Memory tricks for the heart, blood vessels, and circulation

The Frank-Starling law states that the more the ventricle is stretched during filling (↑ preload = ↑ EDV), the more forcefully it contracts — up to a physiological limit. Mechanism: more stretch → more optimal actin-myosin overlap (length-tension relationship) → more cross-bridges formed → stronger contraction → higher stroke volume. This ensures the heart pumps exactly what it receives — left output = right output, preventing blood from pooling in either circuit. Preload: the degree of stretch before contraction — determined by venous return and EDV. Afterload: the resistance the ventricle must overcome to eject blood (systemic vascular resistance). ↑ afterload → ↓ SV. Contractility: intrinsic strength of contraction — increased by sympathetic stimulation, Ca2+, positive inotropes (digoxin).

Arteries: thick walls, elastic (large) or muscular (medium) — carry blood AWAY from heart under high pressure. Aorta is most elastic — stretches during systole, recoils during diastole (Windkessel effect). Arterioles: small arteries with abundant smooth muscle — primary resistance vessels, regulate blood flow to capillary beds. Vasodilation/constriction controlled by sympathetic tone, local metabolites, hormones. Capillaries: single layer of endothelium + basement membrane — site of ALL exchange (O2, CO2, nutrients, waste). Continuous, fenestrated, sinusoidal types. Venules: collect blood from capillaries. Veins: thin walls, large lumen, valves — carry blood TOWARD heart under low pressure. Veins hold ~64% of blood volume — capacitance vessels. Skeletal muscle pump and respiratory pump aid venous return.

The heart has four chambers in two functional pumps. Right side (pulmonary circuit): deoxygenated blood enters right atrium (RA) from superior and inferior vena cava → tricuspid valve → right ventricle (RV) → pulmonary semilunar valve → pulmonary arteries → lungs (gas exchange). Left side (systemic circuit): oxygenated blood returns from lungs via pulmonary veins → left atrium (LA) → mitral/bicuspid valve → left ventricle (LV) → aortic semilunar valve → aorta → body. Left ventricle has thicker wall (pumps against higher systemic resistance). Both sides contract simultaneously. Pericardium: double-walled sac surrounding heart — fibrous outer layer + serous inner (parietal and visceral/epicardium).

SA node (sinoatrial node): right atrium near SVC — the pacemaker. Fires at 60-100 bpm. Sets heart rate. Signal spreads across both atria → atria contract. AV node (atrioventricular node): at junction of atria and ventricles — intentional delay (0.1 sec) allows atria to finish emptying into ventricles before ventricles contract. Only electrical pathway between atria and ventricles. Bundle of His (AV bundle): through interventricular septum. Bundle branches: left and right — travel down each side of septum. Purkinje fibers: spread through ventricular walls from apex upward — rapid conduction causes ventricles to contract from bottom up, ejecting blood upward into great vessels. ECG: P wave (atrial depolarization) → QRS complex (ventricular depolarization) → T wave (ventricular repolarization).

Diastole (relaxation and filling): AV valves open → blood flows passively from atria into ventricles. Atrial systole (atrial kick) completes filling — adds last 20-30%. Isovolumetric contraction: ventricles contract, pressure rises → AV valves close (Lub = S1) → both sets of valves closed briefly. Ventricular ejection: pressure exceeds arterial pressure → semilunar valves open → blood ejected. Stroke volume (~70 mL). Isovolumetric relaxation: ventricles relax → semilunar valves close (Dub = S2) → pressure drops until below atrial pressure → AV valves open again. Heart sounds: S1 (Lub) = AV valve closure. S2 (Dub) = semilunar valve closure. S3 = ventricular gallop (heart failure). S4 = atrial kick into stiff ventricle.

Blood pressure = Cardiac Output × Systemic Vascular Resistance. CO = HR × SV. SVR = determined by arteriole diameter (vasoconstriction raises BP, vasodilation lowers). Normal: 120/80 mmHg. Systolic (120): pressure during ventricular contraction. Diastolic (80): pressure during ventricular relaxation. Pulse pressure = systolic − diastolic = 40 mmHg. Mean arterial pressure (MAP) = diastolic + 1/3 pulse pressure ≈ 93 mmHg. Hypertension (>130/80): ↑ CO, ↑ SVR, or both. Long-term: baroreceptor reflex (carotid sinus and aortic arch detect stretch → adjust HR and SVR). RAAS (renin-angiotensin-aldosterone system): low BP → renin → angiotensin II → vasoconstriction + aldosterone → Na+ retention → ↑ blood volume → ↑ BP.

Fluid movement across capillary walls is determined by four Starling forces. Capillary hydrostatic pressure (CHP): blood pressure inside capillary pushes fluid OUT into interstitium — higher at arterial end (~35 mmHg), lower at venous end (~15 mmHg). Plasma oncotic pressure (COP): proteins (albumin) in blood pull fluid IN — ~25 mmHg throughout. Net result: arterial end — filtration (fluid leaves capillary). Venous end — reabsorption (fluid returns). Excess filtered fluid drained by lymphatics. Edema: occurs when filtration > reabsorption + lymphatic drainage. Causes: ↑ CHP (heart failure), ↓ COP (low albumin — liver disease, malnutrition), blocked lymphatics, increased capillary permeability (inflammation).

The heart receives its own blood supply from two coronary arteries that branch off the aorta just above the aortic valve. Left coronary artery (LCA): divides into Left Anterior Descending (LAD) — supplies anterior LV, interventricular septum, "widow maker" — most critical and Circumflex — supplies lateral and posterior LV. Right coronary artery (RCA): supplies RA, RV, SA node, AV node (in 85% of people — "right dominant"). Coronary arteries fill during diastole (heart relaxed) — that is when myocardium is not compressed. During systole, myocardial contraction squeezes vessels closed. MI (myocardial infarction): blocked coronary artery → ischemia → cell death. LAD occlusion most deadly.

Hepatic portal circulation: blood from GI tract (stomach, intestines, spleen, pancreas) drains into portal vein → liver (first pass metabolism — detoxifies absorbed substances, processes nutrients). Then drains into hepatic veins → IVC. Portal hypertension: liver disease → increased resistance → varices (dilated collateral veins in esophagus and rectum). Pulmonary circulation: only arterial blood that is deoxygenated (right side) — pulmonary arteries carry deoxgenated blood to lungs. Unique: hypoxic vasoconstriction (low O2 → constrict → redirect blood to better-ventilated areas). Fetal circulation: fetus bypasses lungs — foramen ovale (RA→LA) and ductus arteriosus (pulmonary artery→aorta). Close at birth when lungs inflate.