Question

Explain all the process of excretion and nutrition.

Answer 1

The processes of nutrition:

1. Ingestion, for which may be substituted more or less satisfactorily, rectal injection, inunction of fats, subcutaneous injection of salts, dextrose, etc., in solution, as well as gav-age, or feeding through a fistula;

2. Digestion, which does not apply to inorganic matters nor to organic foods already in the state suitable for absorption. Digestion is commonly divided into mechanic, including the comminution and softening of food, and chemic;

3. Absorption, through animal membrane, into the blood or lymph vessels;

4. Assimilation, which literally implies that the food is rendered like the body itself, and which, therefore, refers strictly only to the utilization of certain foods to repair or form tissue; the term is, however, usually extended to indicate the final utilization of any organic, or even inorganic, substance. Following assimilation, after a shorter or longer period,

5. Catabolism, which, like digestion, applies only to certain organic substances and which usually occurs in successive steps, often involving several organs;

6. Elimination of ultimate waste products, by the skin, lungs, kidneys and, to some extent, through the liver directly or even through the alimentary mucous membrane, independently of the liver. Substances which escape absorption in the alimentary canal are wasted rather than waste products.

Excretion:
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys and skin.

Excretion in animals:

1) Many unicellular organisms like Amoeba throw out their wastes by diffusion from their body surface. Protozoan’s have no organs for excretion. As they live in an aquatic habitat, their wastes are eliminated by diffusion through the plasma membrane.

2) Simple multicellular organisms like Hydra throw out solid waste matter through their mouth. Higher multicellular organisms have well-defined specialized excretory organs. These organs could be simple tubular structures as in flatworms and leech.

3) The excretory organs of insects (e.g., grasshopper, cockroach and housefly) are also tubular. They remove nitrogenous wastes from the body fluid and help in maintaining the water balance in the body.

4) In vertebrates, the main organs of excretion and maintenance of water balance are the kidneys.

Excretion in human beings:

Although the kidneys are the main organs of excretion, the skin, lungs and liver also help in excretion.

Skin:

Our skin has sweat glands, through which we excrete small amounts of water, urea and salts.

Liver:

The liver excretes bile, which contains bile pigments. These are produced by the breakdown of old RBCs in the liver. As hemoglobin breaks down, its iron is retained, while the pigment (haem) is excreted with the bile. The liver also excretes cholesterol.

Lungs:

The lungs help in getting rid of carbon dioxide, formed as a result of cellular respiration, through exhalation.

Kidney: Kidney is a bean-shaped organ which lies near the vertebral column in the abdominal cavity. The kidney is composed of many filtering units; called nephrons. Nephron is called the functional unit of kidney.

Nephron: It is composed of a tangled mess of tubes and a filtering part; called glomerulus. Glomerulus is a network of blood capillaries to which renal artery is attached. The artery which takes blood to the glomerulus is called afferent arteriole and the one receiving blood from the glomerulus is called efferent arteriole. Glomerulus is enclosed in a capsule like portion; called Bowman’s capsule. The Bowman’s capsule extends into a fine tube which is highly coiled. Tubes from various nephrons converge into collecting duct; which finally goes to the ureter.

Filtration in Glomerulus: Filtration happens because of very high pressure inside the glomerulus. The lumen of efferent arteriole is smaller than that of afferent arteriole. Due to this, the blood entering the glomerulus experiences very high pressure and due to this, the waste products are filtered out through the thin membrane of capillaries in the glomerulus. The filtered blood is sent to the systemic circulation through efferent arteriole and the filtrate goes to the Bowman’s capsule. That is how urine is formed inside the kidneys. Reabsorption of water and some other filtrates takes place in the tubular part of the nephron. This increases the concentration of urine. The human urine is mainly composed of water and urea.