Blood Flow 1 Flashcards
(52 cards)
1
Q
Blood flow-Ohm’s law: watch lecture at 29 mins
A
2
Q
What is the equation for pressure and how is it determined using arterial and venous pressure?
A
Flow x resistance
Pressure = Pa - Pv (Pa means arterial pressure, Pv venous pressure)
3
Q
Pressure is a ___
A
Gradient
4
Q
What is the driving force for pressure?
A
The pressure gradient (the pressure on the venous side is typically around 0 and varies a lot on the arterial side)
5
Q
A
6
Q
What is the equation for blood pressure?
A
Blood pressure = cardiac output x TPR (total peripheral resistance)
7
Q
A
8
Q
What is total peripheral resistance (TPR) due to?
A
Mostly due to degree of vasoconstriction in the arterioles
9
Q
How would vasodilation affect TPR?
A
It would decrease
9
Q
How would vasoconstriction affect TPR?
A
It would increase
10
Q
TPR is also called ___
A
SVR (systemic vascular resistance)
11
Q
Pouseille’s Law (resistance)
A
12
Q
The longer the tube, the ___ the resistance
A
Greater
13
Q
How does having more red blood cells (higher hematocrit) affect blood viscosity?
A
It would increase it
14
Q
How does viscosity affect resistance?
A
It increases it
15
Q
Resistance is inversely proportional to ___
A
- The radius to the fourth power
- As the radius gets smaller, resistance gets much larger
16
Q
Resistance in parallel
A
- There isn’t just one capillary connected to the aorta- many arteries branch off –> many arterioles –> many capillaries
- As the radius of these blood vessels gets smaller and smaller, the number of parallel pathways through which blood can flow gets greater
- This makes it difficult to determine where the most resistance is in the system (it’s an interplay between the radius of the blood vessels getting smaller and the number of parallel pathways getting bigger)
17
Q
How can you calculate the total resistance of blood vessels in parallel?
A
Total resistance is less than 1
17
Q
Pressure trace
A
- Starts in the left ventricle and goes out to the periphery
- Pressure drops when there’s more resistance??
- The solid line is the average pressure
- More resistance in the arterioles so pressure drops. Resistance at capillaries so pressure drops.
18
Q
Where is the biggest pressure drop/where there is the most resistance?
A
- In the arterioles
- Capillaries have more parallel pathways
19
Q
Capacitance of blood vessels (RC circuits) (very different in arteries and veins)
A
Capacitance of arteries:
- Affects pulse pressure
- Affects flow due to change in resistance
Capacitance of veins:
- Affects stroke volume
- DOES NOT AFFECT RESISTANCE!
20
Q
Is there much resistance to flow in veins?
A
Not much because the internal diameter is very big
21
Q
How do changes in the walls of veins affect blood flow back to the heart?
A
- If the walls of the vein are very flaccid, the blood is not effectively pushed back to the heart
- When the walls tense up a bit, blood gets back to the heart much faster
- This is one of the biggest things that changes during exercise
22
Q
Capacitance of arteries
A
- (a) When the ventricle stretches, it pushes blood into the closed set of tubes with resistance downstream. Some of that energy is absorbed in the walls.
- (b) When the ventricle relaxes and fills with blood coming from the atria, the aortic valve closes, that pressure that was loaded into the walls discharges in between beats to keep flow going between beats.
23
Blood pressure over time graph- why does blood pressure not spike really high and go really low
Because of the stretchiness of the arteries
24
Graph showing changes in blood pressure over years
With age, capacitance of arteries decrease as elasticity is lost (arteries are less ‘stretchy’ and more ridged)
- Systolic higher
- Diastolic similar
- Mean Arterial Pressure (MAP) a bit higher
Pulse pressure higher
25
How do diastolic, mean, and systolic pressure change with age?
- Systolic increases greatly
- There's a mild increase in mean pressure and diastolic pressure
26
Why is the pulse/systolic pressure so high for someone in their 80s?
The walls of the arteries become less elastic
27
Diagram showing how smooth muscle and precapillary sphincter muscles regulate the resistance
28
What is the role of smooth muscle in walls of small arteries and precapillary sphincter muscles?
They regulate the radius of blood vessels, and therefore resistance
29
Change in radius due to constriction of arterial smooth muscle
- Resistance is inversely proportional to the radius to the fourth power
- So small changes in radius have a big impact on resistance
30
Diagram of changes in capillary radius when precapillary sphincters contract
Key points:
- What's not depicted in these images is the cells
- Precapillary muscles control their own supply of nutrients. They can't absorb oxygen from the blood that's flowing through the lumen of the blood vessel that they surround. The oxygen must go to the capillary bed, diffuse out of the capillary, and then come back and feed that muscle with oxygen.
- If the muscle constricts, it's cutting off its own oxygen supply. Unlike other muscles, they are very sensitive to changes in oxygen concentration. You cut off the oxygen just a little and they become much weaker. This makes them dialate, decreasing resistance. This is called autoregulation.
31
How is muscle tone in precapillary sphincters regulated?
By:
- Local metabolic factors
- Sympathetic innervation
32
Summary of auto-regulation in precapillary muscles
- Precapillary sphincter muscles regulate their own oxygen supply.
- When they constrict, less blood flows to local capillaries and oxygen levels drop.
- When O2 levels drop, they can no longer manufacture ATP at the same rate and they become weak and dilate.
33
Due to what metabolic factors does precapillary sphincter muscles dilate?
- Decreased oxygen
- Increased temperature
- Increased CO2
- Decreased pH
- Increased adenosine
34
Active hyperemia
- E.g. if you were to start lifting weights
- If all the cells surrounding the capillaries become really active and start using up all oxygen, oxygen levels drop and the sphincter muscles dilate
- 'Active' refers to the cells of the surrounding tissue becoming active
35
Reactive hyperemia
- E.g. if you put a clamp on your arm and cut off blood supply
- The overall metabolic rate of the tissue stays the same, but there's no oxygen being delivered
- Oxygen levels drop and CO2 levels rise (not being removed)
- When you remove the occlusion (clamp), there will be less resistance in the arm because all the precap sphincter muscles have dilated. Blood will rush to the arm --> increased blood flow (hyperemia)
36
Types of control of blood flow
1. Local control
2. Upstream control- nitric oxide
3. Humoral control
- Vasodilation: histamine, epinephrine (B2 receptor)
- Vasocontriction: epinephrine (alpha receptor), ADH, angiotensin
37
Diagram showing neural, hormonal, and local controls of blood flow (focus mostly on the ones on the last slide)
38
Nitric oxide effect on smooth muscle
- Nitric oxide can be synthesized and released by endothelial cells
- It can diffuse through the cell membrane into the smooth muscle lining it and cause relaxation
39
Upstream artery story
- Muscle is connected an artery and is under local control
- When activity increases in the muscle, there's a message sent to the artery that's outside the environment (not embedded in the tissue that's become active) (upstream artery)
- The upstream artery dilates and becomes bigger. How does it know how to do this? Could be neural, but there was no evidence of this
- Answer: in the local tissue, there's relaxation of precapillary sphincter muscles. This decreases resistance in the tissue, which increases the flow in the upstream artery
- Increase in shear forces on endothelial cells in artery due to increased flow causes cells to release nitric oxide and the smooth muscle relaxes more
40
What happens if you increase the diameter of the artery upstream?
The velocity of blood flow slows down--> less shear force (negative feedback)
41
Graph showing pressure drop at arteriole level
- Vasodilation causes less of a pressure drop at the arteriole level
- Vasoconstriction causes more of a drop
42
Where is most of the blood in the body?
In the veins of the systemic circulatory system
43
How does blood move through the veins if there's no pressure gradient?
- Veins are separated into chambers
- Movements of joints and muscles push in on the walls of these chambers, causing. transient increase in pressure
44
Capacitance of walls of veins
- The higher the capacitance of the walls of veins, the lower the increase in pressure.
- The lower the capacitance (stiffer), the greater the pressure increase and the greater the flow of blood back to the heart.
- Increasing sympathetic tone will stimulate the smooth muscle in the walls of veins, decreasing their capacitance
45
Baroreceptors
- The brain monitors blood pressure in the blood vessels that feed the brain
- The nervous system does not pay attention to flow, but rather to BLOOD PRESSURE
- There are high pressure sensors in the cardiovascular system (carotid artery and aorta)
46
Overview of what changes happen during the Valsalva maneuver
- When you create a lot of pressure in your thoracic cavity and you cut off blood flow in the large vein (superior & inferior vena cava), it prevents blood from getting back to the heart
- Th heart has less blood to pump so stroke volume decreases, so pressure goes down in the carotid sinuses
- To keep pressure from crashing, there is an increase in:
- heart rate
- contractility
- tension in the walls of veins
- peripheral resistance
- Eventually, all of the blood flows back to to the heart and there is tremendous filling of the ventricle, so a big preload (Frank Starling), huge stroke volume, high heartbeat, high resistance, so blood pressure overshoots
- Sympathetic goes down, parasympathetic goes up and heart rate slows down dramatically for a few beats
- Blood pressure then gets back to the set point and beats at the normal resting rate again
47
Graph of baroreceptor action potential frequency
- Baroreceptors are stretch receptors surrounding the carotid sinus and in the walls of the aorta
- If mean arterial pressure increases, there's increased stretch, which causes increased action potential
48
Changes in the body during strenuous exercise
- Blood flow to abdominal organs is cut in half, even though the total cardiac output is tripled
- Blood flow to kidney is cut off
- Huge increase in blood flow to muscle (x12), skin, heart
- Dilation by local control, skeletal muscle having lower oxygen levels, higher CO2, pH more acidic, temperature goes up
- Sympathetic stimulation causes sphincter muscles to tense up, increasing resistance to abdominal organs and kidney (not local factors, but rather sympathetic)
- Sympathetic could also cause constriction in blood vessels in muscles, but there's so much local override that it supersedes any effect by the sympathetic NS
- Upstream arteries dilate
49
Percent changes in blood flow during exercise
Cardiac output goes up but the vascular system is pushing more blood back to the heart as well (hard to visualize)
50
Does capacitance of veins affect resistance?
No, because
