Respiratory System in Mammals

The  most complex respiratory system is found in mammals. Most vertebrates, including birds, lizards and mammals use the lungs as their only respiratory organs where gaseous exchange takes place.

LUNGS

[Photo credit: Patrick J. Lynch, medical illustrator]


The lungs is a pair of reddish, elastic organ, on each side of the heart of a mammal, located within the thoracic cavity of the body. The thorax is supported by a bony framework and consists of the thoracic cavity. It is made up of the sternum in the front, back bone or vertebral column at the back, twelve pairs of ribs at the sides and a dome-shape muscular wall, referred to as diaphragm.

The thorax is separated from the abdominal cavity by the diaphragm. The ribs are attached to the back bone and the breastbone thus forming a bony cage. They are connected to one another by muscles known as the intercostal muscles. Inside the thoracic cavity lie the heart and the lungs.

Anthropods and terrestrial vertebrates are successful group of land animals that use lungs as the gaseous exchange organs. Lungs are closely linked to circulatory system, so that oxygen can easily be transported over long distances. This enables vertebrates to attain the large sizes seen in many types of mammals.

THE RESPIRATORY SYSTEM OF MAN

The respiratory system of man is typical of the respiratory system of mammals in general. It includes the nostrils and mouth, the pharynx, the larynx or voice box, the trachea or wind pipe, the bronchus, the bronchioles and the alveoli.

Movement of Air into the Lungs
The following are the path taken by air to the lungs and it moves:

Air can enter the body either through the nose or mouth. The nose and mouth are separated by the palate, so you can breathe through the nose when you are eating. It is better to breath through your nose because of some reasons. Before the air inhaled through the nose reaches the lungs, it becomes damp, warm and filtered.

Both the nasal cavity and the mouth open into the pharynx, a short passageway, which branches at the end in two directions. One leads to the digestive tract and the other to the larynx and air pathways.

The larynx leads to the windpipe or trachea. A flat piece of tissue, the epiglottis covers the entrance to the larynx and trachea when food is swallowed into the esophagus(oesophagus) which lies just behind the trachea. The trachea branches into two - the bronchi (singular: bronchus). Both the trachea and bronchi have rings of cartilage which straighten them and prevent them from collapsing when the air pressure of air inside is low.

Each bronchus leads to the lungs where it branches into small tubes called bronchioles. The bronchioles lead to numerous air sacs called alveoli(singular: alveolus). The alveoli are richly supplied with blood capillaries and are sites or surfaces where gaseous exchange takes place.

The alveoli communicate with the atmosphere through a continuous air passage leading from them to the bronchioles, bronchi, trachea, larynx, pharynx and nostrils.

MECHANISM OF RESPIRATION IN MAMMALS

The mechanism of breathing in mammals including man involves two phases or stages. These are:

1. External Respiration (breathing).
2. Internal or Tissue or Cellular Respiration.

Both external respiration and tissue respiration have been explained previously on this blog. Click here to read them.

Process of Inhalation or Inspiration in Man

The following processes are involved during inhalation in man:

1. The thoracic cavity first increases in volume.
2. The diaphragm contracts and becomes flattened.
3. The intercostal muscle contracts.
4. The sternum is moved forward.
5. The ribs are moved upwards and outwards.
6. There is an increase in the volume and a decrease or fall in the pressure of the thoracic cavity.
7. Consequently, air from outside is drawn into the lungs or alveoli through the nose, trachea, bronchus and bronchioles leading to an increase in the size of the lungs.

Gaseous exchange takes place after inspiration. Diffusion of oxygen in the inspired air into the blood capillaries of alveoli and carbon dioxide and water vapor out of the blood capillaries take place. Oxygen is able to diffuse into the blood capillaries, carbon dioxide and water vapor are able to diffuse out because the alveoli possess all the conditions necessary for all respiratory surfaces.

Process of Exhalation or Expiration in Man

The following processes are involved during exhalation in man:

1. The thoracic cavity first decreases in volume.
2. The diaphragm relaxes and assumes its dome-shape.
3. The intercostal muscles relax.
4. The sternum now moves backwards or inwards.
5. The ribs are moved downwards and inwards
6. There is a decrease in the volume and an increase in the pressure of the thoracic cavity.
7. Consequently, air containing waste products like carbon dioxide and water vapor from inside the alveoli or lungs are forced out through bronchioles, bronchi, trachea, and finally to outside through the nose. This finally leads to a decrease in the size of the lungs.

During expiration, the air in the lungs is never completely exhaled or removed. The air left in the lungs is called residual air while that which is exchanged with each breathe is called tidal air.

The approximate composition of inhaled and exhaled air is shown in the table below.

Air component	Inhaled air	Exhaled air	Reason for difference
Oxygen	21%	16%	Oxygen is absorbed across respiratory surface, then used by cells in respiration.
Carbon dioxide	0.03%	4.1%	Carbon dioxide is made by cells as a waste product of respiration, and is released across the respirator surface.
Nitrogen	78%	78%	Nitrogen gas is not used by cells.
Water vapor	Variable	Saturated(higher)	Respiratory surfaces must be kept moist. Some of these moisture evaporates and are lost as air is breathed out.
Temperature	Variable	Always higher	Air is warm as it passes through the respiratory passages.

Categories: Biology, Respiration, Respiratory System