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Functions of the Mammalian Kidney

The principal organs of the mammalian excretory system are the two kidneys. The mammalian excretory system keeps the volume of fluid in the body at correct levels and maintains the proper concentrations of solutes such as sodium and potassium ions. It also removes metabolic waste. Both kidneys lie at the back of the abdominal cavity, at about waist level, where they are protected to some extent by the spine.

  1. Ultrafiltration and Active Transport

    • Mammalian kidney function involves two processes, ultrafiltration and active transport. Ultrafiltration is filtration under pressure in which blood is squeezed to separate a fluid called "glomerular filtrate" or simply called "the filtrate." Active transport modifies the filtrate, secreting some substances and reabsorbing others according to the needs of the body. The result is that blood flows back into the body without much of its harmful waste. This waste is a solution containing urea, salts and various other chemicals which is known as urine.

    Control of Blood Volume and Pressure

    • Kidney regulates water reabsorption as well as blood volume. A drop in blood volume leads to a drop in blood pressure which is detected by stretch receptors or muscle spindles. Muscle spindle is a sensory end organ in a muscle that is sensitive to stretch in the muscle. Impulses from these detectors pass to the brain which then secrets a hormone called anti-diuretic hormone (ADH). AHD acts on the kidney to cause more water to be retained and so blood pressure increases.

    Acid/Base Balance

    • Kidney helps maintaining a relatively constant acid/base balance. This balance is important because enzymes are very sensitive to changes in acid/base balance and even a small change can inhibit their activity, as a result serious consequences for an organism. Under normal circumstances, the kidney is engaged in a constant battle against accumulating acid, although this depends on several factors, including diet, cell respiration and digestion.

    Water Balance

    • Kidney can reduce the volume of urine produced in response to bodily requirements. It achieves this by increasing the permeability to water of the distal tubule (this is the convoluted portion of the mammalian kidney), thus allowing more water to pass from the filtrate and into the blood.

    Salt Balance: The Control of Sodium Chloride Levels

    • The concentration of sodium ions in body fluids is controlled by the kidney. When sodium ions are actively transported, a negative ion, normally chloride automatically follows to maintain electrolytic balance (balance of positively and negatively charged ions). The kidneys are responsible for the whole process.

    Osmoregulation in Desert Animals

    • Desert animals can survive on dried food and no drinking water for long periods. Their ability to control their water loss is remarkable. Their kidney consists mainly of juxtamedullary nephrons (for instance, excretory units of kidney) which allow them to produce very concentrated urine and so they can lose their metabolic waste without losing significant amounts of water.

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