Breathing and Exchange of Gases

The process of breathing and exchange of gases is a marvel of biological engineering, ensuring that every cell in our body receives the oxygen it needs for metabolism and effectively disposes of the carbon dioxide it produces. This intricate system involves several stages, from the bulk movement of air to the microscopic diffusion of gases across membranes.

I. Mechanisms of Breathing (Ventilation)

Breathing is a mechanical process involving the coordinated action of the respiratory muscles, rib cage, and diaphragm, leading to changes in thoracic cavity volume and, consequently, lung pressure. This pressure difference drives air movement.

A. Inspiration (Inhalation): This is an active process.

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Diaphragm Contraction: — The diaphragm, a dome-shaped muscle separating the thoracic and abdominal cavities, contracts and flattens, moving downwards.
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External Intercostal Muscles Contraction: — The external intercostal muscles, located between the ribs, contract, pulling the ribs upwards and outwards.
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Thoracic Volume Increase: — The combined action of the diaphragm and external intercostals increases the volume of the thoracic cavity in antero-posterior and dorso-ventral axes.
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Lung Expansion: — As the thoracic cavity expands, the lungs, which are intimately associated with the thoracic wall via the pleura, also expand.
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Intra-pulmonary Pressure Decrease: — The expansion of the lungs increases their internal volume, which in turn decreases the intra-pulmonary pressure (pressure within the lungs) to below atmospheric pressure.
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Air Inflow: — Due to this pressure gradient, air from the atmosphere rushes into the lungs until the intra-pulmonary pressure equals the atmospheric pressure.

B. Expiration (Exhalation): This is generally a passive process during normal quiet breathing.

1
Diaphragm Relaxation: — The diaphragm relaxes and returns to its dome-shaped position.
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External Intercostal Muscles Relaxation: — The external intercostal muscles relax, allowing the ribs to move downwards and inwards.
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Thoracic Volume Decrease: — The relaxation of these muscles reduces the volume of the thoracic cavity.
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Lung Contraction: — The elastic recoil of the lungs causes them to contract, reducing their internal volume.
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Intra-pulmonary Pressure Increase: — The reduction in lung volume increases the intra-pulmonary pressure to above atmospheric pressure.
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Air Outflow: — Air is expelled from the lungs until the intra-pulmonary pressure again equals the atmospheric pressure.

C. Forced Breathing: During strenuous exercise or conditions like asthma, accessory muscles (e.g., internal intercostals, abdominal muscles) are recruited to aid in more forceful inspiration and expiration.

II. Exchange of Gases

Gas exchange occurs via simple diffusion, driven by pressure gradients (partial pressures) of gases. The efficiency of diffusion is governed by factors like the thickness of the membrane, solubility of gases, and surface area.

A. Alveolar Gas Exchange (External Respiration): Occurs in the lungs between alveoli and pulmonary capillaries.

Partial Pressures:

* In alveolar air: $P_{O_2} = 104,\text{mmHg}$, $P_{CO_2} = 40,\text{mmHg}$ * In deoxygenated blood (pulmonary artery): $P_{O_2} = 40,\text{mmHg}$, $P_{CO_2} = 45,\text{mmHg}$

Diffusion:

* Oxygen diffuses from alveoli ($P_{O_2} = 104,\text{mmHg}$) into the blood ($P_{O_2} = 40,\text{mmHg}$). * Carbon dioxide diffuses from the blood ($P_{CO_2} = 45,\text{mmHg}$) into the alveoli ($P_{CO_2} = 40,\text{mmHg}$).

Respiratory Membrane: — This membrane is extremely thin (less than 1 mm) and consists of the alveolar epithelium, the endothelial lining of alveolar capillaries, and the basement membrane between them. This thinness, coupled with a vast surface area (approx. 100 $m^2$), facilitates rapid and efficient gas exchange.

B. Tissue Gas Exchange (Internal Respiration): Occurs in the body tissues between systemic capillaries and tissue cells.

Partial Pressures:

* In oxygenated blood (systemic artery): $P_{O_2} = 95,\text{mmHg}$, $P_{CO_2} = 40,\text{mmHg}$ * In tissue cells: $P_{O_2} = 40,\text{mmHg}$ (due to continuous consumption), $P_{CO_2} = 45,\text{mmHg}$ (due to continuous production)

Diffusion:

* Oxygen diffuses from the blood ($P_{O_2} = 95,\text{mmHg}$) into the tissue cells ($P_{O_2} = 40,\text{mmHg}$). * Carbon dioxide diffuses from the tissue cells ($P_{CO_2} = 45,\text{mmHg}$) into the blood ($P_{CO_2} = 40,\text{mmHg}$). * The blood leaving the tissues is now deoxygenated and rich in carbon dioxide, returning to the heart and then to the lungs.

III. Transport of Gases

Blood is the primary medium for transporting oxygen and carbon dioxide.

A. Oxygen Transport:

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Hemoglobin Binding (97%): — The vast majority of oxygen is transported by hemoglobin (Hb), a red-colored iron-containing pigment in red blood cells. Each Hb molecule can bind up to four molecules of $O_2$ to form oxyhemoglobin ($HbO_2$). The binding is reversible and depends on $P_{O_2}$.

* Factors affecting $O_2$ binding (Oxygen-Hemoglobin Dissociation Curve): * **$P_{O_2}$:** High $P_{O_2}$ (lungs) favors binding; low $P_{O_2}$ (tissues) favors dissociation. * **$P_{CO_2}$:** High $P_{CO_2}$ (tissues) shifts the curve to the right (Bohr effect), favoring $O_2$ release.

*$H^+$ concentration (pH):** High $H^+$ (low pH, tissues) shifts the curve to the right, favoring $O_2$ release. * Temperature: High temperature (tissues) shifts the curve to the right, favoring $O_2$ release.

* 2,3-BPG: A byproduct of glycolysis in RBCs, increases $O_2$ release.

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Dissolved in Plasma (3%): — A small amount of oxygen dissolves directly in the plasma.

B. Carbon Dioxide Transport:

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Bicarbonate Ions (70%): — Most $CO_2$ is transported as bicarbonate ions ($HCO_3^-$). In RBCs, $CO_2$ reacts with water in the presence of carbonic anhydrase to form carbonic acid ($H_2CO_3$), which quickly dissociates into $H^+$ and $HCO_3^-$. $HCO_3^-$ then moves into the plasma, and chloride ions ($Cl^-$) move into the RBCs to maintain electrical neutrality (chloride shift).

*

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Carbaminohemoglobin (20-25%): — $CO_2$ binds directly to the amino groups of hemoglobin to form carbaminohemoglobin. This binding is more favorable when $P_{CO_2}$ is high and $P_{O_2}$ is low (Haldane effect).
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Dissolved in Plasma (7-10%): — A small fraction of $CO_2$ dissolves directly in the plasma.

IV. Regulation of Respiration

Breathing is a rhythmic process regulated by the nervous system to match the body's metabolic demands.

A. Neural Regulation:

Respiratory Rhythm Centre (Medulla Oblongata): — The primary center, responsible for generating the basic rhythm of breathing. It contains inspiratory and expiratory neurons.
Pneumotaxic Centre (Pons): — Located in the pons, it can moderate the functions of the respiratory rhythm center. It sends signals to inhibit inspiration, thereby reducing the duration of inspiration and increasing the respiratory rate.
Apneustic Centre (Pons): — Also in the pons, it prolongs inspiration, leading to deep, prolonged breaths. Its effect is usually overridden by the pneumotaxic center.
Chemosensitive Area (Medulla): — Adjacent to the rhythm center, highly sensitive to $CO_2$ and $H^+$ concentrations in the blood. An increase in $CO_2$ or $H^+$ stimulates this area, leading to an increase in respiratory rate and depth to expel $CO_2$.
Peripheral Chemoreceptors (Aortic Arch and Carotid Artery): — These receptors are primarily sensitive to changes in $P_{O_2}$, but also to $P_{CO_2}$ and $H^+$. A significant drop in $P_{O_2}$ (below 60 mmHg) stimulates these receptors, increasing respiratory activity. Their role in normal breathing is minor compared to central chemoreceptors.

B. Chemical Regulation: The most potent stimulus for regulating breathing is the concentration of $CO_2$ and $H^+$ ions in the blood. Oxygen plays a less significant role under normal physiological conditions.

V. Disorders of the Respiratory System

Asthma: — An allergic reaction causing inflammation and constriction of bronchi and bronchioles, leading to difficulty in breathing (wheezing).
Emphysema: — A chronic disorder where alveolar walls are damaged, primarily due to cigarette smoking, leading to a decrease in the respiratory surface area. This results in shortness of breath.
Occupational Respiratory Disorders: — Caused by prolonged exposure to dust in certain industries (e.g., asbestosis, silicosis). The body's defense mechanisms cannot cope, leading to inflammation and fibrosis (proliferation of fibrous tissue), causing serious lung damage. Workers in such industries often wear protective masks.

VI. Common Misconceptions

Breathing vs. Respiration: — Many students confuse these terms. Breathing is the physical act of moving air, while respiration is the cellular biochemical process of energy production.
Oxygen Transport Only by Hemoglobin: — While hemoglobin carries the vast majority, a small but significant amount of oxygen is dissolved in plasma.
Carbon Dioxide is Just a Waste Product: — While primarily a waste product, $CO_2$ also plays a crucial role in regulating blood pH and stimulating respiratory centers.
Regulation by Oxygen: — While oxygen levels are monitored, the primary chemical regulators of breathing rate are carbon dioxide and hydrogen ion concentrations, as they directly impact blood pH.