Circulatory system and BP

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Describe the main features of the different blood vessels

Explain how arteries function as a pressure reservoir

Explain how the sounds of Korotkoff are used to measure blood pressure

Distinguish between pulse pressure, arterial blood pressure and mean arterial blood pressure

Describe the mechanisms by which blood flow to different organs/tissues can be regulated

Describe the factors regulating venous return

Describe the factors that affect mean arterial blood pressure

Describe both short and long term control of blood pressure

Large Artery

  • Functions as a pressure reservoir
  • Elastin, for elasticity.
  • Collagen, for tensile strength
  • Muscles, for constriction and dilation

Arteriole

  • Main resistant vessels

Capillary

  • Only has endothelium

Large Vein

  • Functions as blood reservoir
  • Endothelium
  • Elastin fibers
  • Smooth muscle
  • Collagen fibers

Reconditioning organs receive more blood than what they need to perform homeostatic adjustments to blood.


Can withstand temporary reduction in blood flow.

  • Flow rate: Volume of blood passing through per unit time. Proportional to pressure gradient and inversely proportional to resistance
  • Blood pressure: Force exerted by the blood against a vessel
  • Pressure gradient: Difference in pressure between the beginning and end of a vessel.
  • Resistance: Friction between the blood and vascular wall.
  • Arteries serve as rapid-transit passageways to the organs and as a pressure reservoir (during diastole).
  • Heart contracts to pump blood into arteries and relaxes to refill with blood from veins

Blood pressure is the force exerted by blood against a vessel wall

  • Depends on volume of blood within the vessel; and
  • Distensibility of the vessel walls (capacity to swell from pressure inside)

Systolic pressure

  • Maximum pressure when blood is ejected into the arteries

Diastolic pressure

  • Minimum pressure when blood is draining into the rest of the vessel during diastole



    Indicated as systolic/diastolic (120/80 mmHg)

The green line in notes is the curve pressure

Korotkoff Sounds

  • First sound: Systolic Pressure
  • Last sound: Diastolic Pressure
  1. Wrap around the arm and inflates. No sound heard coz blood simply cannot flow through.
  2. Pressure slowly released, upon hearing the first sound, that's the peak systolic pressure
  3. Pressure continues to be released. At the last sound, that's the diastolic pressure. After this, there should be no more sounds.

Pulse: Difference between systolic and diastolic


  • Can be felt over arteries
  • Strong pulse means big difference between systolic and diastolic pressure

Mean Arterial Pressure: Main driving force for blood flow


  • Average pressure driving blood forward
  • Is the pressure that is monitored and regulated by body’s blood pressure reflexes (homeostasis) 
  • MAP = Diastolic pressure + 1/3 pulse pressure; or MAP = 2/3 diastolic + 1/3 systolic

Example: if car drives 80km/h for 40min and 120km/h for 20min. Average speed is 93km/h and not 100km/h.

Capillaries: Ideally suited to serve as site of exchange

  • Very thin walled, extensive branching and proximity of almost every cell to a capillary
  • RBC in a single file
  • Blood velocity in capillaries is very slow as compared to arteries
  • Total flow rate (~5L/min) is the same throughout circulatory tree (due to increased surface area)

Has a single layer endothelial cell


  • Allows lipid soluble substances to pass through
  • Allows small water soluble substances to pass through, like sodium, potassium

Pre Capillary Sphincters: Tissues that are metabolic active have more capillaries


  • Many capillaries are not open under resting conditions
  • Capillaries have no smooth muscles...
  • Thus, pre-capillary sphincters serve to control blood flow
  • Sensitive to local metabolic activity changes

↑ metabolic activity → sphincter relax → more open capillaries → increased blood flow to active tissues

Regulation of Blood Flow and Distribution

Vasoregulation


Vasodilation
Increased blood flow in the arteriole wall
↓ O2
↑ CO2
↑ Nitric oxide (very potent)
↓ Sympathetic activity
Histamine
Heat


Vasoconstriction
Decreased blood flow in the arteriole wall
↑ O2
↓ CO2
↑ Endothelin (very potent)
↑ Sympathetic activity
Cold
Vasopression and angiotensin II (potent vasoconstrictor)

Slide 20

Venous Return


  • Veins act as a blood Reservoir. To increase venous return, the body "squeezes the reservoir" hence decreasing venous capacity.


  • Blood returning to the heart

  • Veins have
    • Low resistance, less elastic recoil, less smooth muscle
    • Slow transit time through it (act as a blood reservoir)
    • Storage, as moving not as quickly to the heart to be pumped out
  • Venous capacity (amount of blood veins can hold)
    • Depends on distensibility of vessel walls and other external pressures such as skeletal muscles squeezing it 
  • During exercise
    • Stored blood needed – increase in venous return

Various Conditions that Constrict or Dilate Vessels

BP stays largely constant in arteries, but decreases as it goes to the arterioles, capillaries, venules and veins.

Slide 17


  • Blood flow rate is constant
  • Velocity is lowest as blood flows to capillaries
  • Cross sectional area is highest as blood flows to capillaries

Local control of arteriolar (arterioles)
radius

The fraction of the total CO delivered to each organ varies depending on demands for blood

Differences in flow to organs are determined by differences in vascularization and differences in resistance offered by arterioles supplying each organ

Extrinsic Control of arteriolar radius


  • Extrinsic control of arteriolar radius is important in regulating blood pressure, and influence of total peripheral resistance (TPR, total resistance offered by systemic peripheral vessels) on mean arterial pressure


  • Sympathetic fibers supply arteriolar smooth muscle everywhere


  • Increased sympathetic activity leads to generalized vasoconstriction

  • Decreased sympathetic activity leads to generalized vasodilation


  • Cardiovascular control center control the sympathetic output

  • Adrenal hormones
    • Norepinephrine produces generalized vasoconstriction
    • Epinephrine reinforces local vasodilatory mechanisms in tissues
  • Vasopression and angiotensin II
    • Vasopressin maintains water balance
    • Angiotensin II regulates salt balance

Local controls overriding
sympathetic vasoconstriction


E.g. Riding bicycle (exercise)
→ increased sympathetic activity
→ generalized vasoconstriction, but...
→ local metabolic activity in leg muscle induces vasodilation (override)
→ more blood to leg muscle
→ lesser blood to arm muscles and viscera etc


No parasympathetic innervation
to arterioles

Blood Pressure


  • Affected by cardiac output (heart rate, stroke volume) and


    total peripheral resistance (arteriol radius, blood viscosity)


  • Needs to be closely regulated


    • High enough (adequate pressure) to maintain blood flow and tissue perfusion (fluid exchange)
    • Not too high to overwork heart and cause vascular damage and rupture of small vessels

Venous return affected by sympathetic innervation

  • Vasoconstriction leads to higher venous return, as it is "squeezed" more.

Skeletal Muscle activity

  • When muscle contracts during activity
    • Compress veins
  • Decreases venous capacity
    • Increasing venous return

Gravity


  • Lying down, equal

  • Standing up


    • Vessels below heart subject to gravity (pressure caused by weight of blood from heart to vessel)
    • Distensible veins yield under hydrostatic pressure, leading to ↑ capacity
    • In leg, post-capillary blood pools in extended veins, hence ↓ VR, ↓CO
    • Blood to brain reduced, resulting in fainting
    • Walking helps to push blood to heart

  • Postural Hypotension


    • Triggers sympathetic venous vasoconstriction, driving some of the pooled blood forward
    • Skeletal muscle pump “interrupts” column of blood
    • Sympathetic venous vasoconstriction cannot completely compensate without skeletal pump

  • Valves stop blood from going backwards:


    • One-way valves spaced 2-4 cm away
    • Also help counteract gravity (minimize back flow)
  • Varicose veins
    • Incompetent venous valves (can no longer support the column of blood above it and collapse)
    • Aggravated by frequent, prolonged standing

Other factors:


  • Respiratory activity
    • Pressure within chest cavity is 5 mmHg less than atmospheric pressure
    • External pressure gradient between the lower veins (atmospheric pressure) and chest veins (less than atmospheric pressure)
  • Cardiac suction
    • Ventricular contraction
      • AV valves closed but drawn downward, enlarging atrial cavity so atrial pressure transiently drops lower than 0mmHg – sucking more blood into atria
    • Ventricular relaxation
      • AV valves open, rapid expansion of ventricles creates a suction effect sucking blood from atrium and veins

See the charts

Short Term Regulation of BP (Baroreceptor Reflex)


  • Any change in Mean Arterial Pressure will trigger Baroreceptor Reflex

BP ↓

  • Detected by baroreceptor - Cardiovascular center
    • ↑ sympathetic and ↓ parasympathetic
    • ↑ HR
    • ↑ SV,
    • Arteriole vasoconstriction (↑ total peripheral resistance)
    • Venous vasoconstriction (↑ CO)
    • ↑ BP

Long Term Regulation of BP


  • Regulating blood volume
  • ↓ blood volume results in ↓ arterial BP
  • Major compensation is reabsorption of fluid by kidney (salt and water balance)
  • Renin-Angiotensin system of the kidneys