In this blog post, we will look at the importance of oxygen and carbon dioxide concentrations in the blood, the mechanisms that regulate them, and their effects on the health and homeostasis of the body.
Oxygen that enters the blood in the lungs is transported through the heart to each tissue in the body, where it is used to produce energy. Carbon dioxide, a waste product of metabolism, is transported through the blood to the heart and then to the lungs, where it is exhaled from the body. Gas exchange between the blood and the alveoli, and between the blood and the tissues, occurs through diffusion based on differences in partial pressure, with gases diffusing from areas of high partial pressure to areas of low partial pressure. Among the blood vessels that carry blood, those that carry blood from the heart to the lungs and various tissues are called arteries, and those that carry blood from the lungs and various tissues to the heart are called veins. Blood that flows from the lungs to various tissues after gas exchange in the lungs is called arterial blood, and blood that flows from the tissues to the lungs after gas exchange in the tissues is called venous blood.
The partial pressure of oxygen in the alveoli is 100 to 110 mmHg, and the partial pressure of oxygen in the venous blood in the surrounding capillaries is 40 mmHg, so oxygen in the alveoli diffuses into the venous blood in the capillaries surrounding the alveoli. At this point, the oxygen-rich blood flows through the heart to each tissue in the body. The partial pressure of oxygen in the arterial blood flowing through the capillaries of each tissue is 100 mmHg, and the partial pressure of oxygen in the tissue is 40 mmHg on average, so the oxygen in the arterial blood diffuses into the tissue. The blood that has released oxygen flows through the heart to the lungs.
However, oxygen has low solubility in water, so only about 1.5% of the oxygen transported from the lungs to the tissues is dissolved in plasma. About 98.5% is transported in the form of oxygen haemoglobin, which is bound to haemoglobin in red blood cells. When haemoglobin binds with oxygen, it takes the form of iron oxide, which is one of the reasons why blood is red. In this process, haemoglobin maintains a structure that allows it to transport oxygen very efficiently. This process is essential for maintaining the physiological balance of the human body, and the oxygen-carrying capacity of blood is an important indicator of overall health.
The curve showing the oxygen saturation of haemoglobin according to the partial pressure of oxygen is called the oxygen dissociation curve. The horizontal axis of the oxygen dissociation curve represents the partial pressure of oxygen in the blood, and the vertical axis represents the oxygen saturation of haemoglobin. The sum of oxygen saturation, which is the degree to which haemoglobin binds to oxygen at a given oxygen partial pressure, and oxygen dissociation, which is the degree to which haemoglobin dissociates from oxygen, is 100%. This curve is a gentle S-shape, and when the partial pressure of oxygen decreases, the amount of oxygen dissociated from oxygen haemoglobin is greater in the 0 to 40 mmHg range than in the 40 to 100 mmHg range. The oxygen affinity of haemoglobin indicates the tendency of haemoglobin to bind with oxygen. Factors that affect oxygen affinity include the partial pressure of oxygen, blood pH (hydrogen ion concentration index), and temperature.
When the metabolism of a tissue becomes active, the pH of the blood in the surrounding capillaries decreases due to an increase in carbon dioxide. When the pH of the blood decreases, the oxygen affinity of haemoglobin decreases, promoting the release of oxygen to the surrounding tissues. In other words, when the partial pressure of oxygen is the same, more oxygen is released from oxygen haemoglobin in areas with lower pH. In addition, physical activity such as exercise causes the temperature of tissues to rise, making it easier for oxygen to dissociate from the blood flowing through the capillaries surrounding those tissues, resulting in more oxygen being released to those tissues than before exercise.
Changes in blood pH are generally caused by changes in the concentration of carbon dioxide in the blood. Carbon dioxide reacts with water in the blood to form carbonic acid, which is then broken down into hydrogen ions and bicarbonate ions. An increase in the concentration of hydrogen ions lowers the pH of the blood, which acts as an important factor in promoting oxygen release from tissues. This process also plays an important role in maintaining the acid-base balance in the body.
Meanwhile, carbon dioxide, which is a waste product of metabolism in each tissue, is also diffused into the blood and transported. The partial pressure of carbon dioxide in tissues is 46 mmHg on average, and the partial pressure of carbon dioxide in arterial blood is 40 mmHg, so carbon dioxide in tissues diffuses into the blood flowing in the capillaries surrounding the tissues. Approximately 7% of the carbon dioxide transported from tissues to the lungs is dissolved in plasma, and approximately 23% is transported in the form of carbaminohemoglobin, which is bound to haemoglobin in red blood cells. Haemoglobin that is not bound to oxygen binds more easily to carbon dioxide than haemoglobin bound to oxygen, forming carbamino haemoglobin, so venous blood is more useful than arterial blood for transporting carbon dioxide using haemoglobin.
Approximately 70% of carbon dioxide is transported in the form of bicarbonate ions. Carbon dioxide diffused from tissues mainly combines with water in red blood cells through the action of carbonic anhydrase to form carbonic acid, which is then ionised into hydrogen ions and bicarbonate ions. At this point, hydrogen ions mainly bind to haemoglobin, while bicarbonate ions diffuse into the blood plasma and are transported to the lungs. The opposite reaction occurs in the capillaries surrounding the alveoli. In other words, bicarbonate ions move to red blood cells and recombine with hydrogen ions to form carbonic acid, which is then converted to carbon dioxide and water by carbonic anhydrase. The carbon dioxide produced in this process diffuses into the alveoli and is exhaled from the body. Enzymes in red blood cells play an important role in this process, and if the function of these enzymes is impaired, gas exchange efficiency is reduced.
In addition, the concentration of carbon dioxide directly affects the speed and depth of breathing. High concentrations of carbon dioxide stimulate breathing, increasing carbon dioxide exhalation through the lungs, which is essential for maintaining the acid-base balance in the body. Conversely, low concentrations of carbon dioxide cause respiratory depression, activating a feedback mechanism that increases the concentration of carbon dioxide in the blood. These respiratory control mechanisms play an important role in maintaining homeostasis.
Therefore, the gas exchange process in the human body is highly regulated, with various physiological factors interacting to maintain appropriate concentrations of oxygen and carbon dioxide. This is essential for maintaining overall body function and health, and even small imbalances can have serious consequences.
