Circulatory System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Circulatory System — Key Facts for MDCAT

Types of Circulatory Systems:

System	Description	Examples
Open circulatory system	Blood flows freely in body cavity; not confined to vessels	Insects, arthropods, molluscs
Closed circulatory system	Blood circulates within blood vessels	Earthworms, vertebrates (fish, amphibians, reptiles, birds, mammals)

Human Circulatory System (Double Circulation): Blood passes through the heart twice per complete circuit:

1. Systemic (body) circulation: Left ventricle → aorta → body → right atrium
2. Pulmonary (lung) circulation: Right ventricle → pulmonary artery → lungs → left atrium

Heart Structure (Four-Chambered):

Chamber	Type	Receives From	Sends To
Right atrium	Chamber	Vena cava (deoxygenated blood)	Right ventricle
Right ventricle	Chamber	Right atrium	Pulmonary artery
Left atrium	Chamber	Pulmonary veins (oxygenated)	Left ventricle
Left ventricle	Chamber	Left atrium	Aorta (to body)

Valves (prevent backflow):

• Tricuspid valve: Between right atrium and right ventricle
• Bicuspid (mitral) valve: Between left atrium and left ventricle
• Semilunar valves: Pulmonary valve (right ventricle → pulmonary artery) and Aortic valve (left ventricle → aorta)

Cardiac Cycle:

• Systole = contraction phase
• Diastole = relaxation phase
• One complete cardiac cycle = ~0.8 seconds at rest (72 bpm)
• lub-dub heart sounds: lub = AV valves closing; dub = semilunar valves closing

⚡ Exam tip: The LEFT ventricle has the thickest wall because it must generate enough pressure to pump blood through the entire body (systemic circulation). The right ventricle only pumps blood to the lungs (pulmonary circulation) — less work required. Remember: Deoxygenated blood is always shown in blue; oxygenated in red.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Circulatory System — Complete Study Guide

Blood Vessels:

Vessel	Function	Structure
Arteries	Carry blood away from heart	Thick, elastic walls; small lumen; high pressure
Veins	Carry blood toward heart	Thin walls; large lumen; valves; low pressure
Capillaries	Exchange of materials	Single cell layer; very thin; site of exchange

• Aorta: Largest artery; carries oxygenated blood from left ventricle
• Vena cava: Largest veins; returns deoxygenated blood to right atrium (superior and inferior)
• Pulmonary artery: Only artery carrying deoxygenated blood
• Pulmonary vein: Only vein carrying oxygenated blood

Blood Composition:

Component	Description	Function
Plasma	55% of blood; pale yellow fluid	Transport nutrients, wastes, hormones
Red Blood Cells (RBCs)	Biconcave discs; no nucleus in mammals	Carry oxygen (haemoglobin)
White Blood Cells (WBCs)	Larger than RBCs; have nucleus	Immune defence
Platelets	Cell fragments; no nucleus	Blood clotting
Haemoglobin	Protein in RBCs; iron-containing	Binds O₂; 1 Hb + 4 O₂ → Hb(O₂)₄

Blood Groups (ABO and Rh):

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Receive From
A	A antigen	Anti-B antibodies	A, O
B	B antigen	Anti-A antibodies	B, O
AB	A and B antigens	Neither	A, B, AB, O (universal recipient)
O	Neither antigen	Both A and B antibodies	O only (universal donor)

Rh Factor:

• Rh⁺ (positive): Has D antigen on RBC surface
• Rh⁻ (negative): Lacks D antigen
• Erythroblastosis fetalis: Rh⁻ mother carrying Rh⁺ fetus → mother produces anti-D antibodies → affects subsequent pregnancies

Heartbeat and Conduction:

• SA node (sinoatrial node): “Pacemaker” in right atrium; sets heart rate (60–100 bpm at rest
• AV node (atrioventricular node): Delays impulse between atria and ventricles
• Bundle of His: Impulse from AV node to ventricles
• Purkinje fibres: Distribute impulse through ventricular muscle
• ECG (electrocardiogram): P wave (atrial depolarisation), QRS complex (ventricular depolarisation), T wave (ventricular repolarisation)

⚡ Common mistakes: Confusing which blood vessels carry oxygenated vs deoxygenated blood — remember the EXCEPTIONS: pulmonary artery (deoxygenated), pulmonary vein (oxygenated). Thinking the left ventricle is on the left side of the image — actually it is on the LEFT side of the body, but the RIGHT side of the image (the patient’s right).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Circulatory System — Advanced Notes

Blood Pressure:

• Systolic pressure: Maximum during ventricular contraction (systole) — normal ~120 mmHg
• Diastolic pressure: Minimum during ventricular relaxation (diastole) — normal ~80 mmHg
• Written as: 120/80 mmHg
• Blood pressure = Cardiac output × Peripheral resistance
• Hypertension: Persistently elevated blood pressure (>140/90 mmHg)

Cardiac Output (CO): $$CO = Stroke \text{ } Volume \times Heart \text{ } Rate$$ Normal resting CO: ~5 L/min During exercise, CO can increase to 20–25 L/min

Coronary Circulation:

• Coronary arteries supply blood to the heart muscle (myocardium)
• Left coronary artery → left ventricle wall; Right coronary artery → right ventricle wall
• Coronary artery disease → angina → myocardial infarction (heart attack)
• Blockage of coronary artery → ischemia → tissue death

Circulatory Pathways in Different Vertebrates:

Animal	Heart Chambers	Circulation Type
Fish	2 (1 atrium, 1 ventricle)	Single circulation; 1 circuit
Amphibians	3 (2 atria, 1 ventricle)	Double incomplete; 2 circuits mixed
Reptiles	3 (incomplete ventricular septum)	Double incomplete
Birds and Mammals	4 (2 atria, 2 ventricles)	Double complete; 2 circuits separated

Lymphatic System:

• Lymph: Fluid that leaks from capillaries → enters lymph capillaries → lymphatic vessels
• Lymph nodes: Filter lymph; house WBCs (lymphocytes)
• Functions: Return interstitial fluid to blood; absorb fats (lacteals in small intestine); immune defence
• Thoracic duct: Largest lymphatic vessel; drains into left subclavian vein

Haemoglobin Types:

• Adult HbA (HbA₁): α₂β₂ — 97% of adult haemoglobin
• Adult HbA₂: α₂δ₂ — 2% of adult
• Fetal Hb (HbF): α₂γ₂ — in fetus; has higher oxygen affinity (to extract O₂ from maternal blood)
• Sickle cell haemoglobin (HbS): α₂β₂ with mutant β-chain; causes sickle cell anaemia

Blood Disorders:

Disorder	Cause	Symptoms
Anaemia	Iron deficiency, B12 deficiency, blood loss	Fatigue, pallor, shortness of breath
Leukemia	Cancer of WBCs	Overproduction of abnormal WBCs
Hemophilia	X-linked recessive; factor VIII deficiency	Uncontrolled bleeding
Thalassemia	Genetic defect in α or β globin chain	Anaemia, bone deformities
Sickle cell anaemia	Point mutation in β-globin gene; HbS	Misshapen RBCs, pain crises

MDCAT Question Patterns: MDCAT Pakistan circulatory system questions frequently test: (1) distinguishing arteries, veins, and capillaries, (2) blood flow through the heart (sequence of chambers and valves), (3) ABO and Rh blood groups and transfusion compatibility, (4) cardiac cycle and heart sounds, (5) ECG components, (6) composition and functions of blood, (7) lymphatic system functions, (8) differences in vertebrate circulatory systems. 2–3 questions per paper. Blood groups and heart structure are very high-yield.

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