Dictionary of Keel, Dictionary of Kidney, Dictionary of Keratin, Malipighian body, organ of vertebrates,Body Fluids,Human Kidney, Inner Medulla, Cortex, Mass of Tubules, Malipighian Body,Glomerulus

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Home >>Zoology Dictionary >> Keel - Kidney

Your Ad Here	Keel - Downward projection of breastbone of birds to which the large flight muscles are attached. Very reduced in flightless birds. Keratin - Tough protein formed in the outer layer of the skin and which also forms hair, feathers, nails, claws, hooves and horns. Kidney - Excretory organ of vertebrates. They are conerned with the removal of waste, conservation of water, and the maintainance of the pH and composition of the body fluids. The detailed structure varies from species to species according to the conditions in which it lives (see Excretion) but the basic structure is similar to that given here for the human kidney.
A lengthwise section through a kidney shows two main zones: an outer cortex and an inner medulla. The whole is encased in a protective, fatty capsule. Within the cortex and medulla are masses of tubules. They make up the bulk of the kidney tissue and join up with larger collecting tubes that eventually join the ureter which channels urine from the kidney to the bladder. Each kidney tubule has a rich blood supply: the renal artery and renal vein (together with the ureter) are prominent vessels entering or leaving the kidney. The blind end of each kidney tubule lies in the cortex and may be likened to a champagne glass, the walls and stem of which are hollow and one cell thick. The 'bowl' of the tubule is called Bowman's capsule and it surrounds an elaborate knot of blood capillaries called the glomerulus. Each Bowman's capsule and glomerulus together form the Malipighian body there are at least a million such structures in a single human kidney!
The hollow stem of the champagne glass is the upper part of the kidney tubule. This descends into the medulla where it narrows before turning back into the cortex increasing in diameter again. The thin portion of the tubule is known as the loop of Henle and is primarily concerned with the absorption of water. The ascendinglimb of the tubule joins a collecting duct which joins with others before eventually dischargin it urine into the ureter. In effect each Bowman's capsule is a tiny filter. Blood containing waste substances, proteins, sugars, etc., passes into the kidneys and, under pressure a solution is driven out of the capillaries of the glomerulus through the walls of the capsule into its hollow interior. The solution in the capsule is blood plasma minus the large molecules that are too big to pass through the capillary wall.	Your Ad Here
From the capsule the fluid passes along the tubule. Many of the substances in it are reabsorbed through the tubule wall into the blood capillaries surrounding it. Organic molecules such as some salts and water are absorbed in the descending or proximal limb of the tubule, whilst more water and salts are absorbed in the ascending or distal limb. Certain poisonous substances that have been rendered harmless (detoxicated) are released into the urine by the tubules, together with potassium and hydrogen.
Your Ad Here	Measurements of the amounts of various substances present in the blood and in the urine show striking differences. The concentration of urea the principal waste substance in the urine is normally seventy times as great as it is in the blood, yet, surprisingly enough, some urea is reabsorbed, since a certain blood concentration is needed. Ninety times more sulphate, nine times more potassium and 25 times more uric acid occur in the urine than in the blood. On the other hand no glucose, fatty substances, prote us or bicarbonate ions normally occur in the urine, whereas considerable quantities occur in the blood. The normal daily output of urine is between two and a half to three pints, yet it is calculated that a total of nearly 300 pints of thuid is filtered by the kidneys during that time. They not only reabsorb a vast volume each day they also regulate its content so that the correct levels of essential substances are maintained in the body fluids.

The function of the various parts of the tubule has been analysed by inserting tiny pipettes into them and withdrawing small quantities of the fluid. Differences in the concentrations of substances in the fluid in different parts are strong evidence as to the function of a particular part. The content of the urine varies considerably with the type of diet that a person eats, however. For example, the nitrogen content of the urine is much higher in someone taking a protein rich diet than in another who is eating starch rich food, mainly due to the increased production of urea.

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