HUMAN EXCRETORY SYSTEM
The human excretory system consists of a pair of metanephros kidney, a pair of ureters, a urinary bladder and a urethra.
The two kidneys are present on either side of the vertebral column in the lumbar region of the body. In case of man right kidney is slightly at a lower level than the left kidney as the right abdominal cavity is occupied by the liver. Each kidney is bean shaped, dark red and in adult human it measures about 10-12 cm in length, 5-7 cm in wide and 4 cm. thick. It weighs about 150-170 gm. in adult male and 135-150 gm. in female. The outer margin of each kidney is convex and the inner margin is concave. The inner concave margin possesses a depression known as hilus. The renal vein and renal artery pass in and out of kidney at the hilus. The ureter is originated from the hilus. Inner to the hilus is a broad funnel shaped space called the renal pelvis with projections called calyces. Each kidney is enclosed in a thin, tough fibrous capsule.
The kidney can be distinguished into two distinct regions in its longitudinal section. The outer dark red zone is called the cortex or cortical substance and the inner pale red zone is called the medulla or medullary substance. The medulla is divided into few conical masses or labules called pyramids. The pyramids are projecting into the calyces. The cortex innervates the pyramids as renal columns called the columns of Bertini.
The kidney is a complex structure consisting more than one million of uriniferous tubules or nephrons. The nephron is the structural and functional unit of the kidney as the entire process of urine formation occurs in the nephron. The nephron arises in the cortex and traverses into the medulla and ultimately terminates in the cortex by communicating with the collecting tubule. A number of collecting tubules unite together to form a larger tube termed as duct of Belini which ultimately opens into the renal pelvis.
Each nephron of human kidney is about 35 to 55 mm in length and 25 to 60 um in diameter. Each nephron begins as double walled blind sac termed as Bowman’s capsule. It is lined by squamous epithelium. The capsule contains a tufts of capillaries known as glomerulus. Bowman’s capsule and glomerulus together constitute the Malpighian corpuscles or renal capsule. The tuft of capillaries of the glomerulus are formed by the afferent arterioles-a fine branch of renal artery. Blood from the glomerulus is carried away by the efferent arterioles whose diameter are smaller than the afferent arterioles. Basement membrane of the capsule acts as a dialyzing membrane.
The cavity of the Bowman’s capsule continues into the proximal convoluted tubule (PCT). It lies in the cortex part of the kidney. The proximal convoluted tubule is followed by U-shaped loop of Henlee. The loop of Henlee consists of descending and ascending limbs. The ascending limb continues as highly coiled distal convoluted tubule (DCT).
Number of distal convoluted tubules open into the collecting tubules or collecting ducts. The collecting tubules from different nephrons join one after another to constitute the large duct known as duct of Bellini which ultimately forms the ureter. The malpighian corpuscles, proximial convoluted tubules and distal convoluted tubules are present in the cortical regions of the kidney while the medulla contains the loop of Henlee.
The nephrons may be of two types-some nephrons possess very short loop of Henlee which extends very little to the medullary region and so these are known as cortical nephrons and the other type of nephrons having long loop of Henlee run into the medullary regions and are known as juxta medullary nephrons.
Kidney has rich blood supply. The blood flows into the kidney through two consecutive sets of capillaries, the capillaries of the glomerulus and the capillaries that supply the renal tubules. The renal arteries after emerging the kidney through the hilus branch into smaller and smaller vessels to form the arterioles. These arterioles are called afferent arterioles. Each of these arterioles breaks into capillarles in Bowman’s capsule and form the glomerules. There are about 50 capillaries in each glomerules.
These capillaries rejoin to form the efferent arterioles which carry the blood out of the Bowman‘s capsule. As the diameter of the efferent arteriole is smaller than the afferent arteriole, it causes the raising of glomerular pressure. After leaving the glomerulus, the efferent arteriole again breaks into capillaries around the convoluted tubules which form the vasa recta. The blood from the convoluted tubule is drained by venules which unite to form the renal vein. The renal vein comes out of kidney through the hilus.
The blood pressure is found to be the highest (about 70 to 90 mm. Hg) in the glomerular capillaries. This amount of blood pressure is not found in any other capillaries of the body. This high blood pressure facilitates the glomerular filtration.
ROLE OF OTHER ORGANS IN HUMAN EXCRETORY SYSTEM
(ACCESSORY EXCRETORY ORGANS)
In addition to the kidney, vertebrates possess other excretory organs to remove the excretory products from the body. These are :
1. Skin or integument. In number of aquatic animals where excretory organs are not well developed, excretory substance are removed from the body by diffusion through the general body surface. Ammonia is mainly diffused from the body through the integument. Higher animals like mammals including man possess two types of glands namely sebaceous glands and sweat glands in their skin.
Average Composition of Urine (of an adult normal man taking a mixed diet in 24 hours)
1.Nitrogen (total) – 23-35 gm
2.Urea – 25-30 gm
3.Creatine – 60-150 gm
4.Creatinine – 1.2-1.7 gm
5.Uric acid – 0.5-0.8 gm
6.Ammonia – 0.3-1.0 gm
7.Hippuric acid – 0.1-1.0 gm
8.Amino acid – 150-200 mg
9.Vitamins. hormones and enzymes – traces
10.Allamoin – traces
1.Chloride (As NaCl) – 10-15 gm
2.Chloride – 6-9 gm
3.Phosphate – 0.8-1.3 gm
4Sulphate – 0.8-1.3 gm
5.Sodium – 4-5 gm
6.Potassium – 2.5-3.00 gm
7.Calcium – 0.1-0.3 gm
8.Magnesium – 0.1-0.2 gm
9.Iodine – 50-200 mgm
10.Lead – 50 ugm
11.Arsenic – 50 pgm
12.Iron – 0.006 gm
13.Water – 1000-1500 ml
The sebaceous glands of the skin are located near the hairs, the duct of which opens into the hair follicles. These are holocrine glands and composed of stratified epithelium. It secretes the sebum which mixes with the sweat on the surface and making the skin softer and lubricating the hair. Some excretory substances like waxes, fatty acids, sterols are excreted from the body in the form of sebum.
The sweat glands are embedded in the sub-cutaneous layer of the skin and are distributed evenly over the body surface. The glands secrete sweat. The sweat is responsible for elimination of certain breakdown products of metabolism. help in the loss of body heat and maintaining the osmotic concentration of the body fluid by eliminating the water and salts. However, small amount of sodium chloride, urea, lactic acid are also eliminated form the body along with the sweat.
2. The liver removes the bile pigments namely biliverdin and bilirubin formed by the decomposition of the haemoglobin 0f the worn out RBC through the bile. These pigments pass into the alimentary canal along with the bile and finally removed with the foecal matter. The liver eliminates cholesterol, steroid hormones, some vitamins and drugs via bile. Moreover, ammonia produced from the deamination of amino acids combines with carbon dioxide to form urea. The process is known as ornithine cycle. Uric acid is also produced in liver in uricotelic animals. The damaged liver cannot remove the bile pigments and so the pigments accumulate in the blood and cause jaundice; The excess pigments import yellowish tinge to the skin.
3.Lungs eliminate carbon dioxide (18 litres/day) and water in the form of water vapour formed during respiration. Water vapour and C02 are removed by the lungs during expiration.
Calcium, magnesium and iron ions are excreted internally by the epithelial cells of the intestine which are later excreted along with the faecal matter.
DISORDERS OF THE HUMAN EXCRETORY SYSTEM
In uremia, urea level of the blood rises abnormally which may cause the failure of the kidney function. As a result metabolic waste are accumulating in the blood which may cause many physiological disorders. In such conditions, artificial measures are adopted to remove all these waste products from the blood. The process is called hemodialysis and the apparatus used for this is known as hemodialiser or artificial kidney. In hemodialysis, the blood of the patient is taken out from a convenient artery and cooled to 0°C. Then an anticoagulant (like heparin) is added to the blood to prevent coagulation.
The blood is then pumped into the hemodialiser. The blood flows in the apparatus through tubes bounded by collophane membrane. This membrane is impermeable to protein but permeable to urea, uric acid, creatine. Thus the membrane filters the permeable substances from the blood into a salt solution where the tubes are immersed. Then the blood free from these metabolic wastes is warmed to body temperature and are mixed with antiheparin to restore its normal coagulability. This purified blood is then pumped back into the body of the patient through a vein. The Whole process is commonly known as dialysis.
The acute renal failure in human can be corrected by transplantation of damaged kidney by a functional kidney. A kidney from a donor, preferably a close relative having the same blood group may be used for transplantation to minimise its chances of rejection by the immune system of the host.
Formation of stone calculi is another problem that occurs in most of the cases. The stone in the kldney is mainly formed by the oxalate and phosphate crystallised salts within the kidney. The calculi In the kidney are mostly removed by surgical procedure.
Inflammation or malfunctioning of the glomerulus of the kidney may lead to a condition known as glomerulonephritis which ultimately results severe metabolic disorder within the body.