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There are four eliminative channels comprised of organs which help keep our bodies free from toxins, the intestinal, respiratory, urinary and integumentary systems are all responsible for some part of the detoxification process. This month’s newsletter topic is the urinary system.
The urinary system is an important eliminative system to keep functioning for good health. It consists of the kidneys, ureters, bladder and urethra which form what is commonly known as the urinary tract which acts as the body’s plumbing system to drain urine from the kidneys, store it, and then release it during urination. In addition to filtering and eliminating wastes from the body, the urinary system also maintains the homeostasis (balance) of water, ions, pH, blood pressure and calcium.
The kidneys are a pair of bean-shaped organs found along the posterior (back) wall of the abdominal cavity. The right kidney is slightly lower than the left kidney as it sits below the liver which is much larger on the right side. The kidneys are surrounded by a layer of adipose (connective) tissue which holds them in place and helps protect them from physical damage. The kidneys filter metabolic wastes, excess ions, and chemicals from the blood to form urine.
The kidneys contain around a million tiny structures call nephrons. Nephrons are the functional units of the kidney which filters blood to produce urine. Arterioles in the kidneys deliver blood to a bundle of capillaries surrounded by a capsule called a glomerulus. As the blood flows through the glomerulus, much of the blood plasma is pushed out of the capillaries into the capsule, allowing blood cells and a small amount of plasma to continue flowing through the capillaries. The liquid in the capsule flows through a series of tubules lined with filtering cells and surrounded by capillaries. The cells surrounding the tubules selectively absorb water and substances from the filtrate in the tubule and return it to the blood in the capillaries. At the same time, waste products present in the blood are secreted into the filtrate. By the end of this process, the filtrate in the tubule has become urine containing only water, waste products, and excess ions. The blood exiting the capillaries has reabsorbed all of the nutrients along with most of the water and ions that the body needs to function.
There is a pair of tubes carrying urine from the kidneys to the bladder called ureters. They are around 10 to 12 inches long and run parallel to the spinal column. Gravity and peristalsis of smooth muscle tissue inside the walls of the ureters move toward the bladder. The ends of the ureters extend slightly into the bladder and are closed at the entry point by valves which prevent the urine from flowing back toward the kidneys.
After urine has been produced by the kidneys, it is transported through the ureters to the bladder. The urinary bladder is a sac-like hollow organ used for the storage of urine and is located along the body’s midline at the inferior end of the pelvis. Urine entering the bladder from the ureters slowly fills the hollow space of the bladder and stretches its elastic walls. The walls of the bladder allow it to stretch to hold anywhere from 600 to 800 milliliters (20 to 28 oz) of urine. Stretch receptors in the walls send signals to the brain and spinal cord signaling the relaxation of the involuntary internal urethral sphincter and the sensation of needing to urinate. Urination may be delayed as long as the bladder does not exceed its maximum volume, but increasing nerve signals lead to greater discomfort and desire to urinate.
The urethra is the tube through which urine passes from the bladder to the exterior of the body. The female urethra is around 2 inches long and ends in back of the clitoris and front of the vaginal opening. In males, the urethra is around 8 to 10 inches long and ends at the tip of the penis. The urethra is also an organ of the male reproductive system as it carries sperm out of the body through the penis.
Urination is the process of releasing urine from the bladder through the urethra and exiting the body. The process begins when the urethral sphincter muscles relax allowing urine to pass through the urethra. Once the sphincter relaxes, the smooth muscles in the walls of the bladder contract to expel urine from the bladder.
Homeostasis is defined as the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes.
The kidneys maintain the homeostasis of several important internal conditions by controlling the excretion of substances out of the body.
The kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, and chloride ions into urine. In cases where these ions reach a higher than normal concentration, the kidneys can increase their excretion out of the body to return them to a normal level. Conversely, the kidneys can conserve these ions when they are present in lower than normal levels by allowing the ions to be reabsorbed into the blood during filtration.
pH is a measure of hydrogen ion concentration; a measure of the acidity or alkalinity of a solution. ... Aqueous solutions at 25°C with a pH less than seven are acidic, while those with a pH greater than seven are basic or alkaline
The kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism of dietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions into urine for elimination from the body. The kidneys also conserve bicarbonate ions, which act as important pH buffers in the blood.
The concentration of a solution expressed as the total number of solute particles per liter.
The cells of the body need to grow in an isotonic (denoting or relating to a solution having the same osmotic pressure as some other solution, especially one in a cell or a body fluid) environment in order to maintain their fluid and electrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.
The kidneys monitor the body’s blood pressure to help maintain homeostasis. When blood pressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume of blood in the body. The kidneys are able to reduce blood volume by reducing the re-absorption of water into the blood and producing watery, dilute urine. When blood pressure becomes too low, the kidneys can produce the enzyme renin to constrict blood vessels and produce concentrated urine, which allows more water to remain in the blood.
Production of Hormones
The kidneys produce and interact with several hormones that are involved in the control of systems outside of the urinary system.
Calcitriol is the active form of vitamin D in the human body. It is produced by the kidneys from precursor molecules produced by UV radiation striking the skin. Calcitriol works together with parathyroid hormone (PTH) to raise the level of calcium ions in the bloodstream. When the level of calcium ions in the blood drops below a threshold level, the parathyroid glands release PTH, which in turn stimulates the kidneys to release calcitriol. Calcitriol promotes the small intestine to absorb calcium from food and deposit it into the bloodstream. It also stimulates the osteoclasts of the skeletal system to break down bone matrix to release calcium ions into the blood.
Erythropoietin, also known as EPO, is a hormone that is produced by the kidneys to stimulate the production of red blood cells. The kidneys monitor the condition of the blood that passes through their capillaries, including the oxygen-carrying capacity of the blood. When the blood becomes hypoxic, meaning that it is carrying deficient levels of oxygen, cells lining the capillaries begin producing EPO and release it into the bloodstream. EPO travels through the blood to the red bone marrow, where it stimulates hematopoietic cells to increase their rate of red blood cell production. Red blood cells contain hemoglobin, which greatly increases the blood’s oxygen-carrying capacity and effectively ends the hypoxic conditions.
Renin is not a hormone itself, but an enzyme that the kidneys produce to start the renin-angiotensin system (RAS). The RAS increases blood volume and blood pressure in response to low blood pressure, blood loss, or dehydration.
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