The natural sinus rhythm of the heart that gives a resting heart rate is affected by several factors. The cardiovascular centers in the brainstem control the sympathetic and parasympathetic effects on the heart via the vagus nerve and the sympathetic trunk.[43] These centers receive input from several receptors, including baroreceptors that sense the dilation of blood vessels, and chemoreceptors that sense the amount of oxygen and carbon dioxide in the blood and the pH of the blood. These centers, through a number of reflexes, help regulate and maintain blood flow.[9]
Baroreceptors are stretch-sensitive receptors found in the aortic sinus, the carotid bodies, the vena cava, and other sites including the pulmonary vessels and the right side of the heart. Baroreceptors are stimulated by the amount of their stretch,[44] which is affected by blood pressure, amount of physical activity, and the relative distribution of blood. In the case of elevated pressure and stretching, the rate of impulses sent by the baroreceptors increases, so the cardiovascular centers reduce sympathetic stimulation and increase parasympathetic stimulation. While in hypotension, the baroreceptor impulse rate decreases and the cardiovascular centers increase sympathetic stimulation and decrease parasympathetic stimulation. [9] A similar reflex called the atrial reflex or Bainbridge reflex is associated with varying rates of blood flow into the atria. The appendicular venule extends the walls of the atria where specialized pressure receptors are located. However, when baroreceptors increase their rate of pulse due to their stretching due to the increased pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase the heart rate. The opposite is also true. [9] Chemoreceptors are located in the carotid body or near the aorta in the aortic body, and they respond to the levels of oxygen and carbon dioxide in the blood. Where low oxygen or high carbon dioxide stimulates the release of these receptors.[45]
