People are generally healthier when their bodies are in a state of homeostasis, and their bodies send numerous messages to promote homeostasis. For example, when low blood sugar appears to be developing, people tend to feel hungry, because their brains tell them to eat. Specific cravings can emerge in response to nutritional deficiencies, with the brain essentially creating a shopping list, which will help the body reach a state of balance. People may also feel driven to drink water, exercise, or engage in other activities.
The Kidneys
The kidneys are a pair of vital organs that perform many functions to keep the blood clean and chemically balanced. The kidneys are bean-shaped organs, each about the size of a fist. They are located near the middle of the back, just below the rib cage, one on each side of the spine. The kidneys are sophisticated reprocessing machines. Every day, a person’s kidneys process about 200 quarts of blood to sift out about 2 quarts of waste products and extra water. The wastes and extra water become urine, which flows to the bladder through tubes called ureters. The bladder stores urine until releasing it through urination.
The kidneys remove wastes and water from the blood to form urine. Urine flows from the kidneys to the bladder through the ureters.
Wastes in the blood come from the normal breakdown of active tissues, such as muscles, and from food. The body uses food for energy and self-repairs. After the body has taken what it needs from food, wastes are sent to the blood. If the kidneys did not remove them, these wastes would build up in the blood and damage the body. The actual removal of wastes occurs in tiny units inside the kidneys called nephrons. Each kidney has about a million nephrons. In the nephron, a glomerulus—which is a tiny blood vessel, or capillary—intertwines with a tiny urine-collecting tube called a tubule. The glomerulus acts as a filtering unit, or sieve, and keeps normal proteins and cells in the bloodstream, allowing extra fluid and wastes to pass through. A complicated chemical exchange takes place, as waste materials and water leave the blood and enter the urinary system.
In the nephron, tiny blood vessels intertwine with urine-collecting tubes. Each kidney contains about 1 million nephrons.
At first, the tubules receive a combination of waste materials and chemicals the body can still use. The kidneys measure out chemicals like sodium, phosphorus, and potassium and release them back to the blood to return to the body. In this way, the kidneys regulate the body’s level of these substances. The right balance is necessary for life.
Most people believe that the filtering of wastes is the kidney's most important homeostatic role. However, in addition to removing excess salt, water, toxins, etc., they also release hormones. This is actually the best illustration of the kidneys and homeostasis, because filtering activity is constant. The two main hormones that the kidneys are responsible for releasing are:
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Erythropoietin—erythropoietin (EPO) is a hormone produced by the kidney that promotes the formation of red blood cells by the bone marrow.
The kidney cells that make erythropoietin are specialized so that they are sensitive to low oxygen levels in the blood that travels through the kidney. These cells make and release erythropoietin when the oxygen level is too low. The low oxygen level may indicate anaemia, a diminished number of red blood cells, or haemoglobin molecules that carry oxygen through the body.
Oxygen is low --> kidneys release erythropoietin --> erythropoietin stimulates the production of red blood cells in the bone marrow --> increase in red blood cells increases the oxygen-carrying capacity of the blood --> oxygen levels return to normal --> kidneys stop releasing erythropoietin
- Renin—is an enzyme secreted by the kidneys to regulate the body's balance of fluids and electrolytes. This enzyme is part of the renin-angiotensin system, which regulates blood pressure. Variations in renin levels can be caused by a variety of factors, from a high-salt diet to conditions such as Cushing's disease.
The kidneys produce renin in response to exercise, stress, or a fall in blood pressure. When the enzyme enters the bloodstream, it interacts with angiotensinogen, producing two results. The first is the contraction of the arterioles, which causes the blood pressure the rise. The second result is an increase in aldosterone production, which leads to increased retention of sodium. When the blood pressure has been stabilized, the renin and aldosterone are metabolised, and the body stops producing them.
Blood pressure becomes too low --> kidneys release renin --> rennin stimulates the contraction of the blood vessels --> this increases the resistance to blood flow which translates into an increase in blood pressure --> blood pressure returns to normal --> kidneys stop releasing rennin
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Calcitriol, the active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance in the body. Calcitriol is normally produced in the kidneys and controls absorption of calcium and phosphate from the intestine. In chronic renal failure the kidneys do not produce enough calcitriol which results in less calcium and phosphate being absorbed.
What is anaemia?
A person whose blood is low in red blood cells has anaemia. Red blood cells carry oxygen to tissues and organs throughout the body and enable them to use the energy from food. Without oxygen, these tissues and organs—particularly the heart and brain—may not do their jobs as well as they should. For this reason, a person who has anaemia may tire easily and look pale. Anaemia may also contribute to heart problems.
Anaemia is common in people with kidney disease. Healthy kidneys produce a hormone called erythropoietin, or EPO, which stimulates the bone marrow to produce the proper number of red blood cells needed to carry oxygen to vital organs. Diseased kidneys, however, often don’t make enough EPO. As a result, the bone marrow makes fewer red blood cells. Other common causes of anaemia include blood loss from haemodialysis and low levels of iron and folic acid. These nutrients from food help young red blood cells make haemoglobin, their main oxygen-carrying protein.
Healthy kidneys produce a hormone called erythropoietin, or EPO, which stimulates the bone marrow to make red blood cells needed to carry oxygen throughout the body. Diseased kidneys don’t make enough EPO, and bone marrow then makes fewer red blood cells.
References
Books
Health and Social Care Book 1, Beryl Stretch & Mary Whitehouse et al., 2007
Websites
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All images from: [25.01.2010]