Question

Explain how to flow through a blood vessel is determined. Explain what a “pressure difference” is and why it matters to flow. Define resistance and explain how resistance affects flow. List and discuss the 3 factors which influence the resistance through vessels. For each factor explain the mechanism by which it affects the resistance. Which one of these 3 as the largest impact on resistance? Explain why it has the largest impact on flow. Which one of these 3 can readily be adjusted by the body?

Answer 1

Blood is ejected from the left ventricle and flows into the aorta, which branches into arteries, arterioles, and eventually capillaries.

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as the cardiac output (CO).

Flow-through a blood vessel is determined by the Mean Volume of blood flow.

Mean Volume of blood flow: This is the volume of blood that flows into the circulatory system in a given unit of time. It is the product of mean velocity and the cross-sectional area of the vascular bed. So,

Q = V A

Where Q = Quantity of blood.

V = Velocity of blood flow.

A = Cross-sectional area of the blood vessels.

Factors maintaining Volume of blood flow:

1. Pressure gradient.
2. Resistance to blood flow.
3. The velocity of blood flow.

Pressure gradient: The volume of blood flowing through any blood vessel is directly proportional to the difference between the pressure at either end of the blood vessel.

This difference in pressure is known as pressure gradient and is expressed as follows,

Pressure gradient = P1 - P2

where P1 = Pressure at the proximal end of the vessel.

P2 = Pressure at the distal end of the vessel.

The pressure gradient at different areas of vascular bed is

• Between 2 ends of the aorta: 120 mmHg - 100 mmHg = 20 mmHg.
• Between the beginning of arteries and end of arterioles: 100 mmHg - 30 mmHg = 70 mmHg.
• Between arterial and venous ends of capillaries = 30 mmHg - 15 mmHg = 15 mmHg.

And this pressure gradient explains the flow of blood from the aorta to capillaries.

Resistance:

Resistance is the friction or the tension or hindrance against which the blood flows.

• though it exists in all blood vessels to some extent it is remarkable in peripheral vessels, particularly the arterioles.

The volume of blood flow is inversely proportional to the resistance, thus making resistance an important factor affecting the blood flow.

R = Pressure gradient / Volume of blood flow

Factors influencing resistance:

The radius of the blood vessel: Peripheral resistance is inversely related to the radius of the blood vessel i.e, lesser the radius more will be the resistance.

• The smaller arteries and arterioles have higher resistance and confer the main blood pressure drop across major arteries to capillaries in the circulatory system.
• The radius of the arterioles is the least because of the sympathetic tone.
• This sympathetic tone of the walls of the arteriole provides resistance against the flow of the blood.
• The high resistance observed in the arterioles, which factor largely in the ∆P is a result of a smaller radius of about 30 µm.
• The smaller the radius of a tube, the larger the resistance to fluid flow.
• Immediately following the arterioles are the capillaries.
• Following the logic observed in the arterioles, we expect the blood pressure to be lower in the capillaries compared to the arterioles.
• Since pressure is a function of force per unit area, (P = F/A), the larger the surface area, the lesser the pressure when an external force acts on it.
• Though the radii of the capillaries are very small, the network of capillaries has the largest surface area in the vascular network.
• The larger the total cross-sectional area, the lower the mean velocity as well as the pressure.
• Vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure.
• Vasodilators increase the size of blood vessels, thereby decreasing arterial pressure.
• Radius can be changed when vessels are plugged by plaques of fatty lipids known as atherosclerosis, or hardening of the arteries, thus increasing resistance to blood flow.

The viscosity of blood Flow: Blood viscosity is a measurement of the thickness and stickiness of an individual's blood.

• this creates friction of blood flow against the wall of blood vessels.
• The volume of blood flow is inversely proportional to the viscosity of blood.
• The number of RBC's is the main factor determining the viscosity of blood.
• another factor is a plasma protein, mainly albumin.

Length of the blood vessel; Since pressure is a function of force per unit area, (P = F/A), the larger the surface area, the lesser the pressure when an external force acts on it.

So, greater the length of the vessel lesser the resistance, as the surface area will be more.

Vessel length does not change appreciably, neither doe the viscosity of blood change much however, it can be significantly altered by changes in hematocrit, temperature, and by low flow states.

Therefore the most important of these factors is the radius of the vessel and has the largest impact. As the radius (r) of a tube decreases, the resistance (R) of the tube to fluid flow increases dramatically because R is proportional to 1/r4.

Though it has largest impact it can even be adjusted easily with the help of vasoconstrictor and vasodilators.