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Blood Vessels &
Hemodynamics

Explore how pressure, flow, and resistance govern blood movement through your cardiovascular system.

Anatomy & Physiology

What Are Blood Vessels?

Blood vessels are the tubular structures that carry blood throughout the body. They form a closed circuit: the heart pumps blood into arteries, which branch into tiny capillaries for exchange, then collect into veins that return blood to the heart.

Arteries
Carry blood away from the heart under high pressure. Thick, elastic walls.
Capillaries
Microscopic vessels where gas and nutrient exchange occurs. Walls one cell thick.
Veins
Return blood to the heart under low pressure. Contain valves to prevent backflow.

Vessel Wall Layers

Lumen Tunica Intima Endothelium + basement membrane Tunica Media Smooth muscle + elastic fibers Tunica Externa Connective tissue (collagen) Cross-Section View

What Is Hemodynamics?

Hemodynamics is the study of blood flow dynamics: the physical principles that govern how blood moves through vessels. Three interrelated variables control all blood flow:

Pressure (\( P \))
The force blood exerts on vessel walls. Measured in mmHg. Created by heart contraction.
Flow (\( F \))
Volume of blood passing a point per unit time. Measured in mL/min.
Resistance (\( R \))
Opposition to blood flow from friction. Determined by vessel length, radius, and viscosity.

The Core Equation: \( F = \frac{\Delta P}{R} \)

Flow (F) mL/min = Pressure Gradient
\( \Delta P = P_1 - P_2 \)
 / Resistance (R) Analogous to Ohm's Law: I = V / R

Poiseuille's Law: Resistance Factors

1
Vessel radius (\( r \)) is dominant: Resistance is inversely proportional to \( r^4 \). Halving the radius increases resistance 16-fold.
2
Viscosity (\( \eta \)): Thicker blood (e.g., polycythemia) increases resistance. Normally relatively constant.
3
Vessel length (\( L \)): Longer vessels have more friction, increasing resistance. Relatively fixed in adults.

The Power of Radius

Normal Vessel r = 1 R = 1x Flow = F Constricted (vasoconstriction) r = 0.5 R = 16x Flow = F/16 Dilated (vasodilation) r = 1.2 R = 0.48x Flow = 2.07F

Arteries vs Veins

Feature Arteries Veins
Direction Away from heart Toward heart
Pressure High (80-120 mmHg) Low (2-10 mmHg)
Wall thickness Thick tunica media Thin tunica media
Lumen size Narrow, round Wide, irregular
Valves None (except aortic/pulmonary) Present throughout
Blood volume ~15% of total ~60% of total

Pressure Gradient Through the System

Pressure (mmHg) 0 25 50 75 100 120 Aorta Arteries Arterioles Capillaries Venules Veins Vena Cava Largest drop here Primary resistance vessels

Capillary Exchange Dynamics

Arteriole End Venule End Filtration Reabsorption 35 mmHg 25 mmHg 15 mmHg Blood Hydrostatic Pressure (BHP) Osmotic Pressure (constant ~26 mmHg): pulls fluid IN Net filtration at arteriole end Net reabsorption at venule end

Venous Return Mechanisms

Veins operate at low pressure (2-10 mmHg), so the body uses several assist mechanisms to push blood back to the heart:

1
Skeletal muscle pump: Contracting leg muscles squeeze veins and push blood upward toward the heart.
2
Respiratory pump: During inhalation, decreased thoracic pressure creates a suction effect that draws blood toward the heart.
3
Venous valves: One-way valves prevent backflow, ensuring blood moves only toward the heart.
4
Venoconstriction: Sympathetic stimulation narrows veins, reducing volume and pushing blood forward.

Blood Pressure Regulation

Blood Pressure
\( \text{BP} = \text{CO} \times \text{TPR} \)
 Cardiac Output (CO)
\( \text{CO} = \text{HR} \times \text{SV} \)
 Heart Rate (HR) Stroke Vol. (SV) Total Peripheral Resistance (TPR) Vessel Radius (vasoconstriction) Viscosity (blood thickness) Key Regulatory Mechanisms Baroreceptor reflex : Chemoreceptors : RAAS : ADH

How to Solve Hemodynamic Problems

1
Identify the variables: What is given? (Pressure, flow, radius, viscosity?) What are you solving for?
2
Choose the right equation: Use \( F = \frac{\Delta P}{R} \) for basic flow. Use Poiseuille's law if radius or viscosity changes.
3
Consider the \( r^4 \) relationship: If radius changes, compute the resistance factor. Halving \( r \) means \( R \) increases by \( 2^4 = 16 \).
4
Apply BP = CO x TPR: For whole-system questions about blood pressure, cardiac output, or total resistance.
5
Check units and direction: Flow is always from high to low pressure. Verify your answer makes physiological sense.

Example 1: Basic Flow

1
Identify: \( P_1 = 100 \) mmHg, \( P_2 = 20 \) mmHg, \( R = 40 \) PRU
2
Find \( \Delta P \): \( \Delta P = 100 - 20 = 80 \) mmHg
3
Apply: \( F = \frac{\Delta P}{R} = \frac{80}{40} = 2 \) mL/min

Example 2: Radius Change

1
New radius factor: \( r_{\text{new}} = 0.7r \)
2
Resistance change: Since \( R \propto \frac{1}{r^4} \), new \( R = \frac{R_0}{(0.7)^4} = \frac{R_0}{0.2401} \approx 4.16 R_0 \)
3
New flow: \( F_{\text{new}} = \frac{F_0}{4.16} = \frac{10}{4.16} \approx 2.4 \) mL/min

Common Mistakes

✕ Confusing Flow and Pressure
Pressure is the force on walls. Flow is the volume moving per time. High pressure does not always mean high flow: resistance matters too.
✕ Forgetting the \( r^4 \) Rule
Students often treat radius and resistance as proportional (\( r^1 \)) rather than \( r^4 \). A 50% radius drop = 16x more resistance, not 2x.
✕ Mixing Up Arteries and Arterioles
Arterioles (not large arteries) are the main resistance vessels and primary regulators of blood flow to tissues.

Question 1 of 5

According to the flow equation \( F = \frac{\Delta P}{R} \), what happens to blood flow if the pressure gradient doubles while resistance stays the same?

Question 2 of 5

Which type of blood vessel is considered the primary resistance vessel and most strongly regulates blood flow to organs?

Question 3 of 5

If the radius of a blood vessel is reduced to one-third of its original size, resistance increases by approximately:

Question 4 of 5

At the arteriole end of a capillary, blood hydrostatic pressure is 35 mmHg and blood colloid osmotic pressure is 26 mmHg. What is the net filtration pressure?

Question 5 of 5

A patient's cardiac output is 5 L/min and total peripheral resistance is 20 mmHg/L/min. What is their mean arterial blood pressure?