The Self-Regulatory Mechanisms of the Heart and Blood Vessels

The circulatory system, composed of the heart, blood vessels, capillaries, and lymphatic vessels, is the most important organ system of our body. Over long evolution, it has developed not only a fully functional structure but also powerful self-regulatory mechanisms, so that as the environment changes our body does not, under external stimuli, destroy its own homeostasis.

For example, near our aortic arch and at the carotid bifurcation there are both baroreceptors and chemoreceptors. These baroreceptors and chemoreceptors let us automatically adjust and maintain the body's own homeostasis when blood pressure or the oxygen, carbon dioxide, or hydrogen-ion content of the blood changes sharply.

The aortic arch is the first arched structure formed after the aorta originates, not far from our heart. Before the aortic arch, the very first segment of the aorta (the ascending aorta) gives off two branches, the left and right coronary arteries, whose blood directly supplies the heart itself. Thus the aortic arch is the first turning point the blood pumped by the heart passes through. At this turning point, the body has evolved two monitor-like tissues.

The first is the outer coat of the aortic arch wall, which is rich in nerve endings that sense changes in blood pressure. This is a baroreceptor; when blood pressure is too high or too low, it sends out signals that automatically adjust blood pressure to keep it at homeostasis.

The second is the aortic bodies. Located beneath the aortic arch, near the arterial ligament, are two or three millet-sized bodies; these are the aortic bodies. They are chemoreceptors that sense changes in the partial pressure of carbon dioxide, the partial pressure of oxygen, and the concentration of hydrogen ions in the blood. When the blood oxygen partial pressure falls or the carbon dioxide partial pressure rises, they reflexively, through nervous-system regulation, promote deeper and faster breathing to maintain the balance of oxygen and carbon dioxide in the blood.

Even with these two monitors, the body still does not feel secure enough. It has evolved two further monitors with essentially the same function at the carotid bifurcation: the carotid sinus and the carotid body.

The carotid bifurcation is the second branching of the arterial system, not far from the aortic arch. Here the common carotid artery divides into two branches: the internal carotid artery and the external carotid artery. These two arterial branches directly supply our brain, so their importance is self-evident.

The carotid sinus is located at the very beginning of the carotid bifurcation, a dilated portion at the end of the common carotid artery and the start of the internal carotid artery. The outer coat of the carotid sinus wall is also rich in free nerve endings; these too are baroreceptors. When blood pressure rises, the sinus wall dilates, stimulating the baroreceptors within it; through nervous-system regulation this reflexively causes the heart rate to slow and peripheral vessels to dilate, lowering blood pressure.

The carotid body is an oval body connected by connective tissue behind the common carotid bifurcation; it is a chemoreceptor. Its function is the same as that of the aortic bodies: it senses changes in the partial pressure of carbon dioxide, the partial pressure of oxygen, and the concentration of hydrogen ions in the blood. When the blood oxygen partial pressure falls or the carbon dioxide partial pressure rises, it reflexively promotes deeper and faster breathing through nervous-system regulation, maintaining the balance of oxygen and carbon dioxide in the blood.

From these structures we can deeply appreciate why biologists often say evolution is the optimal solution. Our body formed such structures over the course of evolution because of the need to cope with environmental stimuli. These structures keep our body from being too fragile and let us self-adjust and remain in homeostasis when exposed to environmental and internal stimuli. TCM often says the human body holds a great medicine within, which can help the body heal itself; clearly our ancestors observed this too.

But their functions are, after all, limited. Genetic mutations, long-term environmental stimulation, and long-term unhealthy habits can also weaken the protection of these barriers and destroy our organism. For example, long-term smoking can damage these baroreceptors and chemoreceptors, making them insensitive and causing lesions such as hypertension. So we should not, merely because we possess these natural endowments, ignore the impact of bad habits on our health.