Integrated responses of the circulation

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Effects of posture

How does gravity influence intravascular pressures?

  • When recumbent, gravitational force is constant
  • when standing, dependent (below the heart) extremities have pressure increased by ρgh, whereas areas above the heart are decreased
  • both arterial and venous supplies are affected by the same force, so no net hinderance of blood flow around circulation

Why do postural changes (i.e., gravitational stresses) create a ?problem? on the venous side of the circulation and not on the arterial side?

  • Because of venous pooling
  • change in transmural pressure of veins will result in pooling of up to 500mL (venous capacity increases)

Describe the effects of upright posture on cardiac output, arterial pressure, and capillary pressure in dependent extremities (areas below the level of the heart)

  • Venous pooling of blood happens
  • causes reduction in cardiac filling, and therefore output and arterial pressure
  • capillary pressure also increases, resulting in net filtration
  • filtration results in swelling, as with ankles

How does upright posture affect the circulation in those areas above the level of the heart?

  • Arterial and venous pressure decrease
  • intracranial venous pressure becomes subatmospheric
  • because cerebral veins are enclosed in a rigid, fluid-filled skull, estravascular pressure falls also, and intracranial veins do not collapse
  • therefore, perfusion pressure in brain is kept the same

List three factors that tend to oppose venous pooling and compensate for postural changes. Which factors act to reduce the loss of fluid from the vascular system?

Factors that oppose venous pooling

  • neurogenic mechanisms - baroreceptor reflexes (heart effects, TPR changes, etc)
  • extravascular support - contraction of postural muscles around veins decreases venous capacity
  • muscle pump - contraction of skeletal muscles tend to ?milk? venous blood toward the heart

Factors that reduce loss of fluid

  • neurogenic mechanisms increase arteriolar tone to increase TPR; reduce capillary pressure, therefore reducing tendency toward filtration
  • muscle pump contributes the most toward reduction of capillary pressure

List circulatory conditions that may cause syncope. Describe the physiological basis for syncope in each case

  • Vasovagal syncope - neurogenic vasodilation and bradycardia
  • postural hypotension - instability of vasomotor reflexes due to nervous system dysfunction or pharmacological factors
  • carotid sinus syncope
  • hypovolemia - low blood volume
  • mechanical reduction of venous return - cough; valsalva maneuver; varicose veins
  • obstruction of ventricular outflow - aortic stenosis
  • pump failure - MI
  • arrythmias - electrical instability; bradyarrhythmias, tachyarrythmias

Exercise

Describe the concept of ?maximum oxygen consumption? in the assessment of cardiovascular fitness

  • a good measure of cardiovascular fitness
  • limit of oxygen consumption by an individual under certain controlled conditions
  • VO2max - 35-50ml/kg min in untrained; 50-65+ml/kg min in trained

Using the Fick Principle, write an equation which describes the response of the circulation to maximal exercise

  • VO2max = COmax(a - vO2 difference)max
  • limitation of oxygen uptake is circulation-related; this is why fick principle means anything here

What cardiac factors are responsible for changes in cardiac output during exercise?

Heart rate

  • linear increase with work load to maximum of ~ 200 bpm
  • major factor in untrained persons

stroke volume

  • increases by ~ 50% at 40% of VO2max
  • due to end diastolic volume and contractility

How do arterial pressures change during exercise? How are these changes explained?

  • Systolic arterial pressure - dramatic increase due to increased stroke volume and faster rate of ejection
  • diastolic arterial pressure - slight increase, decrease, or no change
  • MAP - increase of 15 - 20 mmHg

What is the importance of the redistribution of cardiac output during exercise? Which vascular beds participate in this response?

  • Redistribution of cardiac output is important so that perfusion can reach organs/muscles in need
  • increased resistance in abdominal organs partially offsets the tremendous decrease in resistance in working muscles, helping keep arterial pressure sufficiently elevated
Organ Cardiac output (%) during exercise Changes in flow from resting level
Muscle and skin Up to 80-85% up up
Heart 5% up
Brain 3-4% no change
Abdominal organs down as VO2 up down as VO2 up

How is skin blood flow controlled during exercise?

  • Skin blood flow increases during exercise in response to temperature fluctuations
  • increases due to decreased sympathetic control of cutaneous vessels

What mechanisms are involved in the overall responses to exercise?

  • Decreased parasympathetic activity, increased parasympathetic activity, and local control of blood flow explain most of the cardiovascular responses to acute exercise
  • central motor command - origin of increased sympathetic activity; inhibits parasympathetic activity
  • peripheral reflex mechanisms - fine tuning of the sympathetic response.

What is the importance of increased extraction of oxygen during exercise? What mechanisms are involved?

  • Increased extraction happens due to increased temperature, larger capillary surface, and [H+] increase
  • increased extraction also happens in the viscera, where there is less flow

Summarize the major changes associated with training

  • VO2max can be increased by training; decreased by bedrest
  • stroke volume, peripheral resistance and ability to extract oxygen
  • training brachycardia (reduced heart rate) with increased stroke volume