Kidney

A kidney is one of two bean-shaped organs that filter the blood to remove waste products and excess water, forming urine. The kidneys also play crucial roles in maintaining the body's fluid and electrolyte balance, regulating blood pressure, and producing hormones important for red blood cell production and bone health.

• Each kidney contains about 1 million nephrons, which are the functional filtration units (see nephron). Blood enters each nephron's glomerulus where filtration occurs, and the filtrate is refined through the nephron's tubules into urine.
• Key hormones produced by the kidneys: erythropoietin (EPO), which stimulates red blood cell formation in bone marrow, and renin, which initiates the renin-angiotensin-aldosterone system to help regulate blood pressure. (The kidneys also activate vitamin D to its calcitriol form.)
• Kidney function is often measured by glomerular filtration rate (GFR). Healthy adult kidneys filter roughly 180 liters of blood plasma per day, yet about 99% of the filtrate is reabsorbed, leaving 1-2 liters of urine output in a day.

• A classic context is a patient with chronic kidney disease presenting with anemia - this is due to reduced erythropoietin production by failing kidneys. Recognizing the kidney's endocrine function can be key to such questions.
• Questions may also address the RAAS: e.g., low blood pressure or low sodium delivery to the kidney triggers renin release from the juxtaglomerular cells - expect to identify the kidney's role in blood pressure regulation.
• Renal physiology questions often involve understanding how kidneys concentrate urine or maintain pH. For example, knowing that the kidney can excrete hydrogen ions and reabsorb bicarbonate to balance blood pH is important.