Summary
Highlights
The heart is a vital organ in the cardiovascular system, responsible for pumping blood to transport nutrients, oxygen, and remove waste. The cardiovascular system comprises the heart, blood vessels, and blood. The heart acts as two distinct pumps: the right side for pulmonary circulation to the lungs and the left side for systemic circulation to the rest of the body.
The heart's primary functions include generating blood pressure, routing blood effectively through vessels, ensuring one-way blood flow, and regulating blood supply. Physically, the heart is about the size of a fist, weighs less than one pound, and is located between the lungs with its apex pointing slightly to the left, which is why heartbeats are typically felt on the left side.
The heart is enclosed by the pericardium, consisting of an outer fibrous pericardium and an inner serous pericardium (parietal and visceral layers). The epicardium covers the heart's surface, and the pericardial cavity, filled with pericardial fluid, acts as a cushion. Externally, key structures include the coronary sulcus, anterior and posterior interventricular sulci, superior and inferior vena cava, pulmonary veins, pulmonary trunk, and aorta.
The heart has four chambers: the superior Atria (left and right) that receive blood from veins, and the inferior Ventricles (left and right) that pump blood out to arteries. The atria are thin-walled for minimal contraction, while the ventricles are thick and strong for forceful pumping. The interatrial and interventricular septa divide these chambers. Valves like the tricuspid and bicuspid (mitral) separate atria and ventricles, while semilunar valves separate ventricles from arteries, all ensuring one-way blood flow. Papillary muscles and chordae tendineae control valve action.
The cardiac skeleton, made of fibrous connective tissue, supports the atrioventricular and semilunar valves. Blood flow through the heart starts with deoxygenated blood entering the right atrium from the superior and inferior vena cava, then to the right ventricle, and to the lungs via the pulmonary artery. Oxygenated blood returns from the lungs to the left atrium via pulmonary veins, then to the left ventricle, and finally to the body through the aorta. This process ensures efficient oxygenation and circulation.
The heart's own blood supply comes from the coronary arteries, originating from the aorta. The left coronary artery supplies the anterior heart wall and left ventricle, while the right coronary artery supplies the right ventricle. Cardiac veins drain blood from the cardiac muscle. The heart wall has three layers: the outermost epicardium (visceral pericardium) of connective and adipose tissue, the middle myocardium composed of cardiac muscle, and the innermost endocardium, a smooth inner surface.
Cardiac muscles are characterized by a single, centrally located nucleus, branching cells, rich mitochondria for energy, and striations. Intercalated discs connect cardiac muscle cells, enabling coordinated contraction. Heart stimulation involves a precise sequence: atria contract first, followed by ventricles. Action potentials spread across the atrial wall then to the ventricles, causing a synchronized pumping motion essential for blood circulation.
The heart's coordinated movement is controlled by a specialized conduction system. This system includes the Sinoatrial (SA) node (pacemaker) in the right atrium, the Atrioventricular (AV) node (also in the right atrium), the atrioventricular bundle, right and left bundle branches, and Purkinje fibers. This system ensures rapid and efficient delivery of action potentials throughout the heart, particularly to the ventricular walls for forceful contraction.
The cardiac cycle describes all events during one heartbeat, with atria acting as primers and ventricles as primary pumps. Heart sounds ('lub-dub') are produced by the closure of heart valves: the first sound ('lub') from atrioventricular valves closing, and the second ('dub') from semilunar valves closing. Key measurements include stroke volume (70 mL/contraction) and heart rate (average 72 beats/minute).
Heart function is regulated by the nervous system through mechanisms like the baroreceptor reflex (monitoring blood pressure in aorta and carotid arteries via the medulla oblongata) and chemoreceptor reflex (detecting chemical changes affecting heart rate and stroke volume). Common heart diseases include coronary artery disease (narrowing of coronary arteries) and myocardial infarction (heart attack due to artery blockage), leading to muscle death.
Treatments for heart diseases include angioplasty (opening blocked vessels), stent insertion (keeping vessels open), and coronary bypass (rerouting blood around blocked arteries). Microscopically, cardiac muscle cells exhibit branching, central nuclei, and intercalated discs. The heart wall layers (epicardium, myocardium, endocardium) have distinct microscopic appearances, with Purkinje fibers found predominantly in the endocardium of the ventricles.