Final answer:
The (b) kidney is the primary source of erythropoietin (EPO), a hormone that stimulates the production of red blood cells in the bone marrow in response to low oxygen levels.
Step-by-step explanation:
The primary source of erythropoietin (EPO) is provided by the kidney. This hormone is produced by the interstitial fibroblast cells of the kidneys in response to low oxygen levels in the blood. EPO plays a crucial role in the body by stimulating the bone marrow to produce more erythrocytes, otherwise known as red blood cells (RBCs), which carry oxygen throughout the body.
EPO is also a part of the endocrine function of the kidneys. Around 85 percent of circulating EPO is produced by the kidneys; the liver provides the remainder. Its production increases under conditions such as moving to higher altitudes where oxygen pressure is lower, starting an aerobic exercise program, or when erythrocytes are lost due to bleeding or disease. The risks of erythropoietin doping include blood thickening, increased heart strain, and a higher risk of blood clots and subsequently heart attacks and strokes.
Failing kidneys can lead to a deficiency in EPO production, resulting in anemia, which can diminish performance and be life-threatening. This is because anemia makes it difficult for the body to cope with increased oxygen demands or to supply oxygen adequately even under normal conditions.