The function of kidney

The kidneys main functions are to filter the blood of toxins and control the water balance of the body. To do this the kidneys filter the blood. They are very efficient at doing this so that there is a need to reabsorb useful material that is filtered back into the blood. Both of these processes are carries out by the nephrons.

Filtration It is the Bowman’s capsule and glomerulus that carry this out. The blood in the glomerulus is at high pressure and so a process of ultra filtration occurs. The pressure is high in the glomerulus since the afferent vessel is a direct branch from the renal artery. In addition the efferent vessel is narrower than the afferent vessel which increases the pressure. The basement membrane of the capillary acts as the dialysing membrane (ie it only allows small molecules through)

AFFERENT VESSEL EFFERENT VESSEL The basement membrane goes around the capillary

The filtrate The filtered blood is referred to as the filtrate All of the filtrate moves into the tubule of the nephron through the spaces between the podocytes This filtrate contains lots of materials that now need to be reabsorbed The body produces around 180 litres of filtrate a day, most of this is reabsorbed

Filtrate moves into the tubule through the spaces between the podocytes

Composition of the filtrate Filtered blood therefore contains water, glucose, ions and urea There are no cells or protein since they are too big too be filtered

Reabsorption Most of the water is reabsorbed in the PCT (70%) This is not variable Sodium ions and glucose are reabsorbed in the PCT by active transport, the water follows by osmosis All of the glucose is reabsorbed in the PCT The rest of the filtrate travels through the loop of Henle into the DCT and the collecting duct Most of the remaining sodium is reabsorbed during this journey The remaining water is selectively reabsorbed depending on the water potential of the blood

Loop of Henle To reabsorb the remaining water a diffusion gradient has to be set up between the filtrate in the DCT and collecting duct and the surrounding tissue. This is achieved by the loop of Henle. The loop of Henle operates a counter current multipier system.

Counter current multiplier The two arms of the loop flow in opposite directions Each arm has different permeabilities to water and sodium The result is a build up of sodium ions at the apex of the loop This produces a very negative water potential that draws water from the collecting ducts

Direction of flow Movement of water by osmosis Na + pumped out actively Na + moves out passively by diffusion Passive movement of Na +

Increase in concentration of ions and therefore more negative water potential

Direction of flow Increase in concentration of ions and therefore more negative water potential Passive movement of Na + Movement of water by osmosis Na + moves out passively by diffusion Na + pumped out actively

The collecting ducts These have variable permeability to water Their permeability is controlled by the hormone ADH The presence of ADH increases the permeability by opening pores ADH is stored in the pituitary gland Release of ADH is controlled by the hypothalamus

Hypothalamus This monitors the blood’s water potential If the body is lacking water then the water potential of the blood becomes more negative The hypothalamus causes the pituitary to release ADH The ADH opens pores in the collecting duct so water moves out due to the effect of the loop of Henle This water is reabsorbed into the blood stream If the body has too much water then the blood’s water potential will be less negative No ADH is released More water is lost in the urine

Urine If ADH is produced then a small amount of concentrated urine is produced If ADH is not produced then a large amount of dilute urine is produced The difference in colouration of the urine is caused by the concentration of pigments that are the break down products of haemoglobin

Composition of urine Main ingredient is water Urea Minerals (sodium, potassium, chloride) Normally no glucose since this is all reabsorbed in the PCT

This is the end of this section