What is it, how do I remember it all and how on earth do you pronounce it? We can help …
What is it?
The heart is a specially shaped muscle about the size of your ﬁst, containing a series of chambers that move blood throughout the body. These chambers are the right and left atria, and the right and left ventricles. The heart is surrounded by a double layer serous membrane called the pericardium. The outer layer is a tough ﬁbrous connective tissue called the parietal pericardium that anchors the heart to surrounding structures, for example the diaphragm. The inner layer, the visceral pericardium or epicardium, is fused to the heart surface but there is a potential cavity between the layers called the pericardial cavity. The middle layer of the heart wall, the myocardium, is made of cardiac muscle and the heart is lined by epithelium, called the endocardium.
The movements of the heart, called the cardiac cycle, can be divided into two phases called systole and diastole. Usually when discussing heart movement we refer to ventricle activity. When the ventricles are full of blood, the heart contracts (systole). Because the bicuspid and tricuspid valves are one-way valves, as the ventricular pressure increases the valves move so blood cannot return back into the atria. As the pressure increases, the only way for the blood to travel is through the pulmonary semilunar valve (on the right side of the heart) and the aortic semilunar value (on the left side of the heart) to the pulmonary trunk, which divides into the left and right pulmonary arteries and to the aorta.
Specialised cardiac cells can create and distribute an electrical current that causes a controlled and directed heart contraction. Cardiac muscles don’t always rely on nerve impulses or hormones to contract because they can contract on their own. This unique ability is known as autorhythmicity. This electrical current passes along a pathway (sino-atrial node, atrio ventricular node, Bundle of His, left and right bundle branches, purkinje fibres) that collectively allow blood to leave the heart at each beat.
Five quick facts
- The cardiovascular system also helps maintain proper ﬂuid balance of the body and assists in the control of body temperature.
- Each heart beat pumps out approximately 70ml of blood to the lungs and 70ml of blood to the rest of the body. The amount of blood pumped per minute is therefore heart rate [HR] (number of beats per minute) multiplied by the volume of blood pumped to either the lungs or body (70ml) [stroke volume]. This is called cardiac output [HR x SV]: 70 beats x 70ml = 4,900 ml/min. Cardiac output is recorded in litres/minute, so 4,900/1,000 = 4.9 l/min
- The conduction system is the heart’s own electrical system that works together to pump blood around the body. There are five key areas, the sino-atrial node (pacemaker), the atrio-ventricular node; the Bundle of His; the left and Right bundle branches; and purkinje fibres.
- Pulmonary arteries take blood away from the heart. They are the only arteries in the body that carry deoxygenated blood away from the right side of the heart to travel to the lungs to become oxygenated. The pulmonary veins collect oxygenated blood and returns it to the left side of the heart; these are the only veins to carry oxygenated blood
- The vasculature of the heart takes approximately 5 per cent of oxygenated blood from each heartbeat to ensure there is a blood-rich environment so plenty of oxygen and nutrients are available to the heart muscle itself.
Hint to learning the system
Although the heart is a single organ, it is easier to understand its function if you think of it as two separate pumps working together. The right side of the heart is responsible for receiving blood and sending it to the lungs to pick up oxygen and get rid of carbon dioxide. The left side of the heart receives blood from the lungs and pumps it around the body. A good way of learning about the cardiovascular system is to draw the four chambers, valves and arteries (pulmonary and aorta). Once you have done this, drawn the outline of the heart with four chambers, then add the main parts of the conduction system.
- aorta (AY or tar)
- atrioventricular node (ay tree oh vehn TRIK yoo lahr)
- atrium; atria (AY tree um; AY tree ah)
- autorhythmicity (aw to rith MISS city)
- Bundle of His (HISS)
- diaphragm (DYE uh fram)
- diastole (dye ASS toe lee)
- endocardium (ehn doh KAR dee um)
- epicardium,(epp ee KAR dee um)
- epithelium (epp ee THEE lee um)
- myocardium (my oh KAR dee um)
- parietal (pah REE eh tall)
- pericardium (pear ee KAR dee um)
- pulmonary semilunar (pull moe NAR ee)
- purkinje fibres (per KIN gee)
- sino-atrial (sigh no AY tree all)
- systole (SISS toh lee)
- tricuspid (try CUS pid)
- visceral (VISS er all)
- ventricle (VEN trik lz)
Martin Steggall is the co-author of Anatomy and Physiology for Nursing and Healthcare Professionals 2nd Ed.