Gross Anatomy And Physiology Of The Kidneys Notes
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Gross Anatomy and Physiology of the Kidneys
1. Location The kidneys are paired organs which reside in the dorsal abdomen, one on each side. In domestic species the right kidney is always located more cranially than the left. The cranial pole of the right kidney fits into a fossa of the liver which secures its position. The left kidney does not have this association and so is more mobile. Each kidney lies within a splitting of the sublumbar fascia. On the upper pole of each kidney, embedded in the renal fascia, is an adrenal gland.
2. Gross anatomy The surface of the kidneys is smooth and convex, except for an indentation of the medial border. This leads to a concealed space known as the renal sinus. The renal sinus contains the dilated origin of the ureter, known as the renal pelvis, and vessels and nerves that pass to and from the renal hilus. The kidney parenchyma is contained within a tough fibrous capsule. The capsule restricts the kidney's ability to expand. It can be stripped easily from healthy kidney parenchyma but adheres to underlying scarred areas. The capsule has two distinct layers - an outer layer of fibroblasts and collagen fibres, and an inner layer of myofibroblasts. The parenchyma is divided into and outer cortex and an inner medulla. The cortex is a redbrown colour and finely granular. The medulla has a dark purple coloured outer zone. Medullary rays are aggregations of straight tubules and collecting ducts. They extend from the outer zone into the cortex. The inner zone is paler and radially striated, and extends towards the renal sinus.
There are clear species differences in the medulla of the kidney. In some species, it is arranged as discrete pyramidal masses, each associated with a portion of cortex topping the base of the pyramid. The apex of the pyramid points towards the renal sinus and forms a papilla that fits into a cup-like expansion, or calix, of the renal pelvis. Each medullary pyramid and its associated cortex form a renal lobe. Kidneys arranged in this way are multipyramidal or multilobar. In some species, such as cattle, the boundaries between the lobes are also marked by fissures seen on the surface of the kidneys. All mammalian kidneys go through a multilobar stage in development. In some species the number of lobes is later drastically reduced, and in others the pyramids fuse to form a single medullary mass with a shell of cortex surrounding it. The functional units within the kidneys are called nephrons or renal tubules. They are epithelial tubules supported by connective tissue interstitium. Each nephron has a blindending expansion invaginated by a cluster of capillaries known as a glomerulus. The glomerulus and its epithelial lining together constitute a renal corpuscle. The renal corpuscles are scattered throughout the cortex and give it its granular appearance. The last part of the nephron is a long tubule divided into segments. The first portion, the proximal convoluted tubule, is located close to the corpuscle from which it arises. The tubule then straightens and enters a medullary ray. The tubule then forms a hairpin loop, also called the loop of Henle. The first descending part of the loop is narrow and runs through the medulla to the papilla before turning back on itself. The ascending limb is thicker and runs back into the medullary ray. After leaving the medullary ray, the tubule forms a distal convoluted portion that is also close to the original corpuscle. A short portion of the tubule runs to join a collecting tubule within the medullary ray. Each collecting tubule serves many
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