Circulation

The human cardiovascular system pumps ~5 L of blood per minute through ~96 000 km of vessels. The AAMC MCAT content outline tests blood vessel structure, the human heart, the cardiac cycle, and the lymphatic system. This is a high-yield area for the Bio/Biochem section.

Key exam topics. Four key subtopics — Blood Vessels, Human Heart, Cardiac Cycle and Phases of Heartbeat, and Lymphatic System.

Blood Vessels

Three main types of blood vessels carry blood through the body: arteries, veins, and capillaries. Their walls share three layers but differ markedly in thickness and elasticity.

Tunica intima: Inner lining of simple squamous endothelium resting on a basement membrane and elastic tissue.
Tunica media: Middle layer of smooth muscle and elastic fibers — thickest in arteries.
Tunica adventitia (externa): Outer layer of fibrous connective tissue.

Arteries

Carry blood away from the heart. Thick muscular and elastic walls withstand high pressure (~120/80 mmHg in the aorta). Lumen is narrow; pulse is felt over them. Carry oxygenated blood, except the pulmonary artery (deoxygenated to lungs) and umbilical artery (fetus). Atherosclerosis — build-up of fatty plaques — narrows arteries and causes coronary heart disease.

Veins

Carry blood back to the heart. Thinner walls, larger lumen, and semilunar valves at intervals to prevent back-flow. Operate under low pressure (~5-10 mmHg). Carry deoxygenated blood, except the pulmonary veins (oxygenated to left atrium) and the umbilical vein. Faulty valves → varicose veins; clot in deep veins → deep vein thrombosis (DVT).

Capillaries

Microscopic vessels (~5-10 µm diameter, just wider than a red blood cell). One-cell-thick endothelium with no muscle — allows efficient diffusion of O₂, CO₂, glucose, ions, and waste between blood and tissue fluid. Form vast capillary beds.

Artery vs Vein vs Capillary
Feature	Artery	Vein	Capillary
Direction of flow	Away from heart	Towards heart	Connects arterioles to venules
Wall thickness	Thick — muscular & elastic	Thin	One cell thick (endothelium only)
Lumen size	Narrow	Wide	~5–10 µm (RBC squeezes through)
Valves	Absent (except aortic / pulmonary semilunar at heart)	Present — prevent backflow	Absent
Pressure	High (~120/80 mmHg in aorta)	Low (~5–10 mmHg)	Falls along its length
Blood content	Mostly oxygenated (except pulmonary artery, umbilical artery)	Mostly deoxygenated (except pulmonary vein, umbilical vein)	Mixed — site of gas / nutrient exchange
Pulse felt?	Yes	No	No
Disorders	Atherosclerosis, aneurysm	Varicose veins, DVT	Edema (leakage)

Pulmonary vs systemic circulation

Pulmonary circuit: right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium.
Systemic circuit: left ventricle → aorta → body tissues → venae cavae → right atrium.

Human Heart

The human heart is a four-chambered, double-pump muscular organ located in the mediastinum, slightly to the left of the midline. It is enclosed in a double-walled pericardium (fibrous + serous layers).

Wall layers

Epicardium — visceral pericardium (outermost).
Myocardium — cardiac muscle (thickest); the actual contractile layer. Thickest in the left ventricle.
Endocardium — smooth inner lining continuous with blood-vessel endothelium.

Four chambers and four valves

Right atrium (RA) and right ventricle (RV) handle deoxygenated blood; left atrium (LA) and left ventricle (LV) handle oxygenated blood. The interventricular septum prevents mixing.

Tricuspid valve — between RA & RV (3 cusps).
Bicuspid (mitral) valve — between LA & LV (2 cusps).
Pulmonary semilunar valve — at the base of the pulmonary artery.
Aortic semilunar valve — at the base of the aorta.

Cardiac muscle & conducting system

Cardiac muscle is striated, branched, uninucleate, and joined by intercalated discs with gap junctions, allowing rapid electrical conduction. Cardiac muscle is myogenic — contraction is generated within the heart itself, by the conducting system:

SA node (sinoatrial node) — in right atrium wall, the natural pacemaker (~70-80 impulses/min).
AV node (atrioventricular node) — delays the impulse ~0.1 s so atria empty before ventricles contract.
Bundle of His → left and right bundle branches → Purkinje fibers — spread excitation through ventricles.

Coronary circulation

The myocardium is supplied by left and right coronary arteries branching off the aorta. Blockage of a coronary artery causes ischemia (angina pectoris) or, if total, myocardial infarction (heart attack).

Cardiac Cycle and Phases of Heartbeat

One cardiac cycle is the sequence of events in one heartbeat. At a resting heart rate of 75 beats/min, each cycle lasts ~0.8 s.

Three phases of one cycle

Atrial systole (~0.1 s) — SA node fires; atria contract; remaining ~30% of blood is pushed into the ventricles through open AV valves.
Ventricular systole (~0.3 s) — ventricles contract; AV valves close ("lub" sound, S₁); pressure rises; semilunar valves open; blood is ejected into the aorta and pulmonary artery.
Diastole / joint relaxation (~0.4 s) — all four chambers relax; semilunar valves close ("dub" sound, S₂); atria fill passively from venae cavae and pulmonary veins.

Heart sounds & ECG basics

The "lub-dub" heard via stethoscope corresponds to AV-valve and semilunar-valve closure. The electrocardiogram (ECG) records the heart's electrical activity:

P wave — atrial depolarization (atrial contraction).
QRS complex — ventricular depolarization (ventricular contraction); masks atrial repolarization.
T wave — ventricular repolarization (relaxation).

Cardiac output

CO = SV × HR. Stroke volume (SV) is ~70 mL; resting heart rate (HR) ~70-75/min. Cardiac output ~5 L/min — equal to the entire blood volume each minute.

Blood pressure & its regulation

Blood pressure is the force blood exerts on arterial walls, written as systolic / diastolic (normal ~120/80 mmHg). Systolic pressure is the peak during ventricular systole; diastolic is the trough during ventricular relaxation.

Pulse pressure = systolic − diastolic (~40 mmHg) — felt as the pulse.
Mean arterial pressure (MAP) ≈ diastolic + ⅓(pulse pressure) ≈ 93 mmHg. MAP = CO × total peripheral resistance (TPR).
Pressure is highest in the aorta and falls steadily along arteries → arterioles (the main resistance vessels) → capillaries, reaching its lowest in the venae cavae.

Short-term regulation is by the baroreceptor reflex: stretch receptors in the carotid sinus and aortic arch signal the medulla, which adjusts heart rate and vessel diameter via the autonomic nervous system. Long-term regulation is by the kidneys through the renin–angiotensin–aldosterone system (RAAS) and ADH, which control blood volume.

Common cardiovascular diseases

Hypertension — persistently raised blood pressure (>140/90 mmHg).
Atherosclerosis — lipid plaques narrow arteries.
Angina pectoris — chest pain due to reduced coronary flow.
Myocardial infarction — death of heart muscle from coronary occlusion.
Stroke — cerebral artery blockage or rupture.
Heart failure, valvular disease (rheumatic), arrhythmias.

Lymphatic System

The lymphatic system is a network of vessels, nodes, and organs that drains tissue fluid, transports fats, and supports immunity. ~10% of the fluid filtered out of capillaries does not return directly to venules but enters lymphatic capillaries as lymph.

Components

Lymph capillaries: Blind-ended, highly permeable vessels in tissues; collect interstitial fluid.
Lymph vessels: Larger vessels with valves to ensure one-way flow toward the heart.
Lymph nodes: Small bean-shaped organs that filter lymph and house lymphocytes (B and T cells). Swell during infection.
Thoracic duct & right lymphatic duct: Two main collecting ducts that empty lymph into the subclavian veins, returning it to the bloodstream.
Lymphoid organs: Spleen (filters blood, recycles RBCs), thymus (T-cell maturation), tonsils, Peyer's patches, bone marrow (B-cell origin).

Functions

Drains excess interstitial fluid → prevents edema.
Absorbs dietary lipids and fat-soluble vitamins from the small intestine via lacteals in villi.
Houses immune cells; filters pathogens; mounts adaptive immunity.

Disorders: lymphedema (swelling from blocked drainage), lymphoma (e.g., Hodgkin's), lymphadenitis (infected nodes).

Common trap. The pulmonary artery carries deoxygenated blood, while the pulmonary vein carries oxygenated blood — the opposite of every other artery and vein. Examiners ask this almost every year.

Memory aid for valves. "LAB Rat" — Left = bicuspid; Atria above; Bicuspid is mitral; Right = tricuspid. Or simply: "Tri before you Bi" (right side first when learning).

Worked MCQs

Five MCQs that capture the high-yield testing patterns for this chapter.

Q1. The pacemaker of the human heart is the:

AV node
SA node
Bundle of His
Purkinje fibers

The sinoatrial (SA) node, located in the upper wall of the right atrium, spontaneously generates ~70-80 impulses per minute, setting the heart rhythm. The AV node is a relay; the bundle of His and Purkinje fibers conduct impulses through the ventricles.

Q2. Which valve lies between the left atrium and left ventricle?

Tricuspid valve
Bicuspid (mitral) valve
Aortic valve
Pulmonary valve

The bicuspid (mitral) valve has two cusps and is on the left side. The tricuspid valve (3 cusps) is on the right side. The aortic and pulmonary valves are semilunar, located at the bases of their respective arteries.

Q3. The "lub" (S₁) heart sound is caused by closure of:

Semilunar valves
Atrioventricular valves
Aortic valve only
Pulmonary valve only

S₁ ("lub") is produced by simultaneous closure of the tricuspid and bicuspid (AV) valves at the start of ventricular systole. S₂ ("dub") follows when the aortic and pulmonary semilunar valves close at the start of diastole.

Q4. In an ECG, the QRS complex represents:

Atrial depolarization
Atrial repolarization
Ventricular depolarization
Ventricular repolarization

The QRS complex marks ventricular depolarization (and ventricular contraction). The P wave is atrial depolarization; the T wave is ventricular repolarization. Atrial repolarization is masked by the QRS.

Q5. Which lymphatic structure absorbs digested lipids from the small intestine?

Thoracic duct
Spleen
Lacteal
Tonsils

Each intestinal villus contains a central lacteal — a lymph capillary that absorbs fatty acids and monoglycerides as chylomicrons, giving the lymph a milky appearance (chyle). The thoracic duct then returns this fat-rich lymph to the bloodstream.

Quick Recap

Arteries: thick muscular walls, high pressure, no valves; carry oxygenated blood (except pulmonary artery).
Veins: thin walls, valves, low pressure; carry deoxygenated blood (except pulmonary vein).
Capillaries: one-cell-thick endothelium; site of exchange.
Heart: 4 chambers; AV valves (tricuspid R, bicuspid L) + 2 semilunar valves; LV myocardium thickest.
SA node = pacemaker → AV node → bundle of His → Purkinje fibers.
Cardiac cycle: atrial systole (0.1 s) → ventricular systole (0.3 s) → diastole (0.4 s).
ECG: P (atrial depol.), QRS (ventricular depol.), T (ventricular repol.).
Lymphatic system: drains tissue fluid, absorbs fats via lacteals, immune defense via nodes/spleen/thymus.

Test yourself. Take a timed practice test or browse topic-wise MCQs to lock these concepts in.