Control of Blood Vessels

Force per unit area.

kg/m² or dyne/cm²

mmHg

pressure

What happens when pressure increases inside a fluid-filled vessel with distensible walls?

walls will be distended (stretched out)

diameter of the vessel will increase

increase in arterial pressure distends the aorta and large blood vessels

arteries during systole

A volume per time unit

Flow

What is a typical value for cerebral blood flow?

750 ml/min

What is a typical cardiac output in an adult human at rest?

5 L/min

How is blood carried in the circulatory system?

In a closed system of vessels that begins and ends at the heart

What are the three major types of blood vessels

Arteries, capillaries, and veins

Contact tissue cells and directly serve cellular needs.

Capillaries

Thick wall

• High elastin content (recoil)

• Pressure reservoir of the circulation

• Internal pressure: 120/80 mm Hg

aorta and large arteries

High smooth muscle content

• Can constrict their lumen when activated

• Resistance vessels

• Internal pressure: 40-90 mm Hg

small arteries and arterioles

Thin wall

• Low elastin and smooth muscle content

• Volume stores of the circulation

• Internal pressure: 0-10 mm Hg

Veins

Δ Volume / Δ Pressure.

The higher the compliance, the easier the stretching

Compliance

High compliance – walls stretch and expand in response to pressure and recoil

• Function as a pressure reservoir

• propel blood forward while the ventricles are relaxing

Arteries

What happens to arteries in atherosclerosis?

Reduced compliance → aneurysms and rupture

Resistance vessels

Constrict to direct and control blood flow to organs

major factor in determining mean arterial pressure

arterioles

What are the equations for flow and pressure?

Flow = ΔPressure / Resistance

• ΔPressure = Flow × Resistance

What do arterioles provide?

The greatest resistance to blood flow

What allows arterioles to control diameter and resistance?

They contain smooth muscle.

What is responsible for arterial tone?

Spontaneous smooth muscle contraction.

physiological and pathological factors

Contraction of smooth muscles → decrease in diameter → increase in resistance

vasoconstriction

Relaxation of smooth muscle → increase in diameter → decrease in resistance

vasodilation

What is the main determinant of blood flow?

Radius

Flow from arterioles → capillaries→postcapillary venule

microcirculation

arise from a single arteriole

Material exchange vessels – exchange between blood and interstitial fluid

One cell thick

Capillary beds

What is the relationship between blood velocity and cross-sectional area?

Speed (cm/sec) is inversely related to cross-sectional area.

Where is velocity slowest?

Where total cross-sectional area is greatest — in the capillaries

Why is slow blood flow in capillaries important?

Slow flow facilitates exchange

What are the two main mechanisms of capillary exchange?

1. Diffusion
2. Bulk flow

Movement of solutes down their concentration gradient.

• O₂ and nutrients: blood → interstitial fluid → cells

• CO₂ and wastes: cells → interstitial fluid → blood

diffusion in capillaries

Movement of fluids (blood, interstitial fluid) along pressure gradients.

• Filtration: from capillaries into interstitial fluid

• Reabsorption: from interstitial fluid into capillaries

bulk flow

Thin-walled collectors of blood.

Venules

What allows capillary beds to drain into venules?

Low pressure in venules

What is transferred to veins?

Low oxygenated blood.

receive low oxygenated blood from venules

have thinner walls than arteries

Veins

What helps move blood up the limbs and back to the heart?

Contraction of skeletal muscles (massaging action) (skeletal muscle pumps).

Valves in veins prevent backflow

Where is the largest portion of blood (5 L) at rest located?

In systemic veins and venules (blood reservoir)

Reduces the volume of blood in reservoirs

allows greater blood volume to flow where needed.

Venoconstriction

The highest pressure resulting from blood forced into the arteries during ventricular systole

systolic pressure

below systolic pressure; Pressure in the arteries during ventricular diastole

diastolic pressure

Pressure in cardiac system

decreases as it flows through the system; Aorta→arteries→arterioles→capillaries→venules→veins

What is the flow equation for a vascular bed?

F = ΔP / R (same as for a single tube).

R= resistance of the whole vascular bed

What connects to the entrance of each vascular bed?

An artery that connects to the aorta

What connects to the exit of each vascular bed?

A vein that connects to the vena cava

The pressure difference across the vascular bed is equal to MAP - VP (same for all vascular beds).

ΔP (perfusion pressure)

How does blood choose which path to flow?

Blood flows along the path of least resistance.

Organs with the lowest resistance receive the highest flow.

Systolic Pressure (SP) - Diastolic Pressure (DP).

Pulse pressure

What is the equation for Mean Arterial Pressure (MAP)

MAP = DP + 1/3(pulse pressure)
or MAP = DP + 1/3(SP - DP)

Calculate MAP given SP = 120 mmHg, DP = 80 mmHg.

MAP = 80 + 1/3(120 - 80) = 80 + 1/3(40) = 80 + 13.33 = 93 mmHg

Given MAP = 93 mmHg, VP = 3 mmHg, cerebral vascular R = 0.10 PRU, calculate cerebral blood flow

F = ΔP / R = (93 - 3) / 0.10 = 90 / 0.10 = 900 ml/min

The resistance of the whole systemic circulation (many vascular beds in parallel)

Total peripheral resistance (TPR)

What is the pressure at the entrance of systemic circulation?

MAP (Mean Arterial Pressure).

What is the pressure at the exit of systemic circulation?

RAP (Right Atrial Pressure) or CVP (Central Venous Pressure)

What is the flow through the systemic circulation?

CO (Cardiac Output)

What is the equation for cardiac output in terms of MAP, RAP, and TPR?

CO = (MAP - RAP) / TPR

Given MAP = 93 mmHg, VP = 3 mmHg, CO = 6 L/min, calculate TPR

R = ΔP / F = (93 - 3) / 6000 ml/min = 90 / 6000 = 0.015 PRU