CVS 5 (Control of cardiac output + peripheral circulation) Flashcards
How does the pressure difference between the ends of a vessel affect the flow of blood?
Pressure difference is proportional to flow:
- the greater the pressure difference, the greater the flow
Define flow:
The volume of fluid passing a given point per unit time
Define velocity:
The rate of movement of fluid particles along the tube
At a given flow, how is velocity related to the cross sectional area of a tube?
Velocity is inversely proportional to the cross sectional area (at a given flow):
-As the cross sectional area increases, velocity decreases
What kind of vessel has a small cross sectional area, but high velocity?
Large arteries or veins
What kind of vessel has a large cross sectional area, but low velocity?
Capillaries
What is the approx. cross sectional area of the aorta (in cm)?
~ 2.5 cm
Describe laminar flow:
Fluid travels in thin parallel layers, which each pass over each other.
Velocity is greatest in the middle of the vessel, and decreases towards the edges, where flow is stationary.
Where in a healthy body is laminar flow most likely to become turbulent flow, and why?
Ascending aorta, due to the high velocity of blood.
What kind of flow generates sound waves?
Turbulent flow
Define turbulent flow:
A fluid flow in which the velocity at a given point varies erratically in magnitude and direction
What is the relationship between viscosity and velocity?
Velocity is inversely proportional to viscosity:
- as viscosity increases, velocity decreases
How is the cross sectional area of a tube related to velocity?
Velocity is proportional to the cross sectional area of a tube:
- As cross sectional area increases, velocity increases
Whose Law relates flow (Q) to the viscosity (n) of the fluid, the pressure gradient across the tubing (P), and the length (L) and diameter(r) of the tubing?
Poiseuille’s Law
When does Poiseuille’s Law NOT apply?
In vessels smaller than arterioles
What may cause Hyperviscosity Syndrome?
- High plasma protein levels
- High RBC count
- High WBC count
How may Hyperviscosity Syndrome be treated?
- Plasmapheresis (if high plasma protein levels)
- Phlebotomy (if high WBC/RBC count)
How can pressure be calculated from flow and resistance?
Pressure = Flow x Resistance
How is resistance related to viscosity?
Resistance is proportional to viscosity:
- as viscosity increases, resistance increases
Do arteries have high or low resistance? Why?
Low resistance - small pressure difference between the ends of the vessels
Do arterioles have high or low resistance? Why?
High resistance - large pressure difference between the ends of the vessels
Do capillaries have high or low resistance?
Individually - high resistance
In total - low resistance (as connected in parallel)
Do veins and venues have high or low resistance?
Low resistance
When does flow tend to become turbulent?
- High flow velocity
- Low viscosity
- Irregular lumen of vessel
Define bruit:
An audible vascular sound associated with turbulent blood flow
Define capacitance:
The ability to store blood
Which blood vessels have the highest capacitance? Why?
Veins - most distensible
What is meant by the ‘Windkessel effect’?
The elastic arteries of the body smooth the pressure wave between systole and diastole, by storing blood between heart beats (capacitance)
What is the approx. pressure in arteries? (in mmHg)
95 mmHg
What is the approx. pressure in arterioles? (in mmHg)
60 mmHg
What is the approx. pressure in capillaries? (in mmHg)
35 mmHg
What is meant by ‘pulse pressure’?
What is the typical pulse pressure? (in mmHg)
The difference between systolic and diastolic blood pressures
~ 40 mmHg
What is the time period of systole (approx.)?
~ 0.3 s
What is the time period of diastole (approx.)?
~ 0.55 s
What is the time period of the cardiac cycle (approx.)?
~ 0.85 s
Why is vasomotor tone high at rest?
Low demand = low blood flow
Which branch of the nervous system controls vasomotor tone?
Sympathetic branch of Autonomic Nervous System
Which neurotransmitter is released on blood vessels to control vasomotor tone? Which receptor does it bind to?
Noradrenaline
alpha-1 adrenoceptors
How is vasodilation achieved?
- Reduce sympathetic innervation of blood vessel
- Production of vasodilator metabolites by metabolically active tissues (H+/K+/CO2/Adenosine/Lactate)
Name 5 vasodilator metabolites produced by metabolically active tissues:
1) H+
2) K+
3) CO2
4) Adenosine
5) Lactate
Why is vasodilation caused by local mediators very short-lived?
Vasodilation increases blood flow, which disperses and removes the local vasodilator metabolites
What is Reactive Hyperaemia?
The transient increase in blood flow following a brief period of ischaemia, due to increased amounts of vasodilator metabolites produced locally (to try to restore blood flow)
What is TPR?
Total Peripheral Resistance: sum of arteriolar resistances throughout body
What is the usual range of Central Venous Pressure (in mmHg)?
~ 2-6 mmHg
What 2 main factors determines arterial pressure?
1) Cardiac output
2) Total Peripheral Resistance
How does total-body exercise (initially) affect the Total Peripheral Resistance?
TPR falls as muscles require more blood = vasodilation of arterioles
If TPR falls and cardiac output remains constant, what happens to the arterial and venous pressures?
Arterial pressure falls
Venous pressure rises
If TPR rises and cardiac output remains constant, what happens to the arterial and venous pressures?
Arterial pressure rises
Venous pressure falls
If cardiac output rises and TPR remains constant, what happens to the arterial and venous pressures?
Arterial pressure rises
Venous pressure falls
If cardiac output falls and TPR remains constant, what happens to the arterial and venous pressures?
Arterial pressure falls
Venous pressure rises
Cardiac output is calculated via:
Heart Rate x Stroke Vol.
Define Stroke Volume:
The amount of blood pumped out of the left ventricle per beat:
End Diastolic Vol. - End Systolic Vol.
Describe the relationship between venous pressure and ventricular volume:
Venous pressure is directly proportional to ventricular volume:
- If venous pressure increases, the ventricular volume will increase
How is End-Diastolic Volume related to the Stroke Volume?
EDV is proportional to Stroke volume:
- If EDV increases, the Stroke volume increases (to a limit set by actin+myosin filaments)
What is Starling’s Law of the heart?
The Stroke volume increases with the filling of the heart (End-Diastolic vol.)
Define contractility:
The capability of causing contraction
Which 2 factors determines the force of contraction of the heart?
1) End-Diastolic Volume
2) Contractility
Which branch of the nervous system can affect cardiac contractility? What is its effects?
Sympathetic system
Increases cardiac contractility
What happens to the stroke volume if arterial pressure falls?
Stroke volume increases
During heart failure, HR automatically increases. Why should beta-blockers be administered?
Beta-blockers reduce sympathetic innervation
= slows HR
= increases diastolic time
= increases stroke volume
What is meant by ‘heart failure’ (or left ventricular failure)?
Left ventricle fails to contract forcefully enough to maintain a normal cardiac output and peripheral perfusion.
Where are the arterial baroreceptors located?
1) Carotid sinus (bifurcation of internal and external carotid arteries)
2) Aortic arch
Where are the low-pressure baroreceptors located?
1) Great veins
2) Pulmonary vessels
3) Walls of right atria and ventricles of heart
Describe the Bainbridge reflex:
Low pressure baroreceptors detect change in venous pressure:
- Alter parasympathetic and sympathetic innervation to SA node
- Alter heart rate
In which 2 ways can low pressure baroreceptors offset a change in venous pressure?
1) Bainbridge reflex (alter autonomic innervation of SA node)
2) Renal reflex (hormonal control of salt and water)
What will happen to the arterial and venous pressures if the heart stops?
Arterial pressure falls
Venous pressure increases
What is the physiological range for central venous pressure?
3 - 8 mmHg
Which 2 factors can change central venous pressure?
1) Cardiac output
2) Total peripheral resistance
Why do some people faint if they stand up too quickly?
Hypotension due to mass movement of blood to lower extremities
- Venomotor tone is not altered quickly enough to maintain adequate cerebral perfusion
Define pre-load:
The end diastolic volume that stretches the right or left ventricle of the heart prior to contraction
Define after-load:
The “load” that the heart must eject blood against, dependant on aortic and pulmonary pressures
In which area of the brain are the cardiac an vasomotor centres?
Medulla oblongata
