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Structure of Kidney

a. Renal Capsule
Each kidney is encased in a transparent, fibrous membrane called a renal capsule. This membrane protects the kidney againts trauma and infection. The capsule is composed of tough fibres, chiefly collagen and elastin (fibrous proteins), that helps to support the kidney mass and protect the vital tissue from injury. The capsule receives its blood supply ultimately from the interlobar arteries, small vessels that branch off from the main renal arteries. These vessels travel through the cortex of the kidney and terminate in the capsule. This membrane usually 2 to 3 milimeters thick.
kidney structure

The capsule surrounds the outer walls and enters into a hollow region of the kidney known as the sinus. The sinus contains the major ducts that transport urine and the arteries and veins that supply the tissue with nutriens and oxygen. The capsule connects to these structure within the sinus and lines sinus wall.
In a normal person, the capsule is light reddish-purple in colour, translucent, smooth, and glistening. It can usually be easily stripped fro the rest of the kidney’s tissue. A diseased kidney frequently sends fibrous connections from the main body of tissue to the capsule, which makes the capsule adhere more strongly. Difficulty in removing the capsule is noted at autosy as an indication that the kidney was deseased.

b. Renal Cortex
Renal cortex is the outermost layer of the kidney. It is situated between Renal Capsule and Medulla. Upper part of nephron which is Glomerulus and Henle’s loop are situated in this layer. Renal cortex is a strong tissue that protect the inner layer of the kidney. The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections (cortical columns) that extend down between the pyramids. It contains the renal corpuscles and the renal tubules except for parts of the loop of Henle which descend into the renal medulla. It also contains blood vessels and cortical collecting ducts. The renal cortex is the part of the kidney where ultrafiltration occurs.
c. Renal Medulla (Renal Pyramids)
Renal Medulla lies beneath the Cortex. It is an area that contains between 8 and 18 cone-shaped section known as pyramids, which are formed almost entirely of bundles of microscopic tubules. The tips of these pyramids point toward the centre of the kidney. These tubules transport urine from the cortical, or outer, part of the kidney, where urine is produced, to the calyces, or cup-chaped cavities in which urine collects before it passes through the ureter to the bladder. Space between the pyramids filled by cortex and forms structures called renal columns.
The tips of each pyramid, called the papilla, point toward to the calyx at centre of the kidney. The surface of the papilla has a sievelike appearance because of the many small openings from which urine droplets pass. Each opening represents a tubule called the duct of Bellini, into which collecting tubules within the pyramid converge. Muscles fibres lead from the calyx to the papilla. As the muscle fibres of the calyx contract, urine flows through the ducts of Bellini into the calyx. The urine then flows to the bladder by way of the renal pelvis and ureter.
d. Renal Pelvis
Renal Pelvis is extend in the center of each kidney as the tube through which urine flows from the kidney to the urinary bladder. The shape of renal pelvis is like a funnel that is curved to one side. Renal pelvis is almost completely enclosed in the deep indentation on the concave side of the kidney, the sinus. The large end of the pelvis has roughly cuplike extension, called calyces. The calyces’ are cavities in which urine collects before it flows on the urinary bladder.
Renal pelvis is lined with a moist mucous-membrane layer that is only a few cells thick; the membrane is attached to a thicker coating of smooth muscle fibres, which, in turn, is surrounded by a layer of connective tissue. The mucous membrane of the pelvis is somewhat folded so that there is some room for tissue expansion when urine distends the pelvis. The muscle fibres are arranged in a longitudinal and a circular layer. Contractions of the muscle layers occur in periodic waves known as peristaltic movement. This movement push urine from the pelvis into the ureter and bladder. The lining of the pelvis and of the ureter is impermeable to the normal substances found in urine; thus, the walls of these structures do not absorb fluids.
e. Renal Vein and Renal Artery
Two of the body’s crucial blood vessels, renal vein and renal artery. This two vessel are branch of from the abdominal aorta (the abdominal portion of the major artery leading from the heart) and enter into each kidney by attach to the concave part of the kidney.
At the inner concavity of each kidney there is an opening, known as the hilum, through which the renal artery passes. After passing through the hilum, the renal artery divides ordinarily into two large branches, and each branch divides into a number of smaller arteries, which bring blood to the nephrons, the functioning units of the kidney. Blood that has been processed by the nephrons ultimately reaches the renal vein, which carries it back to the inferior vena cava and to the right side of the heart.
The renal arteries deliver to the kidneys of a normal person at rest 1.2 litres of blood per minute, a volume equivalent to approximately one-quarter of the heart’s output. Thus, a volume of blood equal to all that found in the body of an adult human being is processed by the kidneys once every four to five minutes. Although some physical condition can inhibit blood flow, there are certain self-regulatory mechanisms inherent to the arteries of the kidney that allow some adaptation to stress.
When the total body blood pressure rises or drop, sensory receptors of the nervous system located in the smooth muscle wall of the arteries are affected by the pressure changes, and, to compensate for the blood pressure variations, the arteries either expand or contract to keep a constant volume of blood flow.

f. Nephron
 The most important function of kidneys is to remove waste substances from the blood. Nephrons are the functional unit of the kidney in performing this task. Nephrons produce urine in the process of removing waste and excess substances from the blood. There are about 1.000.000 nephrons in each human kidney. These remarkable structure extend between the cortex and the medulla. Under magnification, nephrons look like tangles of tiny vessels or tubules, but each nephron actually has an orderly arrangement that makes possible filtration of wastes from the blood. Each nephron in the mammalian kidney is about 30-55 mm long. At one end of nephron is closed, expanded and folded into a double-walled cuplike structure. This structure, called the corpuscular capsule, or Bowman’s capsule. This capsule enclose glomerulus, the nephron’s primary structure in filtering function.

kidney function

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