The heart
The heart is a powerful, muscular organ and a vital component of the mammalian circulatory system. Located in the thorax between the lungs, it is roughly the size of a clenched fist and typically weighs around 300g in an adult human. The heart is protected by a tough, fibrous sac called the pericardium, which anchors it within the thorax and prevents overfilling with blood, while also containing fluid to reduce friction during its beating.
Structure of the Heart
The human heart is a four-chambered organ, divided into two halves by a muscular wall called the septum, which prevents the mixing of oxygenated and deoxygenated blood.
Atria: The two upper chambers are the atria (singular: atrium). They are thin-walled and receive blood from the veins under relatively low pressure. The right atrium receives deoxygenated blood from the body via the vena cava, and the left atrium receives oxygenated blood from the lungs via the pulmonary veins.
Ventricles: The two lower chambers are the ventricles. Their walls are thick and muscular because they pump blood out of the heart into the arteries. The left ventricle has a significantly thicker, more muscular wall than the right ventricle. This is because the left ventricle pumps blood to the entire systemic circulation (rest of the body) at high pressure, overcoming greater resistance, while the right ventricle pumps blood only to the nearby lungs at lower pressure.
Valves: The direction of blood flow through the heart is maintained by valves, which prevent the backflow of blood.
Atrioventricular (AV) valves (bicuspid/mitral on the left, tricuspid on the right) are located between the atria and ventricles. Tendons attached to these valves prevent them from turning inside out due to ventricular pressure.
Semilunar (SL) valves (aortic and pulmonary) are located at the entrance to the aorta and pulmonary artery, respectively. They snap shut when blood tries to flow back into the ventricles.
Cardiac Muscle: The heart is made of specialized muscle called cardiac muscle. It is myogenic, meaning it can contract and relax rhythmically without external nerve signals.
Coronary Arteries: The heart muscle itself requires a constant supply of oxygenated blood for respiration, which is delivered by the coronary arteries that branch from the aorta. Blockage of these arteries can lead to angina or a heart attack (myocardial infarction).
The Cardiac Cycle
The cardiac cycle is the ongoing sequence of contraction (systole) and relaxation (diastole) of the atria and ventricles that ensures continuous blood circulation. A complete cycle in humans typically lasts around 0.8 seconds.
The cycle involves:
Atrial Systole: The ventricles are relaxed, and the atria contract, decreasing their volume and increasing pressure to push blood into the ventricles. The AV valves are open.
Ventricular Systole: After a slight delay, the ventricles contract, significantly increasing their pressure. This forces the AV valves shut to prevent backflow into the atria and pushes open the SL valves, ejecting blood into the pulmonary artery and aorta.
Cardiac Diastole: Both atria and ventricles relax. Pressure in the ventricles drops, causing the semilunar valves to snap shut, preventing arterial blood from flowing back into the heart. Blood then flows from the veins into the atria and passively trickles into the ventricles.
Control of Heartbeat
The heart's beat is myogenic, meaning it can contract and relax without direct nerve signals.
Initiation: The rhythm is set by the sinoatrial node (SAN), a small mass of tissue in the wall of the right atrium, often called the "pacemaker". The SAN sends out regular waves of electrical activity, causing the atria to contract simultaneously.
Conduction Pathway: A band of non-conducting collagen tissue prevents these electrical waves from passing directly from the atria to the ventricles. Instead, the impulse is transmitted to the atrioventricular node (AVN), located in the septum. The AVN introduces a slight delay, ensuring the atria have time to fully empty their blood into the ventricles before the ventricles contract. The AVN then passes the waves to the bundle of His (a group of muscle fibers), which conducts them to the apex (bottom) of the heart. From there, Purkyne tissue (fine muscle fibers) carries the electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously from the bottom up, pushing blood out of the heart.
Regulation: The heart rate is unconsciously controlled by the cardiovascular control centre in the medulla oblongata of the brain. This centre receives inputs from:
Baroreceptors (pressure receptors) in the aorta and carotid arteries, which detect high or low blood pressure.
Chemoreceptors in the aorta, carotid arteries, and medulla oblongata, which monitor oxygen, carbon dioxide, and pH levels in the blood.
Depending on the stimulus, the medulla sends impulses via parasympathetic neurones (secreting acetylcholine to slow heart rate) or sympathetic neurones (secreting noradrenaline to speed up heart rate) to the SAN, thereby adjusting the heart rate.
Cardiac Output
Cardiac output (CO) is the volume of blood pumped by the heart per minute. It is calculated by the formula: Cardiac Output = Stroke Volume × Heart Rate. Stroke volume is the volume of blood pumped during each heartbeat. Cardiac output increases during exercise due to increases in both heart rate and stroke volume.
Heart Dissection
Dissecting a mammalian heart (often a pig or cow's heart) allows for direct observation of its structure. Key features to identify include the four main vessels, the atria and ventricles, the coronary arteries, and the internal structures of the atrioventricular and semilunar valves. The differences in wall thickness between chambers and arteries/veins are observable and relate directly to their functions.
Heart and Cardiovascular Disease
The heart's proper function is central to overall health. Many cardiovascular diseases (CVDs), which affect the heart and blood vessels, often begin with atheroma formation (fibrous plaques forming in artery walls). These plaques can restrict blood flow, increase blood pressure, and lead to serious conditions like myocardial infarction (heart attack) if a coronary artery is completely blocked, damaging or killing heart muscle.
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