Respiratory System in Man, Animals, Fish, Birds, Reptiles & Insects etc.

Living organisms need energy for their activities. This energy is provided within the cell by the Respiratory System or the process of Respiration. Respiration is a universal process. The breakdown of complex carbon compounds and the release of maximum usable energy within the cell is called respiration.

The term respiration is used in two ways in biology:

  • The exchange of respiratory gases (CO2 and O2) between the organism and its environment is called external respiration. It is the most familiar term.
  • The step-by-step breakdown of the C-chain molecules and the release of energy within the cell are called cellular respiration.

Respiratory System in Man

The respiratory system of man consists of lungs and air passages. These air passages carry fresh air to the respiratory sites.

Air passageways or Respiratory Systemrespiratory system of man

Air passageways or Respiratory System consists of:

  • Nostrils
  • Nasal Cavities
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveolar Ducts
  • Alveolar Sacs

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Respiration in Animals

Respiratory surfaces in animals are the site where gaseous exchange takes place. The respiratory surfaces in most animals show the following features:

  1. Large surface and moisture: the surface area should be extremely large. It is kept moist as in the lungs of the land vertebrates and in the gills of the fishes.
  2. Thin Epithelium: There should little distance for diffusion. In most animals, the epithelium separates air and blood. This epithelium is only two-celled thick. So, the distance for diffusion is very short.
  3. Ventilation (exchange of gases): Ventilation maintains a steep diffusion gradient. There is a big difference in the concentration of the gases at two points for diffusion.
  4. Capillary Network: the respiratory site should possess an extensive network of capillaries. Blood should cross over these capillaries all the time. The speed of the blood should be adequate. Similarly, a steep concentration gradient is maintained. This helps in the rapid diffusion of oxygen.

Respiration in Hydra

Hydra has no specialized organs for respiration. The exchange of gases occurs through the entire body surface. This body’s surface remains in contact with water. During the exchange of gasses; oxygen is absorbed and carbon dioxide is removed. The exchange of gases also takes place through the cells lining the digestive cavity. So, the surface lining of the enter-on acts efficient respiratory surface.Gas Exchange in Hydra

Respiration in Earthworms (Reptiles)

Earthworms have no specialized respiratory organs. The exchange of gases occurs mainly through the skin. The skin is richly supplied with blood capillaries. The skin is kept moist by the secretion of the epidermis mucous gland cells and the coelomic fluid.

  1. Absorption of Oxygen: oxygen is dissolved on the wet surfaces of earthworms. This oxygen passes through the cuticle and epidermal cells into the blood. This oxygen combines with hemoglobin to form oxyhemoglobin in the blood.

The blood moves through the tissues. Oxyhemoglobin releases oxygen in these tissues. The earthworm’s blood does not come in direct contact with the tissue cells. So, the oxygen is dissolved in the tissue fluid or coelomic fluid. Finally, this oxygen is diffused into the tissue cells from the tissue fluid or coelomic fluid.

  1. Removal of CO2: The blood removes carbon dioxide from the tissues. The plasma of the blood carries CO2 to the skin. Finally, this CO2 is removed from the skin.

Respiration in Cockroach (Insects)

The cockroach has specialized organs for respiration. Blood is not involved in the transport of gases. There are the following respiratory organs:

  1. Tracheae: The respiratory system of cockroaches is composed of a branching system or air tubules called tracheae. These tracheae are lined by chitin.
  2. Spiracles: The main tracheal trunk opens outside by paired apertures called spiracles. These spiracles are present on the lateral sides of the body. There are 10 pairs of spiracles in the cockroach. Two pairs are present in the thorax and the remaining 8 pairs are present in each of the eight abdominal segments.
  3. Tracheoles: the main tracheae divide and subdivide and form Tracheoles. The Tracheoles end in blind ducts. These blind ducts surround the tissues and organs. So, they supply oxygen to the cells directly.

Mechanism of Respiration:

Air is pumped in and out of the body by the expansion and relaxation of the abdominal muscles. The process of respiration is divided into inspiration (inhalation) and expansion (exhalation).

  1. Inspiration: The abdomen expands and the first four pairs of spiracles open. Air rushes into the tracheae through these spiracles. A concentration gradient is set between the tracheae and spiracular openings. So, oxygen is diffused from the outer air into the tracheae. The air enters into the Tracheoles from the tracheae. Finally, the exchange of gases between the Tracheoles and tissues takes place and oxygen is supplied to the cells.
  2. Expiration: The abdomen contracts and the first four anterior pairs of spiracles are closed and the last six posterior pairs of the spiracles are opened. This forces air out of the body from the tracheae.

Respiratory System in Fish

  1. Gills: respiration in fish takes place by Gills. The gills are present in pairs. There may be four to five pairs of gill in the fish. They are present on the fourth side of the body. They are located at the junction of the head and trunk. The gills are highly modified for the exchange of gases in aquatic animals.
  2. Gill Slits: The gills open through gill slits. These gill slits are visible on the surface of the pharynx in cartilaginous fishes. The gill stills are placed in the bronchial cavities in the bony fishes. These bronchial cavities are covered by an operculum.

Mechanism of Respiration:

  1. Inspiration: The gills have a large surface area for the gaseous exchange. Water enters through the mouth. It passes over the gills and moves out of the body through the gill’s openings. The surfaces of gills are ventilated (oxygenated) by this constant flow of water. The heart pumps the blood directly to the gills. Oxygenation of blood takes place in the gills. Blood carries this oxygenated blood to all parts of the body.
  2. Expiration: Deoxygenated blood is brought from different parts of the body into the heart. The heart of the fish has a single circuit. So, blood flows only in one direction. The blood passes through different chambers of the heart and enters the posterior side of the heart. Finally, it is pumped into the gills and the gills remove CO2.

Respiration in Frog (Amphibian)

Respiration takes place through the lungs, skin, and buccal chambers. These organs are richly supplied with blood vessels.

  1. Cutaneous Respiration: The gaseous exchange through the skin is called cutaneous respiration.
  2. Buccal Respiration: The gaseous exchange in the buccal cavity is called buccal respiration.
  3. Pulmonary Respiration: The gaseous exchange through the lung is known as pulmonary respiration. The lungs of the frog are simple.

When they are fully expanded, they look like sacs like balloons. There are thin-walled chambers in the inner surface of the lungs. These chambers increase the surface area. The walls of these air chambers are richly supplied with capillaries. These blood-containing areas of the lungs are the main sites of the gaseous exchange. The process of respiration is divided into two parts.

  • Inspiration: the intake of air is known as inhalation or inspiration. The nostrils are opened and the mouth is closed. So, air enters the nostrils. The nostrils are closed after the entry of air. The floor of the buccal cavity is raised and the air is pushed into the lungs.
  • Expiration: The removal of the consumed air from the lungs is called the expiration of exhalation. Expiration occurs in the reverse order of inspiration. After the gaseous exchange of consumed air is removed out of the lung through the nostrils.

Respiratory System in Birds

The birds have the most efficient and elaborate (complex) respiratory system. The birds are very active animals with high metabolic rates. Thus, they need a large amount of oxygen.


There is a one-way flow of air through the lungs. The air is renewed after inspiration. Parabronchi are present in the lungs of birds instead of alveoli. These Parabronchi are tiny thin-walled ducts. These Parabronchi open at both ends and air is constantly ventilated (oxygenated). The walls of the Parabronchi are the chief sites for the gaseous exchange.

There is a countercurrent flow of air and blood. In this case, the direction of the flow of blood is opposite to the direction of the flow of the air in the Parabronchi. The countercurrent exchange increases the amount of oxygen entering the blood. Thus, no stale air remains in the lungs. So, the lungs in birds are very efficient.

Air Sacs

The lungs have developed several extensions called air sacs. These air sacs reach all parts of the body. They also penetrate into the bones. There are nine air sacs in most birds. The ribs articulate and are rotated forward and upward. As a result, these air sacs are inflated by atmospheric pressure. The inflated air sacs act as bellows. They send air into the Parabronchi for the exchange of gases.

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