Homeostatic Control and Gaseous Exchange Mechanisms

Homeostatic Control and Modification of Breathing Rate

Strenuous physical activity increases the energy needs of the body. This increased demand for energy leads to a requirement for a greater intake of oxygen ($O_2$) and results in the production of more carbon dioxide ($CO_2$). When the levels of $CO_2$ in the blood rise, the medulla oblongata responds by increasing both the heart rate and the breathing rate. This physiological adjustment ensures that more blood flows to the lungs, allowing more $O_2$ to be taken up by the body and more $CO_2$ to be expired from the system.

Gaseous Exchange in the Lungs

Gaseous exchange within the lungs is facilitated by the specific conditions at the interface between the alveoli and the surrounding capillaries. The capillaries surrounding an alveolus contain a high concentration of $CO_2$ and a low concentration of $O_2$. In contrast, the air inside the alveolus has a high concentration of $O_2$ and a low concentration of $CO_2$. This concentration gradient is the driving force that causes diffusion to occur.

During this process, oxygen dissolves in the moisture lining the alveolus and diffuses into the bloodstream. Once in the blood, the oxygen forms a loose combination with the haemoglobin found within the erythrocytes, also known as red blood corpuscles, to form oxyhaemoglobin. While most oxygen is transported in this manner, a minimal amount of oxygen may dissolve in the blood plasma, where it is transported in solution form. Simultaneously, $CO_2$ diffuses out of the bloodstream and into the alveolus to be exhaled.

Gaseous Exchange at the Tissues

When the blood in the capillaries reaches the tissues, there is a high concentration of $O_2$ and a low concentration of $CO_2$. The tissue fluid, however, has a higher concentration of $CO_2$ and a lower concentration of $O_2$. This concentration gradient triggers the diffusion process at the tissue level.

Oxygen diffuses from the blood into the tissue fluid and subsequently into the cells for use in metabolic processes. At the same time, $CO_2$ produced by cellular respiration diffuses out of the cells and into the tissue fluid, eventually moving into the blood to be transported back to the lungs for expiration.