The movements of breathing are innervated from a nervous centre situated in the medulla oblongata, in the grey substance of the floor of the fourth ventricle a little above the point of the calamus scriptorius. The part above this may be sliced away without stopping the respiratory movements, but they completely cease when the part indicated is destroyed.
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It is vital that the flow of blood through the kidney be at a suitable rate to allow for filtration. This rate determines how much solute is retained or discarded, how much water is retained or discarded, and ultimately, the osmolarity of blood and the blood pressure of the body.
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Several hormones have specific, important roles in regulating kidney function. They act to stimulate or inhibit blood flow.
Some of these are endocrine, acting from a distance, whereas others are paracrine, acting locally.
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Having reviewed the anatomy and microanatomy of the urinary system, now is the time to focus on the physiology. You will discover that different parts of the nephron utilize specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.
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The kidney is covered by a fibrous capsule, which is slightly attached at its inner surface to the proper substance of the organ by means of very fine bundles of areolar tissue and minute blood vessels.
From the healthy kidney, therefore, it may be easily torn off without much injury to the subjacent cortical portion of the organ. At the hilus of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter.
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The urinary system’s ability to filter the blood resides in about 2 to 3 million tufts of specialized capillaries—the glomeruli—distributed more or less equally between the two kidneys. Because the glomeruli filter the blood based mostly on particle size, large elements like blood cells, platelets, antibodies, and albumen are excluded. The glomerulus is the first part of the nephron, which then continues as a highly specialized tubular structure responsible for creating the final urine composition. All other solutes, such as ions, amino acids, vitamins, and wastes, are filtered to create a filtrate composition very similar to plasma. The glomeruli create about 200 liters (189 quarts) of this filtrate every day, yet you excrete less than two liters of waste you call urine.
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