The kidney produces twoof its own. The first, erythropoetin, is a hormone produced by specialized within the kidney substance. The hormone has an effect on the bone marrow, and stimulates this tissue to produce new red . Patients with some types of anaemia have an increased level of erythropoetin in their circulation. Chronic lung disease, in which the contains low levels of oxygen, also leads to secretion of erythropoetin. The key stimulus to the secretion of the hormone is lack of oxygen in the blood flowing to the kidney. High levels of this hormone can lead to an overproduction of red blood , a condition called polycythaemia. The second hormone is renin, which is produced by a small cluster of cells in the kidney known as the juxtaglomerular apparatus.
It is secreted when blood pressure falls in the renal artery. This hormone sets off a series of reactions which lead to the kidney retaining more salt and water. This increases the volume of blood and tends to raise blood pressure.