Describe the structure and function of the human kidney.

The human kidney is a bean-shaped organ located in the abdominal cavity, typically found on either side of the spine. It is responsible for maintaining homeostasis by regulating the volume and composition of body fluids. Each kidney is approximately 10–12 cm long, 5–7 cm wide, and 2–3 cm thick, weighing around 120–150 grams.

Structure of the Human Kidney

The kidney can be divided into three main regions:

1. Renal Cortex: The outer layer containing the glomeruli and convoluted tubules.
2. Renal Medulla: The inner region composed of renal pyramids, which contain the loops of Henle and collecting ducts.
3. Renal Pelvis: A funnel-shaped structure that collects urine and channels it into the ureter.

The functional unit of the kidney is the nephron, of which there are about 1–1.5 million per kidney. Each nephron consists of:

• Bowman’s capsule: A cup-shaped structure that surrounds the glomerulus.
• Glomerulus: A network of capillaries where blood filtration occurs.
• Proximal Convoluted Tubule (PCT): Reabsorbs water, glucose, and essential ions.
• Loop of Henle: Creates a concentration gradient for water reabsorption.
• Distal Convoluted Tubule (DCT): Fine-tunes electrolyte and pH balance.
• Collecting Duct: Transports urine to the renal pelvis.

Functions of the Human Kidney

The kidneys perform several vital functions:

1. Excretion of Waste Products: Filters out urea, uric acid, and creatinine from the blood.
2. Regulation of Electrolytes: Maintains balance of sodium, potassium, and calcium ions.
3. Maintenance of pH: Regulates acid-base balance by excreting hydrogen ions and reabsorbing bicarbonate.
4. Blood Pressure Regulation: Secretes renin, which helps control blood pressure via the renin-angiotensin-aldosterone system.
5. Red Blood Cell Production: Secretes erythropoietin, stimulating bone marrow to produce red blood cells.
6. Water Balance: Adjusts urine concentration to maintain proper hydration.

The kidneys work continuously to filter about 180 liters of blood plasma per day, producing around 1–2 liters of urine as waste. This ensures the body’s internal environment remains stable despite external changes.