Heart
The heart is a muscular organ that continuously pumps blood. It is made up of:
Right and left atrium
The atria are thin-walled because they only pump blood to the ventricles inferior to them. The cardiac muscles in the atria only have to overcome the resistance to open the atrioventricular valves that close again to prevent the back-flow of blood from ventricles.
Right and left ventricle
The ventricles have thicker muscular walls in order to generate a greater force of contraction.
The resistance to blood flow in the pulmonary circulation is less than that of the systemic circulation because the lungs are large, sponge-like structures. However, the resistance to blood flow is greater in the systemic circulation because tissues organs are comparatively more dense.
The wall of the left ventricle is comparatively thicker than that of the right ventricle as it is responsible for pumping blood into the systemic circulation while the right ventricle is only responsible for pumping blood into the pulmonary system.
Movement of blood through the systems
Essentially, the right atrium and ventricle receive deoxygenated blood, this blood is pumped through the pulmonary trunk to the lungs where gaseous exchange takes place and the blood is oxygenated.
Oxygenated blood returns to the left atrium of the heart, passing to the left ventricle where it is pumped through the aorta into the systemic circulation to supply tissues with oxygen and other nutrients.
Valves guard the orifices between the atria and ventricles and the ventricles and pulmonary trunk and aorta to prevent back-flow of blood as pressure changes occurs in the chambers and vessels.
Diastole and Systole
Blood returns continuously to both sides of the heart when the ventricles relax (diastole). The atrioventricular valves open as the atria contract and blood flows into the ventricles.
When the ventricles are nearly full, the next cycle of contraction (systole) begins. The ventricles contract in a wave which passes upwards from the apex of the heart toward the aortic and pulmonary openings. An increase in pressure is created which closes the atrioventricular valves and prevents back-flow of blood into the atria. Because these valves are closed, returning blood accumulates in the large veins and atria.
Once the intraventricular pressure exceeds that in the large arteries, the semilunar valves at the exits into the aorta and pulmonary trunk are forced open and blood is ejected from the heart.
As soon as contraction ceases, the intraventricular pressure drops below that of the great vessels and the semilunar valves close.
Further relaxation lowers ventricular pressure to fall below that of the atria, now refilled from the systemic and pulmonary circulations, the atrioventricular valves open as the cycle begins again.
Clinical Notes:
Penumopericardium occurs in a pneumothorax patients wherein air may enter along their connective tissues
Hemopericardium occurs when there is blood in the pericardial cavity
Pericarditis occurs when there is pericardial inflammation that causes chest pains
Pericardial effusion occurs when there is fluid in the pericardial cavities
Cardiac tamponade is an emergency which occurs when there is compression of the heart due to extensive pericardial effusion and there is an inability of the elastic fibrous pericardium to accommodate greater volumes. It is characterized by hypotension, tachycardia, jugular venous distension, and muffled heart sounds. It will eventually result in cardiogenic circulatory shock.
