Human Heart

*Heart

The heart is a muscular pump that keeps blood moving through the circulatory system. It sits in the thoracic cavity, slightly left of centre, and beats about 60‑100 times a minute at rest.

Key parts
Atria – the two upper chambers (right and left) that receive blood from the veins.
Ventricles – the two lower chambers that pump blood out: the right ventricle to the lungs, the left ventricle to the rest of the body.
Septum – the wall that separates the right and left sides, preventing mixing of oxygen‑rich and oxygen‑poor blood.
Valves – one‑way doors that keep flow moving forward: tricuspid (right AV), pulmonary (right outflow), mitral (left AV), and aortic (left outflow).
Coronary arteries – vessels that supply the heart muscle itself with oxygen‑rich blood.
Conduction system – the electrical network that coordinates the heartbeat: SA node (pacemaker), AV node, bundle of His, bundle branches, and Purkinje fibers.

Core functions
1. Pump – generates pressure to move blood through the pulmonary and systemic circuits.
2. Distribution – delivers oxygen and nutrients, removes waste products.
3. Regulation – adjusts output (stroke volume and rate) to meet the body’s needs (e.g., during exercise).

Key terms you’ll see
Cardiac cycle – the sequence of contraction (systole) and relaxation (diastole).
Stroke volume – amount of blood ejected per beat.
Cardiac output – stroke volume × heart rate (L/min).
Myocardium – the thick muscular layer of the heart wall.
Pericardium – the fibrous sac that encloses the heart.
Endocardium – the thin inner lining of the heart chambers and valves.
Aorta – the main artery carrying oxygen‑rich blood from the left ventricle to the body.
Pulmonary artery – carries oxygen‑poor blood from the right ventricle to the lungs.

*Heart & Circulation

Blood makes a continuous loop: it picks up oxygen in the lungs, gets pumped out to the body, and returns de‑oxygenated to the heart to start again. The heart itself is the pump that keeps the two circuits moving.

1. Pulmonary circuit (right side)
1. Right atrium receives oxygen‑poor blood from the superior & inferior vena cava.
2. Blood passes the tricuspid valve into the *right ventricle*.
3. The right ventricle contracts → pushes blood through the pulmonary valve into the *pulmonary artery*.
4. The pulmonary artery carries the blood to the lungs, where it picks up O₂ and drops off CO₂.
5. Oxygen‑rich blood returns via the pulmonary veins to the *left atrium*.

2. Systemic circuit (left side)
1. Left atrium empties through the mitral (bicuspid) valve into the *left ventricle*.
2. The left ventricle contracts → forces blood through the aortic valve into the *aorta*.
3. The aorta branches into the arterial tree, delivering O₂‑rich blood to all tissues.
4. After delivering oxygen, blood returns through veins → superior & inferior vena cava → back to the right atrium, and the cycle repeats.

Key players in the loop
Valves (tricuspid, pulmonary, mitral, aortic) keep flow one‑way.
Coronary arteries branch off the aorta to supply the heart muscle itself.
Cardiac cycle = systole (contraction) + diastole (relaxation).
Cardiac output = stroke volume × heart rate (how much blood the heart pumps per minute).