Bio2305 Vascular Physiology

BIO2305 Vascular Physiology
Perfusion = blood flow through tissues or organs
Physiology of systemic circulation determined by

• 
  Dynamics of blood flow
  
• 
  Anatomy of circulatory system
  
• 
  Regulatory mechanisms that control heart and blood vessels
  

Blood volume

• 
  Most in the veins (2/3rd)
  
• 
  Smaller volumes in arteries and capillaries
  

Dynamics of blood circulation involves interrelationships between

• 
  Pressure
  
• 
  Flow
  
• 
  Resistance
  
• 
  Control mechanisms that regulate blood pressure
  
• 
  Blood flow through vessels
  

Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period:

• 
  Is measured in ml per min.
  
• 
  Equivalent to cardiac output (CO), considering the entire vascular system
  
• 
  Relatively constant when at rest, varies widely through individual organs, according to immediate needs
  

Blood Pressure (BP)

Force per unit area exerted on the wall of a blood vessel by its contained blood

• 
  Expressed in millimeters of mercury (mm Hg)
  
• 
  Measured in reference to systemic arterial BP in large arteries near the heart
  

The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

Blood Flow: Pressure changes

• 
  Flows down a pressure gradient
  
• 
  Force of heart contraction
  
• 
  Highest at the heart (driving P), decreases over distance
  
• 
  Compliance (distensibility) of vessel
  
• 
  Decreases 90% from aorta to vena cava
  

Blood flow directly proportional to pressure differences and inversely proportional to resistance.
Blood flow (F) is directly proportional to the difference in blood pressure (ΔP) between two points in the circulation, flows down a pressure gradient

Flow = ΔP /R
F = flow rate of blood through a vessel

ΔP = pressure gradient

R = resistance of blood vessels
R is more important than ΔP in influencing local blood pressure


Resistance factors

Constant factors:

- Blood viscosity – thickness or “stickiness” of the blood; hematocrit, [plasma proteins]

- Blood vessel length – the longer the vessel, the greater the resistance encountered

Dynamic factor:

blood vessel diameter, changes in vessel diameter significantly alter peripheral resistance

Resistance varies inversely with the fourth power of vessel radius (one-half the diameter)
R = L n For example, if the radius is doubled, the resistance is 1/16 as much

r⁴

L = length of the vessel

n = viscosity of blood

r = radius of the vessel

Blood flow & cross-sectional area

At the capillary bed:

• 
  vessel diameter decreases
  
• 
  but number of vessels increase, increasing total cross-sectional area
  
• 
  velocity slows down so that capillaries can unload O2 and nutrients
  

Vascular Tree

Closed system of vessels consists of:

• 
  Arteries - carry blood away from heart to tissues
  
• 
  Arterioles - smaller branches of arteries
  
• 
  Capillaries
  

• 
  Smaller branches of arterioles
  
• 
  Smallest of vessels across which all exchanges are made with surrounding cells
  

• 
  Venules
  

• 
  Formed when capillaries rejoin
  
• 
  Return blood to heart
  

• 
  Veins
  

• 
  Formed when venules merge
  
• 
  Return blood to heart
  

1. Role of Arteries

Elastic or conducting arteries

• 
  Largest diameters, high pressure fluctuations
  
• 
  Provides pressure reservoir
  
• 
  Elastic recoil propels blood after systole
  

Muscular or medium arteries

• 
  smooth muscle allows vessels to regulate blood supply by constricting or dilating
  

2. Role of Arterioles

• 
  Transport blood from small arteries to capillaries
  
• 
  Controls the amount of resistance
  
• 
  Greatest drop in pressure occurs in arterioles which regulate blood flow through tissues
  
• 
  No large fluctuations in capillaries and veins
  

Blood Pressure-

Force exerted by blood against a vessel wall

Depends on

• 
  Volume of blood forced into the vessel
  
• 
  Compliance (distensibility/elasticity) of vessel walls
  

Systolic pressure

• 
  Peak pressure exerted by ejected blood against vessel walls during cardiac systole (ventricular contraction)
  
• 
  Averages 120 mm Hg
  

Diastolic pressure

• 
  Minimum pressure in arteries when blood is draining off into vessels downstream, lowest level of arterial pressure during ventricular cycle
  
• 
  Averages 80 mm Hg
  

Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls)

Pulse pressure – the difference between systolic and diastolic pressure
Mean arterial pressure (MAP) – average pressure that propels the blood to the tissues

MAP = diastolic pressure + 1/3 pulse pressure
Blood Pressure Measurement

• 
  Critical closing pressure
  

Pressure at which a blood vessel collapses and blood flow stops

• 
  Laplace’s Law
  

Force acting on blood vessel wall is proportional to diameter of the vessel times blood pressure

• 
  Blood pressure cuff is inflated above systolic pressure, occluding the artery.
  
• 
  As cuff pressure is lowered, the blood will flow only when systolic pressure is above cuff pressure, producing the sounds of Korotkoff.
  
• 
  Korotkoff sounds will be heard until cuff pressure equals diastolic pressure, causing the sounds to disappear.
  

Different phases in measurement of blood pressure are identified on the basis of the quality of the Korotkoff sounds.

Average arterial BP is 120/80 mm Hg.

Average pulmonary BP is 22/8 mm Hg.
Pulse Pressure

• 
  Increases when stroke volume increases or vascular compliance decreases
  
• 
  Pulse pressure can be used to take a pulse to determine heart rate and rhythmicity
  

• 
  Effect of Gravity - In a standing position, hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart
  

Role of Veins

• 
  Veins have much lower blood pressure and thinner walls than arteries
  
• 
  To return blood to the heart, veins have special adaptations
  

• 
  Large-diameter lumens, which offer little resistance to flow
  
• 
  Valves (resembling semilunar heart valves), which prevent backflow of blood
  

Venous BP

• 
  Venous BP is steady and changes little during the cardiac cycle
  
• 
  The pressure gradient in the venous system is only about 20 mm Hg
  
• 
  Veins have thinner walls, thus higher compliance.
  
• 
  Vascular compliance
  

• 
  Tendency for blood vessel volume to increase as blood pressure increases
  
• 
  More easily the vessel wall stretches, the greater its compliance
  
• 
  Venous system has a large compliance and acts as a blood reservoir
  

• 
  Capacitance vessels - 2/3 blood volume is in veins.
  

Venous Return

• 
  Venous pressure is driving force for return of blood to the heart.
  
• 
  EDV, SV, and CO are controlled by factors which affect venous return
  

• 
  Venous BP alone is too low to promote adequate blood return and is aided by the:
  
  • 
    Respiratory “pump” – pressure changes created during breathing squeeze local veins
    
  • 
    Muscular “pump” – contraction of skeletal muscles push blood toward the heart
    
• 
  Valves prevent backflow during venous return
  

Capillary Network

• 
  Blood flows from arterioles through metarterioles, then through capillary network
  
• 
  Venules drain network
  
  • 
    Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow
    
  • 
    Capillary wall consists mostly of endothelial cells
    
  • 
    Types classified by diameter/permeability
    
  • 
    Continuous do not have fenestrae
    
  • 
    Fenestrated have pores
    

True capillaries – exchange vessels

• 
  Oxygen and nutrients cross to cells
  
• 
  Carbon dioxide and metabolic waste products cross into blood
  

Atriovenous anastomosis – vascular shunt, directly connects an arteriole to a venule

Capillary Exchange and
Interstitial Fluid Volume Regulation

• 
  Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries
  
• 
  A net movement of fluid occurs from blood into tissues – bulk flow.
  
• 
  Fluid gained by tissues is removed by lymphatic system.
  

Exchange of Fluid between Capillaries and Tissues

Distribution of ECF between plasma and interstitial compartments

• 
  Is in state of dynamic equilibrium.
  
• 
  Balance between tissue fluid and blood plasma.
  

• 
  Hydrostatic pressure:
  

• 
  Exerted against the inner capillary wall.
  
• 
  Promotes formation of tissue fluid.
  
• 
  Net filtration pressure.
  

• 
  Colloid osmotic pressure:
  

• 
  Exerted by plasma proteins.
  
• 
  Promotes fluid reabsorption into circulatory system.
  

Fluid Movement

Starling force = ( Pc + i) - (Pi + p)
Pc = Hydrostatic pressure in the capillary
i = Colloid osmotic pressure of the interstitial fluid
Pi = Hydrostatic pressure in the the interstitial fluid
p = Colloid osmotic pressure of the blood plasma.
Lymphatic System

Extensive network of one-way vessels

Provides accessory route by which fluid can be returned from interstitial to the blood

• 
  Lymph - interstitial fluid that enters a lymphatic vessel
  
• 
  Lymph vessels
  
  • 
    Formed from convergence of terminal lymph vessels (initial lymphatics)
    
  • 
    Eventually empty into venous system near where blood enters right atrium
    
  • 
    One way valves spaced at intervals direct flow of lymph toward venous outlet in chest
    

Functions

• 
  Return of excess filtered fluid
  
• 
  Defense against disease
  

• 
  Lymph nodes have phagocytes which destroy bacteria filtered from interstitial fluid
  

• 
  Transport of absorbed fat
  
• 
  Return of filtered protein
  

Intrinsic Regulation of Blood Flow (Autoregulation)

• 
  Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters
  
  • 
    Response to increased rate of metabolism
    
  • 
    Intrinsic receptors sense chemical changes in environment
    
  • 
    Vasodilator substances produced as metabolism increases
    
    • 
      Decreased 02:
      
    • 
      Increased C02:
      
    • 
      Decreased pH - Lactic acid.
      
    • 
      Increased adenosine/K+ from tissue cells
      

• 
  Myogenic control mechanism:
  

Occurs because of the stretch of the vascular smooth muscle - maintains adequate flow.

• 
  A decrease in systemic arterial pressure causes vessels to dilate.
  
• 
  A increase in systemic arterial pressure causes vessels to contract
  

• 
  Endothelium secretions:
  
  • 
    Nitric Oxide - Vasodilation
    
    • 
      NO diffuses into smooth muscle:
      
    • 
      Activates cGMP (2nd messenger).
      
  • 
    Endothelin-1 – vasoconstriction
    
• 
  Histamine release
  
• 
  Heat/cold application
  
• 
  
  

Extrinsic Regulation of Blood Flow

• 
  Sympathoadrenal
  
  • 
    Increase cardiac output
    
  • 
    Increase TPR: Alpha-adrenergic stimulation - vasoconstriction of arteries in skin and viscera
    
• 
  Parasympathetic
  
  • 
    Parasympathetic innervation limited, less important than sympathetic nervous system in control of TPR.
    
  • 
    Parasympathetic endings in arterioles promote vasodilation to the digestive tract, external genitalia, and salivary glands
    

Blood Pressure Regulation

Pressure of arterial blood is regulated by blood volume, TPR, and cardiac rate.

• 
  MAP=CO ´ TPR
  
• 
  Arteriole resistance is greatest because they have the smallest diameter.
  
• 
  Capillary BP is reduced because of the total cross-sectional area.
  
• 
  3 most important variables are HR, SV, and TPR.
  
  • 
    Increase in each of these will result in an increase in BP.
    
• 
  BP can be regulated by:
  
  • 
    Kidney and sympathoadrenal system
    

Short-Term Regulation of
Blood Pressure

• 
  Baroreceptor reflexes
  
  • 
    Change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure
    
• 
  Chemoreceptor reflexes
  
  • 
    Sensory receptor