Tag: mechanism of respiration

When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity in conjunction with the abdominal muscles which act as an antagonist paired with the diaphragm's contraction. The internal intercostals assist in expiration by pulling the ribcage down.

When we breathe in, or inhale, the diaphragm contracts (tightens) and moves downward. This increases the space in the chest cavity, into which the lungs expand. The intercostal muscles between the ribs also help enlarge the chest cavity. They contract to pull the rib cage both upward and outward when we inhale. As the lungs expand, air is sucked in through the nose or mouth. The air travels down the windpipe and into the lungs. After passing through the bronchial tubes, the air finally reaches and enters the alveoli (air sacs).

Carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body. About 30% of CO$ _2$ is transported as carbamino haemoglobin. 

Internal respiration refers to gas exchange across the respiratory membrane in the metabolizing tissues. External respiration refers to gas exchange across the respiratory membrane in the lungs. Pulmonary ventilation is the process by which oxygen enters and carbon dioxide exits the alveoli. Respiration is the process by which oxygen and carbon dioxide diffuse in and out of the blood. 

What most people don’t know is that it is, in fact, the level of CO$ _2$ rather than oxygen that usually drives breathing rate. Your body’s ability to detect specific concentrations of CO$ _2$ is extremely sensitive. If the level of CO$ _2$ in your blood increases by a mere 10%, your rate of breathing will double. The answer comes from the way CO$ _2$ is transported. Instead of being bound to hemoglobin, the CO$ _2$ mostly just dissolves in your blood. When the CO$ _2$ dissolves, it combines with the water in your blood to form carbonic acid. As this acid builds up, it dramatically decreases the pH of your blood, throwing it out of balance. Your brainstem detects this pH change, and speeds up your breathing rate in order to to get rid of the extra CO$ _2$. 

The correct sequence of events in the initiation of respiration is:
(i)    The contraction of external intercostal muscles raises the ribs and sternum
(ii)    The volume of thorax increases in the dorsoventral axis
(iv)    Diaphragm contraction
(vi)    The volume of thorax increases in the anterior-posterior axis
(iii)    Intrapulmonary pressure decreases

The relaxation of external intercostal muscles and diaphragm leads to return of diaphragm, ribs, and sternum to resting position thus restoring thoracic cavity to pre-inspiratory volume. The pressure in the lungs increases and the air is exhaled.
So, the correct answer is option A.

During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs.

During expiration, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.