Homeostasis ccea

• Kidneys regulate the internal environment by constantly adjusting the compositon of the blood - organs of excretion and osmoregulation.
• Excretion - removal from the body of toxic waste products of metabolic processes. E.g mammals - CO2 produced during respiration is excreted from lungs. Kidneys excrete nitrogen containing compounds mostly urea, produced in the breakdown of excess amino acids annd nucleic acids in the liver. Kidneys also excrete a little creatinine (waste product produced from the degradation of creatine phosphate - molcule of major importance in ATP generation) in muscles. 
• Homeostasis - maintenance of steady states within the body. Kidneys have a homeostatic function in regulating the water content of blood. Kidneys control the water potential of body fluids (osmoregulation) under the influnece of antidiuretic hormome. 

Kidneys receive 25% of the cardiac output via the aorta and renal arteries. This shows the importance of their role in regulating the composition of the blood.

• There are two layers inside a kidney - the outer cortex and inner medulla, surrounding a central cavity the pelvis. The medulla is sub-divided into a number of pyramids. The kidney contains over one million microscopic tubules called nephrons, each of which have a rich blood supply.