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Thursday, 17 January 2019

January 17, 2019

Gills and Tracheal system

 • 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.  
January 17, 2019

Gaseous Exchange in Lower Animals

 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.


 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.


 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.  
January 17, 2019

Gaseous Exchange Structures

 • 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.  
January 17, 2019

Respiratory Mechanisms

 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.  
January 17, 2019

Gaseous Exchange in Plants

 Gases enter and leave the plant through (i) stomata, (ii) lenticels, and (iii) root hairs.

 • Opening and closing of stomata regulate gaseous exchange, especially in leaves. This is brought about by changes in the solute concentration of guard cells, making them turgid (stoma is open) or flaccid (stoma is closed).  
January 17, 2019

Mammalian Respiratory System

 • The mammalian respiratory system consists of a pair of lungs enclosed in the thorax and connected to the air outside by a series of branched air tunes (trachea, bronchi and bronchioles) and air pathways (nasal cavity, pharynx and larynx).

 • The rib cage, intercostal muscles and diaphragm work together to draw air into (inspiration) and out (expiration) of the lungs.

 • Gaseous exchange occurs in the richly vascularized alveoli, which form the gaseous exchange surface. When oxygen in the alveolar air diffuses in to the blood in the capillaries, it combines with haemoglobin in the red blood cells, forming oxyhaemoglobin. These enhances the diffusion of oxygen into the blood by maintain a steep diffusion gradient.