Kidney Dialysis and Maintaining the Body's Chemistry*
In order for blood to perform its essential functions of bringing nutrients and oxygen to the cells of the body, and carrying waste materials away from those cells, the chemical composition of the blood must be carefully controlled.
Blood contains particles of many different sizes and types, including cells, proteins, dissolved ions, and organic waste products. Some of these particles, such as proteins like hemoglobin, are essential for the body. Others, such as urea (a waste product from protein metabolism), must be removed from the blood or they will accumulate and interfere with normal metabolic processes. Still other particles, including many of the simple ions dissolved in the blood, are required by the body in certain concentrations that must be tightly regulated, especially when the intake of these chemicals varies.
The body has many different means of controlling the chemical composition of the blood. The largest responsibility for maintaining the chemistry of the blood falls to the kidneys, a pair of organs located just behind the lining of the abdominal cavity. It is the job of the kidneys to remove the harmful particles from the blood and to regulate the blood's ionic concentrations, while keeping the essential particles in the blood (Figure 1).
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Figure 1
This is a schematic diagram illustrating the kidneys' ability to separate particles in the blood in order to maintain optimal body chemistry. Blood enters the kidney through the renal artery. In the kidney, the blood undergoes filtration and dialysis to separate the particles that will be removed from the body (through the ureter to the bladder) from those that will be returned to the circulating blood (through the renal vein).
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The kidneys meet these challenges through a remarkably elegant system. Essentially, kidneys act like dialysis units for blood, making use of the different sizes of the particles and specially-maintained concentration gradients.
Blood passes through the membrane-lined tubules of the kidney. Particles that can pass through the membrane pass out of the tubules by diffusion, thus separating the particles that remain in the blood from those that will be removed from the blood and excreted. The dialysis mechanism used by the kidneys allows them to function effectively over a very wide range of conditions.
For example, sodium intake can vary from one tenth to ten times the average consumption, with only minimal fluctuations in blood-plasma sodium concentrations. Even when the kidneys are severely damaged, the kidneys can still effectively maintain the body's chemistry as long as at least ten percent of their functional units are working. Nonetheless, damage to the kidneys can cause the functional capacity to drop below this level, and fatal illness will develop unless an artificial system is employed to perform the work of the kidneys.
Sources:
*This information from chemistry.wustl.edu/~edudev/LabTutorials/Dialysis/Kidneys.html
Authors: Rachel Casiday and Regina Frey
Department of Chemistry, Washington University
St. Louis, MO 63130
References:
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