The kidneys are two bean-shaped organs in the human body that play a major role in filtering blood and maintaining overall balance in the body. They are part of the excretory system, also called the urinary system, and are located at the back of the abdominal cavity, on either side of the spine, just below the rib cage.

Structure of the Kidney:

Each kidney is about the size of a fist and has three main regions:

1. Cortex: The outer part of the kidney. It contains the initial parts of the filtering units.

2. Medulla: The inner region that has cone-shaped structures called renal pyramids, where urine is concentrated.

3. Pelvis: A funnel-shaped space that collects urine and passes it into the ureter, a tube leading to the bladder.

Main Functions of the Kidney:

1. Filtration of Blood: The kidney removes waste products like urea, creatinine, and excess salts from the blood. This process takes place in tiny structures called nephrons.

2. Formation of Urine: After filtration, the waste is turned into urine and sent to the bladder.

3. Regulation of Water Balance: The kidney adjusts the amount of water reabsorbed into the blood, depending on how much water the body needs.

4. Maintaining Electrolyte Balance: It controls the levels of sodium, potassium, calcium, and other ions in the body.

5. Acid-Base Balance: The kidney helps maintain the pH of the blood by controlling the levels of hydrogen and bicarbonate ions.

6. Regulation of Blood Pressure: The kidneys release a hormone called renin, which helps regulate blood pressure.

7. Production of Hormones: The kidney also produces erythropoietin, a hormone that stimulates the bone marrow to make red blood cells.

8. Detoxification: It helps remove toxins and drugs from the body.

How Nephrons Work:

Each kidney contains about 1 million nephrons, and each nephron has a glomerulus (a small bunch of capillaries) and a tubule. Blood flows through the glomerulus, where filtration happens. The useful substances like glucose, amino acids, and some water are reabsorbed in the tubule, while waste is passed as urine.

Path of Urine:

1. Urine is formed in the nephrons.

2. It passes into collecting ducts.

3. From there, it enters the renal pelvis.

4. Then it flows through the ureter.

5. It is stored in the urinary bladder.

6. Finally, it leaves the body through the urethra during urination.

Why the Kidney Is Important:

Without the kidneys, waste would build up in the blood, and the body would lose its balance of water, salts, and pH. This could lead to serious health problems and death. People with damaged kidneys may need dialysis, a machine that performs the kidney’s job artificially.