Module 10: Cardiovascular and Lymphatic System
This lesson aligns with the AAMC’s official MCAT content outline and falls under Foundational Concept 4, specifically Content Category 4A and 4B. The cardiovascular and lymphatic systems are essential to understanding homeostasis, circulation, and immune surveillance. On the MCAT, you’ll be tested on blood flow, heart anatomy, blood pressure regulation, gas exchange, and the lymphatic system’s role in fluid balance and immunity. Mastery of these topics is critical for reasoning through biologically integrated passages in the Chemical and Physical Foundations of Biological Systems (C/P) and Biological and Biochemical Foundations of Living Systems (B/B) sections.
This module focuses on the MCAT-relevant anatomy and physiology of the cardiovascular and lymphatic systems. Core topics include heart anatomy and function, the cardiac cycle, blood vessels and pressure, blood components, systemic and pulmonary circulation, lymphatic vessels, immune fluid dynamics, and regulation of fluid and solute transport.
Overview of The Cardiovascular and Lymphatic Systems for The MCAT
Circulatory System Function
The circulatory system delivers oxygen and nutrients to tissues and removes carbon dioxide and metabolic waste. It also facilitates immune surveillance, temperature regulation, and hormonal distribution. The system is composed of two major subsystems:
- Cardiovascular system: Heart and blood vessels
- Lymphatic system: Lymph vessels and nodes that return interstitial fluid to circulation and support immunity
MCAT Focus: Understand the difference between systemic vs. pulmonary circuits, artery vs. vein structure, and pressure/flow regulation.
Heart Anatomy and Flow
The heart is a four-chambered pump:
- Right side: Receives deoxygenated blood from the body and pumps it to the lungs
- Left side: Receives oxygenated blood from the lungs and pumps it to the body
Blood Flow Pathway: Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic valve → aorta → body
MCAT Tip: Know the names and order of valves, especially the difference between AV valves (tricuspid, bicuspid) and semilunar valves (pulmonary, aortic).
Cardiac Cycle and Electrical Conduction
The cardiac cycle consists of:
- Systole: Ventricular contraction and blood ejection
- Diastole: Ventricular relaxation and filling
Mnemonic: “We May Contract Cysts But We Don’t Relax Until We Die”
- We May Contract Cysts = Ventricular Contraction (Systole)
- But We Don’t Relax Until We Die = Ventricular Relaxation (Diastole)
Valve dynamics:
- Systole: AV valves (tricuspid, mitral) close; semilunar valves (aortic, pulmonary) open → blood is ejected
- Diastole: Semilunar valves close; AV valves open → ventricles fill
Electrical conduction coordinates contraction:
- SA node (pacemaker): Initiates impulse
- AV node: Brief delay allows atrial contraction
- Bundle of His → Purkinje fibers: Spread impulse to ventricles
MCAT Tip: ECG traces electrical activity. The SA node sets the rhythm; the AV node introduces a delay. Conduction defects can impair coordination.
MCAT Tip: Understand that systolic pressure reflects pressure during contraction and diastolic pressure reflects pressure during relaxation. Blood pressure is typically given as:
Blood Pressure = Systolic/Diastolic (e.g., 120/80 mmHg)
Cardiac Output
Cardiac Output (C)) = Heart Rate (HR) x Stroke Volume (SV)
This is the volume of blood pumped by the heart per minute and is calculated using the equation:
Where:
- Heart Rate (HR): Number of heartbeats per minute (bpm)
- Stroke Volume (SV): Volume of blood pumped from one ventricle per beat (mL/beat)
- Cardiac Output (CO): Total blood volume pumped per minute (mL/min or L/min)
MCAT Tip: Both HR and SV are influenced by autonomic input. Sympathetic stimulation increases both, raising CO; parasympathetic input primarily decreases HR, lowering CO.
Blood Vessels and Pressure
| Vessel Type | Structure | Function |
|---|---|---|
| Arteries | Thick, muscular, elastic | Carry blood away from heart (high pressure) |
| Arterioles | Smooth muscle | Control blood flow and resistance |
| Capillaries | Thin-walled, single-cell layer | Site of exchange |
| Venules | Collect blood from capillaries | Start return flow |
| Veins | Thin-walled, valves present | Return blood to heart (low pressure) |
Pressure is highest in arteries and lowest in veins. Veins rely on skeletal muscle contraction and valves to return blood.
Blood Composition and Function
- Plasma: Water, electrolytes, hormones, proteins (e.g., albumin, fibrinogen)
- Red blood cells (RBCs): Transport oxygen via hemoglobin
- White blood cells (WBCs): Immune defense (neutrophils, lymphocytes, etc.)
- Platelets: Clotting (form plug, activate fibrin)
Hematocrit is the percentage of blood volume occupied by RBCs.
Circulatory Pathways
- Systemic circulation: Left heart → body → right heart
- Pulmonary circulation: Right heart → lungs → left heart
Portal systems (capillary beds in series):
- Hepatic portal system: GI tract → liver (nutrient processing)
- Hypophyseal portal system: Hypothalamus → anterior pituitary (hormone delivery)
Blood Pressure Regulation Blood pressure (BP) is regulated by:
- Cardiac output (CO): Heart rate × stroke volume
- Peripheral resistance: Arteriolar constriction/dilation
- Baroreceptors: Sense pressure; activate autonomic responses
- Hormones: ADH, aldosterone (increase BP); ANP (decrease BP)
MCAT Tip: Vasoconstriction increases BP; vasodilation decreases it. RAAS system raises BP during low volume states.
Lymphatic System Functions:
- Drains interstitial fluid into venous circulation
- Transports fats (chylomicrons) from intestines
- Filters pathogens via lymph nodes
| Structure | Function |
|---|---|
| Lymph vessels | Return fluid to subclavian veins |
| Lymph nodes | Immune surveillance (contain B and T cells) |
| Lacteals | Absorb fats in small intestine |
Lymph flow is unidirectional, propelled by valves and muscle contractions.
MCAT Summary
- Heart: 4 chambers, valves control direction
- Flow: RA → RV → lungs → LA → LV → body
- Electrical system: SA node → AV node → ventricles
- Systole: AV valves close, semilunar valves open
- Diastole: Semilunar valves close, AV valves open
- Mnemonic: “We May Contract Cysts But We Don’t Relax Until We Die”
- Blood vessels: arteries (away), veins (toward), capillaries (exchange)
- Blood: RBCs, WBCs, plasma, platelets
- CO = HR × SV (key for flow and BP)
- BP = Systolic / Diastolic (e.g., 120/80 mmHg)
- BP regulation: baroreceptors, hormones (ADH, RAAS, ANP)
- Lymph: returns fluid, absorbs fat, immune defense