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AN ACCOUNT OF NITROGENOUS EXCRETION IN MAMMALS Excretion is the disposal of the waste products of metabolism, example ammonia and urea from

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Introduction

AN ACCOUNT OF NITROGENOUS EXCRETION IN MAMMALS Excretion is the disposal of the waste products of metabolism, example ammonia and urea from deamination of amino acids and disposal of substances present in excess e.g. water, salts. Excretion is the elimination of the unwanted products of metabolism and of metabolites present in excess within the organism. Animals excrete nitrogenous waste products such as ammonia, urea or uric acid. These or similar products are produced by the deamination of any excess protein that has been eaten. Nitrogenous compounds are formed in metabolism of proteins and nucleic acids. Ammonia is lost by diffusion from small aquatic organisms, urea is excreted by organs of nitrogenous excretion example the kidney. Excretion involves eliminating any excess or toxic substance taken in with the diet, including water and salts. Excretion plays an important part in the process by which the internal environment is regulated to maintain more or less constant condition (homeostasis). Green plants do not excrete nitrogenous waste; on the contrary, they re-use any breakdown products of nitrogen metabolism. Excretion in animals is mainly concerned with nitrogenous waste, and to a less extent with other waste products of metabolism. The three compounds of importance in nitrogenous excretion of animals are ammonia, urea and uric acid. ...read more.

Middle

* Secretion- other substances, not required by the body, are added to the filtrate from the body by active transport from the cells of the tubule wall. * Differential permeability- parts of the walls of the tubule are impermeable to water, ions or urea; these substances thus cannot diffuse or be transported back into the blood in those regions. Hormones control the permeability of the walls of the collecting duct and the distal convoluted tubule. * Urine storage- the urine passes to the bladder via long tubes, the ureters, and is stored there. The bladder is a dispensable sac with a wall of smooth muscle lined by a special type of epithelium, known as a transitional epithelium. The bladder can hold 400-500 ml of urine. Once 200 ml or so has been collected stretch receptors are stimulated, and this stimulation initiates the desire to discharge the urine. The urine passes to the exterior through the urethra. Spincter muscles close the exit from the bladder. Urine formation is carried out in the following way: * Ultrafiltration in the renal capsule - ultrafiltration occurs from the capillaries of the glomerulus into the lumen of the renal capsule, due to the pressure of blood in the kidney and the sieve-like quality of the walls of the glomerular capillaries and the renal capsule. ...read more.

Conclusion

The cells of the distal tubule also regulate the plasma concentrations of ions such as sodium, chloride and calcium. Whenever necessary these ions are removed from the urine into the blood by active transport across the tubule wall. * Water reabsorbtion in the collecting duct - the permeability to water of the walls of the collecting duct is controlled by a hormone know as antidiuretic hormone (ADH), secreted by the pituatry gland. When the walls are made permeable water is reabsorbed from the urine in the collecting duct by osmosis, as a result of the high salt concentration in the medulla, through which the collecting duct runs. When the walls are impermeable, however, no water is lost from the urine as it travels down the collecting duct.... Control by ADH secretion leads to the production of hypotonic or hypertonic urine, according to the body's varying demand for water. Urine formed by the kidneys is temporarily stored in the bladder. Emptying of the bladder, known as micturition, normally occurs well before the limit capacity is reached. Micturition takes place by contraction of the bladder wall and relaxation of the spinchter muscles at the neck of the bladder. The bladder wall consists of smooth muscle, which is under the control of the autonomic nervous system. Micturition is a reflex response to stretching of the walls. The reflex becomes suppressed by voluntary nervous action. ?? ?? ?? ?? 1 ...read more.

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