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Man is a highly evolved vertebrate and so the excretory organs are well developed.
Kidneys
The kidneys are the principal excretory organs in man through which the nitrogenous metabolic wastes are eliminated in the form of urine.
- The kidneys function as a pair of filters through which about one litre of blood circulates each minute. The entire blood in the body passes through them in 5 - 6 minutes. In a day it filters 1800 litres of blood which is 400 times the blood volume.
- Tea, coffee and alcohol are beverages that increase the formation of urine. They are termed diuretics.
- The human kidney contains more than 1.2 million nephrons.
- When a kidney is damaged or diseased, the other kidney is able to compensate for it. So a person can lead a normal life with only one kidney.
- A normal adult excretes 1 - 1.8 litres of urine per day.
- An artificial kidney (dialysing machine) is used for dialysis. When the kidneys of a patient stop functioning, life can be prolonged through dialysis.
- Kidney transplants have had good success rates and have given fresh lease of life to patients.
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The kidneys are reddish brown, bean-shaped organs situated in the abdominal cavity, one on either side of the vertebral column in the lumbar region of the body. They lie asymmetrically, the right kidney being lower than the left as the right side of the abdominal cavity is occupied by the liver. Each kidney is 10 cm long, 6 cm wide and 4 cm thick and weighs 200 - 250 g in adults. A thin, tough, fibrous whitish capsule envelops each kidney. The outer surface of each kidney is convex while the inner surface is concave. Associated with the kidneys are the following:
Renal Artery
The renal artery enters the kidney at the notch on the concave side called the hilus renalis. As it is a branch from the aorta it brings oxygenated blood carrying toxic nitrogenous wastes into the kidneys.
Renal Vein
The renal vein drains away deoxygenated blood which is free of toxic substances into the inferior vena cava. This vein arises from the hilus of each kidney.
Ureter
The ureter, a tube, runs from each kidney downwards into the lower part of the abdomen connecting each kidney to the urinary bladder. Its function is to transport the urine from the kidneys to the urinary bladder. The oblique entrance of the ureter into the bladder acts like a valve, preventing the back flow of urine from the bladder into the ureter.
Urinary Bladder
This is a large muscular storage sac that collects urine from both the kidneys through the ureters. As the urine gets drained into the bladder its volume increases. The mouth of the bladder is guarded by a tight ring of muscle called the sphincter which regulates the opening or closing of the bladder. When the sphincter relaxes, urine is released out through the urethra.
Urethra
This is a short muscular tube that carries urine at intervals from the urinary bladder to the outside. The base of the urethra is also guarded by a sphincter which keeps the urethra closed except while passing urine.
A longitudinal section of the kidney shows three distinct regions:
- Renal Cortex
- Renal Medulla
- Renal Pelvis
Renal Cortex
This is the outer dark red layer. It contains the malphigian corpuscles, the proximal and distal parts of the renal tubule.
Renal Medulla
This forms the inner pale zone and contains the Henle's loop and the collecting tubules, present in the form of renal pyramids. The conical pyramids project into the renal pelvis.
Renal Pelvis
This is a large funnel-shaped region behind the renal medulla. Urine flows into the renal pelvis through minute openings at the tips or papillae of the pyramids. The urine that is collected is passed down to the ureters into the bladder.
Each kidney consists of a large number of microscopic filtering units called nephrons. There are more than 1,250,000 nephrons in each kidney. Each nephron is about 30 - 40 mm. in length, originating in the cortex and extending into the medulla.
The nephron is the structural and functional unit of the kidney. Each nephron functions as an independent unit and produces a miniscule quantity of urine. The nephron can be differentiated into the following regions:
This consists of two parts:
Bowman's Capsule
This is a cup - shaped structure which is double walled in the hollow of which is a net work of capillaries called the glomerulus.
Glomerulus
This is a knotted mass of blood capillaries formed by the afferent arteriole (incoming) and the efferent arteriole (outgoing).
This is the remaining part of the nephron, continuous with the Bowman's capsule. It is lined with ciliated epithelium and differentiated into the following regions:
Proximal Convoluted Tubule (PCT)
This is the region behind the Bowman's capsule and consists of a coiled tube that descends to form the Henle's loop. The proximal convoluted tubule is present in the cortex region.
Henle's loop
This is continuous with the proximal convoluted tubule and is U-shaped having a narrow descending limb and a thick ascending limb. This part of the nephron descends from the cortex into the medulla region.
Distal Convoluted Tubule (DCT)
This is another coiled and twisted tubule that continues from the ascending limb of loop of Henle found in the renal cortex.
The distal convoluted tubule continues to form the collecting tubule.
Collecting Ducts
Several collecting tubules fuse to form large collecting ducts which pass downwards from the cortex to the medulla region.
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Ducts of Bellini
Several collecting ducts join to form the larger ducts of Bellini in the medulla region. Running through the pyramids they finally open in the renal pelvis draining all the urine collected from the various nephrons. The renal pelvis is an expanded region which opens into the ureter.
Vasa Rectae
The efferent arteriole that exits from the glomerulus does not merge into a vein but breaks up into a system of capillaries called vasa rectae or peritubular capillaries which envelop the proximal convoluted tubule, the Henle's loop and distal convoluted tubule. These capillaries rejoin and drain into the venule. Venules join together to form the renal vein which drain blood from the kidneys.
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