CAM201 Lecture Notes - Lecture 5: Afterload, Vasodilation, Arteriole
Learning Objectives
• Describe how blood flow is regulated including the concept of autoregulation
• Define cardiac output and describe its determinants
• Understand the importance of the Frank-Starling mechanism in matching cardiac output to
venous return and tissue blood flow requirements
• Identify the determinants of stroke volume, including preload, contractility and afterload
• Revise the control of heart rate
Cardiac Output
• At rest:
• SV = 80 ml/beat
• HR = 60 beats/min
• CO = 4.8 L/min
• This is the average resting CO
• CO can increase by a factor of 5-7 fold during maximal exercise
• You can also think of CO as the total blood flow through the systemic or pulmonary circulation
• Over time:
1. Blood flow through the systemic circulation (left) must be equal to blood flow through
the pulmonary circulation (right)
2. CO must be equal to venous return (except for minor adjustments over a few beats)
Total Systemic Blood Flow
• Organ circulations are arranged in parallel with each other
• CO - the sum of all of the organ/tissue flows
• Tissue blood flow (supply) is adjusted according to the metabolic demands of the tissue
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Regulation of Organ Blood Flow
Mechanisms of Matching Flow to Demand
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1. Special receptors: skeletal muscle arterioles dilate in response to moderate levels of
adrenaline
2. Metabolic control: diffusion of metabolic waste (adenosine, ADP, CO2, K+ etc) from active
cells causes vasodilation
3. Oxygen deficiency: oxygen removal from blood by active cells reduces local blood oxygen ->
vasodilation
4. Nitric oxide: shear stress in arteries and arterioles causes their lining endothelial cells to
release Endothelium Derived Relaxing Factor (EDRF) -> vasodilation
5. Longer-term control mechanisms: capillary density changes (angiogenesis in skeletal muscle
and some other organs)
Arterial Pressure and Blood Flow
Autoregulation of Blood Flow
• Why is flow not a passive reflection of pressure changes?
• METABOLIC MECHANISM: a restatement of the metabolic and oxygen mechanisms shown
before
• MYOGENEIC MECHANISMS: stretch-induces a reflex in vascular constriction, prominent in
arterioles
Cardiac Output and Organ Flow
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Document Summary
Identify the determinants of stroke volume, including preload, contractility and afterload: revise the control of heart rate. Total systemic blood flow: organ circulations are arranged in parallel with each other, co - the sum of all of the organ/tissue flows, tissue blood flow (supply) is adjusted according to the metabolic demands of the tissue. Autoregulation of blood flow: why is flow not a passive reflection of pressure changes, metabolic mechanism: a restatement of the metabolic and oxygen mechanisms shown before, myogeneic mechanisms: stretch-induces a reflex in vascular constriction, prominent in arterioles. Blood flow regulation at tissue level: called vasomotion, vasomotion is the rhythmic distribution of blood flow in a tissue driven by changes in metabolism. In effect, flow can be directed to a capillary group or away from it. In essence, some capillaries receive no blood flow for periods of time: this is most apparent in skeletal muscle at rest where capillary blood flow only acts to meet nutrient demand
