Systole

• Upon systole, the right atrium sends the venous blood to the right ventricle, which drives this blood to the lungs via the pulmonary arteries. Between the right atrium and ventricle is an atrioventricular valve, called the tricuspid valve, which closes when the ventricle contracts, preventing the backflow of blood into the atrium.

• After hematosis in the lungs, the arterial blood reaches the left atrium through the pulmonary veins. The contraction of the left atrium sends the blood to the left ventricle, whose systole directs the blood, under high pressure, into the aorta towards the entire body.

• Between the left atrium and the ventricle, there is an atrioventricular valve. Which also closes during ventricular systole, preventing the reflux of blood into the atrium.

The semilunar valves

• At the exit of the aorta, there is the aortic valve. And at the exit of the pulmonary artery is the pulmonary valve. When the ventricles relax, the semilunar valves close, preventing blood reflux back to the ventricles.

• The characteristic sounds emitted by the heart are due to the closing of the valves mentioned above.

• The aorta has a series of branches that originate from other arteries, such as the carotid and coronary arteries. The carotid arteries carry blood to the head, while the coronary arteries supply the heart itself, carrying O2 and nutrients to the myocardial cells.

• There are situations in which the coronary arteries can become blocked, leading the individual to suffer myocardial infarction.

• A heart attack is a death due to O2 deficiency, resulting from blockage of the flow of oxygenated blood to the heart tissue.

• The condition called atherosclerosis is linked to cases of myocardial infarction. One factor to be observed in the development of atherosclerosis is blood cholesterol levels. Cholesterol plays several roles in the human body. It acts, for example, in the structure of cell membranes and as a precursor of sex hormones.

• In the bloodstream, the transportation of cholesterol happens by two lipoproteins: LDL (low-density lipoprotein) and HDL (high-density lipoprotein). LDL supplies the body's cells with cholesterol, while HDL collects the excess cholesterol present in the tissues and takes it to the liver, where it participates in the formation of bile.

• Individuals who have a high concentration of LDL relative to the have a higher risk of developing atherosclerosis. In this condition, the arterial lining is damaged, causing inflammation that results in the formation of plaque that gradually grows and deposits on the arterial wall, clogging the vessel.

• People that develop atherosclerosis, and don't treat it, have a higher risk of suffering a heart attack. And a cerebral vascular accident (stroke).

Heartbeat Control

• The human heart generates the impulses that determine its beating. Heartbeats happen according to the rhythm of electrical impulses originated by a set of specialized cardiac muscle cells that determines a structure called the sinoatrial node.

• The electrical impulses generated by the sinoatrial node propagate rapidly through the heart muscle, resulting in the simultaneous contraction of the atria, and reach a cluster of cells, called the atrioventricular node, located in the wall between the atria.

The lymphatic circulation

• The lymphatic capillaries act to collect the excess tissue fluid, a fluid whose composition is similar to the blood plasma. The lymphatic capillaries gather into vessels of larger caliber, which reach the blood vessels near the heart, releasing the lymph into the blood.

• Lymph nodes are organs in which are found many defense cells, such as lymphocytes and macrophages. If the lymph detects foreign agents, the lymph node retains them. And a defense response is triggered.

• Besides the lymph nodes, other component organs of the lymphatic system are the thymus, the spleen, and the tonsils, structures that also act in the body's defense.

• Therefore, one of the functions of the lymphatic system is the defense of the body. And its components are also part of the immune system.