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Circulatory System
Blood Flow Dynamics

Master the structure of the heart, the path of blood through systemic and pulmonary circuits, and the physics of hemodynamics governed by \( Q = \frac{\Delta P}{R} \).

MCAT Biology

The Circulatory System at a Glance

The circulatory system is a closed, double-loop system that delivers oxygen and nutrients to tissues while removing carbon dioxide and metabolic waste. It consists of the heart (a four-chambered pump), blood vessels (arteries, veins, capillaries), and blood itself.

Pulmonary Circuit
Heart → Lungs → Heart: exchanges \(\text{CO}_2\) for \(\text{O}_2\)
Systemic Circuit
Heart → Body → Heart: delivers \(\text{O}_2\) and nutrients to tissues

Four-Chambered Heart Anatomy

Right Atrium Right Ventricle Left Atrium Left Ventricle Tricuspid Mitral To Lungs To Body From Lungs From Body

Complete Path of Blood Flow

Body Vena Cava RA RV Lungs Pulm. Veins LA LV Aorta Tricuspid Mitral

Types of Blood Vessels

Arteries
Carry blood away from heart. Thick, elastic walls to handle high pressure. Branch into arterioles.
Capillaries
One-cell-thick walls for gas and nutrient exchange. Site of diffusion between blood and tissues.
Veins
Return blood to the heart. Thinner walls, contain valves to prevent backflow. Lower pressure.

Hemodynamics: Poiseuille's Law

Blood flow obeys principles analogous to Ohm's Law for electric circuits. The fundamental relationship is:

Pressure Changes Across the Vasculature

Pressure (mmHg) 120 100 60 30 0 Aorta Arteries Arterioles Capillaries Venules Veins Biggest Drop!

Arteries vs Veins

Arteries Veins
Direction Away from heart Toward heart
Wall thickness Thick, elastic Thin, less elastic
Pressure High (80-120 mmHg) Low (2-10 mmHg)
Valves None (except semilunar) Present throughout
Lumen Narrow Wide
Blood volume ~15% of total ~65% of total

The Cardiac Cycle

Each heartbeat consists of a coordinated sequence of systole (contraction) and diastole (relaxation). The entire cycle lasts about \( \frac{60}{72} \approx 0.8 \) seconds at rest (72 bpm).

1
Atrial systole: Atria contract, pushing remaining blood into ventricles through open AV valves (tricuspid and mitral).
2
Isovolumetric contraction: Ventricles begin contracting with all valves closed: pressure rises rapidly, volume stays constant.
3
Ventricular ejection: Semilunar valves (aortic and pulmonary) open when ventricular pressure exceeds arterial pressure. Blood is ejected.
4
Isovolumetric relaxation: Ventricles relax, semilunar valves close (second heart sound, "dub"). All valves closed again.
5
Passive ventricular filling: AV valves reopen. Blood flows passively from atria into ventricles. Cycle repeats.

Cardiac Output and Its Determinants

Preload
Degree of ventricular stretch at end of diastole. Increased venous return → more stretch → greater SV (Frank-Starling law).
Contractility
Intrinsic strength of contraction, independent of preload. Increased by sympathetic stimulation and catecholamines.
Afterload
Resistance the ventricle must overcome to eject blood. Higher afterload (e.g., hypertension) → decreased SV.

The Frank-Starling Mechanism

End-Diastolic Volume (Preload) Stroke Volume Normal Sympathetic Heart Failure

How to Solve Hemodynamics Problems

1
Identify what changes: Is the question about vessel radius, blood viscosity, vessel length, heart rate, or stroke volume?
2
Choose the right equation: Use \( Q = \frac{\Delta P}{R} \) for flow, \( \text{CO} = \text{HR} \times \text{SV} \) for cardiac output, or \( R = \frac{8\eta L}{\pi r^4} \) for resistance.
3
Track the direction of change: If radius doubles, \( R \) decreases by a factor of \( 2^4 = 16 \), so flow increases 16-fold (assuming constant \( \Delta P \)).
4
Consider compensatory mechanisms: The body adjusts heart rate, contractility, and vessel tone via the autonomic nervous system (baroreceptor reflex).
5
Check units and reasonableness: Normal CO is ~5 L/min. Normal MAP is ~93 mmHg. If your answer is wildly different, re-examine.

Example 1: Vasoconstriction Effect

If an arteriole's radius is halved due to sympathetic vasoconstriction, by what factor does resistance change? Assume viscosity and length remain constant.

Example 2: Cardiac Output Calculation

A patient has a resting heart rate of 80 bpm and a stroke volume of 65 mL. During exercise, HR increases to 150 bpm and SV increases to 100 mL. What are the resting and exercise cardiac outputs?

Example 3: Mean Arterial Pressure

A patient's systolic blood pressure is 130 mmHg and diastolic is 80 mmHg. Calculate the mean arterial pressure (MAP) and determine if it is normal.

Common Mistakes to Avoid

✕ Confusing Direction
Arteries carry blood away from the heart, not "oxygenated blood." The pulmonary artery carries deoxygenated blood.
✕ Ignoring the 4th Power
Students forget that radius is raised to the 4th power in Poiseuille's equation. A small radius change causes a massive resistance change.
✕ Mixing Up Sides
The left ventricle pumps to the systemic circuit (body), not the lungs. Right side = pulmonary.
✕ MAP Formula Error
MAP is not the simple average of SP and DP. It is weighted: \( \text{DP} + \frac{1}{3}(\text{SP} - \text{DP}) \).

Summary of Key Equations

Concept Equation Key Variable
Blood Flow \( Q = \frac{\Delta P}{R} \) \( \Delta P \), \( R \)
Resistance \( R = \frac{8\eta L}{\pi r^4} \) \( r \) (radius, 4th power)
Cardiac Output \( \text{CO} = \text{HR} \times \text{SV} \) HR, SV
MAP \( \text{DP} + \frac{1}{3}(\text{SP} - \text{DP}) \) Weighted toward DP
Stroke Volume \( \text{SV} = \text{EDV} - \text{ESV} \) Preload, Contractility, Afterload

Question 1 of 5

If the radius of an arteriole is doubled, by what factor does blood flow through it change (assuming constant pressure gradient)?

Question 2 of 5

Which vessel type holds the greatest percentage of total blood volume at any given time?

Question 3 of 5

A patient's blood pressure is \(140/90\) mmHg. What is their approximate mean arterial pressure?

Question 4 of 5

According to the Frank-Starling mechanism, what happens to stroke volume when venous return increases?

Question 5 of 5

Where does the largest pressure drop occur in the systemic circulation?