CST - Heart and Blood Vessel Lesson
Heart and Blood Vessel
The heart is one of the most important muscles in the human body. It is a hollow, muscular organ about the size of a fist (although the size of your heart can vary depending on age, size, and condition of your heart). The human heart is located slightly to the left of the center in the thoracic cavity between the lungs and is protected by the rib cage. This organ works like a machine to pump blood and, in turn, transport important nutrients around the body and remove wastes.
Tissue Layers
A double-walled protective sac called the pericardium surrounds the heart. The double membrane of the pericardium contains fluid that nourishes the heart. The pericardium functions to protect and anchor the heart as well as prevent it from overfilling with blood.
The muscular wall of the heart is composed of three layers:
Layer | Location | Function |
Epicardium (visceral pericardium) | Outer layer | Protective outer layer of heart wall |
Myocardium | Middle layer | Middle layer that contains muscle tissue responsible for contraction |
Endocardium | Inner layer | Inner layer that covers heart valves and merges with blood vessels |
Interior Structure
Chambers
A human heart acts as a double pump, moving blood through the body in two different circuits: the systemic (body) and the pulmonary (lungs) circuit. It is composed of four chambers: two atria and two ventricles. The atria are two small, thin-walled chambers located at the top of the heart where blood first enters. Each atrium then pumps blood to its corresponding ventricle. The ventricles are two muscular, V-shaped chambers that pump blood out of the heart.
Valves
Valves in the heart allow blood to move only in a single direction. There are four valves in the human heart that determine the pathway of blood flow:
Valve Type | Valve Name | Location |
Atrioventricular (AV) | Mitral (bicuspid) valve | Between the left atrium and ventricle |
Tricuspid valve | Between the right atrium and ventricle | |
Semilunar (SL) | Aortic valve | Between the aorta and the left ventricle |
Pulmonary valve | Between the pulmonary artery and right ventricle |
AV valves are attached to the chordae tendineae (heart strings), which anchor the valves to the muscles of the heart and enable them to open and close with the contraction and relaxation of the heart. The SL valves do not have chordae tendineae and are more similar to the valves in the veins. The familiar "lub dub" sound of a heartbeat is actually the sound of these valves closing: the closure of the AV valves is heard as "lub;" the closure of the SL valves is heard as "dub." Heart murmurs are sounds produced when blood is able to escape back through a closed valve. This can be caused by narrowing or leaking valves, or the presence of abnormal blood passages in the heart.
The Septum
The left and right sides of the heart are separated by a septum, dividing the heart into two functionally separate and anatomically distinct units.
Coronary Circulation
Coronary circulation is the circulation of blood in the blood vessels of the heart muscle. The vessels that deliver oxygen-rich blood to the myocardium are known as coronary arteries. The vessels that remove the deoxygenated blood from the heart muscle are known as cardiac veins. The flow of blood through these vessels is vital for the normal functioning of the heart. When these vessels become blocked, the myocardium becomes oxygen-deprived and will begin to die. The scientific name for this is a myocardial infarction, but it is more commonly known as a heart attack.
Learn about heart attacks in the animation below.
Heart Contraction and Blood Flow
Each heartbeat has two basic parts: diastole and systole. During diastole, the atria and ventricles relax and begin to fill with blood. At the end of diastole, the heart's atria contract (atrial systole) and pump blood into the ventricles. The atria then begin to relax. Next, the heart's ventricles contract (ventricular systole) and pump blood out of the heart.
Learn about the contraction and blood flow of the heart in the learning object below.
Electrical System of the Heart
The beating of the heart is caused by the rhythmic contraction of cardiac muscle cells (found only in the heart). These cells are involuntary and each has the ability to contract on their own. However, it is the heart's electrical system (known as the cardiac conduction system) that coordinates their contractions to create the necessary pump action required to move blood through the pulmonary and systemic circuits.
There are three main parts to the cardiac conduction system:
Name | Location |
Sinoatrial (SA) node | Right atrium |
Atrioventricular (AV) node | Interatrial septum near tricuspid valve |
His-Purkinje System | Ventricle walls |
Learn about how the electrical system of the heart works in the learning object below.
Review
IMAGES CREATED BY GAVS OR OPENSOURCE