<P> Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself . Baroreceptors fire at a rate determined by how much they are stretched, which is influenced by blood pressure, level of physical activity, and the relative distribution of blood . With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation . As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation . There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria . Increased venous return stretches the walls of the atria where specialized baroreceptors are located . However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart rate . The opposite is also true . Chemoreceptors present in the carotid body or adjacent to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels . Low oxygen or high carbon dioxide will stimulate firing of the receptors . </P> <P> Exercise and fitness levels, age, body temperature, basal metabolic rate, and even a person's emotional state can all affect the heart rate . High levels of the hormones epinephrine, norepinephrine, and thyroid hormones can increase the heart rate . The levels of electrolytes including calcium, potassium, and sodium can also influence the speed and regularity of the heart rate; low blood oxygen, low blood pressure and dehydration may increase it . </P> <P> Cardiovascular diseases, which include diseases of the heart, are the leading cause of death worldwide . The majority of cardiovascular disease is noncommunicable and related to lifestyle and other factors, becoming more prevalent with ageing . Heart disease is a major cause of death, accounting for an average of 30% of all deaths in 2008, globally . This rate varies from a lower 28% to a high 40% in high - income countries . Doctors that specialise in the heart are called cardiologists . Many other medical professionals are involved in treating diseases of the heart, including doctors such as general practitioners, cardiothoracic surgeons and intensivists, and allied health practitioners including physiotherapists and dieticians . </P> <P> Coronary artery disease, also known as ischemic heart disease, is caused by atherosclerosis--a build - up of fatty material along the inner walls of the arteries . These fatty deposits known as atherosclerotic plaques narrow the coronary arteries, and if severe may reduce blood flow to the heart . If a narrowing (or stenosis) is relatively minor then the patient may not experience any symptoms . Severe narrowings may cause chest pain (angina) or breathlessness during exercise or even at rest . The thin covering of an atherosclerotic plaque can rupture, exposing the fatty centre to the circulating blood . In this case a clot or thrombus can form, blocking the artery, and restricting blood flow to an area of heart muscle causing a myocardial infarction (a heart attack) or unstable angina . In the worst case this may cause cardiac arrest, a sudden and utter loss of output from the heart . Obesity, high blood pressure, uncontrolled diabetes, smoking and high cholesterol can all increase the risk of developing atherosclerosis and coronary artery disease . </P>

What kind of tissue makes up most of the heart