“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.