What are Kidneys?

Neha Mittal   by Neha Mittal, MS, Biotechnology    Last updated on April 19, 2021,

What are kidneys?

The kidneys are a pair of bean-shaped organs on either sides of the body in vertebrates, below the ribs and behind the belly. Each kidney is about 4 to 5 inches long, about the size of a large fist.

The kidneys' work is to filter your blood. They remove wastes, keep the body's fluid balance, and maintain the right levels of electrolytes in the body. The blood in body passes through them many times a day.


the kidneys: anatomy and functioning


Kidneys receive blood from renal arteries. Blood exits into the paired renal veins. Each kidney is attached to the ureter (a tube that carries excreted urine to the bladder).

The two kidneys together filter about 110 to 142 liters of blood to produce about 1 to 2 liters of urine. The urine flows from the kidneys to the bladder through the ureters. Ureters are located on each side of the bladder. The bladder stores the urine.

The muscles of the bladder remain in relaxed state while the bladder fills with urine. When the capacity of the bladder is full, a signal is sent to the brain to tell a person to excrete. When the bladder empties, the urine flows out of the body through the urethra (a tube through which urine is passed out). Urethra is located at the bottom of the bladder. Men have a long urethra. In women, it is shorter.

Why are our kidneys important?

Kidneys are important and vital organs of our body. They keep the composition of the blood stable, which lets the body function. They are important for many reasons such as the following:

  • They prevent the wastes and extra fluid to build in the body
  • They keep the levels of electrolytes stable, such as potassium, sodium, phosphate
  • They produce hormones that help in the regulation of blood pressure, formation of red blood cells, and to keep the bones stay strong

The kidneys are great chemical factories that perform a variety of functions such as the following:

  • remove waste products from the body
  • remove drugs from the body
  • keep a balance of the body's fluids
  • release hormones that regulate blood pressure
  • produce an active form of vitamin D to promote health of the bones
  • control the production of red blood cells

Functioning of the kidneys: How do kidneys work?

Our kidney is not a single filter. Each kidney is made up of about a million filters. These are called nephrons. Each nephron filters a small amount of blood. The nephron has a filter (called glomerulus) and a tubule.

Video Credit:Animated Anatomy


The glomerulus allows the fluid and waste products to pass through it. However, it prevents the blood cells and large molecules such as proteins from passing out. The filtered fluid passes through the tubule, which sends important minerals back to the bloodstream and removes the waste products. The final product that comes out through this two step filtration process is urine.

The production of urine involves highly complex steps of excretion and re-absorption. This process is important to maintain a balance of body chemicals, fluids, and electrolytes.

The kidneys perform their life-sustaining job of filtering of the blood. Any disease that can affect their functioning can be serious and life threatening. Chronic kidney disease is one of them that is characterized by kidney abnormality, or marker, such as protein in the urine and a decreased kidney function for three months or longer. Read about chronic kidney disease.

Neha Mittal

Neha Mittal completed graduation in Biotechnology from Graphics Era University, Dehradun and Masters in Biotechnology from Amity University, Noida. In the past, she worked with Sharon Biomedicine Ltd., DUKES Products Ltd., and Forest Research Institute, Dehradun as researcher in biotechnology.


Neha loves singing and dancing in leisure time. She also enjoys traveling and maintains a collection of great novels.

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