Chemoreceptors

“Chemo-“ refers to the chemical composition of the blood, so chemoreceptors are special nerve cells or receptors that sense changes in the chemical composition of the blood. That information is sent from the chemoreceptors to the brain to help keep the cardiovascular and respiratory systems balanced.

Alright, according to their location, chemoreceptors can be classified into two types: peripheral and central ones. Now, the peripheral chemoreceptors are so named because they live outside the brain. They are actually tiny bodies, or clusters of nerve cells and include the aortic body which sits along wall of the aortic arch, and the carotid body which is located at the point where each common carotid artery splits in the internal & external carotid arteries, running alongside the neck. Both the aortic and carotid bodies are bathed in arterial blood- and they carefully monitor changes in the concentration or partial pressure of oxygen, PO2 for short, but also in the partial pressure of carbon dioxide, PCO2 for short, as well as the concentration of hydrogen ions, which determines blood pH. The aortic body sends this information along to the vagus, or tenth (X) cranial nerve, and the carotid body sends this information along to the glossopharyngeal, or ninth (IX) cranial nerve. These two large nerves travel up towards the respiratory centers which are in the brainstem. The respiratory centers are groups of neurons, located in the pons and medulla oblongata, that are responsible for the autonomic or involuntary control of breathing. The respiratory centers also communicate with the cardiovascular centers. The cardiovascular centers are areas in the lower one-third of the pons and medulla oblongata of the brainstem, responsible for the autonomic or involuntary control of the cardiac and vascular function. They do that by coordinating the sympathetic and parasympathetic branches of the autonomic nervous system. There are two main cardiovascular centers - the first is the vasomotor control center, which controls the diameter of the blood vessels, using the sympathetic nerve fibers to cause vasoconstriction. The second is the cardiac control center, which is further divided into the cardiac accelerator and cardiac decelerator centers. The cardiac accelerator center speeds up the heart rate and increases cardiac contractility through the sympathetic outflow tract, while the cardiac decelerator center slows down the heart rate through the parasympathetic outflow tract. Notice that both the sympathetic and parasympathetic system affect the heart rate, but that only the sympathetic system has an effect on the diameter of the blood vessels and the contractility of the heart muscle.

Okay, so, let’s say that you have hypoxemia, meaning a decrease in the arterial partial pressure of oxygen, PO2, along with hypercapnia, an increase in the arterial partial pressure of carbon dioxide, PCO2, and acidemia, a decrease in blood pH. As blood pulses through the aortic arch and the common carotid, these changes get detected by the aortic and carotid bodies, which start firing more nerve impulses up through the vagus and glossopharyngeal nerves towards the cardiovascular centers of the brainstem. In response, the vasomotor center increases the vasoconstrictive effect of the sympathetic nervous system, causing arterioles to narrow, raising the total peripheral arterial resistance. In addition, the vasomotor center limits blood flow to peripheral organs, like the muscles, to help conserve oxygen for the brain and heart, and decreases the metabolic rate to help reduce carbon dioxide production. There’s also increased constriction of veins, which returns more blood to the heart rather than allowing it to pool in the periphery. Increased venous return means there’s more preload, and that also increases cardiac output. Now another effect, is that the cardiac accelerator center gets inhibited, reducing the sympathetic effect on the heart, so that it can work slower and less forcefully, in other words decreasing the heart rate and contractility, while the cardiac decelerator center is activated, boosting the parasympathetic effects on the heart, which again slows down the heart rate. This last bit might seem counterintuitive, but stay tuned.

Now, it turns out that peripheral chemoreceptors also send impulses through the vagus and glossopharyngeal nerves which reach the respiratory centers of the brainstem. And they cause you to suck in more oxygen and blow off more carbon dioxide, by getting the diaphragm and chest wall muscles to breathe faster and deeper. Now, this causes airways to get stretched, and that’s detected by another type of receptors, called pulmonary stretch receptors, which are located in the smooth muscles lining the trachea and central bronchi. Once stretched, these receptors start firing through the vagus nerve back to the pons and medulla, where they tell the respiratory centers to slow down. At the same time, though, they stimulate the cardiac accelerator center and, thus, the sympathetic effect on the heart, increasing the heart rate and contractility, while inhibiting the cardiac decelerator center and thus, the parasympathetic effects on the heart, which again helps the heart rate speed up. In other words, they reverse the direct effects that the peripheral chemoreceptors had on the cardiac centers. Ultimately, this makes the heart work faster and stronger, letting the cardiac output CO and blood pressure BP rise, which is essential to push more blood to the lungs, so that it gets supplied with oxygen and freed from the excess carbon dioxide.