All events occurring during cardiac activity (cardiac cycle) are regulated by:
The heart rate, ventricular contractility and diameter of blood vessels are regulated by the cardiovascular centre. The cardiovascular centre consists of a collection of neurones and is located in the medulla oblongata. Functionally, these neurones are classified into four groups:
- Cardiostimulatory centre which stimulates the heart function.
- Cardioinhibitory centre which inhibits the heart function.
- Vasoconstrictor centre which decreases the diameter of blood vessels.
- Vasodilator centre which increases the diameter of blood vessels.
The cardiovascular centre gets impulses from the cerebral cortex, limbic system (a part of the brain that is responsible for olfaction, feeding behaviour, motivation, sexual behaviour and generation of emotions) and hypothalamus. It also receives stimulations from baroreceptors (which monitor blood pressure in major arteries and veins), chemoreceptors (which monitor the chemical changes in the blood) and properioceptors. Properioceptors are receptors found in joints and muscles. They send impulses to the cardiovascular centre during physical exercises which in turn sends a response to cope with the condition.
The output from the cardiovascular centre is through the sympathetic and parasympathetic autonomic nervous system to the heart. The sympathetic innervation of the heart is from the spinal nerves via the cervical and upper thoracic (T1-T4) ganglia. The sympathetic fibres reach the SA node (node which generates the cardiac impulses, located in the right atrial wall), AV node (part of the impulse conducting system of the heart, located in the lower part of the interatrial septum) and most portions of the myocardium.
Stimulation of the sympathetic fibres results in the release of norepinephrine at their cardiac synapses, which in turn binds to beta-1 receptors in the cardiac muscle. This binding leads to certain effects. First, it increases the rate of spontaneous depolarisation on the SA and AV, accelerating the heart rate. Second, on the cardiac muscle fibres, it enhances calcium ions inflow causing an increase in the contractility and a greater ejection of blood. In maximal sympathetic stimulation, the heart rate becomes very high; the stroke volume will be low due to the very short filling time and a low preload.
The parasympathetic innervation of the heart is via the vagus nerves which innervate the SA node, AV node and atrial myocardium. The parasympathetic stimulation of the heart releases acetylcholine, which decreases the heart rate and slows the rate of spontaneous depolarisation (an inhibitory effect).
Certain conditions such as hypoxia (low O2 level), acidosis (low pH), and alkalosis (high pH), affect the heart rate and contractility due to changes in the blood level of certain chemicals. Two major types of chemicals affect the physiology of the heart:
- Hormones: epinephrine and norepinephrine from the adrenal gland increase the heart rate and contractility. Stress, physical activity and excitement stimulate the adrenal gland to secrete more hormones, increasing the activity of the heart. Thyroid hormones from the thyroid gland also increase the heart rate and contractility.
- Ions: certain ions have a direct effect on cardiac function. High blood levels of potassium ions and sodium ions decrease the heart rate and contractility. High sodium levels prevent calcium inflow during cardiac action potentials (sequential changes in transmembrane potential that result in the propagation of electrical impulses in excitable cells, such as nerves and muscles), causing a decrease in the force of the muscle contraction. Excess in potassium levels blocks generation of the action potentials. A moderate increase in the extracellular level of calcium increases its level inside the muscle cell, accelerating the heart rate and strengthening the heart beat.
- Gender: adult females have higher resting heart rates than adult males.
- Exercise: physical activities increase the heart rate and contractility.
- Body temperature: increased body temperature increases the heart rate. A decrease in the body temperature decreases the heart rate and the force of contraction.