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Homeostasis in Humans

Many aspects of our body’s internal environment e.g. body temperature, the concentration of glucose in the blood and tissue fluid, and the osmotic concentration and volume of the blood, are kept at almost constant levels. This maintenance of a constant internal environment is called homeostasis. Homeostasis ensures that the body cells are provided with a relatively constant environment so that they work efficiently and optimally, even though there might be changes outside the body.

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Negative feedback mechanisms are used to keep a fluctuating feature within narrow limits. In fact, any particular feature maintained by a negative feedback mechanism cannot be kept really constant. There is usually some fluctuation about a set point. This is caused by the time taken for the information to be transmitted to the effectors and for the actions of the effectors to take effect. The longer this time delay, the greater the fluctuations will be.

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Homeostatic Role of the Kidney

The kidney is a red-brown, bean-shaped organ about 10 cm long, 6 cm wide and 2.5 cm thick. The pair of kidneys is attached to the back of the abdominal cavity, to either side of the backbone, just behind the stomach and the liver. It receives oxygenated blood from the aorta through the renal artery, and deoxygenated blood is carried away in the renal vein to the inferior vena cava.

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A tube, the ureter runs from each kidney to the base of the bladder in the lower abdomen, and carries urine. A section through the kidneys shows a reddish-brown, outer region, the cortex, an inner greyish zone, the medulla, and a hollow space called the pelvis from which the ureter leads. Cones or pyramids of kidney tissue were projects into the pelvis.

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Each kidney contains many minute filtration units called nephrons. A nephron consists of a twisted hollow tube, closed at one end and opened at the other, and a network of associated blood vessels. The renal artery enters the kidneys branches to form a series of arterioles, and these distribute blood to each nephron as a spherical coiled ball of blood takes place here.

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The blood that leaves the glomerulus then flows through another capillary bed surrounding the tubular portion the nephron. The blood vessels then coverage and form a system of veins that merge to form the large renal vein by which the blood leaves the kidney. The blind end of the kidney tubule is greatly expanded to form a double-walled funnel, called Bowman’s capsule. The wall of Bowman’s capsule is made from a thin single layer of flattened cells. The cup-like portion of the capsule almost completely surrounds the glomerulus.

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Leading form the Bowman’s capsule is a tubule whose lumen is continuous with the capsular space. The first part of the tubule, the proximal convoluted tubule is highly coiled. It leads to as U-shaped loop of Henle which leads to another coiled section, the distal convoluted tubule. This portion opens into a collecting duct, along with several other nephrons. The ducts collect at the pelvis of the kidney, releasing their contents into the ureter, which conveys the urine to the bladder for temporary storage.

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The glomeruli, Bowman’s capsules, and the convoluted portions of the kidney tubules are located in the cortex, whereas the straight portions of the tubules, Henle’s loop, and the urine-collecting ducts are located in the medulla.

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Formation of Urine

Blood is conveyed to each glomerulus by an afferent arteriole, a branch of the renal artery. The capillaries of the glomerulus have very thin walls and minute pores that act as filters. The pores are of such a size that blood cells and proteins cannot leave, but water and smaller molecules flow out. Blood pressure that is higher than the osmotic pressure of the blood plasma, forces a cell-free, protein-free fluid from the glomerulus into the tubule. This type of filtration under pressure is called ultra-filtration.

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The pressure in the blood is due to a) the force generated by contraction of the heart, and b) the efferent arteriole having a smaller diameter than the afferent arteriole, thus increasing the resistance to blood flow. Thus the fluid that passes out of the glomerulus is similar to the blood serum. The remainder of the blood that does not enter the tubule leaves the glomerulus through the efferent arteriole, which empties into a second network of capillaries called peritubular capillaries. Urea is still secreted into the proximal convoluted tubule from the peritubular capillaries. After the glomerular filtrate enters Bowman’s capsule, it passes into the tubular system of the nephron.

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As the glomerular filtrate flows through the proximal convoluted tubule, almost all the water, sodium ions, chloride ions, most of the bicarbonate; and all of the glucose, vitamins, and amino acids are reabsorbed into the surrounding network of capillaries. The cells lining the proximal convoluted tubule have millions of microvili to increase the re-absorption surface area, thus facilitating the rapid re-absorption of water and other essential materials.

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Water is reabsorbed by osmosis, whereas active transport is involved in the case of glucose, amino acids and vitamins. Energy for active uptake is provided by the numerous mitochondria located within the tubule cells themselves. The tubular fluid, now free of amino acids, glucose, and vitamins pass from the proximal convoluted tubule to the descending loop of Henle, and forum there to ascending loop.

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A sodium chloride concentration gradient is maintained in the tissue fluid surrounding the loop of Henle. The concentration is highest in the medulla (at the hairpin turn in the loop of Henle) and lowest in the cortex (at the region of the convoluted tubules). Concentration of the filtrate by osmotic reabsorption of water occurs as it flows in the collecting duct.

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As a result, the fluid in the duct, now called urine, eventually becomes isotonic with the fluid surrounding the duct, and at the end of the duct the urine is not only much more concentrated than the original glomerular filtrate but is also hypertonic to the blood plasma. The composition of urine is modified by the addition of large waste molecules, urea, uric acid and ions during tubular secretion at the distal tubules.

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Osmoregulation

Water is lost from the body in urine, faces, sweat and exhaled air. It is gained by eating and drinking. These activities will change the osmotic concentration of the blood. The maintenance of the osmotic concentration and volume of the blood, or osmoregulation is carried out by the kidneys using a negative feedback mechanism. The distal convoluted tubules and the collecting ducts are relatively impermeable to water in the absence of antidiuretic hormone (ADH). When little ADH is secreted, a large part of the filtrate is not reabsorbed in the distal convoluted tubule and the collecting duct, and becomes part of the urine.

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ADH is produced by the hypothalamus secreted from the posterior pituitary gland. The primary stimulus for ADH secretion is an increase in osmotic pressure of the blood. Osmoreceptors in the hypothalamus respond to the increase in osmostic pressure of the blood by stimulating an increase in ADH secretion. ADH causes the walls of the distal convoluted tubule and collecting, ducts to become highly permeable to water, so that water leaves the tubules and seeps back into the bloodstream. This loss of water concentrates the urine.

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If, on the other hand, the Osmoreceptors are not stimulated, as when the water in the body is at the normal level, ADH is not released, and the tubules remain impermeable to water. The result of an absence of ADH is the formation of dilute urine, since salt is reabsorbed but water is not. Diabetes insipidus is caused by a deficiency in the supply of ADH; reabsorption of water is low, and the urine produced is copious and quite dilute. Persons with this condition have an incessant thirst and suffer from a salt imbalance in the body fluids.

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Diabetes mellitus is characterized by the presence of sugar in the urine. This is caused by a failure to regulate properly the amount of glucose in the blood due to a amount of glucose, and only a friction of this can be reabsorbed. The excess glucose in the tubule increases the osmotic pressure of the contents and thus reduces reabsorption of water in the collecting ducts. The person will excrete large quantities of water and sugar.

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Body Temperature Regulation

Humans keep their internal temperature almost constant by maintaining a balance between heat gain and heat loss. The negative feedback loop which controls temperature centers on the hypothalamus constantly monitors the temperature of the blood that flows through it. On detecting a change in the temperature by thermoreceptors in the skin, and the hypothalamus itself, the hypothalamus responds by sending out nerve impulses to various part of the body, causing a decrease or increase in heat producing or heat-losing activities.