External respiration occurs in the lungs, and it is the first step in the journey of oxygen inside the body. When we inhale, oxygen enters our lungs and diffuses across the thin alveolar membrane. The alveoli are tiny, air-filled sacs that provide a vast surface area for gas exchange. Oxygen in the alveoli has a higher partial pressure (105 mmHg) than in the adjacent pulmonary capillaries (40 mmHg), establishing a pressure gradient. As a result, oxygen molecules move from the alveoli into the blood, binding with hemoglobin in red blood cells.

Simultaneously, carbon dioxide, produced as a waste product of cellular metabolism, moves from the blood into the alveoli. The PCO2 of blood is slightly higher (45 mmHg) than in alveolar air (40 mmHg), driving this exchange. Once the exchange is complete, oxygenated blood returns to the heart to be pumped throughout the body, while CO2 is expelled during exhalation.

Internal respiration, also known as systemic gas exchange, occurs at the level of our body's tissues. Here, the oxygenated blood pumped by the left ventricle travels through systemic arteries, eventually reaching systemic capillaries. The goal is to provide oxygen to tissue cells and collect their waste product, carbon dioxide. Systemic capillaries have higher PO2 than tissue cells because oxygen is used for ATP production in cells, prompting oxygen to diffuse from the capillaries into the cells. Conversely, CO2 production by active metabolism, such as during cellular respiration, results in higher PCO2 in cells than in systemic capillaries, causing its diffusion from cells into these capillaries.

Factors Affecting the Rates of Diffusion in Internal and External Respiration

The gradient of Partial Pressure: Greater differences in partial pressures enhance gas diffusion rates. The partial pressures of O2 and CO2 in alveolar air also rely on lung airflow. Certain drugs, like morphine, slow respiratory rate, reducing O2 and CO2 exchange. At higher altitudes, both total atmospheric and O2 partial pressure decline, causing slower O2 diffusion into the blood. The resultant high-altitude sickness results in breathlessness, headache, fatigue, insomnia, nausea, and dizziness due to decreased blood oxygen levels.

Surface Area Available: The large surface area of alveoli and the abundance of capillaries surrounding them allow for efficient gas exchange. Conditions that reduce this surface area, such as emphysema, slow the exchange rate.

Diffusion Distance: The thin respiratory membrane and the single-file passage of red blood cells through narrow capillaries minimize diffusion distance. Accumulation of interstitial fluid between the alveoli, as seen in pulmonary edema, hinders gas exchange by extending the distance for diffusion.