Control of Blood Flow and Nutrient Exchange Flashcards
What physiological principle explains the need for blood flow?
Fick’s principle.
What is Fick’s principle?
Oxygen consumption = Blood flow x (arterio-venous oxygen difference)
O2 Consumption = Blood Flow x (aO2 - vO2)
What component of Fick’s principle is affected by cardiovascular system function?
Blood flow
Which systems in the body affect the aO2 component of Fick’s principle?
- Respiratory system
2. Hematological system (Hb)
What symptoms result from decreased oxygen supply?
Dizziness, fainting, and shortness of breath.
What are the 3 determinants of blood flow (cardiac output)?
- Changes in pressure gradients
- Changes in resistance
- Changes in stroke volume and heart rate
What are the 5 determinants of stroke volume?
- Preload (Frank-Starling)
- Contractility
- Blood volume
- Afterload
- Venous return
How is blood flow calculated?
Blood flow = Pressure gradient / resistance of conduit
F = (P1-P2)/R
How does resistance affect upstream and downstream pressures?
Increased resistance always causes higher pressures upstream and lower pressures downstream.
How is the pressure gradient calculated?
Pressure gradient = Flow x Resistance
What is right atrial pressure normally?
0 mmHg
How is MAP calculated?
MAP = DBP + ⅓(SBP-DBP)
Diastolic blood pressure plus ⅓ pulse pressure
How is pulse pressure calculated?
SBP - DBP
How is cardiac output calculated?
CO = SV x HR
What is MAP minus right atrial pressure equivalent to?
MAP - RAP = CO x TPR (= cardiac output x total peripheral resistance)
What are some of the extrinsic factors that cause vasoconstriction?
- ANS - SNS - NA (alpha-1 receptors)
2. Circulating Angiotensin II
What are some of the extrinsic factors that cause vasodilation?
- ANS - SNS - Na (beta-2 receptors)
2. ANS - PNS - Ach-NO
What are some of the intrinsic (local) factors that cause vasoconstriction?
- Endothelial factors: endothelin
- Local metabolites: oxygen
- Prostaglandins
- Myogenic responses
What are some of the intrinsic (local) factors that cause vasodilation?
- Endothelial factors (NO)
- Local metabolites - adenosine & carbon dioxide
- Prostaglandins
- Histamine
What are determinants of stroke volume based on?
Frank-Starling effects
Determinants of stroke volume: What is the effect of postural changes?
Results in changes in gravity which affect venous return
Determinants of stroke volume: What are some things that affect intra-thoracic pressure?
- Breathing
- Holding breath
- Valsalva maneuver
Determinants of stroke volume: What is the effect of increased intra-thoracic pressure?
Decreases venous return = decreased filling volumes = decreased stroke volume
Determinants of stroke volume: What is the effect of the skeletal muscle pump?
Increases venous return = increased filling volumes = increased stroke volume
Determinants of stroke volume: What is the effect of changes in blood volume?
Increased blood volume = increased stroke volume and vice versa
Determinants of stroke volume: What is the effect of constriction or dilation of capacitance vessels (veins)?
If capacitance vessels constrict they hold less blood, therefore more blood returns to the heart = increased filling volumes = increased stroke volume
Determinants of stroke volume: What is the effect of decreased blood ejected from ventricle (leaves more blood behind)?
Blood left behind adds to filling volume = increased stroke volume.
Determinants of stroke volume: What is the effect of restriction of cardiac filling by fluid in the pericardial space or stiff wall of the heart?
Restrict expansion of ventricle = decreased filling = decreased stroke volume
Determinants of stroke volume: What is the effect of a heart rate exceeding 180 beats per minute?
Does not allow enough time for heart to adequately fill = decreased filling = decreased stroke volume
What causes increased contractility?
SNS effects, beta-agonists, increased cAMP (PDEI)
What causes decreased contractility?
Decreased oxygen, cell death, pharmacological depressants
How is tension calculated (afterload)?
(Pressure x Radius) / (2 x wall thickness)
What are the results of an increased afterload?
Increased O2 demand
= Reduced systolic function
To maintain blood flow, what two things are needed?
- A well functioning pump (heart)
2. Low resistance
What results in oedema?
- Increased capillary hydrostatic pressure
2. Decreased capillary oncotic pressure
What are the three causes of decreased oxygen delivery to tissues?
- Decreased blood flow (cardiac output)
- Decreased oxygenation of blood (resp)
- Decreased oxygen carrying capacity (haem)
What are the causes of decreased cardiac output?
- Increased resistance (vasoconstriction)
- Decreased blood pressure (heart failure, shock)
- Decreased heart rate (ANS)
- Decreased stroke volume (decreased venous return, decreased blood volume, decreased contractility, increased afterload, decreased filling)
