Circulation- PV2 Lecture 3
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What part of the circulation receives the majority of blood flow?
- Systemic circulation 84%
- Pulmonary circ. 9%
- Heart 7%
Of the systemic circulation, how much is in the veins, arteries, capillaries and arterioles?
- Veins 64%
- Arteries 13%
- Arterioles and capillaries 7%
What do the conduit vessels do and what is an example?
Transport blood to and from REGIONS of the body, ex: aorta and vena cava
What do the distribution vessels do and what's an example?
Distribute and return blood to and from various organs
Ex: femoral artery, brachial vein
What do the resistance vessels do and what's an example?
- Provide resistance in the circ.
- Ex: arterioles
What do the exchange vessels do and what's an example?
Allow for movement of gases, fluids, and nutrients between blood and organs. Ex: capillaries
What do the capacitance vessels do and what's an example?
Act as a reservoir for blood, can hold a large volume at low pressure. Ex: veins.
What are the 3 layers of vessels, going from inside to outside?
- Innermost- tunica intima
- Middle- tunica media
- Outermost- tunica externa / adventicia
Do capillaries have all 3 vessel layers?
No, they only have a single layer of endothelial cells
What does the tunica intima consist of?
A continuous layer of endothelial cells connected from end to end, and a basement membrane
What does the tunica media consist of?
Vascular smooth muscle that can contract to regulate BP and vascular resistance. Also has elastin for elastic recoil.
What organ has tight junctions in its tunica intima?
The BBB, allows only lipid soluble molecules to pass
What does the tunica adventicia consist of?
Connective tissue and small arteries and nerves for the ANS. Also has vasovasorum (tiny vessels that penetrate vessel walls to supply O2 and nutrients to the vessel itself).
What 2 substances are released from the tunica intima and what do they do?
- 1) endothelin- vasoconstriction
- 2) Nitric oxide- vasodilation
What is the significance of the resistance vessels, especially for us in the OR?
They respond to needs like acute blood loss, can direct blood flow to where we need it, when we need it, ex: to legs when jogging
Do the veins contain all 3 vessel layers?
Yes, but they have thin walls and unidirectional valves to promote forward flow
What are the 3 categories of capillaries? Give an example as to where each is found.
- 1) continuous- tight junctions, ex: BBB
- 2) fenestrated- pores btw endothelial cells allow small substances to cross the membrane easily, ex: kidneys, intestinal villi
- 3) discontinuous, AKA sinusoid, very leaky, large gaps, ex: liver
T or F, the systemic and pulmonary circulation both receive the same amount of blood flow / minute, despite the pressures in the pulmonary system being much lower than the systemic circ.?
- PA mean = 16 mmHg
- PAS = 25
- PAD = 8
What are the 2 major functions of the aorta? How does it do this?
- 1) move blood at high pressures away from the heart (systole)
- -Thick muscle layer and lots of elastic fibers
- 2) elastic recoil to support BP btw contractions (diastole)
- -Large diameter which decreases resistance to flow
What is the pulse pressure?
SBP - DBP
What 2 major factors affect pulse pressure?
- 1) stroke volume
- 2) compliance (how distensible arterial tree is)
Does collagen or elastin provide more resistance to flow?
- Collagen offers more resistance (it's more rigid)
- Elastin offers least resistance.
How does increased SV affect pulse pressure?
Increased SV increases aortic PP
How does decreased aortic compliance (due to age or artherosclerotic disease) affect aortic pulse pressure?
It increases it.
T or F, each vessel can dilate or constrict to control local flow, since each tissue has its own metabolic needs.
Which has lower resistance, series or parallel vessels?
What are the units for resistance? What is the usual value of SVR and PVR?
Rx: dynes sec / cm5
- PVR 20-130
What happens to blood flow when the metabolic rate of a tissue increases?
Blood flow increases.
What happens to blood flow when O2 sat decreases
Blood flow increases.
What are the 2 theories of blood flow regulation?
- 1) Vasodilator theory
- 2) Oxygen (and other nutrients lack) theory
Explain vasodilator theory.
- The greater the metabolic rate (or less available O2), the greater the formation of various vasodilator substances in various tissues.
- Ex: of vasodilator substances are adenosine, CO2, histamine, K+, H+
Explain the oxygen (and other nutrients) lack theory.
Precapillary sphinctors and metarterioles open and close several times / minute (vasomotion), duration of open phase is proportional to the metabolic needs of the tissues for oxygen. Duration of open phase correlates with tissues need for O2. So with lack of O2, it stays open, vasodilates.
What constitutes as "distance factors" in the control of blood flow and vascular resistance?
- ANS vasopressin, and angiotensin 2
- Ex: Ach released from the post-ganglion of the PNS causes vasodilation. This is either by direct stimulation or indirectly by NO release
- -Effects of ANS can cause VD or VC, overall effect comes down to number of receptors in that area.
What is the metabolic theory for variations in arterial pressure?
When arterial pressure is too great, is washes out VD released by the tissues, thus the BV constrict
What local factors affect blood flow and vascular resistance?
- Metabolic- active hyperemia
- Myogenic- intrinsic property of vasc smooth muscle, when vessel is stretched it contracts, part of auto regulation
- Endothelial- release of NO (VD), endothelin (VC), and prostacyclin (VD)
What is active hyperemia?
Increased metabolism causes vasodilation and increased blood flow, ex: exercise
What are some of the LT control mechanisms to control blood flow and vascular resistance?
- Increase in vascularity
- -O2 debt or excess
- -Angiogenic growth factors
- -Collateral circulation
What are examples of humoral control substances that cause vasoconstriction?
NE, epi, angiotensin 2, vasopressin, increased Ca++ ion, decreased H+ ion (increased pH)
What are example of humoral control substances that cause vasodilation?
Bradykinin, histamine, increased K+, increased Mg+, increased H+, increased acetate and citrate ions, increased CO2
What provides global neuronal control of the circulation?
The ANS via the vasomotor center.
Where is the vasomotor center located?
Lower 1/3 of pons, bilaterally medulla.
What are the 3 components of the vasomotor center?
- VC area
- VD area
- sensory area
How does the sensory area receive information and how does it respond?
- Receives signals from circ. system via vagus and glossopharyngeal nerves.
- Output controls VD and VC areas of vasomotor center, provides "reflex control," ex: baroreflex
What is vasomotor tone?
A partial state of contraction in the blood vessels. This is due to partial firing of the SNS vasoconstrictor fibers.
What is the most important factor for effective CV function?
Maintenance of systemic arterial BP
What is the single most important mechanism for short term blood pressure control?
What does the baroreceptor reflex system do? How does it work?
- -Decreases BP. A stretch in baroreceptors is caused by increased BP, they signal back to the CNS and via the ANS, BP is decreased.
- -Negative feedback system
Where are the sensory baroreceptors located?
carotid sinus (internal carotid artery) and aortic arch
What is the path of the signal from the sensory carotid sinus to the CNS?
What about the aortic baroreceptors?
- -Carotid sinus- Hering's nerves to glossopharyngeal nerve to tractus solitarius to medulla, VC center is inhibited, and PNS stimulated
- -Aortic- vagus
- -This results in decreased BP, decreased HR, and decreased contractility
At what pressure do the carotid sinuses respond?
What about the aortic receptors?
- -Carotid: P > 60 mmHg, as pressure (MAP) increases they respond progressively more rapidly to a max of 180 mmHg
- -Aortic: P > 90 mmHg
The baroreceptor reflex operates when MAP > 60, but usual MAP is 100, is this a good thing?
Yes, it works where we need it most and modulates postural changes in BP
Do the baroreceptors respond more to a rapidly or slowly changing pressure?
Respond more to rapidly changing pressure.
Are the baroreceptors effective in long term BP control?
No, if BP is elevated over a few days, the baroreceptor adapts to the change, it is a short term mechanism only
How do chemoreceptors affect BP control?
- -Act within the same pathways as the baroreceptors. They are chemoreceptor organs, made up of 2 carotid bodies and 1-3 aortic bodies. A decrease in O2 or increase in CO2 causes carotid body to fire, excites vasomotor center, and increases BP.
- -Play more of a role in respiratory system than CV.
How do the atrial and pulmonary artery reflexes affect BP?
Aorta and PA have low pressure stretch receptors. Similar to baroreceptors in systemic circulation. Volume increases withOUT an increase in pressure.
What is the volume reflex?
Atrial stretch activates kidneys. Dilation of afferent arterioles in kidneys, decreased ADH production (reduces water reabsorption). ANP also released, leads to more water excretion. Help to return blood volume back to normal after pressure overload, indirectly reduce BP.
What is the Bainbridge Reflex?
Increase in atrial volume (stretch) increases HR by 40-60%. Afferent signals are transmitted thru vagus to medulla. Efferent signals are transmitted via SNS and vagus to INcrease HR and contractility.
What is the CNS ischemic response?
- Severely decreased CBF (ischemia), or build up of LA or CO2 (both acidic), stimulate vasoconstrictor center response, causes dramatic increase in BP.
- Not activated until BP falls below 60 mmHg, emergency response. Greatest response when BP = 15-20 mmHg.
What is the Cushing Reflex?
A type of CNS ischemic response due to increased ICP. If ICP rises above systemic pressure, no blood flow occurs, and it initiates the ischemic response. This causes BP to rise. BP usually resets to a new equilibrium just slightly higher than the ICP.
What is the Bezold-Jarisch Reflex?
Stimulated by injection of dye into the coronary arteries or ischemia to the LV. Response is decrease BP and decreased HR, mediated by the vagus.
How is MAP calculated?
60% diastolic BP + 40% systolic BP
How do you calculate SVR? Who's law is being applied here?
SVR = MAP-CVP / CO x 80 (80 is conversion factor to get to dynessec/ cm5)
How will increased viscosity affect flow? What factors can increase viscosity?
- Flow will decrease with increased viscosity.
- Factors that increase viscosity include: increased Hct and increased plasma proteins
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