Besides the kidney, the lungs, liver and skin also excrete waste products.

     The lungs excrete carbon dioxide and water , the waste products of cellular respiration.

      The liver cells make bile which is needed for the digestion of fats. The bile contains bilirubin, a greenish yellow pigment which is a break down product of haemoglobin. Bilirubin is harmful if allowed to accumulate in the body. When bile is secreted into the small intestine during digestion of food, the bilirubin is also got rid of . It undergoes changes in the intestine and leaves the body in the faeces, which owes its yellowish brown colour to this pigment. Thus, by excreting bilirubin via bile, the liver acts as an excretory organ.

       The skin of humans contains sweet glands. We control our body temperature through producing sweat. Sweat contains water with sodium chloride and traces of urea dissolved in it. The substances in sweat can be considered as excretory products. However, the only purpose of sweating is to cool the body. Excretion of substances in sweat is thus incidental.  

 An earthworm has a segmented body. Each segment has a pair of tubes called nephridia. These are the excretory organs of the earthworm. Each nephridium has a ciliated funnel, the nephrostome, at one end. This opens into the body cavity of the segment in front. The nephrostome leads into a long coiled tube made up of the following parts:

 • Narrow ciliated tube, •

• Middle ciliated tube,

• Wide, non ciliated tube, and

 • Muscular tube which opens to the exterior via an excretory pore.

     The nephridium thus opens at both ends, unlike the flame cells which are blind tubules opening only at one end to the exterior.

    Each nephridium is surrounded by s capillary network. The waste products, mainly urea are extracted from the blood in the capillaries surrounding the nephridia. Wastes are also removed from the fluid in the body cavity and wastes moves through the long tubes of the nephridia. Along the way, salts and other reabsorbed through the walls of the tubes. This composition and concentration of the body fluid constant.

     The unabsorbed substances, including water collect in the muscular tube as hypotonic urine. The sphincter (ring of muscle) guarding the excretory pore relaxes to allow the urine to escape to the exterior.

     Although the earthworm is terrestrial, it Lives in a moist environment since it needs a moist skin surface for gaseous exchange. Carbon dioxide is excreted by diffusion through the skin. The earthworm cannot survive in a dry environment. Within limits, it can tolerate dry conditions and conserve water by producing very little urine which is hypertonic.  

 The processes involved in the formation of urine are as follows:

 • ultra filtration,

 • selective reabsorption, and

 • tubular secretion.

     As blood circulates through the glomerulus, ultra filtration occurs. Small molecules such ad water, urea, mineral salts, sugar and plasma solutes pass through the one cell thick walls of the capillaries and the Bowman capsule into the capsular space. Bigger molecules like plasma proteins and the blood cells cannot pass through this barrier which thus act as a filter. a high pressure in the glomerulus is essential for the filtration process. This is brought about in the following ways:

 • The blood entering the kidney is already at high pressure because the renal artery branches off from the dorsal aorta at only a short distance from the heart.

 • The blood vessel leaving each glomerulus is narrower than the one entering it, thereby increasing the pressure of the blood in the glomerulus further.

    The fluid that filters into the Bowman's capsule is known as glomerular filtrate. It flows down of the tubule. As it passes through the proximal part of the tubule and the Henle loop, selective reabsorption takes place. In this process, water and useful substances like sugar, amino acids and salts are reabsorbed into the surrounding blood capillaries. The filtrate then moves into the distal part of the tubule. Here, large waste molecules like creatinine are secreted into the tubules. If necessary, ions (hydrogen, potassium and hydrogen carbonate) are secreted into the tubules to keep the osmotic concentration of the blood constant. The fluid that eventually remains in the tubule is concentrated and is known as urine.

    An average of 1.5 litters of urine is produced daily. The amount of urea excreted will depend on the protein content of the daily diet. The filtered blood leaving the kidney by the renal vein contains

 • less oxygen and glucose, and mor carbon dioxide, as a result of cellular respiration; and less nitrogenous wastes, salts and water as a result of excretion.

    The urine formed trickles down the ureter and collects in the bladder. When the bladder is full, it contracts discharging the urine out of the body through the urethra.

Note How the kidney helps to maintain a constant internal environment is going to be discussed in my next topics.  

 Insects are a very successful group of animals, with a remarkable ability to conserve water so that they can be found in the hottest and driest places on earth. Their ability to conserve water can be attributed to several factors:

 • their outer surface is waterproof because of a layer of wax;

• the spiracles prevent water loss from the gaseous exchange surface which is inside the body; and

 • they have an extremely efficient excretory system.

 The excretory organs are the malpighian tubules. They are found between the midgut and the rectum. One end of each tubules opens into the gut, while the other free end floats in the haemocoel.

     Nitrogenous waste products and water which are librated into the haemocoel are absorbed at the distal end of the malpighian tubule. The nitrogenous waste is converted to uric acid as it passes along the malpighian tubule towards the gut. A lot of water Is also reabsorbed so that by the time the uric acid reaches the proximal end of the malpighian tubule it is changed to solid crystals. In the hindgut, more water is reabsorbed by the rectal gland. Thus, the urine which eventually leaves the body is very concentrated, almost a dry solid.  

 The urinary tubule is the functional unit of the kidney. Each urinary tubule begins in the cortex as a cup like structure called the Bowman's capsule. The capsule opens into a short coiled tube, the proximal convoluted tubule. Then it straightens out as it passes into the medulla, where it makes a U-shaped loop, the Henle loop, before reentering the cortex. In the cortex. The tubule becomes coiled again to form the distal convoluted tubule. The tubule bends once more and completes its course in the medulla.

     The tubule widens as it approaches the pelvis. Together with many other tubules, it pours its contents into wider main collecting ducts which eventually join up and open into the pelvis st the apices of pyramids.

       All along its course, the tubule is closely associated with several networks of blood capillaries. The renal artery branches in the kidney. Each branch breaks into a made of blood capillaries in the Bowman's capsule. This knot of capillaries is called the glomerulus. The capsule and glomerulus form the malpighian capsule. The capillaries in the glomerulus rejoin to form a blood vessel leading out of the capsule. This vessel then branches into a capillary network around the urinary tubule before rejoining to form a branch of the renal vein.  

 The paired kidneys are the excretory organs of humans. They remove unwanted nitrogenous substances like urea and other ammonium compounds from the blood. they also maintain the osmotic pressure of the blood by controlling the excretion of water and salts.

       The kidney is supplied with blood vessels. The renal artery enters the kidney. It arises directly from the dorsal aorta and brings oxygenerated blood containing excretory products. The renal vein drains filtered deoxygenated blood from the kidney to the posterior vena cavae. A narrow tube, the ureter, connects the kidney to the urinary bladder. Urine is stored in the urinary bladder. The urinary bladder leads to the urethra which opens to the exterior.


       The kidney has two distinct regions, an outer cortex and an inner medulla. More than a million fine narrow tubules, the urinary tubules pass through both these regions. They open at the tips of triangular shaped masses of tissues called pyramids. The pyramids open into a funnel shaped cavity called the pelvis. The pelvis is continuous with the ureter. The kidney has many tiny capillaries which are branches of the renal artery and vein.  

 The following mechanism are involved in the formation of urine:

 • Active transport : this is the mechanism that is mainly responsible for selective reabsorption of substances that the body needs and excretion of urea. The energy needed comes from cellular respiration.

 • Varying permeability of tubule : various regions of the tubule are selectively permeable to water, ions and urea. For example, the descending limb of the loop of Henle is Kore permeable to water than the ascending limb.

 • Passive diffusion and osmosis ions and water move in and put of the fluid in the tubule by diffusion and Osmosis respectively. These processes are controlled by concentration gradients and selective permeability of the regions of the tubule concerned.

 • Hormonal control : Hormones act on various regions of the tubule to control reabsorption of ions and water.  

Excretion is the removal of waste substances from the body of a living organism. Waste substances include

 • by products formed during metabolic processes in the body; • excess food substances which cannot be stored in the body; and

 • unwanted materials present in food which are absorbed by the body.

  The main waste substances come from metabolic reactions which digested food substances undergo in the cells of the body.

    In Cellular respiration, glucose is oxidalized to produce energy. At the same time, carbon dioxide, water and heat are formed as by products. Carbon dioxide is an unwanted substance in animals. If it is allowed to accumulate, it becomes harmful since it forms carbonic acid which, when in excess, can upset the delicate acid base balance of the body fluids. Carbon dioxide is, therefore , an excretory product. Water is needed by the body. However, excess water has to be excreted as it will dilute the body fluids. Excess heat too is harmful as it raises the temperature of the body, adversely affecting enzymes , which only operate within a narrow temperature range. Therefore, excess heat too has to be dissipated to the external environment. 

     Amino acids are metabolized in cells to produce protoplasmic materials for growth and repairs of tissues. excess amino acids in the diet, however, cannot be stored. They are deaminated to produce

 • carbon fragments which can be converted to carbohydrates and stored , and

 • nitrogenous fragments which give rise to ammonia, the most poisonous of the metabolic by products.

      In aquatic unicellular organisms, ammonia cannot be got rid of rapidly by diffusion. In multicellular animals this is not possible. So ammonia is converted to less toxic substances such as urea and uric acid. Urea is more easily formed than uric acid. It is water and can afford to loose water. Uric acid is practically insoluble and is usually formed by animals that need to conserve water and cannot afford 'water costly' excretion processes. Many animals have special excretory systems to deal with the excretion of nitrogenous wastes. In mammals highly specialized organs extract nitrogenous wastes from the blood and get rid of them from the body.

Note : Egestion and secretion should not be confused with excretion. Egestion is the removal of solid undigested substances in food, not by products of metabolism. secretion is the production of useful substances such as enzymes and hormones by metabolic processes.  

 All parts of a unicellular organism are in close contact with the external environment. This is not the case with most of the cells in a multicellular animal. These cells are bathed in tissue fluids which provide their immediate environment. Known as the internal environment, tissue fluids play an important role in the functioning of the cells of multicellular animals. The conditions in the internal environment, such ad temperature, pH and concentration of solutes, must he maintained at levels that are optimal for the cells. Poisons and unwanted substances can pollute the internal environment and harm the cells, and hence, the animal as a whole.

   To get rid of unwanted materials from the internal environment, multicellular animals have developed excretory systems and mechanisms.  

 Flatworms may be free living or parasitic. The free living worms may be found in freshwater, sea water or in damp places on land. Parasitic worms live inside their hosts bodies in a fluid environment.

     In free living planarians, there are two longitudinal excretory canals with a number of opening to the exterior on the body surface. The canals consist of numerous branched tubules. The fine ends of the tubules end in special hollow structures called flame cells.

      In a flame cell, The nucleus is displaced to one side of the cell and the cytoplasm has a large hollow called the cell lumen. The lumen is continuous with the fine tubules. A bunch of flagella hangs down the lumen.

     Waste products such as ammonia, carbon dioxide and water enter the flame cell from the surrounding cells. It is thought that the flagella help to propel the fluid into the tubules. From there, the fluid passes to the exterior.  

 Excretory systems according to the degree of complexity of the organism; the mode of life of the organism, I.e. its food, degree of activity, etc.; and the external environment. In simple unicellular organisms, the contractile vacuole serves as an adequate excretory and Osmoregulatory structure. The flame cells , nephridia ( singular: nephridium) and malpighian tubules are important excretory structures in many invertebrates, while the highly specialized kidneys are found in all vertebrates. Plants do not have any special excretory organs.  

 Beside excretion of waste products, excretory systems have another very important function helping to maintain a constant internal environment. For living cells to function properly, blood and tissue fluids must be kept at a constant osmotic pressure. The osmotic pressure of these fluids depends on the amount of dissolved substances and water present in them, I.e. the osmotic concentration. this pressure is kept constant by regulating the amount of water and solutes in the body fluids. This mechanism is called water balance or Osmoregulation.

      Osmoregulation and excretion go hand in hand as both are concerned with the maintenance of a constant internal environment (homeostasis).  

 The Amoeba lives in freshwater which is hypotonic to the contents of its cell. Water constantly enters the amoebic cell through its selectively permeable membrane by osmosis. This water would dilute the cell contents unless the Amoeba excretes it. The excess water collects in a contractile vacuole which is a small sac, lined with a membrane.

    The contractile vacuole expands as its contents to the exterior. Thus, the vacuole acts as a kind of water pump for getting rid of excess water. This water has almost no salt in it. For the contractile vacuole to work, energy is needed. Numerous mitochondrial lie close to the vacuole. If an Amoeba is treated with a metabolic poison, the contractile vacuole will have no energy to work. As a result the animal will take in so much water that it will swell and eventual burst.

    Thus, we see that getting rid of excess water is the main problem in freshwater organisms. The Amoeba overcomes this by

 • having a cell membrane with a low permeability to water , and

 • a specialized mechanism (contractile vacuole) just for the removal of excess water. 

     The frequency of the filling and discharging processes of the contractile vacuoles decreases as the salinity of the aquatic medium increases as an Osmoregulatory as it regulates the water content of the cell.

      Excretory products , like ammonia and carbon dioxide, leave the Amoeba by diffusion across the selectively permeable cell membrane to the exterior. this is an adequate mechanism for excretion of metabolic wastes since the surface area to volume ratio of the Amoeba is large.  

 Ventilation of the lungs

In mammals, the structure of the thorax is designed to bring about the ventilation of the lungs. The ribcage, intercostal muscles and diaphragm work together to draw air into and out of the lungs. How this ventilation mechanism works in humans is described.

 Gaseous exchange in the alveoli

Inhaler air in the alveoli is rich in oxygen and poor in carbon dioxide. Blood flowing to the capillaries in the alveoli, k. The other hand, is poor in oxygen and rich in carbon dioxide. These differences in the concentrations of the gases in the alveolar air and blood produce steep diffusion gradients, driving oxygen into the blood stream and carbon dioxide into the alveolar air space. The two layers of cells that separate the air in the alveolar spaces and the blood in the capillaries offer minimal resistance to this gaseous diffusion.

     Oxygen dissolves in the film of moisture lining the alveolar wall before it diffuses into the blood. Diffusion of oxygen is greatly enhanced by the presence of haemoglobin in the red blood cells. This is because as soon as the oxygen diffuses into the blood, it enters the red blood cells and form a loose complex with hemoglobin known as oxyhaemoglobin. This keeps the concentration of free oxygen in blood low, thereby maintaining a steep diffusion gradients even after a large amount of oxygen has diffused into the blood. In fact , blood containing haemoglobin can absorb about 70 times more oxygen than blood without the oxygen carrying pigment.

   The carbon dioxide waste from the cells is transported in the blood plasma ad hydrogen carbonate ions. In the capillaries lining the alveoli, the hydrogen carbonate ions breaks down to carbon dioxide and water. The dissolved carbon dioxide diffuses into the alveolar air space and escapes as a gas.

      Inhaled air contains about 21% oxygen. However, not all this oxygen is removed during gaseous exchange on the alveoli. Exhaled air contains more carbon dioxide, water and best than inhaled air.

 Gaseous exchange in body cells

, Oxygenerated blood from the lungs is pumped by the heart to all parts of the body. In the body cells, which carry out cellular respiration continuously, the concentration of oxygen is low and that of carbon dioxide is high. Oxyhaemoglobin breaks down under these condition to release oxygen. As a result , the diffusion gradients for carbon dioxide and oxygen between the body cells and blood is steep. This causes the oxygen in the blood to diffuse into the body cells and carbon dioxide in the cells to diffuse out into the blood.

 Lung capacity and breathing rate

 Our lungs do not become completely empty. In an average adult, after a normal expiration about 3000cm3 of air still remains in them. Of this volume , about 1500 cm3 can be expelled forcibly. The remaining 1500 cm3 can never be expelled naturally and is known as residual air. Each time a normal inspiration or expiration occurs, about 500cm3 of air moves in or out of the lungs. This is the tidal air, after a normal inspiration, a further 200cm3 of air may be taken in forcibly. Hence, our lungs are capable of holding up to to 5500cm3 of air this volume is known ad the total lung capacity.

    An average person breathes about 16 times a minute. During exercise, this can increase to about 20 to 30 breathe per minute. The depth of breathing can also increase. ( the heart too pumps blood more rapidly during exercise). This allows more oxygen to be absorbed and transported to the active muscles. At the same time. The extra amount of carbon dioxide formed in the muscle cells can also ne got rid of rapidly by the increased rage and depth of breathing.  

 In Amphibians like frog and toad, gaseous exchange occurs through the skin, lining of the buccal cavity and lungs. Only gaseous exchange through the buccal cavity and lungs involves ventilation movements.

 Buccal gaseous exchange

 The buccal cavity in a toad is large and lined by a thin membrane that is richly supplied with blood capillaries. To draw air into the buccal cavity , the mouth is closed, the nostrils are opened , and the floor of the buccal cavity is lowered. This creates a low air pressure in the buccal cavity, causing air from outside to flow through the nostrils. Then, the nostrils and glottis are closed. Gaseous exchange occurs in the buccal cavity between the blood in the capillaries and the inhaled air. To expel air, the floor of the buccal cavity is raised, increasing the air pressure within the cavity. this forces the valves closing the nostrils to open, allowing air from the buccal cavity to flow out. thus the muscular floor of the buccal cavity acts like a pump: it causes air to be sucked in and pumped out alternatively.

Gaseous exchange through the lungs

 Under normal circumstances, the toad gets sufficient oxygen for its needs by gaseous exchange through it's skin and buccal cavity. To draw air into the lungs, air is first sucked into the buccal cavity as just described. Then, the nostrils are closed, the glottis is opened, and the floor of the buccal cavity is raised. This increases the air pressure in the buccal cavity, forcing air into the lungs though the open glottis. The lungs become inflated as air fills the alveoli. Gaseous exchange occurs between the inhaled air in the alveoli and the blood in the capillaries lining the alveoli.

      To expel air from the lungs into the mouth, the floor of the buccal cavity is lowered to create a region of low pressure there. When air fills the buccal cavity, the glottis closes and the floor of the buccal cavity is raised. This causes the valves of the nostrils to open , allowing air flow out of the body.  

 Plants carryout gaseous exchange for

 • Photosynthesis, and

•Cellular respiration

     Photosynthesis is carried out by chloroplast containing cells in the presence of sunlight. Cellular respiration is carried out by all plant cells all the time. Since plants are inactive, their energy requirements are low, so cellular respiration proceeds slowly. Photosynthesis is a very vigorous process, especially in bright sunlight. Thus, actively photosynthesizing plants take in the absence of photosynthesis, plants take in oxygen and give out carbon dioxide like animals. Besides the gases involved in these two processes, water vapour also escaped from plants during transpiration. 
 
      Plants do not have special gaseous exchange structures like complex animals. Instead, gases enter and leave the plant body through

 • Microscopic openings called stomata on the surfaces of green aerials parts of plants;

 • tiny openings called lenticels on old stems and roots; and •root hairs in young roots.
Stomata

 Stomata are found in the epidermal layer in green aerial parts of plants, especially leaves. They occur mainly on the Lowe surface e of dicotyledonous leaves, although on monocotyledonous leaves they are found on both surfaces.

     Intercellular air spaces found throughout the lead are linked to stomata as a leaf is a very thin flattened organ, individual leaf cells are either in direct contact with intercellular air spaces or very close to such air spaces. This ensures that the rate of gaseous exchange meets the high metabolic demands of an actively photosynthesizing leaf.

 Note : As a leaf is led than 1 mm thick, its cells are less than 0.5mm from intercellular air spaces. This presents no problem for diffusion.

      The gases that enter the intercellular spaces of the lead dissolve in the moisture on the walls of the cells lining these spaces. These walls are kept moist by a continuous stream of water that reaches the leaves from the root.

 Opening and closing of stomata

    The opening and closing of stomata control the flow of gases in and out of the leaves. this control is necessary to prevent excessive loss of water as vapour from the plant body via transpiration. Usually stomata are open during the day and closed at night.

       Each stomata or stomatal pore is flanked by two bean shaped guard cells, the only epidermal cells with chloroplast s. The walls of the guard cells next to the pore are thicker than those adjacent to the epidermal cells the thicker walls cannot stretch as much as the thinner walls.

      Changes in the solute concentration of the guard cells cause water to flow in and out of them by osmosis. When the solute concentration of the guards cells is high, water flows into them from the surrounding epidermal cells. As a result, the volume and turgidity of the guards cells increase. The thin walls strectch more than the thicker walls, causing the guard cells increase. The thin walls stretch more than the thicker walls causing the guards cells to become more curved, and so open the stoma. When the solute Concentration of the guard cells is low, hence their volume, decrease, I.e. the guard cells becomes flaccid. As the walls of the guard cells are elastic, they return to their original position. This causes the guard cells to straighten up and close the stoma.

      Recent studies have shown that guard cells can actively pump in ions, especially potassium, from the surrounding cells, thereby increasing their solute concentration. this active transport mechanism needs energy (ATP) which is probably supplied by photosynthesis in the guards cells. When active transport of Ione into the guard cells stops, the ions in them diffuse out, causing water to flow out also. The guard cells thus become flaccid.

 Lenticels

 These are the air pores found in the bark of stem and roots. They appear as scars or small protrusions on the surface of stems and roots.

    Lenticels are formed when stems and roots undergo secondary thickening. Usually a lenticels develops below a stoma, where the cork cambium, instead of forming compact rows of cork cells, produces irregularly shaped cork cells which are loosely arranged with a lot of intercellular spaces. As these cork cells increase in number and size, the epidermis ruptures to form an opening or lenticels thorough which air can diffuse in and out of the plant. The intercellular spaces in the cork cambium and cortex ensure that the living plant cells in the stem and root are in contact with or close to the air from outside.

, Note : Most of the cells in a woody stem, however, are composer of dead cells.

Root hairs

 These provide a large surface area for the absorption of water, mineral salts and oxygen. Oxygen , present in the soil air, dissolves in the soil moisture and diffuses into the root hairs. From here, it diffuses into the other root cells. The carbon dioxide produced by the root cells diffuses out of the root into the soil via the root hairs.  

 • Bony fishes have the most complex gills, composed of gills filament which are made up of richly vascularized transverse plates.

• A continuous one way circulation of water from outside, through the mouth and over the the gill filaments ventilates the gills. Gaseous exchange is enhanced by blood flowing in the opposite direction to water flow.

•  The tracheal system in arthropods consists of a branching system of air tubes (tracheae and tracheoles) which open to the exterior through spiracles.

 • Air flow in the tracheal system is regulated by opening and closing the spiracles. In large and active insects, air is pumped in and out of the tracheal system by alternately flattening and relaxing the body.  

 Sponges and coelenterates All cells in the bodies of sponges and coelenterates are in contact with the water in which they live. Oxygen diffuses into each cell and carbon dioxide diffuses out simultaneously. Soon, however, the water bathing these cells would become deficient in oxygen and saturated with carbon dioxide, bringing gaseous diffusion to a stand still.

      To maintain a high diffusion rate, flagellated cells in the body walls of these animals beat rhythmically to create water currents. The water bathing the cells is continuously replaced by a fresh supply of water rich in oxygen and poor in carbon dioxide. As a result , the diffusion gradient for the gases remains high, enabling gaseous exchange to proceed at a sufficiently high rate.

 Annelids

 In the terrestrial earthworms, gaseous exchange occurs by diffusion through their moist skins. There is no special mechanism to circulate air over their skins. Instead , a rich blood supply to the skin rapidly removes the oxygen that diffuses into the epidermal cells, thus maintaining a sufficiently high diffusion gradient. Simultaneously, the carbon dioxide in the blood capillaries diffuses out of them, to enter the epidermal cells. From here, the gas diffuses out of the body into the external environment.

     When the oxygen rich blood from the skin reaches the various body tissues oxygen diffuses out of the capillaries and enters the individual body cells. At the same time, carbon dioxide waste diffuses out of the body cells and enter the blood capillaries to be transported to the skin , where they can be got rid of to the external environment. 

Note : in annelids, the removal and transport of oxygen from the skin is enhanced by the presence of the oxygen carrying pigment, haemoglobin, in their blood. This type of blood transports oxygen more efficiently than one without such a pigment.

Insects

 Air enters and leaves the body of an insect through its tracheal system. This flow of air is controlled by adjusting the size of the spiracular openings. The spiracles open fully when the carbon dioxide concentration in the body tissues is high. This occurs when the insect is active. When the insects is at rest, the carbon dioxide Concentration in the tissues drops. This triggers the valves guarding the spiracular openings to behave like tiny doors and decreases the size of the openings.

     Usually, oxygen diffuses in and carbon dioxide diffuses out passively through the tracheal system. In a large and active insects, like the locust, air is actively pumped in and out of the tracheae by ventilation movements, also referred to as breathing movements. In this mechanism , dorso ventral muscles (vertical muscles connecting the roof and floor of the body segments) contract the flatten the body. This reduces the volume of the tracheal system and forces air out of the body (expiration). When the muscles relax, the body returns to its normal size. The tracheal system, too, returns to its original (larger) size. This causes air to flow into the body (inspiration). In co ordination with these ventilation movements, the spiracles in the anterior and posterior parts of the body open and close alternately. This causes a one way flow of air through the tracheal system, with air being sucked in through the anterior spiracles and expelled through the posterior ones.

     The oxygen in the air that enters the tracheal system dissolves in the tissue fluid in the fine tracheoles. From here, the oxygen diffuse in the body cells. At the same time, carbon dioxide wastes in the cells diffuse out in to the tissue fluid. In the fine tracheoles, the carbon dioxide in the tissue fluid escapes as a gas that leaves the body through the spiracular openings.  

 • In monerans, protists, fungi, simple multicellular animals and plants, gaseous exchange occurs through the body coverings such as plasma membrane, epidermis and skin

 • Complex animals have specialized respiratory or gaseous exchange structures. These include the gills in aquatic animals, tracheae in terrestrial arthropods and lungs in air breathing vertebrates.

 • Gaseous exchange occurs by diffusion or dissolved gases. To improve the rate of diffusion, respiratory structures must have (i) thin gaseous exchange membranes with large, moist (ii) ventilation mechanisms to maintain steep diffusion gradients, and (iii) a close link with the Organism's transport system.  

 In practically all living organisms, cellular respiration uses oxygen and produces carbon dioxide as waste. These gases enter or leave the bodies of all organisms by diffusion at the gaseous exchange surfaces.

      In a simple multicellular animal, lime the hydra, gaseous exchange occurs directly between the external environ and the individual body cells. In a complex animal, however, gaseous exchange occurs by diffusion at two sites:

 • the surfaces in respiratory organs like the lungs and gills , and

• The surfaces of individual body cells.

     In many multicellular organisms, air or water from the external environment is continuously circulated over the gaseous exchange surfaces. This process is known as ventilation. Mechanisms which brings about ventilation differ according to the complexity of the organisms and the medium in which the organism lives.

   I am going to discuss the various mechanisms which brings about ventilation and gaseous exchange in the respiratory systems of animals and plants.

 Note: Cellular respiration is sometimes referred to as tissue or internal respiration, while ventilation and gaseous exchange are referred to as external respiration.