Gas Exchange in Humans Study Notes

Components of the Human Respiratory System

The human breathing system comprises the respiratory tract and several associated structures. The respiratory tract consists of the nostrils, nasal cavity, pharynx, larynx, trachea, bronchus (plural: bronchi), bronchioles, and air sacs (alveoli). Associated structures providing support and facilitating movement include the ribs, intercostal muscles, the diaphragm, and the rib cage. The thoracic cavity, which houses the lungs and heart, is protected by the sternum and the backbone.

Air Filtration and Protective Mechanisms

Air enters through the nostrils, where small hairs filter large dust particles. The lining of the nasal cavity and the respiratory tract contains two specialized cell types: mucus-secreting cells (goblet cells) and ciliated epithelial cells. Goblet cells secrete mucus, a sticky substance that traps dust and microorganisms. Ciliated epithelial cells possess tiny cilia that move in a back-and-forth beating motion to sweep the trapped mucus toward the pharynx to be either swallowed or coughed out.

Structure of the Trachea and Bronchi

The trachea and bronchi are muscular tubes kept open by C-shaped cartilage to prevent them from collapsing during breathing. This cartilage is absent on the side adjacent to the esophagus to facilitate the expansion of the esophagus during swallowing. Bronchioles, the smaller divisions of the bronchi, do not contain cartilage. The inner linings of the trachea, bronchi, and large bronchioles are also equipped with ciliated epithelial cells and mucus-secreting cells for further filtration.

Gas Exchange in the Air Sacs

In the lungs, bronchioles terminate in cup-like structures called air sacs or alveoli. Each air sac is surrounded by a network of blood capillaries and has an epithelium only one-cell thick to minimize the diffusion distance. A thin water film lines the inner surface of the air sac to dissolve oxygen. Gas exchange occurs via diffusion down concentration gradients: oxygen dissolves in the water film and moves from the air sac into the blood, while $CO_2$ moves from the blood into the air sac. The pulmonary artery carries deoxygenated blood (high $CO_2$, low $O_2$) from the heart to the lungs, while the pulmonary vein carries oxygenated blood (high $O_2$, low $CO_2$) from the lungs back to the heart.

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Mechanics of Ventilation

Ventilation involves inhalation and exhalation, driven by pressure changes in the thoracic cavity derived from the relationship $PV = nRT$. During inhalation, the intercostal muscles and diaphragm muscles contract; the rib cage moves upwards and outwards, and the diaphragm flattens. This increases the volume of the thoracic cavity and lungs, causing the internal air pressure to drop below the atmospheric pressure of $760\,mmHg$, forcing air into the lungs. During exhalation, these muscles relax, the rib cage moves downwards and inwards, and the diaphragm returns to its dome shape. This decreases the thoracic volume, raising lung pressure above atmospheric levels to expel air.

Questions & Discussion

Question: During which time interval does inhalation occur according to the pressure graph? Answer: Inhalation occurs between $0\,s$ and $2\,s$ because the lung pressure is lower than the atmospheric pressure of $760\,mmHg$.

Question: During which time interval does exhalation occur? Answer: Exhalation occurs between $2\,s$ and $4\,s$ because the lung pressure is greater than the atmospheric pressure.

Question: At what time is the lung volume at its largest? Answer: The volume is largest at $2\,s$, the transition point between inhalation and exhalation.

Question: How is the rate of breathing calculated from the graph? Answer: Since one full breath (inhalation and exhalation) take $4\,s$, the rate is calculated as $60 \div 4 = 15\text{ breaths per minute}$.