Excretion in Human: Urinary System.
by Unknown
February 27, 2017
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Excretion
Excretion is the process by which metabolic wastes and other non-useful materials are eliminated from an organism.
Considering the chemical basis of life and its sustainability on metabolic pathways, the wastes are primarily generated at the metabolic level and these are called metabolic wastes. These include urea, produced from the metabolism of amino acids: creatinine, produced from muscle creatine; uric acid, from nucleic acids bilirubin, end products of hemoglobin breakdown and metabolites of various hormones.
Metabolic wastes also include the toxins produced by the body and ingested such as pesticides, drugs, and food additives. The presence of wastes in the body can cause serious hazard thus are removed by the excretory system.
Excretory Organs of Humans
In human’s liver and kidneys is the primary structure of production and elimination of waste products.
Liver
Metabolism of the waste production and elimination is centralized here in the liver. Due to this property, liver works are pivotal to homeostasis and involve interaction with most of the body's organs systems. Liver Supports the excretory role of the kidney by detoxifying many chemical poisons and produce ammonia, urea and uric acids from the nitrogen of amino acids. Removal of salt with water by the sweat glands and of sebum by sebaceous glands seems to be excretory in nature. The removal of water and salts from sweat glands is for the purpose of thermoregulation and of sebum on the skin is for protection against the microorganism. That why in the context of the definition of excretion, the skin may not be considered as the excretory organ. Among the various nitrogenous wastes described earlier urea is major excretory product and liver form it from the waste nitrogen. The metabolic pathways involved in the production of urea are termed as the urea cycle.
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Two ammonia and one carbon dioxide molecules are involved in the cycle to generate one molecule of urea. One ammonia molecule combines with carbon dioxide and already available precursor from previous cycle ornithine to form citrulline, subsequently, another ammonia combines to form arginine. The arginine is split by arginase to form urea and the precursor ornithine for next cycle.
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The liver is not only involved in the synthesis of nitrogenous wastes to assist kidney in their disposal but also has numerous crucial functions of homeostasis importance. These functions belong to synthesis, Storage conversion, recycling, and detoxification categories.
Urea is detoxified form of ammonia in the urea cycle, which can be retained in the body in greater amounts than ammonia and can be eliminated with 10 quantity of water as compared to ammonia.
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Urinary System
A pair of kidney consists of millions of functional units, nephrons. The nephrons have extensive blood via arteries, which leave each kidney via the renal vein. Liver account 1% of body weight but receives 20% of blood supply. Following filtration of blood and each further processing through tubular urine is collected in a central cavity the kidney system pelvis. Urine leaves duct called Ureter. The ureters of both the kidneys drain into urinary bladder through the ureteral orifice. Urine leaves the body through a duct called urethra, which empties near the vagina in females or through the penis in males. Sphincter muscles near the junction the urethra and the bladder controls the urine in the bladder.
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Nephron
The functional units, nephrons, in human kidneys are arranged along two an outer cortex and inner medulla. The nephrons arranged along the cortex are called as cortical, however, those arranged along the border of cortex and medulla with their tubular system looping deep in the inner medulla are juxtamedullary nephrons. These juxtamedullary nephrons are specifically instrumental in the production of concentrated urine. In each nephron, inner end forms a cup-shaped swelling, called Bowman's capsule and it is around a ball of capillaries called glomerulus. Glomerulus circulates blood through capsule as it arrives through afferent arteriole and leaves the capsule by efferent arteries. The blood vessel subdivides again into another network of capillaries, the peritubular capillaries. Bowman capsule continues as extensively convoluted proximal tubule, a loop of Henle and the distal tubule, which empties into collecting tubules. The collecting tubules open into the pelvis. The filtrate from glomerulus passes through these Structures and is processed ultimately for urine formation. The peritubular capillaries intermingle with proximal and distal tubules of the nephron. In juxtamedullary nephrons, additional capillaries extend down to from a loop of vessels vasa recta.
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Filtration:
Blood passing through glomerulus is filtered into Bowman capsule. It is specifically filtered here, unlike at the other parts of the vessels, because glomerulus walls are porous, and the fraction of the blood pressure reaching here provides the filtration pressure. The filtrate appearing in Bowman's capsule is called glomerular filtrate, which contains numerous useful substances such as glucose, amino acids, salt in aqueous solution.
Reabsorption
All useful constituents of the glomerular filtrate are reabsorbed in proximal tubules and when filtrate leaves proximal tubules, it mostly consists of nitrogenous wastes.
Secretion
The tubular epithelium also secretes substances into the lumen, this secretion is very selective and is mainly of hydrogen ions to balance pH value of the filtrate passing through the tubule.
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Concentration of Excretory Products
In restricted supply of water, the conservation of water is the principal function of the body. This is done by the concentration of the filtrate by counter current and hormonal mechanisms. In the sufficient or excess supply of water. Reabsorption of water from the filtrate is reduced, specifically due to inhibition of release of antidiuretic hormones in the presence of hypoosmotic body fluids. The reduction in reabsorption causes large volumes of diluted urine. Mammalian kidney including human is adapted to conserve water by over 99.5% reabsorption of glomerular filtrate.
Gradually increasing osmotic concentration from cortex to inner medulla is the main factor for the production of hypertonic (concentrated) urine into mammals including human.
The interstitial fluid of the kidney is gradually concentrated from cortical to medullary part, thus inner medulla is highly concentrated with the presence of urea and through a mechanism of counter-current multiplier. This mechanism causes a gradual osmotic outflow of water from the filtrate back to the kidney as it passes downward in the descending loop of Henle. Furthermore, ascending loop of Henle does not allow outflow of water from its filtrate, instead actively transport Na ions into kidney interstitial to sustain its high concentration.
Hormones
The active uptake of sodium in the ascending limb or thick loop of Henle is promoted by the action of aldosterone, the hormone secreted from the adrenal cortex. The other site in the nephron, where reabsorption of water takes place is collecting tubules. ADH 9(Antidiuretic Hormone) released from posterior pituitary lobe acts to actively transport water from the filtrate in collecting tubules back to the kidney.
Kidney as Osmoregulatory Organ
The production of varied concentrations of urine depending on the availability water exhibits clearly that kidney functions as an osmoregulatory organ along excretory role of nitrogenous wastes.
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Thats all from my side. Will be back with a new blog on a new topic.
Since then keep smiling, stay blessed.
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Excretion in Human: Urinary System.
Reviewed by Unknown
on
February 27, 2017
Rating: 5
Reviewed by Unknown
on
February 27, 2017
Rating: 5
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