoluted tubule and the collecting duct.
The body responds to a fall in water potential of the blood as follows:
- osmoreceptors in the hypothalamus (in the brain) detect a fall in water potential and water is lost from the osmoreceptor cells which causes the cell to shrink causing the hypothalamus to produce ADH (antidiuretic hormone)
- ADH passes into the pituitary gland from where it is secreted into the capillaries
- ADH passes into the blood to the kidney
- protein receptors on the cell surface membrane of these cells bind to ADH leading to the activation of an enzyme (phosphorylase). This causes vesicles within the cell to move to and fuse with the cell surface membrane
- These vesicles contain pieces of plasma membrane that have numerous water channel proteins (aquaporins) and the number of water channels is significantly increased when they fuse
- this increases the permeability of the collecting duct to urea which passes out lowering the water potential of the interstitial space
- this causes more water to leave the collecting duct by osmosis and reenter the blood
- this prevents the water potential of the blood from getting any lower
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