Lecture 56 - Blood Pressure and its Regulation Flashcards
What questions do we ask when we see a patient with low BP?
- Is it genuine hypertension?
- is the heart rate appropriate for BP?
- What is the cardiac filling?
- does the heart contract normally?
- is the patient abnormally vasodilated?
What are the factors that effect BP?
TPR
CO
Blood volume
Elasticity
Describe the generalised tissue structure of veins and arteries
Tunica intima
Tunica media
Tunica externa
What is special about the structure of arteries?
Elastic tissue
For elastic recoil to move blood along the artery
Describe the structure of arterioles, and how this relates to the function
Smooth muscle
Endothelium
Smooth muscle for resistance
Describe the pressure changes from the heart to veins
Heart: Fluctuates from 5-120 mmHg with beating Arteries: pulse: 120/80 mmHg Arterioles: drops, pulse present Capillaries: around 40 mmHg Veunles: around 20 Veins: 20 down to 0
What is pulse pressure?
Strength of the pressure wave
Sys. - Dys.
What is MAP?
Average pressure responsible for driving blood forward
What unit is used to measure BP?
mmHg
What is driving pressure?
Pressure that pushes blood forward
Generated by the heart
What is transmural pressure?
Pressure difference between inside of vessels and outside of vessels
eg. Brain in skull: skull means that there is high transmural pressure
What is hydrostatic pressure?
Pressure in a column of water due to the height of the liquid
What are the origins of pressure in circulation?
Gravity
Compliance
Viscous resistance
Inertia
When does gravity affect pressure?
Gravity affects hydrostatic pressure when there is a height difference
ie when standing
Does gravity affect driving pressure?
No
What is the difference in pressure of the arteries and veins when standing and lying at any given point on the body?
85 mmHg
What is compliance?
The ‘expandability’ of the walls of the vessel
What sort of container has zero compliance?
What happens when we keep adding liquid to this container?
eg. A steel tube
Pressure increases
Volume does not increase
What happens when we add liquid to a container with infinite compliance?
Increase in volume
Pressure remains at zero
What happens when we add liquid to a container with finite compliance?
Increase in volume
Increase in pressure
(walls can expand)
Change in pressure with a given change in volume is greater if the compliance of the vessel is…
Lower
What is viscous resistance?
Sticky fluid stick to the side of the vessel and thus exert resistance.
What does blood viscosity depend on?
Fibrinogen concentration Hematocrit Vessel radius Linear velocity Temperature
How does vessel radius affect blood viscosity?
Lesser radius, greater viscosity
How does temperature affect blood viscosity?
Greater temperature, lower viscosity
What does the law of conservation of energy say about fluid pressure and velocity?
When blood velocity increases, fluid pressure must decrease
What causes the decrease in pressure of the blood when blood velocity increases?
Inertia
What is ‘Z’?
Impedance
What does vascular impedance depend on?
Flow resistance
Compliance
Inertiance
According to Poiseuille’s law, what is resistance affected by?
Length of tube
Viscocity
Radius of tube
What is conductance?
Inversely proportional to resistance
Bloop flow through a vessel at a given pressure difference
Where is total resistance highest?
Why?
Arterioles
Why is it important for the walls of the arteries to be elastic?
When blood flow in, the wall stretches out.
The elastic recoil then pushes the blood forward.
The elastin stretching is stored potential energy that is converted into kinetic energy
What happens when MAP is too high or low?
Low: dizziness, organ failure
High: stroke, aneurysm
What things determine MAP?
MAP = CO x TPR
(Cardiac output
TPR)
Also:
- total blood volume
- HR
- SV
Why must flow into a segment equal flow out?
If this is not maintained, BP will either be too high or too low.
How is MAP calculated?
1/3(sys - dys) + dys
What is DP?
Pressure gradient
ie P1 - P2
What functions does the movement of blood fulfill?
Heat transport
Gas
Nutrient
Wastes
What things help the movement of blood?
Heat (energy source) Elastic tissue Gravity Skeletal muscle Diaphragm
Of course, heart contraction
What is normal CO?
5 L/min
What is normal SV?
Stroke volume
70 mL
How is SV calculated?
EDV - ESV
End diastolic volume - End systolic volume
What factors affect stoke volume?
- preload
- contractility = inotropy
- afterload
What is preload?
EDV: end diastolic volume
What is there before systole
What is afterload?
ESV: end systolic volume
What is left after systole
Explain the Frank-Starling Law
As EDV increases, SV increases.
The greater the stretch of the cardiac muscle, ie the more blood coming into the heart, the heart will eject more blood in the next stroke.
At any one time, what percentage of blood is in the veins?
How about arteries?
Veins: 64%
Arteries: 13%
What determines preload?
Venous return determines EDV
What factors affect venous return?
Muscle pumps Respiratory pump Neural control (sympathetic tone vasoconstriction)
How does noradrenaline affect EDV?
NA increases EDV and thus SV
What does increased afterload mean?
decreased SV
Increased ESV
What does increased inotropy mean?
Increased SV
Decrease ESV
What does increased proload mean?
Increased SV
Increased EDV
What things control the heart rate?
Neural:
- Intrinsic: autorhythmic cells
- Extrinsic: ANS
Describe the sympathetic effect on HR
Increases contraction of the heart by:
- changing ion permeability
- so that the cells reach threshold potential sooner
- depolarisation
Describe the parasymathetic effect on HR
Decreased contraction rate of the heart by:
- changing ion permeability
- so that the cells take longer to reach threshold
(hyperpolarisation)
Which cells does the sympathetic and parasympathetic act on?
B-adrenoceptors and mAChR
Conduction through the internodal pathways
What happens to MAP when there is a decrease in TPR?
Decrease in MAP
What does an organ do to achieve greater blood supply?
Decreased arterial resistance
Relaxation of arterial smooth muscle
What compensatory mechanisms are effected to compensate for low MAP?
Increase CO
Increased TPR
Increased HR
Increase SV
What happens when CO decreases?
Pooling of blood in the legs
How does the body monitor CO?
pH
CO2
O2
BP
How is BP regulated over the short term?
Neural (ANS)
Baroreceptor reflex
Describe the baroreceptor reflex when standing up
- Decreased stretch detected by baroreceptors
- Central integration
- Activation of Sym. and deactivation of Parasym.
- Increased:
- HR
- Force of contraction
- TPR
4b. Decreased:
- atrioventricular conduction time
Where are the baroreceptors located?
Carotid sinus
Aortic arch
What are the normal effects of the parasympathetic division on the heart
Decreased HR
Increased antrioventricular conduction time
Describe the baroreceptor reflex when lying down
- Excess stretch detected by baroreceptors
- Central integration
- Activation of Parasym. and deactivation of Sym.
- Increased:
- AV conduction time
4b. Decreased:
- TPR
- HR
- Force of contraction
What do chemoreceptors detect?
Low pH
Low O2
What does low O2 and high pH stimulate?
Increased respiration rate
Increased BP
How is BP regulated over the long-term?
- Cardiac Output
- hormonal
- renal - TPR
- hormonal
- vascular structure
How do we increase CO over the long-term?
Increased ECF
Decreased renal salt excretion
How do we increase TPR over the long-term?
Hypertrophy of arterial wall
Vasoconstrictive hormones
Which receptors detect low BP?
Atrial baroreceptors
Describe the function of atrial baroreceptors
Monitor changes in central blood volume
–> Thirst stimulated
