Vascular System • The heart can be thought of 2 separate pumps

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Vascular System

• The heart can be thought of 2 separate pumps

– from the right ventricle, blood is pumped at a low 

pressure to the lungs and then back to the left atria

– from the left ventricle, blood is pumped at a high 

pressure to the rest of the body and then back to 

the right atria

• There are 3 main types of vessels that carry blood 

around the body

– Arteries 



(small arteries)

• carry blood away from the heart

– Capil



• allow for 



of materials between the 

blood and the cells of the body

– Veins



(small veins)

• carry blood back to the heart

Vascular Pathways

• Arteries and arterioles are characterized by a 

divergent pattern of blood flow

– blood leaves each ventricle via a single artery but 

split into numerous and smaller diameter vessels

• Arterioles branch into 




– capillaries are the most numerous blood vessel with 

the smallest diameter

• Venules and veins are characterized by a convergent 

pattern of blood flow

– blood flows out of many capillaries into a single 

venule with a larger diameter

– from the venules, blood flows into veins that are 

larger in diameter which merge into a single vessel 

to deliver blood to the atria

– ~60% of the blood volume at rest is in the veins

Vascular Walls

• All blood vessels are lined with a thin layer of 


, a type of epithelium which is supported 

by a basement membrane

– called the

tunica intima

(or tunica interna)

– only layer of capillary walls

• The walls of most arteries and veins have layers of 

smooth muscle and/or elastic connective tissue called 


tunica media

and fibrous connective tissue called 


tunica externa

, surrounding the endothelium

– the thickness of the tunica media and externa vary 

in different vessels depending on their function or 

the amount of internal (blood) pressure that they 


Smooth Muscle

• Most blood vessels contain 

vascular smooth muscle

arranged in circular layers which is partially contracted 

at all times creating a condition known as muscle tone

• Additional contraction of the smooth muscle results in 


which narrows the diameter of the 

vessel lumen

• Relaxation of the smooth muscle results in 


which widens the diameter of the vessel 


• Neurotransmitters, hormones and paracrine signals 

influence vascular smooth muscle tone which in turn 

will affect 

blood pressure


blood flow


the cardiovascular system

Blood Flow Through Vascular System


blood flow

through any level of the circulation is 

equal to the cardiac output

– if cardiac output is 5 




, the blood flow through all 

systemic capillaries is also 5 




– blood flow through the pulmonary side is equal to 

blood flow through the systemic circulation

• prevents blood from accumulating in either the 

systemic or pulmonary loop

Distribution of Blood Flow

• The distribution of systemic blood varies according to 

the metabolic needs of individual organs and is 

governed by homeostatic reflexes

– skeletal muscles at rest receive 21% of cardiac 

output, but during exercise when they use more O


and nutrients and produce more CO


and wastes 

receive as much as 85% of cardiac output 

• accomplished through the vasoconstriction and 

vasodilation of arterioles supplying blood to 

various regions, organs or tissues of the body

• The ability to selectively alter blood flow to organs is 

an important aspect of cardiovascular regulation

What Determines Blood Flow?

• Blood flow (F) through the vascular system is 



to the pressure gradient (ΔP) between to 

points within the system:  F 

∝ ΔP

– if the pressure gradient increases, flow increases

– if the pressure gradient decreases, flow decreases

– blood pressure is the amount of force blood exerts 

outwardly on the wall of a vessel

• The tendency of the vascular system to 



flow is called its 


(R) and is 



to flow:  F 




– if the resistance increases, flow decreases

– if the resistance decreases, flow increases

• Combining the equations above results in:  F 




Blood Pressure

• Aortic pressure reaches an average high of 120 

mmHg during ventricular systole (

systolic pressure

and falls steadily to a low of 80 mmHg during 

ventricular diastole (

diastolic pressure


– systolic pressure > 120 is called 



– systolic pressure < 100 is called 



• The highly elastic walls of the arteries allows them to 

capture and store the energy of ventricular ejection

– note that the pressure in the aorta drops only to 80 

mmHg (not to 0mmHg as observed in the ventricle) 

which keeps blood constantly moving (never stops)

– energy stored by the arteries can be felt as a 


• Blood pressure decreases as it flows downstream

• A similar blood pressure profile (albeit lower) is 

observed on the pulmonary side of circulation

What Determines Arterial BP?

• Arterial blood pressure is directly proportional to the 

amount of blood found in an artery

– more blood in an artery = higher pressure

– less blood in an artery = lower pressure

• Since arterial pressure is pulsatile, the 

mean arterial 




) is used to represent the driving 

pressure of blood through the vascular system

– MAP = diastolic + 1/3 (systolic – diastolic)

– MAP = 80 + 1/3 (120 – 80) = 93 mmHg in the aorta

• Mean arterial pressure is a balance between blood 

flow into the arteries and blood flow out of the arteries

– if flow in exceeds flow out, pressure increases

– if flow out exceeds flow in, pressure decreases

• Blood flow in is equal to the 

cardiac output 

• Blood flow out is influenced primarily by the 



offered by the arterioles determined mainly 

by their diameter


∝ CO X Resistance


Regulation of Mean Arterial Blood Pressure

• The central nervous system coordinates the reflex 

control of blood pressure

• The main integrating center is a cluster of neurons in 

the medulla oblongata called the 


control center

• Sensory input to the integrating center comes from a 

variety of peripheral sensory receptors stretch 

sensitive mechanoreceptors known as 


in the walls of the aorta and carotid arteries travel to 

the cardiovascular center via sensory neurons

• Responses by the cardiovascular center is carried via 

both sympathetic and parasympathetic neurons and 

include changes in cardiac output and peripheral 

resistance which occur within 2 heartbeats of the 


Baroreceptor Reflex

• The baroreceptors are tonically active stretch 

receptors that fire action potentials continuously at 

normal blood pressures

• When blood pressure increases in the arteries 

stretches the baroreceptor cell membrane, the firing 

rate of the receptor increases 

– in response, the cardiovascular center increases 

parasympathetic activity and decrease sympathetic 

activity to slow down the heart

– decreased sympathetic outflow to arterioles causes 

dilation allowing more blood to flow out of the 


• When blood pressure decreases in the arteries, the 

cardiovascular center increases sympathetic activity 

and decreases parasympathetic activity creating 

opposite responses in the effectors to increase blood 


What Else Determines Mean Arterial BP?

• Although the volume of blood is usually relatively 

constant, changes in blood volume can affect mean 

arterial blood pressure

– if blood volume increases, blood pressure increases

• fluid intake

– if blood volume decreases, blood pressure 


• fluid loss

• Relative distribution of blood between the venous and 

arterial sides of circulation is an important factor in 

regulating arterial blood pressure

– when arterial blood pressure falls, vasoconstriction 

of the veins redistributes blood to the arterial side

Systemic Venous Blood Pressure

• As blood moves through the vessels, pressure is lost 

due to friction between the blood and the vessel walls 

• The low pressure blood in veins inferior to the heart 

(arms, abdominopelvic cavity and legs) must flow 

against gravity to return to the heart

• To assist venous flow, these veins have internal one 

way valves to ensure that blood passing the valve 

cannot flow backward

• The movement of blood through veins is also assisted 

by the 



skeletal muscle

• Veins located between skeletal muscles are squeezed 

during contraction 

• This increases the venous pressure enough to move 

the blood through the valves, back towards the heart

What Determines Resistance in the Vessels?

• For fluid flowing through a tube, resistance is 

influenced by 3 parameters:

– the radius (r) of the tube (half of the diameter)

– the length (L) of the tube

– the viscosity (η) or thickness of the fluid

• Poiseuille’s Law relates these factors to resistance:

– R 

∝ Lη/r


• if the tube length increases, resistance increases 

• if the viscosity increases, resistance increases

• if the tube’s radius increases, resistance 


– Since blood viscosity remains relatively constant 

and blood vessel lengths can’t change, 



is the major determinant of resistance 

• Arteriolar constriction reduces blood flow through that 

arteriole and redirects the flow through all arterioles 

with a lower resistance

– total blood flow through all the arterioles of the body 

always equals cardiac output

Local and Systemic Control of Arteriolar Diameter

• Local control is accomplished by paracrines secreted 

by the vascular endothelium or by tissues to which the 

arterioles are supplying blood

– low  O


and high CO


dilate arterioles which 

increase blood flow into the tissue bringing 

additional O


while removing excess CO


• can be caused by an increase in metabolic 

activity (

active hyperemia

) or by a period of low 

perfusion (

reactive hyperemia


• Systemic control occurs by sympathetic innervation

– tonic release of norepinephrine which binds to α-

adrenergic receptors on vascular smooth muscle 

helps maintain tone of arterioles

– if sympathetic release of norepinephrine decreases, 

the arterioles dilate, if the release of norepinephrine 

increases, arterioles constrict

Capillary Wall Promotes Exchange

• Most cells are located within 0.1 mm of the nearest 

capillary over which diffusion occurs rapidly

• The most common type are continuous capillaries 

– endothelial cells are joined by leaky junctions

• Less common type are fenestrated capillaries

– endothelial cells have large pores (


that allow high volumes of fluid to pass quickly 

between the plasma and interstitial fluid

• Exchange occurs either by:

– movement of substances through the gaps between 

adjacent endothelial cells (paracellular movement)

– movement of substances through/across the cell 

membrane of endothelial cells (transcellular


Capillary Exchange

• Paracellular exchange occurs through endothelial cell 

junctions or fenestrations

– solutes can move by 


– solutes can move by 

bulk flow

which refers to the 

mass movement of a solvent as a net result of 

hydrostatic and or osmotic pressure gradients 

across the capillary wall

• if the direction of bulk flow is out of the capillary 

the fluid movement is called 


• if the direction of bulk flow is into the capillary the 

fluid movement is called 


• Transcellular exchange occurs through the cell 

membrane of endothelial cells

– nonpolar gasses and solutes can move by 


– large polar solutes can move by 

vesicular transport

Capillary Exchange by Bulk Flow

• 2 forces regulate bulk flow in capillaries

– hydrostatic pressure





• lateral pressure component of blood flow that 

pushes plasma out through the capillary pores

• decreases along the length of the capillary as 

energy is lost to friction

– osmotic pressure





• pressure exerted by solutes within the plasma 

• the main solute difference between plasma and 

interstitial fluid is due to 


(present in 

plasma, but mostly absent in interstitial fluid)

–the osmotic pressure created by plasma 

proteins is called 

colloid osmotic pressure

• favors water movement by osmosis from 

interstitial fluid into plasma

• is constant along the length of the capillary

• Net Pressure = 





• Net Pressure

arterial end

= 32mmHg – 25mmHg = 7mmHg

– favors filtration

• Net Pressure

venous end

= 15mmHg – 25mmHg = -10mmHg

– favors absorption

• In most capillaries there is more 

filtration than absorption

• 90% the volume of fluid filtered out at the arterial end is 

absorbed back into the capillary at the venous end

– the other 10% enters lymphatic vessels where it is 

returned back into circulation as the lymph vessels 

empty lymph fluid into blood at the right atrium

Document Outline

  • Vascular System
  • Vascular Pathways
  • Vascular Walls
  • Smooth Muscle
  • Blood Flow Through Vascular System
  • Distribution of Blood Flow
  • What Determines Blood Flow?
  • Blood Pressure
  • What Determines Arterial BP?
  • Regulation of Mean Arterial Blood Pressure
  • Baroreceptor Reflex
  • What Else Determines Mean Arterial BP?
  • Systemic Venous Blood Pressure
  • What Determines Resistance in the Vessels?
  • Local and Systemic Control of Arteriolar Diameter
  • Capillary Wall Promotes Exchange
  • Capillary Exchange
  • Capillary Exchange by Bulk Flow

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