Respiration and Gas Exchange Notes

Comparing Inspired and Expired Air

Table 11.2 summarizes the differences in composition between inspired and expired air:

The air we breathe in contains approximately 20-21% oxygen. Expired air has less oxygen (around 16%) because cells use oxygen during respiration. However, expired air still contains a significant amount of oxygen. This is primarily because expired air mixes with normal atmospheric air in the bronchi and trachea, resulting in a mixture of alveolar air and atmospheric air.

Inspired air contains a small percentage of carbon dioxide (about 0.04%). Body cells produce carbon dioxide during aerobic respiration. This carbon dioxide diffuses from the blood into the alveoli. Consequently, expired air has a higher concentration of carbon dioxide, increasing to approximately 4%.

Breathing Movements

Breathing involves changing the volume of the thorax to move air in and out of the lungs. Increasing the thoracic volume draws air in, while decreasing it forces air out.

Two sets of muscles facilitate breathing:

1. Intercostal muscles: Located between the ribs (Figure 11.8). They raise and lower the rib cage.

2. Diaphragm: A large sheet of muscle and elastic tissue situated beneath the lungs and heart. It stretches across the body.

Key Definitions:

• Breathing: The process of using diaphragm and intercostal muscles to alter the thorax volume, facilitating air intake and expulsion from the lungs.

• Intercostal muscles: Muscles between ribs that control rib cage movement during breathing.

• Diaphragm: A muscle separating the chest and abdominal cavities in mammals, crucial for breathing.

Breathing In (Inspiration)

During inspiration, the diaphragm muscles contract, pulling the diaphragm downwards and increasing the thoracic volume (Figure 11.9). Simultaneously, the external intercostal muscles contract, raising the rib cage upwards and outwards, further increasing the thoracic volume.

The increased thoracic volume reduces the pressure inside the thorax below atmospheric pressure. Air then flows along the trachea and bronchi into the lungs.

Breathing Out (Expiration)

During expiration, the diaphragm muscles relax. The diaphragm returns to its domed shape due to its elastic properties, reducing the thoracic volume. Concurrently, the external intercostal muscles relax, allowing the rib cage to drop back to its normal position, further decreasing the thoracic volume.

Typically, relaxing the diaphragm and external intercostal muscles suffices for breathing out. However, forceful exhalation (e.g., coughing) involves contraction of the internal intercostal muscles, causing a greater drop in the rib cage. Abdominal wall muscles also contract to squeeze additional air out of the thorax.

Exercise and Breathing Rate

All body cells require oxygen for respiration, supplied by the lungs and transported via the blood. During intense activity (e.g., running), muscles demand more energy and, consequently, more oxygen.

Muscle cells combine oxygen and glucose at an accelerated rate to release energy for contraction. Increased oxygen demand leads to deeper and faster breathing and an increased heart rate to expedite oxygen delivery to muscles.

A limit exists to how quickly the heart and lungs can supply oxygen. When this limit is reached, and muscles still require more energy, anaerobic respiration provides extra energy.

\text{glucose} \rightarrow \text{lactic acid} + \text{energy}

Anaerobic respiration yields less energy but can provide a crucial short-term boost.

After exercise, lactic acid accumulates in muscles and blood. The liver breaks down lactic acid through aerobic respiration, requiring extra oxygen. Therefore, breathing and heart rate remain elevated to facilitate lactic acid breakdown and transport.

Oxygen Debt

During strenuous activity, an oxygen debt is incurred. This represents energy 'borrowed' without immediate oxygen 'payment'. The post-exercise period involves 'paying off' this debt by using extra oxygen to break down lactic acid. Breathing and heart rate return to normal once all lactic acid is processed (Figure 11.10).

Key Definition:

• Oxygen debt: The additional oxygen required after anaerobic respiration to break down accumulated lactic acid.

Control of Breathing Rate

The brain regulates breathing rate by monitoring blood pH. Elevated carbon dioxide or lactic acid levels in the blood decrease pH. The brain detects this change and sends nerve impulses to the diaphragm and intercostal muscles, stimulating more frequent and forceful contractions, resulting in a faster and deeper breathing pattern.