Thursday, 6 May 2010
DESCRIBE THE STRUCTURE OF THE HEART AND EXPLAIN THE CARDIAC CYCLE
Image found at http://www.medicalook.com/diseases_images/heart-diseases1.jpg
Structure of the heart
The heart is a hollow, muscular vital organ. It is approximately the size of a clenched fist. Abrahams (2002 pg114) states “Roughly the shape of a pyramid on its side, the heart is said to have a base, three surfaces and an apex”. The heart sits near the left lung, in the cardiac notch. It has to the ability to move slightly during breathing.
The heart contains 4 chambers; 2 on the left and 2 on the right. The chambers are called
RA (right atrium)
RV (right ventricle)
LA (left atrium)
LV (left ventricle)
The left and right sides of the heart are separated by a thick muscular wall called the ‘interventricular septum’. The two sides of the heart needs to separated because they have different functions. The left half of the heart is responsible for pumping oxygenated blood through the systemic circulatory system, whereas the right half of the heart is responsible for pumping deoxygenated blood to the lungs via the pulmonary circulatory system. These occur simultaneously and this is way humans are referred to as having ‘double circulatory system’ due to the heart supplying to different circulatory systems with blood.
The chambers of the heart are separated by valves. Blood flows into the atriums and then past the valves into the ventricles. The valves only allow blood to flow into the ventricles at certain times and they will open to allow this to happen. At other times the valves stay closed. The valves that separate the atriums and ventricles are generically called the atrioventricular valves. The atrioventricular valve that separates the RA and the RV is called the tricuspid. The atrioventricular valve that separates the LA and the LV is called the bicuspid or mitral.
The atria of the heart are for receiving blood and the ventricles of the heart are for pumping blood out of the heart. Due to this fact, atria of the heart have thin walls, whereas the ventricles of the heart have thick walls. In fact the left ventricle has thicker walls than the right ventricle. This is because the left ventricle needs to be pump blood at high pressure so blood efficiently travels around the body. This is a longer route than the blood in the right ventricle has to travel; the heart is positioned within in the thoracic cavity where the lungs are also present.
The blood can be pumped easily out of the heart due to the fact that the majority of the heart is made of muscles. The heart consists of 3 layers; the outer layer is called epicardium, the inner layer is called the endocardium and the middle layer (which is the thickest) is called myocardium. This is cardiac muscle that has the ability to contract simultaneously in various directions. Within the hearts walls are nodes; sino-atrial and the atrioventricular node. These nodes are responsible for the stimulating and keeping the pace of the contractions of the heart.
When blood is in the right and left ventricles, it is held in the ventricles due to another set of valves, generically called semi lunar valves. Blood can only leave the ventricles when the ventricles contract and the valves open.
Blood from the RV, when ventricular contraction occurs passes the semi lunar valve called the pulmonary valve, and enters the pulmonary artery leaving the heart.
Blood from the LV, when ventricular contraction occurs passes the semi lunar valve called the aortic valve and enters the aorta, leaving the heart.
It is the movement of all the valves at specific times that make the ‘lub dub’ sound of our heart beat as they close.
Blood enters the heart via large blood vessels. The RA has 2 large blood vessels that enter it. These are called the vena cava superior and the vena cava inferior. These are naturally delivery deoxygenated blood back to the heart from the body. The LA has only one blood vessel of delivery, this is called the pulmonary vein.
The cardiac cycle is the sequence of events including contraction and relaxation of the heart, which means that blood is pumped around the body. The cardiac cycle can be clearly heard because the common name for this is ‘a heart beat’. When the heart beats once, this is one cardiac cycle. The cardiac cycle usually lasts less than a second.
According to Vascular Concepts (2010) “Each heartbeat or cardiac cycle is divided into 2 phases”.
Blood enters both atria at the same time. The muscle in the heart is relaxed and the atrioventricular valves are open and the semi lunar valves close making a lub sound. This is called diastole phase.
The second phase is the systole phase. The sino-atrial node of the heart signals the atria to contract, forcing more blood into the ventricles. The contraction reaches the atrioventricular node and this signals the ventricles to contract. As they contract, the atrioventricular valves (tricuspid and bicuspid) snap shut (making the dub noise) and the semi lunar valves open, due to the pressure of the blood inside the ventricles. The blood with the contraction is forced out of the heart.
The contraction slowly stops. The heart is relaxed. The atrioventricular valves open allowing blood to flow freely to the ventricles and this is now called the diastole phase. The whole process has begun again.