The normal function of the kidneys

It's very simple: no kidneys, no life. They are your body's tireless waste removers. They deserve a closer look on World Kidney Day.

Here's more about how they function and what they do for you.

Renal function
The work done by your kidneys is called your renal function. Effective renal function is vital to life. Different factors can cause it to decline including inflammation, infections, accidents, certain drugs or poisons and chronic conditions like diabetes and hypertension. Inherited diseases of the kidney also account for kidney disease. However, you can survive even if your kidneys are not functioning 100 percent or with only one kidney intact.

Measuring renal function
The glomerular filtration rate (GFR) is used as a tool to measure your renal function. It is derived from blood and urine tests. This involves collecting and assessing all urine passed in a 24-hour period. Symptoms of renal failure may only appear when more than 50 percent of renal function is lost. By then the disease is often very advanced. The normal GFR of women is lower than that of men and GFR declines with age. Pregnancy may also influence your GFR.

Main functions of your kidney – job description of the kidneys

  • Removing wastes and excess water.

  • Maintaining the chemical balance of your body.

  • Producing important hormones.

Tireless waste remover
Your kidney processes 180 liters of blood daily to sift out two to three liters of waste product and water, called urine.

The waste is generated as end products from food ingested, your metabolism, exercise and environmental factors. This includes foreign substances like drugs and water-soluble toxins. A part of your food is used for energy and functions like self-repair. What the body does not need is returned to the blood and transported to the kidney.

Muscle is made of protein and it’s breakdown leads to products in the blood, called creatinine. The end product of dietary protein like meat is urea. Your kidneys eliminate these substances and other toxins that have not yet been identified. The accumulation of urea, creatinine and other toxins is found if your kidneys fail to function properly and can be detrimental to your health. The kidneys remove these wastes that would otherwise build up and be damaging to your body.

The kidneys also control the volume of blood by removing extra fluid or water. This is very important for your blood pressure and the workload on your heart. Excess water could also dilute chemicals and make the blood chemically unbalanced.

A chemical balancing act
Normal kidneys have the ability to regulate blood volume and composition according to the body’s needs. The kidneys perform this vital function by acting as a constant gauge, controlling the chemical content, concentration and acidity by changes in the urinary excretion.

The kidneys measure out chemicals like sodium, potassium and phosphorus. These electrolytes are returned to the circulation if necessary or the excess is excreted in the urine.

The right chemical balance is essential for life. Your enzymes function best at a specific pH level or ion concentration. The membranes surrounding your cells are charged and need the correct levels of potassium and calcium for them to function.

An important function of the kidney is the production of certain substances which includes:

  • Erythropoietin that stimulates the bone marrow to make more red cells. Red cells carry the oxygen in your blood. Anaemia is common in patients with chronic renal failure.

  • Renin is an enzyme that helps to control blood pressure. It is released when blood pressure is low, which causes blood vessels to narrow and blood pressure to increase.

  • Vitamin D(3) is the active form of Vit D and helps to regulate calcium and phosphate levels in the blood. This is important for the formation of bone and for the chemical balance in the body.

Small margin of error
Build-up of excess fluid or toxic waste products in your blood is very dangerous and can be life threatening. The kidneys remove this waste and water that would otherwise be damaging to your body. Regulating blood volume is important for your blood pressure and the workload on your heart. Biochemical reactions require a specific pH level or ion concentration. The correct levels of potassium, calcium and other electrolytes or minerals are essential for the body to function. Therefore both the volume and the composition of body fluid must be controlled.

Part of a detoxification system
The urinary system is a vital part of your body’s cleansing systems. These include the lungs, skin and intestines. All these organs excrete waste and water to maintain your chemical and fluid balance in the body. The kidneys play a vital role in maintaining an environment that is optimal for the cells of your body.

Dipstick test
Fortunately doctors don’t have to rely on taste to test urine any more. A urine test is part of the routine evaluation of patients and the urine dipstick is an inexpensive tool that can allude to kidney problems. The physical appearance, colour and smell of the urine are also important. Changes in the filtration barrier of the glomerulus can cause bigger molecules like protein to be sifted out and be detected in the urine. This is called proteinuria. Problems with the capillaries of the glomerulus can cause haematuria, where blood cells are detected in the urine. Urine can also be sent to a laboratory for microscopic examination and tests.

Blood tests
If your doctors are concerned about the wellbeing of your kidneys, they can perform blood tests which can determine the level of urea and creatinine in your blood.

Written by Dr K. Coetzee, reviewed by Dr R. Moosa, head of the Renal Unit, Tygerberg Academic Hospital.

Read more:
Beating incontinence
Recognising kidney trouble

National Kidney Foundation

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