The Excretory System
The Excretory System
The main organ is the extcretory system is the kidney. The functional units in the kidney are the nephrons. The function of the excretory system is to manage the acid-base balance of the chicken's body, excrete water and excrete metabolic waste.
In the body of the bird, the kidneys have three lobes and are situated against the back of the bird, under the lungs. The texture of the kidney is soft and fragile, making it easy to damage when removed. Chickens lack a bladder and their urine is actually uric acid. The urine is yellowish in color accompanied by a thick, sticky white paste. The uric acid is not water soluble.
The ureter is a straight and narrow tube that leaves the kidneys on the medial border and opens into the cloaca, which is adjacent to the deferens duct and oviduct of birds of the respective gender.
Diagram 13 shows one lobe of an avian kidney
Avian kidneys have two kinds of nephrons. A reptilian-type, with no loops of Henle are located in the cortex, and a mammalian-type with long or intermediate length loops, are located in the medulla. Nephrons filter the blood plasma to eliminate waste products, but, in doing so, must not lose needed materials (like glucose) or too much water. Blood enters nephrons via small arteries called afferent arterioles. This blood enters the glomerulus (a collection of capillaries) under high pressure and 'filters' through the walls of the capillaries and the walls of a surrounding structure called a capsule. The filtrate that moves from the glomerular capsule into the proximal tubules is basically plasma without protein (the protein molecules are too large). The filtrate contains a lot of important substances. Vitamins and glucose are reabsorbed into the blood in the proximal convoluted tubule.
Other than mammals, birds are the only vertebrates that conserve body water by producing urine osmotically more concentrated than the plasma from which it is derived. However, the ability of birds to concentrate urine is limited compared to mammals. Typically, water-deprived birds produce urine that is 1.4-2.8 times more concentrated than plasma, whereas some mammals can produce urine 20-25 times more concentrated than plasma (but, for most mammals, urine is about 5 - 10 times more concentrated than plasma). This 'concentrating capacity' resides within the medullary cones. Solutes (sodium chloride, or NaCl) are actively transported out the ascending limb of the Loop of Henle, where they become concentrated in the medulla (medullary cones). When urine passes throughout the osmotic gradient in the medulla, water leaves the tubules by osmosis and the urine become concentrated. Because only the looped nephrons contribute to the intramedullary osmotic gradient, the presence of loopless nephrons may limit the ability of the kidneys to produce hyperosmotic urine. Thus, the concentrating ability of avian kidneys is more limited than in mammals.
Water-deprived birds do have a mechanism for reducing the amount of water leaving the kidneys. In response to dehydration, the pituitary gland releases more of a hormone called arginine vasotocin (AVT) into the blood. In the kidneys, AVT causes a reduction in the glomerular filtration rate (the rate at which plasma filters from the glomeruli into the glomerular capsule) so that less water moves from the blood into the kidney tubules. In addition, AVT increases the permeability of the walls of collecting ducts to water by opening protein water channels called aquaporins. As the collecting ducts become more permeable, more water moves by osmosis out of the collecting ducts (because of the higher solute concentration in the medullary cones) and can be reabsorbed by kidney capillaries.
As noted above, the avian kidney has a limited capacity for the conservation of body water and electrolytes via elimination of hyperosmotic urine. This low capacity to concentrate urine is not a liability because urine formed by the kidneys travels along the ureters into the cloaca. From the ureters, urine may move by retrograde peristalsis into the lower intestine (colon) and cecae. Fluid coming from the upper gastrointestinal tract also enters the cloaca. The cloaca, therefore, receives an influx of water from the kidneys and the small intestine. This water can be reabsorbed through the epithelium of the lower intestinal tract to maintain hydration. In the lower intestine and cecae, water and sodium chloride are reclaimed by the process of sodium-linked water reabsorption. Positively-charged sodium ions are actively transported out of the intestine and negatively-charged chloride ions follow them. Water follows the ions by osmosis. As a result, concentrated uric acid is excreted as a relatively dry mixture with feces
The next page will discuss the immune system.
In the body of the bird, the kidneys have three lobes and are situated against the back of the bird, under the lungs. The texture of the kidney is soft and fragile, making it easy to damage when removed. Chickens lack a bladder and their urine is actually uric acid. The urine is yellowish in color accompanied by a thick, sticky white paste. The uric acid is not water soluble.
The ureter is a straight and narrow tube that leaves the kidneys on the medial border and opens into the cloaca, which is adjacent to the deferens duct and oviduct of birds of the respective gender.
Diagram 13 shows one lobe of an avian kidney
Avian kidneys have two kinds of nephrons. A reptilian-type, with no loops of Henle are located in the cortex, and a mammalian-type with long or intermediate length loops, are located in the medulla. Nephrons filter the blood plasma to eliminate waste products, but, in doing so, must not lose needed materials (like glucose) or too much water. Blood enters nephrons via small arteries called afferent arterioles. This blood enters the glomerulus (a collection of capillaries) under high pressure and 'filters' through the walls of the capillaries and the walls of a surrounding structure called a capsule. The filtrate that moves from the glomerular capsule into the proximal tubules is basically plasma without protein (the protein molecules are too large). The filtrate contains a lot of important substances. Vitamins and glucose are reabsorbed into the blood in the proximal convoluted tubule.
Other than mammals, birds are the only vertebrates that conserve body water by producing urine osmotically more concentrated than the plasma from which it is derived. However, the ability of birds to concentrate urine is limited compared to mammals. Typically, water-deprived birds produce urine that is 1.4-2.8 times more concentrated than plasma, whereas some mammals can produce urine 20-25 times more concentrated than plasma (but, for most mammals, urine is about 5 - 10 times more concentrated than plasma). This 'concentrating capacity' resides within the medullary cones. Solutes (sodium chloride, or NaCl) are actively transported out the ascending limb of the Loop of Henle, where they become concentrated in the medulla (medullary cones). When urine passes throughout the osmotic gradient in the medulla, water leaves the tubules by osmosis and the urine become concentrated. Because only the looped nephrons contribute to the intramedullary osmotic gradient, the presence of loopless nephrons may limit the ability of the kidneys to produce hyperosmotic urine. Thus, the concentrating ability of avian kidneys is more limited than in mammals.
Water-deprived birds do have a mechanism for reducing the amount of water leaving the kidneys. In response to dehydration, the pituitary gland releases more of a hormone called arginine vasotocin (AVT) into the blood. In the kidneys, AVT causes a reduction in the glomerular filtration rate (the rate at which plasma filters from the glomeruli into the glomerular capsule) so that less water moves from the blood into the kidney tubules. In addition, AVT increases the permeability of the walls of collecting ducts to water by opening protein water channels called aquaporins. As the collecting ducts become more permeable, more water moves by osmosis out of the collecting ducts (because of the higher solute concentration in the medullary cones) and can be reabsorbed by kidney capillaries.
As noted above, the avian kidney has a limited capacity for the conservation of body water and electrolytes via elimination of hyperosmotic urine. This low capacity to concentrate urine is not a liability because urine formed by the kidneys travels along the ureters into the cloaca. From the ureters, urine may move by retrograde peristalsis into the lower intestine (colon) and cecae. Fluid coming from the upper gastrointestinal tract also enters the cloaca. The cloaca, therefore, receives an influx of water from the kidneys and the small intestine. This water can be reabsorbed through the epithelium of the lower intestinal tract to maintain hydration. In the lower intestine and cecae, water and sodium chloride are reclaimed by the process of sodium-linked water reabsorption. Positively-charged sodium ions are actively transported out of the intestine and negatively-charged chloride ions follow them. Water follows the ions by osmosis. As a result, concentrated uric acid is excreted as a relatively dry mixture with feces
The next page will discuss the immune system.
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