Kidney – Class 11 | Chapter – 19 | Biology Short Notes Series PDF

Kidney: The kidneys are among the most vital organs of the human body. Malfunction of the kidneys can lead to serious illness or even death. Each kidney has a very complex structure and function.

They have two important functions namely: to flush out harmful and toxic waste products and to maintain balance of water, fluids, minerals and chemicals i.e., electrolytes such as sodium, potassium, etc.

Structure of the Kidney

The kidney produce urine by removing toxic waste products and excess water from the body. Urine formed in each kidney passes through the ureter, flows into bladder before finally being excreted through the urethra.

Most people (males and females) have two kidneys.

    • The kidneys are located at upper and back side of the abdomen, on either side of the spine (see diagram). They are protected from damage by the lower ribs.
    • The kidneys lie deep inside the abdomen so normally one cannot feel them.
    • The kidneys are a pair of bean shaped organs. In adults, a kidney is about 10 cm long, 6 cm wide and 4 cm thick. Each kidney weighs approximately 150-170 grams.
    • Urine formed in the kidneys flow down to urinary bladder and then through the ureters. Each ureter is about 25 cm long and is a hollow tube- like structure made up of special muscles.


  • The urinary bladder is a hollow organ made up of muscles, which lie in the lower and anterior part of the abdomen. It acts as a reservoir of urine.
  • The adult urinary bladder hold about 400-500 ml of urine; when filled to near capacity, a person feels the urge to pass urine.
  • The urine in the bladder is excreted through the urethra during the process of urination. In females, the urethra is relatively short, while it is much longer in males.

Why are the kidneys essential for living?

  • We consume different kinds and quantities and kind of food every day.
  • The quantity of water, salts, and acids in our body also varies every day.
  • The continuous process of converting food into energy produces harmful toxic

The primary function of the kidney is to make urine and purify the blood. Each kidney removes waste materials, and other chemicals which are not required by the body. Most important functions of the kidney are described below.

Removal of waste products

  • Purification of blood by removal of waste products is the most important function of the kidney.
  • The food that we consume contains protein. Protein is necessary for the growth and repair of the body. But as protein is utilized by the body it produces waste products. Accumulation and retention of these waste products is similar to retaining poison inside the body. Each kidney filters blood, and toxic waste products which are eventually excreted in the urine.
  • Creatinine and urea are two important waste products that can easily be measured in the blood. Their “values” in blood tests reflects the function of the kidney. When both the kidneys fail, value of creatinine and urea will be high in blood test.

Removal of excess fluid

  • The second most important function of the kidney is the regulation of fluid balance by excreting excess amount of water as urine while retaining the necessary amount of water in the body, that is essential for living .When the kidneys, fail they lose the ability of removing this excess amount of water. Excess water in the body leads to swelling.

Balance minerals and chemicals

  • The kidneys play another important role of regulating minerals and chemicals like sodium, potassium, hydrogen, calcium, phosphorus, magnesium and bicarbonate and maintains normal composition of body fluid.
  • Changes in the sodium level can affect person’s mental state, while changes in the potassium level can have serious adverse effects on the rhythm of the heart as well as functioning of the muscles. Maintenance of normal level of the calcium and phosphorus is essential for healthy bones and teeth.

Control of blood pressure

  • The kidneys produce different hormones (renin, angiotensin, aldosterone, prostaglandin etc) which help regulate water and salt in the body, which plays vital roles in the maintenance of good blood pressure control. Disturbances in hormone production and regulation of salt and water in a patient with kidney failure can lead to high blood pressure.

Red blood cells production

  • Erythropoietin is another hormone produced in the kidneys, it plays an important role in the production of red blood cells (RBC). During kidney failure, production of erythropoietin is decreased, which in turn leads to decreased production of RBC resulting in low hemoglobin (anemia). This is the reason why in patients with kidney failure, the hemoglobin count does not improve despite supplementation with iron and vitamin preparations.

To maintain healthy bones

  • The kidneys convert vitamin D into its active form which is essential for the absorption of calcium from food, growth of the bones and teeth, and keep the bones strong and healthy. During kidney failure, decreased active vitamin D leads to decreased, growth of bones and they also become weak. Growth retardation may be sign of kidney failure in children.

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By Team Learning Mantras