13.3: Respiration and Gaseous Exchange
The intricate interplay between the cardiovascular and respiratory systems is crucial for efficiently transporting respiratory gases throughout the body. Let us explore the cardiovascular system's multifaceted functions, emphasizing its pivotal role in gas exchange.
Respiration involves the exchange of gases, especially oxygen (O2) and carbon dioxide (CO2), between the alveoli and body cells, a process facilitated by blood circulation. As a result, the cardiovascular system, which involves the heart and blood vessels, is essential for efficient gas exchange. Let us start by examining the heart's anatomy.
The heart, a cone-shaped muscular pump, is the central organ of the cardiovascular system. It has four hollow chambers: two atria on top and two ventricles below. The atria (singular: atrium) are the inlets, receiving blood from the veins, which are the superior and inferior vena cava and the left and right pulmonary veins. The ventricles are the outlets, forcefully pumping oxygenated blood out through the arteries, including the pulmonary arteries and the aorta. These arteries supply oxygen-rich blood to the body and carry deoxygenated blood to the lungs for oxygenation.
The heart's unique structure, with one-way valves at the entrances (tricuspid and mitral) and exits (pulmonary and aortic) of each ventricle, ensures blood flows in the correct direction, preventing backflow and maintaining the efficiency of the system.
The extensive network of blood vessels forms a closed circuit, facilitating blood transport between the heart and body cells. Arteries and arterioles transport blood away from the heart, directing it towards capillaries, while venules and veins return blood to the atria.
Capillaries, the smallest blood vessels, are critical for exchanging substances between blood and body cells. In the lungs, deoxygenated blood—low in oxygen and high in carbon dioxide—travels from the heart's right side to the lungs. Here, in the alveoli, O2 from the inhaled air diffuses into the blood, while CO2, a waste product, diffuses out of the blood into the alveoli to be exhaled. The alveoli are lined with a delicate layer of surfactant fluid, without which the alveoli would collapse. The surfactant, produced by the alveolar cells, reduces the surface tension of the fluid, preventing the alveoli from collapsing and facilitating the exchange of gases. After acquiring oxygen and releasing carbon dioxide, the oxygen-rich blood returns to the heart's left side. This oxygenated blood, now low in carbon dioxide, is circulated throughout the body, delivering oxygen to various tissues.
Understanding oxygen transport also requires recognizing the roles of plasma and red blood cells (erythrocytes). While a small amount of oxygen dissolves in plasma, most is bound to hemoglobin in red blood cells, in the form of oxyhemoglobin, due to hemoglobin's high affinity for oxygen. Hemoglobin also carries carbon dioxide, primarily in the form of carboxyhemoglobin.
During internal respiration, oxygen [O2] and carbon dioxide [CO2] are exchanged between tissue cells and blood. Any abnormalities in blood composition can significantly impact this process. For example, hemorrhage, characterized by excessive bleeding, decreases cardiac output and reduces blood available for oxygen delivery. It can lead to tissue hypoxia, a condition where the body's tissues do not receive enough oxygen. Anemia, another condition, is characterized by a reduction in erythrocytes, which leads to insufficient hemoglobin for oxygen transport and inadequate tissue oxygenation. On the other hand, regular exercise improves the efficiency of the heart muscles and increases the amount of oxygen delivered to the tissues. Other conditions, such as respiratory diseases or high altitude, can also affect gas exchange, highlighting the delicate balance required for maintaining homeostasis.
