The kidney

The kidney
The kidneys are organs that filter waste products out of the blood and excrete then e.g. urea along with water as urine.
Urea is made by the liver. Excess amino acids can’t be stored therefore the liver breaks them down to urea.(ornithine cycle)
OSMOREGULATION: The homeostatic mechanism of the active regulation of the osmotic pressure of bodily fluids to maintain the homeostasis of the body’s level of water, minerals and salts in the blood.
NITROGENOUS EXCRETION: excretion of nitrogen in urine.

The kidney is involved with the homeostatic functions of regulating the chemical properties of the blood. (Water, solvent concentration and pH)

When the blood passes through the kidney nephrons liquid is filtered out of the blood, carrying solutes with it, including urea. Useful nutrients are selectively reabsorbed and the wasted products are removed in urine.
There are two main stages of kidney function:

1) ULTRAFILTRATION
This occurs in the barrier between the blood and the filtrate in the Bowman’s capsule. In the Bowman’s capsule there is a dense network of capillaries called the glomerulus, blood flows into the capillaries.
A glomerulus is enclosed in the sac. Fluids from blood in the glomerulus are collected in the Bowman's capsule (i.e., glomerular filtrate) and further processed along the nephron to form urine. This process is known as ultrafiltration.
A high pressure caused by the blood moving into the capillaries via the afferent wide arteriole and moving out through the narrow efferent arteriole. The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron. This type of high pressure filtration is ultrafiltration.
Blood is forced against the capillary endothelium which has pores in it which allow the blood plasma to reach the basement membrane and small molecules are squeezed about but large ones like protein and blood cells stay in.

2) Selective reabsorption (proximal tubule)
Selective reabsorption takes place in the proximal convoluted tubule (PCT) of the kidney. Blood leaving the Bowman’s capsule has a low hydrostatic pressure and so flows slowly. It has a high concentration as these could not pass out before. It therefore has a low water potential and this enables it to gain water from the proximal tubule. It is the process by which certain substances that are required by the body (such as glucose, amino acids, vitamins and water) but have been filtered out of the blood during ultrafiltration are reabsorbed. As only certain substances are reabsorbed, it is known as selective reabsorption.

substances secreted into the distal convoluted tubule are: h+ ions and potassium ions.
substances actively reabsorbed from the proximal convoluted tubule are sodium ions and glucose.
the kidney tubule has adaptation such as lots of mitochondria and a brush border for atp for the active reabsorption of molecules and a large surface area for absorption.
overview: sodium ions are actively pumped out, glucose reabsorbed, amino acids reabsorbed and chloride reabsorbed, 50 % urea reabsorbed

ADH makes the collecting duct more permeable to water. So more water is taken back into the body by osmosis. more water is taken back into the blood stream and less is lost in urine. It is secreted by the posterior pituitary gland. Osmoreceptors detect changes in the hypothalamus.

Urine: composition varies due to diet and climate.