Blood pressure Flashcards
How would you calculate cardiac output?
CO = HR x SV.
HR can be measured by simple pulse or ECG.
Stroke volume can be detected invasively (tranluminally), or estimated via echo
What is the normal range (or a normal value) for Cardiac Output?
5 litres per minute in a healthy adult
During exercise cardiac output can increase as much as 5-fold
Describe the progression of electrical activity through the heart during a single beat.
Impulse begins in SA node. 4 internodal bundles relay depolarisation from SA node. 3 (anterior, middle, and posterior tracts) go to AV node; Bachmann’s bundle goes to the left atrium. By the time it has emerged from the AV node impulse has been delayed ~ 160 ms. Impulse exits AV node and enters ventricles at bundle of His. This splits into the left bundle branch and right bundle branch (which also continue to have smaller branches), which travel down the septum. At the septum the two branching systems go in opposite directions, superiorly to the right ventricle and superiorly to the left ventricle.
What is the normal range (or a normal value) for End Diastolic Volume?
120 ml
What is the normal range (or a normal value) for End Systolic Volume?
50 ml
What is the normal range (or a normal value) for Stroke Volume?
70 ml
What is the normal range (or a normal value) for Ejection Fraction?
55-70%
Anything below 45% is heart failure
What is the normal range (or a normal value) for Heart Rate?
60-100
What is the definition of stroke volume?
SV = EDV – ESV
What is the definition of ejection fraction?
Percentage of ventricular volume pumped out during a heart beat: SV / EDV
Give an example of a portal system.
Hepatic portal system – Food is absorbed in capillaries of gut, these capillaries assemble into portal vein, portal vein goes to liver and breaks into capillaries.
–Thus, liver sees nutrients (and dietary toxins such as alcohol) at high concentration before it is distributed throughout the body
•Could also mention kidney or hypothalamus-pitutiary
What is the normal cardiac output?
4.9 litres per minute
Name a range of causes of vasodilatation and vasocontraction
CNS,
Autonomic,
Endocrine
eg Exercise
Local regulation of pressure
Immune
Haemostasis
Name two forces inside a blood vessel that determine local vasomotor control
–Hydrostatic pressure
–Shear force
Name five effects of increased sympathetic activity that are associated with increasing blood pressure.
- Constrict BVs
- increased Peripheral Resistance
- Increased pulse Pressure
- increased Heart Rate
- increased Cardiac Output
- Increased ventricular contractility
- Renal artery constriction
- Decreased urine production
How much does the atrial systole add to the ventricular filling?
20% – 25%
What are the definitions of preload and afterload?
Preload is the degree of force (or tension) stretching the ventricular myocytes during the filling phase. it is related to venous return.
Afterload is the degree or force (or tension) the ventricular myocytes must overcome to successfully eject blood during systole. Afterload is increased by obstacles (eg stenotic valves) or by back pressure (eg hypertension)
What is the equation determining how much pressure changes along the length of a blood vessel?
ΔP = flow (Q) x R
Where R equals resistance, Q equals flow, and ΔP equals change in pressure.
Or could use Poiseuille equation:
ΔP = Q x (8 η L / π r4)
Draw a graph illustrating the Frank-Starling Law (Starling’s Principle) relating force of contraction to preload and afterload. Include labels for the axes.
The X-axis could be labelled end diastolic volume or right atrial pressure (preload could not be measured). The Y-axis could be labelled cardiac output cardiac work or stroke volume
What causes pressure overload and what causes volume overload?
Pressure overload is caused by elevated afterload, volume overload is caused by elevated preload.
Would decreased aortic pressure lead to decreased, increased or the same levels of preload in the left ventricle?
Decreased preload
Name two compensatory mechanisms for patients to cope with immediate issues with shock.
- Tachycardia
- Tachypnoea
- Reduced GFR or reduced urine output
What are the unique features of coronary circulation?
It has an incredibly high demand for the weight of the tissue.
Most blood flow occurs during diastole – because during systole the heart muscle contracts so much that it crushes its own blood vessels, thus preventing flow.
Describe circulatory volume vs. circulatory capacity, and explain their relationship to shock.
Successful delivery of blood to guarantee perfusion requires a level of blood pressure that can push blood forward without ripping the blood vessels. Pressure homeostasis requires a match between circulatory volume and circulatory capacity. Circulatory volume is the total amount of plasma fluid, which is determined by fluid intake and the excretion the kidneys. By contrast, circulatory capacity is a term that suggests the total amount of blood that can fit in all the blood vessels in the body, and this depends upon vasodilatation, etc.
How would you calculate peripheral resistance?
Total peripheral resistance = BP / CO. You can get a good estimate of deltaBP from brachial arterial pressure.
How do you calculate pulse pressure?
Pulse pressure = systolic pressure – diastolic pressure
How do you calculate mean arterial pressure?
MAP = P(diastolic) + (0.333 x pulse pressure)
Which vessel is subject to greater wall tension (and what is the result to the structure of the vessel): the aorta or a capillary?
The aorta has a much bigger radius (as well as a much higher pressure), so its wall has to withstand much more tension. Thus the walls of the aorta are much thicker than the walls of the capillary
Where are the arterial baroreceptors, and what nerves are used to send their signals to the central nervous system?
Aortic arch (innervated by the Vagus nerve (CN X))
Carotid sinus of both left and right internal carotid arteries (innervated ultimately by Cranial Nerve IX)
What causes hypertension and what are the controlling mechanisms of high blood pressure?
Mismatch between blood volume
and circulatory capacity.
Autonomic nervous system & brain. Humoural component & endocrine. Local control. OR:
Kidney controls blood volume, blood vessels control circulatory capacity and heart controls pressure
What is the thoracic pump?
It is a physiological mechanism
Whereby the skeletal muscles of respiration increase the pumping of blood through the circulatory system
During inspiration, the intrathoracic pressure is negative (suction of air into the lungs), and abdominal pressure is positive (compression of abdominal organs by diaphragm). This makes a pressure gradient between the infra- and supradiaphragmatic parts of v. cava inferior, “pulling” the blood towards the right atrium.
What are the main features of the pulmonary circulation that differentiate it from the systemic circulation?
The effects of O2, CO2 and PH are opposite of those in the systemic system.
It has a very low vascular resistance.
It makes angiotensin converting enzyme.
High capillary density
Acts as a filter (and protects against emboli
Briefly explain the effects of CO2, pH and O2 on vasomotor activity in the pulmonary circulation
- The effects of O2, CO2, pH are opposite to those in systemic circulation
- In pulmonary system, if O2 is low in a region of the lung, the arterioles constrict
- (likewise if CO2 or H+ are high, vasoconstriction)
- Thus, Poor ventilation leads to reduced perfusion
•
•This minimises the amount of blood that is poorly oxygenated
What are the major cardiac diseases that high blood pressure can lead to?
Aneurysm or stroke. Myocardial infarct, kidney failure, heart failure, cardiac hypertrophy.
What is orthostatic hypotension?
•Low BP on standing
–decreased venous return
–
•Dizziness or syncope
What is cardiogenic shock?
- Critically low perfusion (associated with low blood pressure)
- Caused because the heart is not pumping blood fast enough or with enough force
- Medical emergency
- Affects critically ill patients
- Example: haemorrhagic shock
What would happen to pressure along the length of a blood vessel if the radius became larger?
If the radius of a length of a blood vessel increased, the resistance along that stretch of the blood vessel would decrease, so the flow rate in the vessel would tend to immediately increase, and the pressure would immediately decrease. If the end of that vessel connected to a capillary with a stable amount of back pressure, a new equilibrium would occur in which slightly faster flow and lower pressure would exist
Note that the rate of pressure falling may be lower because the resistance has decreased (although the flow rate has increased)
What would happen to the fall in pressure along the length of a blood vessel if you replaced a resistance artery with a capillary?
The capillary has much higher resistance, so flow would be much slower. At the entrance to the capillary, there would be a large loss in pressure (and kinetic energy) associated with the friction of getting so much fluid into an capillary
What is the relationship between the radius of a blood vessel and the flow rate of plasma going through it?
The relationship between how much fluid will flow through a tube vs the dimensions of the tube and the forces in the tube. ΔP = 8 Q ηL / π r4 where nu = viscosity, r = radius, L = length of tube, and Q = flow
This is called Poiseuille’s Law
Would a capillary be more likely to burst than a resistance artery, and why?
Wall tension is proportional to be pressure and radius. Normally a capillary has a much lower pressure and a smaller radius than an arteriole, so that although it has less wall strength, the tension is usually so much lower that the capillary wall will not rip.
Plainly this depends on the pressure and wall tension. For identical pressures, a capillary has a much smaller radius, so the tension might be lower. However, the higher pressure in the arteriole might be too much for the capillary. The capillary definitely has less wall strength, and for identical amounts of tension, the capillary’s wall would rip first
What would happen to the resistance of a blood vessel if the radius became larger?
The resistance would shrink dramatically (to the power of 4) as the radius becomes larger. This is from Poiseuille’s law.
Which would create more resistance: doubling the number of capillaries (ie in parallel) or doubling their length?
More resistance would be created by doubling the length. Doubling the number in parallel would actually halve the resistance
If you decreased peripheral resistance, what would happen to pressure?
Decreased peripheral resistance leads to decreased pressure
If you increased compliance, what would happen to flow?
The pulse pressure would be smaller (because the wall would absorb energy during systole and release it during diastole).
This would mean the flow would be slower during systole, but might be higher during diastole
Flow slows down in more compliant vessels because the pressure (ie force) can not only push the rest of the blood forward, but it can also stretch the wall, expending energy while essentially storing both the energy and the blood volume in the now stretched vessel.
If you were to imagine an analogy between blood flow through the aortic valve and ion flow across a cell membrane, what would be equivalent to blood pressure, flow, and resistance?
- Ionic flow (current) à blood flow
- Transmembrane voltage à pressure difference across the valve
- Electrical resistance à hydrostatic resistance (both decrease when something opens)
What determines resistance through a blood vessel vs resistance across a cell membrane?
•Blood vessel: decreased by increasing radius, increased by length, obstructions, turbulence (bends), viscosity
•
•Cell membrane: decreased by channel openings, holes; increased by lipid composition
During exercise systolic BP increases while diastolic BP decreases (or stays the same). Why?
During exercise sympathetic activity has the following effects: increased myocardial contractility and increased heart rate (leading to greatly increased cardiac output), and some vasoconstriction (especially to kidney and splanchnic circulation), but also some vasodilatation (esp. to skeletal muscles). The net effect on peripheral resistance is a slight reduction. As a result, during systole the blood pressure is much higher (because of the increased contractility of the heart), but the additional blood flow is easily conducted to the muscles, reducing back pressure, so that during diastole the decreased peripheral resistance leads to a net reduction in pressure
