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Excretory System

Excretion: During catabolic activities of the body, the energy is produced which is utilized in performing different vital activities. Along with these activities certain waste products are also produced. These products are harmful for the body and affect the cellular activities adversely. The elimination of such waste products from the body is known as "excretion". Skin, liver, lungs, large intestine and kidneys are the excretory organs of the body. The excretory organs along with the elimination of waste products also help in performing the osmoregulatory function of the body.

CLASSIFICATION OF ANIMALS 'ON THE BASIS OF THE EXCRETORY PRODUCTS

The catabolic activities of carbohydrates and fats result in the production of CO2 and water as a waste product, whereas the main excretory products are formed due to the breakdown of proteins. On the basis of the production of protein breakdown, the animals are classified into following three types-

(1) Ammonotelic animals-The elimination of excess of nitrogen in the form of ammonia is known as ammonotelism. Ammonia is a toxic substance which has a great solubility in water. The marine animals (non¬chordates) have 0.4 mg to 0.48 mg ammonia per 100 ml of blood. e.g. protozoans, sponges, jelly fish, fresh water fishes, cephalopods and other molluscs and crustaceans.

(2) Ureotelic animals-When main excretory product is urea then phenomenon is ureotelism. The urea is mainly synthesised from ammonia and is less toxic than ammonia. e.g. man and all other mammals, aquatic animals like whales and seals, desert animals such as camels, kangaroos, rats, amphibians, cartilagenous fishes, reptiles (aquatic) etc.

(3) Uricotelic animals-The main excretory product is uric acid. It is synthesised by ammonia in the liver. It is far less toxic than urea and ammonia. e.g. gastropod, reptiles, birds, insects etc. It is mainly an excretory product of desert animals.

Extention in Lower Animals: The protozoans have the contractile vacuoles as the organs of osmoregulation. In coelenterates the excretory products are expelled in the coelenteron. The flame cells are the unit of excretion in flatworms whereas nephridia form the excretory system of the annelids. The green glands are the excretory organs of prawn and crabs but malpighian tubules perform the excretion in the insects and arachnids. In insects the fat bodies and nephrocytes are also performing the function of excretion.

Skin of mammals: They are provided with numerous sweat glands. The secretion of sweat glands has nearly the same composition to that of urine. Some excess salts are also filtered through the blood. Excess amino acids are converted into urea in the liver which is excreted through the kidneys. Bile pigments are the breakdown products of old RBC. Excess of cholesterol is also separated from the blood and mixed with the bile by the liver. Water vapours and CO2 are expelled out by the lungs. Certain heavy- salts which cannot be filtered through the kidneys, liver and sweat glands are directly poured into the large intestine by the blood capillaries. Inactive steroid hormones, other hormones and some vitamins are also excreted by the liver with bile and are expelled out by the large intestine with the faecal matter.

Extention in Higher Animals:

Excretory system also includes two ureters for the conduction of urine to the urinary bladder. One urinary bladder for temporary storage of urine and a urethra for conveying the urine from urinary bladder to the outside.

KIDNEYS

Lower and higher chordates have the specialized organs of excretion known as kidneys; but liver, skin, lungs and intestine also help in the excretion. The kidneys are paired structure. In man these are of metanephros type. These are situated in the abdominal cavity on both sides of the vertebral column Right kidney is posterior to the left one due to the presence of liver towards right side. These are covered by peritoneal membrane. The outer surface is convex but the inner surface is concave and has a shallow depression "the hilum". T.S. of kidney shows the outer cortex and inner medulla parts. There are above 2 million units of excretion 'nephrons' or 'kidney tubules' in both kidneys.

Each nephron consists of Bowman's capsule, a bunch of blood capillaries, the glomerulus, a proximal and a distal convoluted tubule. Both the convoluted tubules are connected by a 'U' shaped narrower tubule known as loop of Henle. The collecting tubule opens into the duct of Bellini which passes the urine into renal pelvis.

Incoming blood vessels or branches of arterioles of renal artery are called afferent arterioles while outgoing are called efferent arterioles.

Efferent arterioles are then branched and form a network of fine vessels around secretory part of the nephron.

These fine vessels then unite and form venules which again unite and form renal vein.

[Structure of human kidney has already been described in the chapter 'Histology of mammalian organs']

PHYSIOLOGY OF EXCRETION

In human beings, urine is filtered at + 15 mm of Hg.

(GHP = + 65 mm of Hg) - glomerular hydrostatic pressure

(BCOP = - 32 mm of Hg) - blood plasma colloidal osmotic pressure

(CHP = - 18 mm of Hg) - capsular hydrostatic pressure

Net effective filtration pressure (NEFP) = + 15 mm of Hg

Plasma filtered in 24 hours = 170 litres.

Urine filtered in 24 hours = 1.5 litres The hormone ADH which is secreted by the neurohypophysis of the pituitary gland with the help of osmoreceptor situated in the hypothalamus region regulates the water reabsorption. The secretion of ADH is also controlled by the intake of water. If intake of water is less, more is the secretion of ADH which will be affecting more reabsorption of water but process is reversed when water intake is high. Reflex action also controls the urine excretion in infants.

Steps-Three steps are involved in the urine formation-

(1) Ultrafiltration-It is performed in Bowman's cup of nephron. Net filtration pressure which is about + 15 mm of Hg is responsible for ultrafiltration.

(2) Reabsorption-Useful substan¬ces which are filtered under pressure into kidney tubules are reabsorbed by the network of blood vessels around them. The filtrate remained after reabsorption is called urine.,

(3) Secretion-This is the secretion of remained excretory matter into the kidney tubules from the network of blood vessels surrounding them.

COMPOSITION OF URINE

Urine is composed of organic and inorganic substances. Following are the quantities of different substances in total amount of urine in 24 hours-

(A) Organic substances

1. Urea-25-30 gram

2. Creatine – 60 – 150 gram

3. Creatinine - 1.2 - 1.7 gram

4. Ammonia - 9.3 - 1.0 gram

5. Hippuric acid - 0.1 - 1.0 gram

6. Amino acid – 150 - 250 mg

7. Uric acid- 0.5 - 0.8 gram

8. Oxalic acid - 10 - 30 mg

9. Vitamins, hormones and enzymes - Traces

(B) Inorganic substances

1. N2 – 25 - 35 gram

2. Chloride – 6 - 9 gram

3. Phosphate - 0.8 - 1.3 gram

4. Sulphate - 0.8 - 1.3 gram

5. Na – 4 - 5 gram

6. Ca - 0.1 - 0.3 gram

7. K - 2.5 - 3.0 gram

8. I2 – 50 – 250 u gram

9. Pb – 250 u gram

10. Mg - 0.1 - 0.2 gram

11. As - 50 u gram