The Excretory System Biology 20
Introduction Follow along on page 376 What dangers exist if your body is unable to regulate the fluid balance of your tissues? What challenged would the body have to respond to if the kidneys failed to work? Explain how the circulatory system and excretory system interact during exercise.
The Excretory System The excretory system regulates the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. The primary functions of the excretory system is to Get rid of wastes Eliminate useless by-products excreted from cells Eradicate harmful chemical build-ups Maintain a steady, balanced chemical concentration
The human excretory system includes the lungs, skin, liver and kidneys. The lungs are responsible for excreting the waste product of cellular respiration carbon dioxide. The liver converts many toxic substances into less harmful substances to be eliminated by the kidneys.
What happens when we eat too much meat? Nitrogenous wastes are formed in the body when amino groups (NH 2 ) are removed from amino acids during protein metabolism in a process called deamination. Amino groups (NH 2 ) combine with hydrogen ions to form ammonia (NH 3 ), which is very toxic. Ammonia is combined with carbon dioxide in the liver to create urea, which the body can tolerate in higher concentrations. Urea is then eliminated from the body via the kidneys (and a small amount from the skin).
The breakdown of nucleic acids, such as DNA and RNA, also produces nitrogenous waste, which is converted by the liver into uric acid. Uric acid is also eliminated by the kidneys.
The Urinary System Urinary System - kidneys filter the blood to form urine, which is excess water, salt, urea and uric acid.
The Nephron The functional unit of the kidneys is the nephron. Nephrons are long, slender tubes where urea and uric acid is filtered out of the blood, along with excess salts, glucose and water.
Afferent arterioles from the renal artery supply the nephrons with the blood to be filtered. These arterioles form a special kind of capillary bed, called a glomerulus, where the blood is filtered. Blood does not leave this capillary bed via venules, but through other arterioles, called efferent arterioles. These arterioles form a second capillary bed, called peritubular capillaries, that wrap around the nephron. This time blood leaves the capillaries via venules and leave the kidney through the renal vein.
The glomerulus is surrounded by part of the nephron, called the Bowman s capsule, in the cortex of the kidney. Fluid containing urea, uric acid, salts, glucose and water leaves the capillaries of the glomerulus and enters the Bowman s capsule. This process is called filtration. This fluid leaves the Bowman s capsule via the proximal tubule where nutrients are reabsorbed from the filtrate back into the blood.
Salt is actively transported out of the filtrate into the cortex of the kidney and water follows due to osmotic pressure. Ammonia in the blood diffuses into the filtrate in the proximal tubule in a process called secretion. Excess hydrogen ions and potassium ions are secreted here as well. Bicarbonate ions are reabsorbed to help maintain blood ph. The filtrate then travels down the loop of Henle into the medulla of the kidney.
Loop of Henle Filtrate from the glomerulus passes down the descending arm of the loop of Henle. Because the surrounding tissue in the outer medulla has a high concentration of salt ions, water passes out of the filtrate by osmosis. This keeps you from losing too much water; a process called osmoregulation. The filtrate is now very concentrated.
As the concentrated filtrate passes up the ascending arm of the loop of Henle, salts diffuse out of the tubule into the medulla. The filtrate then travels through the distal tubule in the cortex of the kidney. More salt and water is reabsorbed and excess hydrogen and potassium ions are secreted. By the end of the distal tubule, the filtrate is now called urine, which empties into a collecting duct.
The collecting duct passes through the medulla of the kidney and more water is reabsorbed by osmosis. All of the collecting ducts merge in the renal pelvis of the kidney and the urine leaves the kidney via a ureter and is transported to the bladder. Stretch receptors in the bladder cause urinary sphincters to relax and urine passes out of the body via a urethra.
Anatomy of the Urinary System
ph Balance Kidneys maintain the ph balance in the body. ( 7.3 to 7.5) Carbonic Acid is produced in cellular respiration which ionized to produce H+ ions in the blood. H+ ions are buffered by bicarbonate ions, preventing a change in ph. This reaction produces carbon dioxide and water. The kidneys then restore the buffer and release bicarbonate back into the blood stream, and excrete the H+ ions.
Hormonal Control of Urine Production Aldosterone increases the reabsorption of Na+ ions in the kidneys, thereby increasing water reabsorption via osmosis. This increases blood volume, and therefore, increases blood pressure. Antidiuretic hormone (ADH), also called vasopressin, increases permeability of the distal tubule and collecting duct to water thereby increasing water reabsorption. It is released when the body is dehydrated and causes the kidneys to conserve water, thus concentrating the urine, and reducing urine volume.