The Renal System Study Guide (page 2)
The chemical reactions of metabolism that occur in the cells of organisms produce waste substances that must be excreted. One-celled organisms can excrete toxic substances by diffusion through their cell membranes or by using specialized organelles called vacuoles, whereas multicellular organisms have developed special organ systems to accomplish the same task. The circulatory and excretory systems work together to eliminate metabolic wastes.
All organisms try to live in a favorable environment, but even such an environment will have conditions that fluctuate over time. Organisms must be able to respond to these changes and yet still maintain a relatively constant internal environment within their bodies. For instance, they must maintain a balance of water, temperature, and salt concentration. The series of physical and chemical processes that work to maintain an internal equilibrium is called homeostasis. We first mentioned homeostasis in the circulatory system lesson. The circulatory system and the filtration/excretion system work very closely together to help maintain homeostasis.
Filtration and Excretion
When you eat food, some of it is indigestible and must be eliminated by the digestive tract. However, the digestible part is broken down and eventually absorbed as very small molecules. These molecules are transported to the cells by the blood. The cells then carry out the biochemical reactions of metabolism, which results in wastes such as carbon dioxide. The carbon dioxide is carried by the blood to the lungs where it is exhaled, but other wastes are produced during cellular metabolism besides carbon dioxide. For the most part, these wastes cannot be eliminated in the lungs. Instead, they must be filtered out of the blood and then excreted.
The Kidneys as Filters
The blood delivers wastes such as ammonia, which comes from the metabolism of amino acids in cells, to the liver where they are converted into urea. The blood then carries the urea to the organs in the lower back called the kidneys. The kidneys are bean-shaped (they look very much like a kidney bean, in fact!) and are about the size of your fist. They are part of the renal system, which also includes the ureters and urethra, two tubes that carry the liquid wastes out of the body. The urinary bladder, a holding place for urine, is also part of this system.
The kidneys are sophisticated filters. They are able to take urea, which is less toxic than ammonia but still not well tolerated in high concentrations, and convert it into urine, which is very soluble in water and can be excreted. The kidneys also regulate the amount of water that is used in this process in order to prevent body dehydration. Kidneys also maintain proper levels of a number of substances in the body (including sodium, potassium, chloride, calcium, glucose sugar, and amino acids) by reabsorbing them from urine before it is excreted. The kidneys also help maintain blood pressure and the acidity (pH) level of the blood. It is obvious that the kidneys are important regulators and maintainers of bodily homeostasis.
How Does a Kidney Work?
In each kidney are at least a million individual units called nephrons. Functionally, nephrons are similar to the alveoli in the lungs. The alveoli are structured to function as gas exchange interfaces, whereas the kidney nephrons are structured to function as fluid interchange points.
Each nephron consists of a bed of capillaries with thin walls surrounded by a tube structure called Bowman's capsule. Filtration of the blood occurs for water, nutrients, and wastes through the capillaries into the Bowman's capsule. Most of the water and nutrients are reabsorbed right away. This concentrates the wastes in the fluid inside the Bowman's capsule tubules. We now call this fluid urine.
The tubules leading away from Bowman's capsule eventually arrive at the collecting duct where even more water may be absorbed. The collecting duct leads to the interior of the kidney where urine collects and flows into the ureters, which take it to the urinary bladder. Urine will collect in the urinary bladder until the urge to urinate is strong enough that the urine is expelled from the body through the urethra.
Monitoring Water Levels
Part of what the kidneys do is regulate the amount of water that circulates in the bloodstream. If the brain detects low levels of water in the blood, then more antidiuretic hormone (ADH) is released. As its name implies, it causes the kidneys to reabsorb water into the bloodstream, thus concentrating the urine and preserving water for the body.
The brain is very good at keeping a balance on many interrelated factors. Our nervous system respond to sensors in the body that keep track of blood sugar, blood pressure, blood carbon dioxide, blood oxygen, blood dissolved salts, and so on. Lack of water affects all these values, which is how the body detects it.
When you drink too much alcohol, you usually urinate more often. This is because alcohol inhibits the action of the ADH signal from the brain. Thus, nothing tells the kidneys to conserve water.
The series of physical and chemical processes that work to maintain an internal equilibrium is called homeostasis. The circulatory system and the filtration/excretion system work very closely together to help maintain homeostasis, and wastes other than carbon dioxide are produced during cellular metabolism. For the most part, these wastes cannot be eliminated in the lungs, and instead, they must be filtered out of the blood by the kidneys and then be excreted.
The kidneys are sophisticated filters. They are able to take urea—which is less toxic than ammonia but still not well tolerated in high concentrations—and convert it to urine, which is very soluble in water and can be excreted. The quantity of water in the body is tightly controlled, and the kidneys play an important role in that necessary function. The amount of water in the blood is controlled in what is called a negative feedback loop.
Practice problems of this concept can be found at: The Renal System Practice Questions
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