Peripheral Nervous System

Consists of 12 pairs of cranial nerves.

31 pairs of spinals nerves plus ganglia and receptor cells.

Attached to each segment of the spinal cord is a pair of spinal nerves that are attached by ventral and dorsal roots.

The dorsal roots contain sensory axons and their neurons are in the dorsal root ganglion.

The ventral roots contain motor axons that come from cell bodies in the gray matter.

Each spinal nerve is a mix of sensory and motor fibers.

Shear Stress

Shear stress is the force required to overcome viscosity and maintain the same blood flow.  It is maximum at the vessel wall.

Shear stress acts on vessel walls to release the vasodilator NO, which helps dialate the burdened vessel.  An increase in shear stress means more NO will be released.

Hypertension and atherosclerosis diminish and eventually eliminate this endothelial dependent vasodilation.

Blood Flow vs. Velocity

Blood Flow (Q) is a volume per unit time.  Blood velocity (u) is speed of movement and is expressed in units of distance moved/time.

Velocity, u, is dependent on and varies with radial position, so average velocity, U, is calculated.

Vasoactive Changes

The dominant factor affecting R is a change in D of a blood vessel.  Diameters can change 2 ways:

Passive 

• Occurs if Ptm in vessel changes due to events unrelated to its own VSM changes. 
• Ex: Pu increases due to vasodilation of an upstream artery causing Ptm to increase which causes a D change that depends on the vessels compliance (C).

Active

• Change in D due to vessel’s own VSM relax or contract.



 The net effect of vasoconstriction (active D reduction) is increased R

Vascular Pressure Concept

As blood moves through a vessel, it loses energy due to friction-like processes.  To maintain flow, this energy loss must be balanced by an equal amount of energy supply.  This energy is supplied by pressure.



Perfusion Pressure

Perfusion pressure is defined as the pressure difference upstream in a vessel minus the pressure downstream in a vessel.

ΔP = Pu – Pd  = Q x R

or for a system:  ΔP= MAP-CVP

The pressure difference across the systemic circulation is 90 mmHg.  This difference is the perfusion pressure for the entire systemic circulation.

Arterial Blood Pressure

Arterial BP depends on average BP and pulse.

Avg BP = MAP depends on:

• blood volume in arteries in relation to C
• CO x TPR
• gravity

Pulse depends on:

• amount and rate ventricular ejection
• C of the arteries receiving the SV
• pulse-wave reflection effects

Ejection of blood from the LV raises the aortic pressure from its lowest (diastolic) to highest (systolic) value.  The difference is defined as pulse pressure.

Vessel Types

ELASTIC ARTERIES

• large arteries, aorta
• conduct flow w/ little energy loss
• distensible due to large elastic component of wall
• transiently store blood during ventricular ejection
• recoil during ventricular relaxation
• maintain flow to peripheral arteries

MUSCULAR ARTERIES

• small arteries
• bring blood to organs
• minor factor in TPR

ARTERIOLES

• regulate flow by changing D
• major factor in TPR

TERMINAL ARTERIOLES

• pre-capillary spinchters
• fine tuning for capillary network pressure, flow and exchange

CAPILLARIES

• blood-tissue exchange
• diameter doesn’t change
• endothelial cells of capillaries release vasoactive substances, control diameter

VENULES/VEINS

• thin walled but have some VSM
• store/mobilize blood volume

Blood Vessel Wall Structure

The walls of blood vessels are composed of elastin, collagen and vascular smooth muscle (VSM).  Elastin is easy to stretch but collagen is much stiffer. VSM is NOT found in capillaries.  This is because capillaries are basically endothelial tubes.

FACTS:

• VSM is not found in capillaries
• arterioles have the greatest % of VSM (thus, they can contract most and have smallest C)
• veins have higher C than arteries

Cardiac Output

CO is the product of HR x SV. 

Each organ is supplied with part of the CO in proportion to the relative resistance of that organ, determined by the resistane of the arterioles supplying that organ.  Organs with higher R get less CO.

Blood flow can be altered by changes in R.  Change in diameter is the most dominant factor affecting R.