Anatomy of the Kidney

• The kidneys are two bean-shaped organs located on either side of the spine, just below the ribs.
• Each kidney is approximately the size of a human fist.

Outer Structure of the Kidney

• Renal capsule: a tough outer covering that protects the kidney from injury.
• Adipose capsule: a layer of fat that further cushions the kidney. Each kidney is encased in this fat to keep it in place.

Inner Structure of the Kidney

• Renal cortex: the outermost part of the kidney where the initial stages of urine formation occur.
• Renal medulla: the inner part of the kidney made up of many cone-shaped sections called renal pyramids.

Kidney's Functional Unit: Nephron

• Each kidney contains around a million microscopic units called nephrons, the functional units of the kidney.
• Nephrons are involved in the formation of urine, which happens through four major processes: filtration, reabsorption, secretion, and concentration.
• Filtration: occurs in the glomerulus, a bundle of tiny blood vessels within a cup-like structure known as Bowman's capsule.
• Reabsorption: essential substances such as glucose, amino acids, and salts are reabsorbed from the fluid in the renal tubules back into the blood.
• Secretion: some waste substances are secreted from the blood into the renal tubules.
• Concentration: as the resulting fluid (now mostly waste and water) passes through the renal tubule, water is reabsorbed leaving concentrated urine.

Kidney's Role in Homeostasis

• The kidneys maintain homeostasis by regulating the volume and composition of blood, including the concentration of waste products, electrolytes, and pH level.
• They accomplish this by selectively excreting or reabsorbing different substances.
• The kidneys also regulate the body’s water balance. When the body is dehydrated, the kidneys reabsorb water into the blood to prevent its loss in the urine. On the contrary, if the body has excess water, the kidneys allow more water to be lost in the urine.
• They also produce hormones. For example, they produce erythropoietin, which stimulates the production of red blood cells. They also release renin, an enzyme that affects blood pressure.