Physiology 7 Flashcards
What is the difference between the resting autonomic tone of the heart and the peripheral blood vessels
Heart: Resting parasympathetic tone is needed to keep heart rate normal
Vessels: There is no resting parasympathetic tone.
What factors affect activity of the vasomotor centre?
Direct stimulation: CO2, hypoxia
Excitatory inputs: From cortex via hypothalamus; Pain; Carotid and aortic chemoreceptors
Inhibitory inputs: Cortex via hypothalamus; Lungs; Carotid, aortic and cardiopulmonary baroreceptors
Where is the vasomotor centre located?
Medulla oblongata
Describe and explain how the Valsalva manoeuvre affects systemic blood pressure
Phase I: Increased BP due to raised intrathoracic pressure
Phase IIa: Fall in BP due to compression of intrathoracic veins and thus reduced CO
Phase IIb: Increased HR and SVR due to hypotensive inibition of the baroreceptors
Phase III: On opening the glottis, CO is restored as intrathoracic pressure falls. BP initially falls however, due to aortic decompression. This is then counteracted by the baroreceptor reflex, normalising BP.
Phase IV: Combination of increased CO and baroreceptor inhibition leads to overshooting BP, causing baroreceptor stimulation and bradycardia - dropping BP
What would a valsalva manoeuvre in a person with autonomic neuropathy show?
Exaggerated BP fall during phase II due to lack of baroreceptor reflex
Lack of BP overshoot and thus reflex bradycardia in phase IV
Which vasoactive substances are released by the vascular endothelium?
Adenosine, bradykinin, histamine, nitric oxide - vasodilators
Thromboxane A2 - promotes plt activation
Prostacyclin - inhibits plt activation and is a vasodilator
Endothelins - very potent vasoconstrictors
What is the Windkessel effect?
The effect exerted by the arteries in converting pulsatile blood flow into constant, steady flow through elastic recoil during diastole
Relatively speaking, how much resistance do the capillaries contribute to SVR?
Very little, due to the very high total cross-sectional area for blood flow
What is the pressure range of the LVOT?
0-120mmHg
What is the pressure range of the arteries?
80-120mmHg
What is the pressure range of the arcade arterioles/collaterals?
60-80mmHg
What is the pressure range of the terminal arterioles?
40-60mmHg
What is the pressure range of capillaries?
15-35mmHg
What is the pressure range of the venules/post-capillary capacitance vessels?
8-12mmHg
Describe the structure of the arterial/venous wall
Tunica intima (squamous epithelial cells and surrounding subendothelial connective tissue)
Tunica media (circular elastic/connective tissue fibres and vascular smooth muscle)
Tunica adventitia (connective tissue, nerves, vasa vasorum)
What is the velocity of the ascending aortic blood flow?
approx. 1m/s (0.93m/s)
What is the role of the arterial system?
To convert high-velocity pulsatile flow to optimal low-velocity steady flow in capillary bed necessary for cellular exchanges
What are the functions of the vascular endothelium?
- Regulating basal vasomotor tone through vasodilators (eg. NO, PGI2, prostacyclin) and constrictors (endothelin, PAF)
- Acting as a non-thrombogenic surface (due to expression of heparan sulphate, protein C, protein S)
- Promoting laminar flow by presenting a smooth surface
- Regulating growth of surrounding connective tissue
What are the actions of Protein C, Protein S and heparan sulphate?
Proteins C and S are directly fibrinolytic and heparan sulphate acts by promoting fibrinolysis via antithrombin III
Along the length of a capillary, where is the vascular permeability greatest?
At the venous end
What is the velocity of capillary blood flow?
approx 0.05-0.1 cm/s
What is the approximate flow of crystalloid through a 20G cannula at 10psi?
59 ml/min
What is the approximate flow of crystalloid through a 18G cannula at 10psi?
103 ml/min
What is the approximate flow of crystalloid through a 16G cannula at 10psi?
225 ml/min
What is the approximate flow of crystalloid through a 14G cannula at 10psi?
290 ml/min
Where is turbulent blood flow found under normal pressures and circumstances?
How does this relate to response to an increase in pressure gradient?
The ascending aorta normally exhibits turbulent flow.
This means that flow is only increased in proportion to the square root of the increase in pressure gradient
Which factors affect blood viscosity?
Haematocrit
Temperature
Vessel diameter
Flow rate
How does vessel diameter affect blood viscosity?
In arterioles and capillaries, cells tend to occupy the central axial stream, effectively reducing the haematocrit (Lindqvist effect)
Effective blood viscosity in capillaries is no greater than for plasma alone
How does flow rate affect blood viscosity
Low flow rates increase interactions between cells and proteins, increasing viscosity
What are the main basal regulators of vascular tone?
NO and endothelins
How does NO cause its effect on vascular smooth muscle?
NO binds to guanylyl cyclase to cause SM relaxation
In what percentage of humans is the coronary blood flow right dominant?
50% right dominant
20% left dominant
30% equal
Into where does the coronary sinus drain?
Right atrium
What is the resting O2 consumption of the heart?
8-10 ml/100g
What is the normal pO2 of the blood in the coronary sinus?
2.4-2.7 kPa
sO2 25 - 40%
What is the normal O2 extraction of the myocardium at rest?
approx 60%
How does coronary blood flow relate to cardiac work?
Increased cardiac work increases oxygen consumption significanty. Threre is a close linear relationship between cardiac O2 consumption and coronary blood flow.
How does the balance of volume and pressure work affect cardiac efficiency?
Increased pressure work reduces cardiac efficiency significantly (ie. higher O2 consumption for a given CO)
Outline the phasic nature of coronary blood flow
In the right coronary circulation, although phasic, the average perfusion pressure is not significantly different between systole and diastole.
In the left ventricular circulation, blood flow during systole is significantly reduced, even reversed due to the high extravascular myocardial pressure. Thus the LV receives most of its perfusion during diastole.
How does tachycardia affect blood flow to the LV?
Reduced diastolic time reduces perfusion. This is counteracted by vasodilatation as a result of increased metabolic activity
How does oxygen extraction from the coronary circulation change with increased myocardial work?
It remains relatively constant, thus increased O2 demand can only be supplied by increased blood flow. This is achieved through change in vascular tone.
How does the coronary vasculature respond to increased metabolic activity?
Decreased coronary vascular resistance
How does the coronary vasculature respond to increased aortic pressures?
Increased coronary vascular resistance
What systems are responsible for regulation of coronary blood flow?
Vascular endothelium
Local metabolism
Neural and neurohumeral factors
What are the main mechanisms through which the vascular endothelium regulates decreased tone?
- Mainly through NO, which increases intracellular cGMP
- Prostacyclin also mediates vasodilatation through increased cAMP
- Bradykinin mediates relaxation through endothelium derived hyperpolarising factor (EDHF)
What are the main mechanisms through which the vascular endothelium regulates increased tone?
- Mainly through endothelin production through the action of endothelin converting enzyme (ECE)
- Also through thromboxane A2
What are the main mechanisms through which increased metabolism increases coronary blood flow?
- Increased adenosine concentrations through reduced regeneration of ATP
- Opening of ATP-sensitive K+ channels
What is the net effect of neural influences on the coronary vascular tone?
Net constrictive effect
in the denervated heart, a vasodilatory effect is noted
