The Cardiovascular System: The Heart
Anatomy, blood flow, conduction, the ECG, the cardiac cycle and cardiac output
By the end of this unit you can…
- ✓Describe the location, coverings and layers of the heart wall
- ✓Name the chambers, valves and great vessels and trace blood through the heart
- ✓Describe the coronary circulation and the structure of cardiac muscle
- ✓Explain the intrinsic conduction system, the cardiac action potential and the ECG
- ✓Describe the cardiac cycle, heart sounds and pressure changes
- ✓Calculate cardiac output and explain the factors that change stroke volume and heart rate
Key terms
Practice →1Location & coverings
The heart is a double pump: the right side pumps oxygen-poor blood to the lungs (pulmonary circuit); the left side pumps oxygen-rich blood to the body (systemic circuit). Arteries carry blood away from the heart; veins carry blood toward it.
The heart is about the size of a fist, lying in the mediastinum between the lungs, with two-thirds left of the midsternal line. Its base (broad, posterior surface) points toward the right shoulder; its apex points to the left hip and rests on the diaphragm at the 5th intercostal space.
- Fibrous pericardium
- Tough outer sac — protects, anchors the heart, prevents overfilling.
- Serous pericardium
- Two layers: parietal (lines the fibrous pericardium) and visceral = epicardium (on the heart surface). The pericardial cavity between them holds serous fluid that reduces friction.
- Epicardium
- Outermost layer of the heart wall (visceral pericardium).
- Myocardium
- Thick middle layer of cardiac muscle — the layer that contracts; reinforced by the fibrous cardiac skeleton.
- Endocardium
- Inner endothelium lining the chambers and covering the valves; continuous with vessel linings.
2Chambers, valves & great vessels
| Chamber | Receives from | Pumps to |
|---|---|---|
| Right atrium | Superior & inferior venae cavae and coronary sinus (O₂-poor) | Right ventricle |
| Right ventricle | Right atrium | Pulmonary trunk → lungs |
| Left atrium | Four pulmonary veins (O₂-rich) | Left ventricle |
| Left ventricle | Left atrium | Aorta → body. Walls ~3× thicker than the right — it pumps against much higher resistance |
The interatrial septum (with the fossa ovalis, remnant of the fetal foramen ovale) and interventricular septum separate the sides. Ventricle walls have trabeculae carneae; papillary muscles anchor the chordae tendineae.
| Valve | Type | Location |
|---|---|---|
| Tricuspid | Atrioventricular (AV) — 3 cusps | Right atrium → right ventricle |
| Mitral (bicuspid) | AV — 2 cusps | Left atrium → left ventricle |
| Pulmonary | Semilunar (SL) | Right ventricle → pulmonary trunk |
| Aortic | Semilunar | Left ventricle → aorta |
Valves open and close in response to pressure differences. When ventricles contract, pressure shuts the AV valves; chordae tendineae and papillary muscles prevent the cusps from everting into the atria. Semilunar valves close when ventricles relax and blood flows back toward them.
3Path of blood flow
4Coronary circulation & cardiac muscle
The heart muscle gets its own blood supply through the coronary arteries, which arise from the base of the aorta and fill when the heart relaxes. Left coronary artery → anterior interventricular artery (LAD) and circumflex artery. Right coronary artery → right marginal and posterior interventricular arteries. Veins (great, middle, small cardiac) drain into the coronary sinus → right atrium.
| Feature | Cardiac muscle |
|---|---|
| Cells | Short, branched, striated; 1–2 central nuclei; ~25–35% mitochondria (very aerobic, fatigue-resistant) |
| Junctions | Intercalated discs — desmosomes (hold cells together) + gap junctions (ions pass, so the heart contracts as a unit — a functional syncytium) |
| Stimulation | Some cells are self-excitable (pacemaker cells) — no nerve needed |
| Refractory period | Long (~250 ms) — prevents tetanic contraction, so the heart can refill |
| Ca²⁺ source | SR + extracellular fluid (Ca²⁺-induced Ca²⁺ release) |
5Intrinsic conduction system
- 1Sinoatrial (SA) node — in the right atrial wall; the pacemaker: depolarizes spontaneously ~75 times/min (sinus rhythm).
- 2Impulse spreads through the atria (via internodal pathways) to the atrioventricular (AV) node — a delay of ~0.1 s lets the atria finish contracting.
- 3AV bundle (bundle of His) — the only electrical connection between atria and ventricles.
- 4Right and left bundle branches — down the interventricular septum toward the apex.
- 5Subendocardial conducting network (Purkinje fibers) — depolarize ventricle cells; contraction starts at the apex and squeezes blood up toward the great arteries.
Pacemaker cells have an unstable resting potential: slow Na⁺ influx (with reduced K⁺ efflux) drifts the membrane toward threshold (the pacemaker potential), then Ca²⁺ influx causes the depolarization. In contractile cells, Na⁺ influx causes rapid depolarization, then a long plateau caused by slow Ca²⁺ channels keeps the cell depolarized ~200 ms.
The cardiac center in the medulla adjusts rate via the ANS: the cardioacceleratory center (sympathetic → ↑ rate and force) and cardioinhibitory center (parasympathetic via the vagus nerve → ↓ rate).
6The electrocardiogram (ECG)
| Wave / interval | What it shows |
|---|---|
| P wave | Atrial depolarization |
| QRS complex | Ventricular depolarization (atrial repolarization is hidden here) |
| T wave | Ventricular repolarization |
| P–R interval | From the start of atrial excitation to the start of ventricular excitation (~0.16 s) — includes the AV node delay |
| S–T segment | Entire ventricular myocardium depolarized (plateau) |
| Q–T interval | Start of ventricular depolarization through repolarization |
7The cardiac cycle & heart sounds
Systole = contraction; diastole = relaxation. One cardiac cycle at 75 beats/min lasts ~0.8 s. Blood always flows from higher to lower pressure, and valves open or close passively as pressure changes.
- 1Ventricular filling (mid-to-late diastole) — AV valves open; ~80% of filling is passive; then atrial systole tops off the ventricles. End-diastolic volume (EDV) ≈ 120 mL.
- 2Isovolumetric contraction — ventricles contract; ventricular pressure exceeds atrial pressure → AV valves close (S1). For a moment all valves are closed.
- 3Ventricular ejection — ventricular pressure exceeds pressure in the aorta and pulmonary trunk → semilunar valves open; blood is ejected. Stroke volume ≈ 70 mL leaves; end-systolic volume (ESV) ≈ 50 mL remains.
- 4Isovolumetric relaxation — ventricles relax; backflow closes the semilunar valves (S2; dicrotic notch in aortic pressure). Then the AV valves open and filling begins again.
| Sound | Cause | Marks |
|---|---|---|
| S1 — “lub” | Closure of the AV valves | Start of ventricular systole |
| S2 — “dup” | Closure of the semilunar valves | Start of ventricular diastole |
8Cardiac output
Cardiac output (CO) = heart rate (HR) × stroke volume (SV). At rest: 75 beats/min × 70 mL/beat ≈ 5.25 L/min — the entire blood volume each minute. Cardiac reserve is the difference between resting and maximal CO (4–5× resting in non-athletes, up to 7× in athletes).
Stroke volume = EDV − ESV (120 − 50 = 70 mL). Three factors regulate it:
- Preload
- Degree of stretch of cardiac muscle before contraction. Frank–Starling law: the more the ventricles are stretched by venous return (higher EDV), the more forcefully they contract. Exercise increases venous return → ↑ SV.
- Contractility
- Contractile strength independent of stretch — increased by sympathetic stimulation, epinephrine, Ca²⁺, thyroid hormone, glucagon, digitalis (positive inotropic agents).
- Afterload
- Pressure the ventricles must overcome to eject blood (mainly arterial pressure). Hypertension increases afterload → ↓ SV and the heart works harder.
| Factor | Effect on heart rate |
|---|---|
| Sympathetic stimulation (NE), epinephrine, thyroid hormone | ↑ (positive chronotropic) |
| Parasympathetic (vagus, ACh) | ↓ — dominant at rest (vagal tone) |
| Atrial (Bainbridge) reflex — stretch of the atria | ↑ |
| Fever, exercise, low blood Ca²⁺ / high K⁺ | Fever ↑; ion imbalances cause arrhythmias |