Blood Flow, Blood Pressure, Cardiac Output

Blood Vessels

Blood Vessels

• Made of smooth muscle, elastic and fibrous connective tissue

• Cells are not electrically coupled

Blood Vessels

Arteries → arterioles → metarterioles → capillaries → venules → veins

Blood Vessels

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• Neurotransmitters, hormones, paracrines, etc. affect vessel diameter

• Endothelial muscle exhibits graded potentials

• Vascular smooth muscle is always partially contracted

Arteries and Arterioles

Arteries and Arterioles

• Arteries and arterioles

• Carry blood AWAY from the heart

• They are resistance vessels

• Blood here is under high pressure

Veins and Venules

• Veins

• Carry blood toward the heart

• They are capacitance vessels; they do NOT provide resistance; they expand to allow blood flow through.

• How is blood pumped back to the heart under such low pressure?

Veins and Venules of note

pulmonary veins and the umbilical vein carry blood rich in oxygen

Capillaries

• Capillaries are one cell layer thick

• Capillaries are sites of exchange

Pressure, Flow, Resistance and Velocity

Basic Principles:

• Pressure = force exerted by a fluid (or a gas) on a container holding the fluid (or gas)

Basic Principles:

• Measured in mm Hg

• the pressure exerted by a column of Hg that is 1mm high

• = 1 torr

• = 1/1760 atm

• = 133.3 Pascals

Basic Principles

Liquids (and gasses) flow from areas of high pressure to areas of low pressure until equilibrium is reached.

The pressure of fluid (or a gas) in motion decreases over distance, due to friction of the fluid as it comes in contact with surfaces and other water molecules.

Basic Principles

Volume and pressure are inversely relatedBasic Principles

• True for vessels too(!)

• Vasoconstriction = decreased volume, increased pressure

• Vasodilatation = increased volume, decreased pressure

Basic Principles

Laws and Equations Based on Basic Principles

Stoke’s Law

• Blood flows from an area of high pressure to of lower pressure until equilibrium is reached

• Flow = volume of fluid passing one point per unit time (gal / hour) or (ml / min)

• Blood flow ∝ ∆P

• ∆P = P1 - P2

• The greater the pressure differential, the faster the flow

Flow

What Reduces Flow? Resistance

• Resistance (R) = tendency of the system to oppose flow

• Flow ∝ 1/R

• Blood flowing through vessels encounters friction from:

• 1. Vessel walls

• 2. Blood cell collisions

Flow ∝ ∆P / R

Resistance to Flow is Determined by

What factor most greatly affects blood flow in the body?

Flow and Resistance

Poiseuille's Law:

• This law describes the relative importance of the three factors affecting resistance:

• R = 8L v / π r4

• 8 / π is a constant, so simplify:

• R ∝ L v / r4

• Flow = volume of blood passing one point per unit time (ml / min)

• Velocity = Flow / total cross-sectional area (distance / time)

• miles / hour

• m / sec

Blood Velocity

Blood Velocity

Blood Velocity and Total Cross-Sectional Area

Damage or weakening of a vessel puts strain on the others

Is Blood Velocity Highly Variable?

During exercise, blood pressure and cardiac output increase significantly. What about blood velocity?

Blood Pressure

• Ventricular contraction creates driving pressure

• Arteries are high-pressure vessels in which blood moves with a pulsatile flow.

• If a patient is bleeding, how might you tell if it was an artery or vein that was cut?

Blood Pressure Measurements

• Systolic Pressure: Avg. high of 120 mm Hg

• Diastolic Pressure: Avg. high of 80 mm Hg

• Sphygmomanometry is read as systolic / diastolic (120 / 80)

• Pulse = rapid pressure increase that occurs when ventricles push blood into the aorta

MAP = Mean Arterial Pressure

• MAP measures the average arterial pressure during cardiac cycle

• MAP = Diastolic P + 1/3 (Systolic - Diastolic)

• Abnormally low MAP indicates impaired blood flow and oxygen delivery

• Abnormally high flow indicates risk of hemorrhage

MAP is determined by:

1. Balance between blood flow into and out of arteries

2. Cardiac output (CO)

Cardiac Output = stroke volume x cardiac rate

3. Arteriole resistance

Remember: R ∝ 1/r4

4. Total blood volume

Kidneys and ANS regulate

Capillary Exchange

• Capillary density is proportional to the metabolic needs of the tissues served

• Velocity of blood flow is lowest at capillaries, allowing diffusion to go to equilibrium

• Total cross-sectional area of ALL capillaries determines velocity

• Two types of capillaries:

• Continuous capillaries

• Cells joined close together

• Fenestrated capillaries

• Cells have large pores

• Found in kidney and intestine

Capillary Exchange

Filtration

Absorption

Defining Blood Flow Direction

Capillary Exchange

• Question: What drives the flow of materials into or out of capillaries?

Capillary Exchange

• Question: What drives the flow of materials into or out of capillaries?

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• Answer: GRADIENTS (what else?!)

• Two opposing gradients:

• 1. Hydraulic pressure decreases along the length of the capillary as energy is lost to friction

• 2. (Colloid) osmotic pressure due to solute differences between the two sides of the capillary

Capillary Exchange

• Net filtration at arterial end, net absorption at venous end

• 85% of the capillary filtrate is directly returned via capillaries

• What happens to the rest?

Capillary Exchange

Angiogenesis

• Angiogenesis = process by which new blood vessels develop

• Understanding angiogenesis can lead to treatment of

• 1. Cancer

• 2. Coronary artery disease

Hypertension

• Hypertension is blood pressure that is in excess for one's age and gender

• Treatment:

• 1. modification of lifestyle is first.

• 2. diuretic drugs

• 3. Beta blockers are used to lower heart rate

Shock

• Shock occurs when low blood volume reaches an area of the body

• Hypovolemic Shock: circulatory shock due to low blood volume (due to bleeding, burns, dehydration)

• Septic Shock: circulatory shock due to sepsis (infection)

• Anaphylactic shock: due to an allergic reaction

• Cardiogenic shock: due to heart problem