Arterial System

Cardiac Cycle
- Systole: Heart contraction causing blood to accelerate.
- Diastole: Heart relaxation causing blood to decelerate.
Pulsatile Flow: Flow directly influenced by the effects of the beating heart, with acceleration and deceleration over the cardiac cycle.

Tunica Externa:
- Outermost layer made of connective tissue.
- Vasa Vasorum: Small artery responsible for arterial wall blood supply.
Tunica Media:
- Made of smooth muscle.
- Responsible for vasoconstriction and vasodilation.
Tunica Intima:
- Innermost layer.
- Made of epithelial tissue, also known as endothelium.

Great Artery:
- Large arteries located close to the heart.
- Have a well-developed tunica externa with more elastic fibrous tissue.
- Example: Aorta, brachiocephalic artery.
Medium/Small Size Artery
Arterioles:
- Have a well-developed tunica media (thick smooth muscles).
- Responsible for vasoconstriction and vasodilation of the blood vessels (control the amount of the blood into the organs).
Capillary:
- Smallest arteries.
- Located deep in tissue and cells.
- Made of one single layer tissue (endothelium).
- Allow exchanges between blood circulation and tissue.

Pressure Energy of the Cardiovascular System
- Total energy pressure of the cardiovascular system is equal to the sum of 3 energies:
  1. Potential energy
  2. Kinetic energy
  3. Gravitational/Hydrostatic

Stored energy of the cardiovascular system.
Created by the pumping action of the heart, causing distention of the vessel walls (store energy).
Represents the intravascular pressure.

Is the hydrostatic energy.
Represents the weight of the column of blood extending from the heart.
At the level of the heart, it is equal to zero.
In a standing position, the highest is at the level of the feet.
In a supine position, it is equal to zero because the body level is the same as the heart level.
$Hydrostatic pressure = blood \ gravity \ x \ acceleration \ due \ to \ gravity \ x \ distance \ from \ the \ heart$

States: “When a fluid flows without a change in velocity from one point to another, the total energy content remains constant, providing no frictional losses.”
Total fluid energy is a balance between potential and kinetic energy.
If velocity increases, then pressure must decrease.
In the presence of stenosis, total blood energy can be:
- Proximal to stenosis: High
- At the stenotic site: Low
- Distal to stenosis: Very low

In the presence of stenosis, to keep flow volume (amount of the blood) constant, the velocity must increase and the pressure must decrease.

Describes flow volume relationship
Flow: is volume (amount) of the blood passing per unit time
$Flow = \frac{\pi \times pressure \ gradient \times radius}{8 \times viscosity \times vessel \ length}$

Pressure is the driving force behind fluid flow.
The movement of the blood between points requires a difference in energy (pressure) between the two points.
The greater the pressure differences between the two points, the greater is the flow.

Is the thickness of the blood.
Unit is poise.
Represents the friction that exists between bordering layers of fluid.
Creates energy “losses” in the vascular system (conversion of friction to heat).
Determined by the level of RBC/hematocrit.

The tendency of a body at rest to stay at rest or a body in motion to stay in motion.
Also causes energy “losses” in the vascular system.
Occurs when blood is forced to change direction or velocity (bifurcation).

Low resistance
- Occurs with dilated distal arteriolar bed.
- Flow is antegrade throughout the cardiac cycle.
- Typical of vessels that supply organs.
High resistance
- Occurs with vasoconstriction of distal arterioles.
- Flow is antegrade during systole, retrograde during diastole, and forwarded component caused by the recoil energy stored during systole and released during diastole.
- Typically found in vessels that perfuse muscles (subclavian, aorta, resting peripheral arteries).

Velocity of the blood represents the rate of blood movement with respect to time/direction.
Velocity of the blood = flow divided by area
Relationship between flow, area, and velocity.

Normal blood flow is laminar (blood moves in a layer patterns) that are parallel to each other.
Laminar Plug Flow: The layers of the blood are with similar speed, seen at the entrance of the blood vessel.
In the middle of the vessel, the central layer is the fastest, and the layers close to the vessel wall are slowest; the flow pattern is called parabolic.
Disturbed flow: is a laminar flow, flow is altered from their straight form, occurs at a bifurcation of the vessels
Turbulence flow: is non-laminar flow, random and chaotic, layers moving at different speeds in many directions, even in circles called eddies, commonly seen in the presence of stenosis; can be predicted using Reynolds number when it is over 2000.

Reynolds number and the likelihood of turbulence are directly proportional to:
- Velocity of blood
- Density of blood
- Radius of blood vessel
And inversely proportional to the viscosity of blood
Because blood density and viscosity are constant, the turbulence of blood flow develops because of changes in the velocity.
Used to predict turbulent flow; a value over 2000 signifies turbulent flow.

Peripheral circulation is controlled by:
- Central nervous system
- Local conditions in the tissue bed
Arterioles vasoconstrict and vasodilate in response to sympathetic nervous system control and local factors such as:
- Oxygen and carbon dioxide levels
- Hydrogen and potassium ions
- Blood pressure

Collateral vessels
- Preexisting pathways that enlarge with a stenosis or occlusion.
- Main mechanism to compensate for stenosis.
- Helps reduce resistance at the stenotic area, providing an alternate pathway for blood to reach the distal vascular bed.
Effects of exercise
- Exercise increases blood flow to at least three to five times resting values in normal limbs.
- With mild-to-moderate disease, blood flow is not able to increase this much.
- As a result, patients who are asymptomatic at rest become symptomatic after exercise.
- Additionally, blood pressure distal to an arterial lesion will decrease; exercise exacerbates this.