Excretion

The process of removal of non-gaseous nitrogenous wastes like ammonia, urea, uric acid etc. along with excess of water, salts and pigments out of body, is called excretion. Main aim of excretion is to keep a constant internal chemical composition called homeostasis.  In simple words, excretion is the elimination of metabolic wastes from the body.

Table Features of Nitrogenous wastes

Name	Toxicity in Tissues	Solubility in water	Advantage	Disadvantage	Examples
1. Ammonia	High	Very good	Production costs little energy.	Needs much water for removal.	Aquatic invertebrates, bony fish, tailed amphibians.
2. Urea	Medium	Fair	Less toxic, can collect in tissues without harm, need less water for removal.	Production costs more energy.	Land invertebrates, cartilaginous fish, amphibians, mammals.
3. Uric acid	Low	Poor	Conserves nearly all body water.	Production needs a lot of energy.	Land insects, land reptiles, birds.

Excretion, Secretion and Defaecation

Defaecation or egestion is the removal of indigestible substances from the body, i.e. the passage of faecal matter through the anus. The difference lies in the fact that the faecal matter is not produced by metabolism. Secretion is the production of useful substance, such as enzymes and hormones, by metabolism

Organs of Excretion in Man

Various organs of excretion are :

1. Skin is one of the accessory excretory organs in man. It is richly supplied with blood vessels. As the blood flows through them, waste products like water, salts etc. diffuse out from the blood into the sweat glands. These waste products then leave out of the body through sweat pores in the skin. Evaporation of sweat helps to keep our bodies cool in summers.
2. Lungs also act as an excretory organ as it helps in eliminating carbon dioxide.
3. Liver is the largest gland present in the human body. It helps in excretion in various ways :

(a)   It converts harmful amino acids into harmless urea by deamination and detoxification.

(b)   It forms bilirubin and biliverdin from the haemoglobin of dead erythrocytes.

(c)   It also helps in the excretion of various toxic substances where they are changed to non-toxic ones like cholesterol and certain vitamins like vitamin C.

4. Kidneys are the main excretory organs in man. They are located on each side of the vertebral column about 10 cm long and6 cm wide. The right kidney is 1.25 cm lower than the left one.

On the basis of excretory products formed, organisms are classifed as :

Ammonotelic organisms are those which excrete ammonia e.g. protozoans, sponges, coelenterates, aquatic arthropods, few amphibians and crocodiles.

Uricotelic organisms excrete uric acid e.g. most of the insects, land snails, land reptiles, birds.

Ureotelic organisms excrete urea e.g. cartilaginous fish like shark, frogs, man and other mammals.

Ascaris and Earthworm are both ammonotelic as well as ureotelic.

The need for Excretion

The excretory processes play an important role in maintaining homeostasis in the body’s internal environment. If the excretory processes fail to eliminate metabolic wastes from the body of an organism, their accumulation in the body may lead to disturbances in osmotic relationships between blood, lymph and tissues, the delicate acid-base balance in the body, and several other physiological processes which may lead to death of the individual.

Nature of Excretory wastes

The wastes formed as a result of various metabolic (anabolic and catabolic) activities can be divided into two main groups-respiratory and nitrogenous waste products .

Respiratory waste products are formed as a result of oxidation of glucose (food) (cellular respiration). These mainly constitute carbon dioxide and water. The carbon dioxide is eliminated from the body through the lungs (during expiration), while water becomes a part of the rest of the water contained in the body .

Nitrogenous waste products are formed as a result of deamination of unwanted amino acids, body’s own proteins and nucleic acids. The three main nitrogenous waste products excreted by animals are ammonia, urea and uric acid. Urea is a highly poisonous waste product and its accumulation beyond a certain concentration in the body may cause death. Urea is eliminated through the kidneys.

In addition, there are other waste materials like salts (NaCl), vitamins and water, that we take directly through food, which need to be excreted out of the body.

Types of Excretion

The proteins and nucleic acids undergo metabolism (catabolism) to produce nitrogenous waste products. The immediate product of deamination of proteins is ammonia. It may be excreted immediately or converted into other compounds, such as urea and uric acid, which differ from each other in their water solubility and nature of toxicity. The exact type of the excretory waste product in any organism is determined by its metabolic capabilities. This in turn is dependent upon the following factors:

(i)      The metabolic capability of the organism, i.e. which enzymes are present,

(ii)     The availability of water to the organism, i.e. its habitat, and

(iii)     The extent to which water loss is controlled by the organism.

Based on the nature of major nitrogenous excretory products, excretion can be classified into three types-ammonotelism, ureotelism and uricotelism.

Excretion in plants :

Every living organism needs excretion for its stability since all the organisms are made up of cells. Cells perform their work all the time with millions of tasks and regulate billions of reactions. Therefore, many toxic substances stored in cells must be removed from the cells for their existence.

Plants do not have excretory organs. Plants produce less waste products. Main waste products of plants are CO₂, O₂, water vapour etc. These substances are excreted by the stomata of leaf and some times other parts of the plant like stem by the process of photosynthesis. Plants excrete carbon dioxide produced as waste during respiration. Plants excrete oxygen as waste product only during the day time when sunlight is there. Water vapour produced as waste by respiration is, however, excreted by plants all the time (day as well as night). Plants also store some of the waste products in their body parts. For example, some of the waste products collect in the leaves, bark and fruits of the plants (or trees). Plants get rid of these wastes by shedding off leaves, peeling off  bark and felling off  fruits. So when dead leaves, bark and ripe fruits fall off from tree, then waste products contained in them are get rid of. In some plants waste get stored in fruits in the form of solid bodies called raphides. These wastes are removed when fruits get detached from the plant. Raphides are solid waste product of a plant stored in its fruits like, yam (zamikand). These waste products i.e., raphides are made up of calcium oxalate crystals. The raphide crystals are needle shaped, which on eating hurt our throat.

For example, when we eat fruit called yam, then our throat hurts. This is due to the presence of needle-shaped raphides in yam fruit. The raphides (made of calcium oxalate) are soluble in acid. So, eating of acidic substances like tamarind helps in reducing the effect of raphides of yam in our throat.

Some of the plant wastes are also very useful to human beings such as → Natural rubber, resins, gums, and essential oils (e.g., sandalwood oil).

Excretion in Amoeba

Amoeba is an ammonotelic organism since the principal excretory product is ammonia. Special excretory organelle in Amoeba is lacking. Like CO₂, ammonia is excreted by diffusion in solution through plasma membrane. The concentration of ammonia is always higher in Amoeba than in the surrounding water.

 

Figure : Contractile vacuole in Amoeba

The water enters through plasma membrane by endosmosis. Water is formed in cytoplasm by metabolism. Surplus water enters contractile vacuole. This surplus water can result in rupturing animal’s body. Thus size of contractile vacuole increases. When the contractile vacuole is fully expanded with water, it moves towards the periphery. As it comes in close contact with the plasma membrane, the contractile vacuole bursts. Thus excess of water (surplus water) is discharged in the surrounding water. This phenomenon of controlling the amount of water in the body is called osmoregulation.

Table Excretory Organs of different animals groups

S.No.	Animal Groups	Excretory Organs
1	Protozoans (e.g. Amoeba, Paramecium)	Plasma membrane.
2	Sponges (e.g. Sycon)	Plasma membrane of each cell.
3	Cnidaria (e.g. Hydra)	Plasma membrane of each cell.
4	Platyhelminthes (e.g. Planaria)	Flame cells.
5	Nemathelminthes (e.g. Ascaris)	H-shaped excretory system of canals and renette cells.
6	Annelids (e.g. Neries, Earthworm)	Nephridia
7	Arthropods (a) Prawn (b) Most insects	Green glands Malpighian tubules.
8	Molluscs (e.g. Unio, Pila)	Kidney like structure
9	Echinoderms  (e.g. Starfish)	Tube feet.
10	Hemichordates (e.g. Balanoglossus)	Glomerulus.

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