Respiration (Gaseous Exchange) in Animals

Introduction:

Whenever we breathe, we take in oxygen via our nostrils (that is, nose) and give out carbon-dioxide as waste product. We can live for weeks with no food, but we will die in a few minutes devoid of oxygen. Oxygen is employed to release energy stored in food which we consume in a procedure termed as respiration. Food includes carbohydrates, fats, proteins, minerals and water. Such food should be broken down to liberate energy in living cells.

The use of oxygen is to liberate energy as ATP (Adenosine triphosphate) in living cells is termed as cellular respiration. Cellular respiration comprises aerobic (in presence of oxygen) and anaerobic (devoid of oxygen) respiration.

Kinds of Respiratory systems and structures:

There are mainly four kinds of respiratory systems in animals as:

a) Body surfaces:  In most of the unicellular organism example: amoeba and paramecium, gaseous exchange occurs via the plasma membrane through simple diffusion.

In flat worms, example: planaria the body surface has raised surface area to volume ration and brings the innermost cells close to the body surface. This lets the worms to get its oxygen need by simple diffusion.

b) Gill:  They are special respiratory organs employed in aquatic atmosphere to absorb the oxygen.

Simple gills:  These are external gills found in tadpoles, sea slugs, aquatic snails and numerous fishes. They are branched, permits increased surface area to be exposed for the gaseous exchange in water. They are rich with blood capillaries.

Complex gills:  These gills are in the gill chambers (that is, opercullar cavities), there are two gills chambers located on each side of the head, just at the back of mouth.

Gills are enclosed by operculum. The direction of water-flow in fish is as expressed as:

Water → mouth → pharynx → gill chambers → out from opercular.

c) Tracheae: Tracheal system is situated in land arthropods. Air enters the body via openings termed as spiracles found all along the sides of the insect. In cockroach there are 10 pairs of spiracles. Spiracles lead to the tubes termed as tracheae, which branch into the trachioles. Trachioles includes fluid in which oxygen dissolves prior to it gets to individuals cell of the body. In flying insects the trachea build up into air sacs. The oxygen diffuses into cells, tissues and muscle.

d) Lungs:

It is mainly found in mammals. They are two, that is, left lungs and right lungs and are surrounded in the horax. In humans, air enters into the nostrils. The nostrils and mouth open into the pharynx, which branches into two; one leads to digestive tract whereas the other leads to the larynx (that is, voice box). The entrance into the larynx is termed as glottis. The larynx leads to the trachea those branches into bronchi which is the air tube which enters the lungs. Smaller air tubes are termed as bronchioles which finish in the air acs or alveoli.

Common features of Diffusion and Respiratory Structures:

The gases enter and leave the cells through diffusion. Gases should be dissolved in water before they can diffuse across the cell membranes. Thus respiratory structures encompass certain properties that enable them to carry out their functions. A few of these are:

1) Respiratory structures should encompass big gaseous exchange surface.

2) The membrane that the gas diffuses through should be thin.

3) They should have ventilation methods which maintains difference in the concentrations of the gases across the membrane (that is, to maintain a steep diffusion gradient).

4) Gaseous exchange system should be linked to the transport (that is, circulatory system).

Cutaneous (Skin) Respiration:

The skin of toad and frog is thin and well supplied along with blood vessels. The skin is kept moist via mucus, secreted by mucus gland. Oxygen diffuses via the moist skin to the blood vessels. The oxygen then joins with an oxygen carrying pigment termed as hemoglobin that is present in the blood. If the oxygen joins with hemoglobin, it forms oxy-hemoglobin. Oxy-hemoglobin is transported to all portions of the body after internal respiration; the carbon-dioxide generated in the cells diffuses to the blood and from the blood to the atmosphere.

This kind of respiration (cutaneous) occurs if the toad or frog is in water. Therefore dissolved oxygen in water diffuses via the skin to the blood capillaries and carbon-dioxide from blood diffuses into water. Cutaneous respiration can as well occur on land provided the skin moist.

Gaseous Exchange in Fish:

The gas exchange is more complicated for fish than for mammals as the concentration of dissolved oxygen in water is less than 1 percent as compared to 20 percent in air. Fish have developed specialized gas-exchange organs termed gills, which are comprised of thousands of filaments. The filaments in turn are enclosed in feathery lamellae which are just a few cells thick and have blood capillaries. This structure provides a large surface area and a short distance for the exchange of gas.

Water flows above the filaments and lamellae and oxygen can diffuse down a concentration gradient the short distance among water and blood, while carbon-dioxide diffuses in the opposite direction as well down its concentration gradient. Each and every gill is covered through a muscular flap (that is, the operculum) on the side of fish's head. The gills are so thin that they can't support themselves devoid of water, therefore when a fish is taken out of water after a while the gills will collapse, the SA/Vol ratio reduces and the fish suffocates.

Fish ventilate their gills to keep up the gas concentration gradient. Dissimilar mammals and birds having their 'push-pull' system of breathing, fish constantly pump water over their gills by moving their opercula and mouth, sucking in water from in front of fish, passing it over the gills and then pushing the 'stale' water behind.

The opercular valve makes sure the one-way flow which the high density of water needs. The gill lamellae are arranged as a sequence of flat plates sprouting from the gill arch. On their upper and lower surfaces there are numerous thin vertical flaps that comprise blood capillaries. The blood flows via such capillaries in the opposite direction to the flow of water above the gills. This is termed as counter-current flow system and provides a highly proficient diffusion pathway as the blood flows all along and picks up oxygen it meets up water that always consists of higher oxygen content than itself and the diffusion of oxygen into blood will be maintained.

Gaseous Exchange in Mammals:

In mammals, the gaseous exchange structure is the lungs which are enclosed in the thorax. Ventilation of the lungs comprises inspiration and expiration of the air. The ribcage, intercostals muscles and diaphragm work altogether to bring on ventilation of the lungs.

Inspiration: Air enters the lungs via the nostrils, leading to rise of the thoracic cavity and inflation of the lungs.

• Then the external intercostals muscles contract and internal intercostals muscles loosens up.
• The ribs are shifted upwards outwards.
• Causing the sternum to shift upwards and outwards.
• The position of back-bone stay fixed.
• The diaphragm contracts, flattening downwards and pushing out the ribs.

Expiration: Air flows out of the lungs to the external atmosphere and the thoracic cavity normalizes and deflates the lungs.

• The internal intercostals muscles contract and the external intercostals muscles relaxes.
• The ribs are shifted downwards and inwards.
• The diaphragm relaxes and curves upwards to the normal shape (that is, dome-like shape)
• This ultimately forces the air out of the lungs into the external atmosphere.

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