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- The walls of the heart consist of three layers:
- The outer epicardium, is another name for the visceral pericardium mentioned above.
- The thick, middle myocardium, is made of muscle tissue and gives the heart its ability to contract.
- The inner endocardium, lines the heart chambers and is the main component of the heart valves.
- The heart consists of four chambers:
- The upper chambers are the right and left atria (singular: atrium).
- The lower chambers are the right and left ventricles.
- The interventricular septum is a muscular wall that separates the right and left ventricles. The interatrial septum separates the right and left atria.
- The atrium and ventricle on each side of the heart are separated by an atrioventricular (AV) valve:
- The right AV valve, or tricuspid valve, separates the right atrium and right ventricle.
- The left AV valve, or bicuspid valve, separates the left ventricle and the left atrium. This valve is also called the mitral valve.
- There are also two semilunar valves:
- The pulmonary valve separates the right ventricle from the pulmonary trunk.
- The aortic valve separates the left ventricle from the aorta.
Physiology
- Circulation through the heart: Blood is pumped by the heart in order to provide oxygen and nutrients to every cell in the body.
- The heart as an organ (coronary blood supply): The heart is an organ, made of cells and tissues which require their own blood supply.
- The heart’s electrical conduction system: The heart is able to independently generate and transmit instructions to the myocardium, in order to make it contract and pump the blood.
Circulation
- The heart pumps blood to two distinct but linked circulatory systems called the pulmonary and systemic circuits.
- The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and drops off carbon dioxide.
- The systemic circuit transports freshly oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.
- Blood that is carrying carbon dioxide and waste products from the body tissues is returned to the right atrium via the superior vena cava and the inferior vena cava.
- From the right atrium, the deoxygenated blood moves through the tricuspid valve into the right ventricle.
- The right ventricle pumps deoxygenated blood through the pulmonary valve into the pulmonary trunk, which splits into the right and left pulmonary arteries, leading toward the lungs. These arteries branch many times before reaching the pulmonary capillaries, where gas exchange occurs: carbon dioxide exits the blood and oxygen enters.
- The pulmonary arteries are the only arteries in the postnatal body that carry deoxygenated blood.
- Freshly oxygenated blood returns from the lungs to the left atrium via the pulmonary veins.
- From the left atrium, the blood moves through the mitral valve into the left ventricle.
- The left ventricle pumps blood through the aortic valve, into the aorta, delivering blood to all parts of the body.
Cardiac Cycle
- Diastole is the portion of the cycle in which the heart is relaxed and the atria and ventricles are filling with blood. The AV valves are open so that blood can move from the atria to the ventricles.
- Systole is the portion of the cycle in which the heart contracts, AV valves slam shut, and the ventricles eject blood to the lungs and to the body through the open semilunar valves. Once this phase ends, the semilunar valves close, in preparation for another filling phase.
Coronary Blood Supply
- Myocardial cells require their own blood supply to carry out their function of contracting and relaxing the heart in order to pump blood.
- Their own blood supply provides nutrients and oxygen and carry away carbon dioxide and waste. These functions are provided by the coronary arteries and coronary veins.
Conduction System
We can detect and record the electrical activity of the heart’s conduction system using an electrocardiogram (ECG or EKG).
- Sinoatrial Node (SA) – known as the heart’s natural pacemaker. The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles. The SA node sets the rate and rhythm of your heartbeat. Normal heart rhythm is often called normal sinus rhythm because the SA (sinus) node fires regularly.
- Atrioventricular Node (AV) – a cluster of cells in the center of the heart between the atria and ventricles, and acts like a gate that slows the electrical signal before it enters the ventricles. This delay gives the atria time to contract before the ventricles do.2
- Atrioventricular bundle (bundle of His) – a collection of cells that carry electrical signals from the AV node to the bundle branches.3
- Left and Right bundle branches – directs the impulse to the respective left and right ventricles. The signals end at the Purkinje fibers.4
- Purkinje Fibers – this network sends the impulse into the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body.
- The SA node fires another impulse. The cycle begins again.
- Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]
- Cleveland Clinic. (n.d.). Heart Beat. Retrieved from https://my.clevelandclinic.org/health/articles/17064-heart-beat[↩]
- Hobart Cardiology & Medical Specialists. (n.d.). Bundle of His. Retrieved from http://hobartcardiology.com.au/the-normal-heart/bundle-of-his/[↩]
- Hobart Cardiology & Medical Specialists. (n.d.). Bundle Branches. Retrieved from http://hobartcardiology.com.au/the-normal-heart/bundle-branches/[↩]