Q.Describe the mechanism of breathing in humans, covering both inspiration and expiration, and explain how changes in thoracic volume bring about the pressure changes that drive airflow.

The average resting breathing rate in a healthy human is roughly 12–16 breaths per minute.

Breathing rate (the number of complete breathing cycles — one inspiration plus one expiration — per minute) is one of the basic respiratory volumes/parameters discussed alongside tidal volume, vital capacity, and residual volume. For a healthy adult at rest, this rate averages about 12 to 16 times per minute. It rises during exercise, fever, high altitude, or emotional stress because the body's oxygen demand and CO₂ output increase, and it is regulated by the respiratory rhythm centre in the medulla, which responds mainly to blood CO₂/H⁺ levels sensed by chemoreceptors.

The normal breathing rate in a healthy adult is 12-16 breaths per minute, so the answer is (B).

The respiratory rate is the number of breaths (one inspiration + one expiration) taken per minute. In a healthy adult at rest, this is normally about 12 to 16 breaths per minute.

The rate rises during exercise, fever or excitement (mainly because of increased CO2 in the blood) and is higher in infants. Under normal resting conditions, however, the standard value is 12-16 breaths per minute.

During inspiration the diaphragm contracts and moves downwards, so the answer is (B).

Inspiration is an active process. The diaphragm (dome-shaped at rest) contracts and flattens, moving downwards, while the external intercostal muscles lift the ribs and sternum. Together these increase the volume of the thoracic cavity and lungs, lowering the intrapulmonary pressure below atmospheric pressure so air rushes in.

During expiration the diaphragm relaxes and becomes dome-shaped again (moving upwards), reducing thoracic volume. So during inspiration the diaphragm contracts and moves downwards.

The statement is False — contraction, not relaxation, of the diaphragm and inter-costal muscles drives inspiration.

Breathing in humans is driven by changes in the volume of the thoracic cavity. During inspiration, the diaphragm contracts and flattens (moving downward), and the external inter-costal muscles contract, lifting the ribs and sternum outward and upward. Together these actions increase the volume of the thoracic chamber, decreasing the pulmonary pressure below atmospheric pressure so air rushes into the lungs. During expiration, the diaphragm and inter-costal muscles relax, the thoracic volume decreases, and air is pushed out — exactly the opposite of what the statement claims for inspiration.

Inspiration happens when the diaphragm and the external intercostal muscles both CONTRACT, enlarging the thoracic cavity and drawing air in — option (a).

Step 1 — Muscles involved in breathing.

Breathing (pulmonary ventilation) is achieved by rhythmic contraction and relaxation of the diaphragm (a dome-shaped muscle separating thorax from abdomen) and the intercostal muscles between the ribs.

Step 2 — What happens during inspiration.

• The diaphragm contracts, so its dome flattens and moves downward, increasing the vertical dimension of the thoracic cavity.
• The external intercostal muscles contract, pulling the ribs and sternum upward and outward, increasing the antero-posterior and lateral dimensions of the thoracic cavity.
• Together this enlarges the thoracic volume. By Boyle's law, an increase in volume causes a decrease in the intra-pulmonary pressure below atmospheric pressure, so air flows from the atmosphere (high pressure) into the lungs (low pressure) — this is inspiration.

Step 3 — Why the other options are wrong.

Relaxation of either muscle (as in options b, c, d) would decrease or fail to increase thoracic volume, which is what happens during (passive) expiration, not inspiration.

Inspiration begins when the diaphragm's muscle fibres contract, flattening its dome shape and enlarging the thoracic cavity so air is drawn in.

Breathing (pulmonary ventilation) works on pressure differences created by changing the volume of the thoracic chamber. The diaphragm is a dome-shaped muscular partition between the thoracic and abdominal cavities.

• During inspiration, the diaphragm contracts, which pulls it downward and flattens its dome shape; simultaneously the external intercostal muscles contract, lifting the ribs and sternum outward. Both actions increase the volume of the thoracic cavity, decreasing intra-pulmonary pressure below atmospheric pressure, so air flows into the lungs.
• During expiration, the diaphragm and external intercostal muscles relax, allowing the diaphragm to return to its dome shape and the thoracic volume to decrease, raising intra-pulmonary pressure above atmospheric and pushing air out.

So the option describing inspiration correctly is that the diaphragm contracts and flattens (not relaxes and domes, which describes expiration).

Expiration (exhalation) is the breathing movement that pushes air out of the lungs into the atmosphere, occurring when the thoracic (chest) volume decreases.

During expiration:

• The diaphragm relaxes and domes upward, and the external intercostal muscles relax, allowing the ribcage to move down and inward.
• This decreases the volume of the thoracic cavity and lungs, which increases the pulmonary (intra-pulmonary) pressure above atmospheric pressure.
• Because gases flow from a region of higher pressure to lower pressure, air is pushed out of the lungs and alveoli, through the bronchioles, bronchi, trachea, and finally out through the nose/mouth into the atmosphere.

This is the reverse of inspiration, where diaphragm/intercostal muscle contraction increases thoracic volume and draws air in.

The diaphragm's contraction and relaxation, together with the intercostal muscles, drive breathing: relaxation of the diaphragm assists expiration.

During inspiration, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity and drawing air in. During expiration, the diaphragm relaxes and returns to its dome (upward-curved) shape, decreasing thoracic volume and helping to push air out, aided by the elastic recoil of the lungs and relaxation of the external intercostal muscles.