Circulation: MDCAT Notes and MCQs
Structure of the heart
- Cone-shaped, in the thoracic cavity between lungs, enclosed in the double-walled pericardium (pericardial fluid reduces friction). Wall: epicardium, myocardium, endocardium.
- 4 chambers: 2 thin-walled atria, 2 thick-walled ventricles; left ventricle has the thickest wall (pumps to whole body).
- Valves: tricuspid (right AV), bicuspid / mitral (left AV) — held by chordae tendineae & papillary muscles; semilunar valves at the bases of aorta and pulmonary artery.
- Blood flow: vena cavae → RA → RV → pulmonary artery → lungs → pulmonary veins → LA → LV → aorta → body. Heart muscle supplied by coronary arteries.
Conducting system
SA node (pacemaker, wall of right atrium) → atria contract → AV node (delay ≈ 0.1 s) → bundle of His → Purkinje fibres → ventricles contract from apex upward.
Cardiac cycle (≈ 0.8 s at 72–75 beats/min)
| Phase | Time | Events |
|---|---|---|
| Atrial systole | 0.1 s | Atria contract; ventricles get the final ≈ 30% filling |
| Ventricular systole | 0.3 s | AV valves close ("lub", 1st sound), semilunar valves open, blood ejected |
| Joint diastole | 0.4 s | Semilunar valves close ("dub", 2nd sound), chambers relax and fill |
Cardiac output = stroke volume (≈ 70 ml) × heart rate (≈ 72) ≈ 5 L/min. Normal BP ≈ 120/80 mmHg.
Blood vessels
| Arteries | Veins | Capillaries |
|---|---|---|
| Carry blood away from heart (oxygenated, except pulmonary artery) | Carry blood to heart (deoxygenated, except pulmonary veins) | Connect arterioles and venules; site of exchange |
| Thick, muscular, elastic walls; narrow lumen; high pressure | Thin walls, wide lumen, low pressure | Wall one cell thick (endothelium only) |
| No valves (except at base) | Valves prevent backflow | No valves |
All three have endothelium (tunica intima); arteries and veins also have tunica media (smooth muscle) and tunica externa (connective tissue).
Lymphatic system
- Returns excess tissue fluid (lymph) to blood; absorbs fats from intestine via lacteals; immune defence.
- Lymph vessels have valves; largest = thoracic duct, drains into the left subclavian vein; right lymphatic duct → right subclavian vein.
- Lymph nodes: filter lymph, contain lymphocytes & macrophages (swell in infection).
- Lymphoid organs: spleen (largest; destroys old RBCs), thymus (T-cell maturation), tonsils, Peyer's patches, bone marrow.
PMDC learning outcomes for this unit
- Human Heart: 11.1 Discuss general structure of human heart
- Cardiac cycle and phases of Heartbeat: 11.2 Describe the phases of heartbeat.
- Blood Vessels: 11.3 List the differences and functions of arteries, veins and capillaries.
- Lymphatic system: 11.4 Describe lymphatic system (nodes, vessels and organs)