Unit 2 Lecture 9: Arterioles & Microcirculation Flashcards
3 facts about arterioles
- Located within individual organ
- Determine blood flow to individual organs (what requires more blood than others)
- Greatest contributors to TPR; important regulators of MAP
What is the second determinant of blood flow?
Radius
Greatest determinant of resistance
How is resistance related to radius?
Resistance is 1/r^4
Note: Larger radius gives a low resistance
r = 2, resistance is 16x less than r = 1
Other than radius, how does resistance increase?
- Tube length (twice as long ⇒ Increasing resistance)
- Viscosity of blood (2x thickness ⇒ Increased resistance by 2 folds)
Note: When you increase tube length you increase the amount of friction that the blood generates as it touches the wall for a longer time due to a longer tube length which therefore increases resistance
At rest, blood flow to individual organs is not equal. how does blood flow look after redistribution during exercise?
Significant increases in some areas while drop in blood flow to other areas
* Skeletal muscle increases over 1000% in terms of CO
* 370% increase in blood to the skin during exercise due to needing to cool the body down
Note: Consider flow to an organ as percent of total cardiac output (CO)
At rest most goes to digestive tract & kidneys
In exercise why does blood flow not increase to brain?
It will increase the pressure in the brain as it is a restricted space & brain has its own control of blood
What does the picture show?
- If we had blood flow to the skeletal muscle and we had high resistance not much blood will actually go to the muscle
- Lowest resistance means that a significant portion of the blood will go to that organ (or vial in this case)
What dictates whether blood flow occurs?
Arterioles (they are what vasoconstrict/dilate to control flow of blood)
Two main cells that make arterioles?
- Endothelial cells
- Smooth muscle cells
Function of smooth muscle cells in arterioles?
- Maintain shape of blood vessel
- Set blood vessel diameter (vasoconstrict/dilate)
- Regulate local blood flow
Endothelial layer is very thin so smooth muscle provides support too
Does smooth muscle contract act like cardiac muscle or skeletal muscle? How is smooth muscle formed?
Initially like cardiac muscle where it has Ca2+ induced-Ca2+ release to enable contraction
- Formation of smooth muscle is not arranged through striations like in skeletal muscle
- Thick and thin filaments are situated in between the lines
What are dense bodies?
Like Z-lines in skeletal muscle they act as anchors for the thick and thin actin and myosin filaments
Based on the arrangement of smooth muscle how does it contract?
When it vasoconstricts it squeezes the thick and thin filaments and allows the smooth muscle to shrink to a smaller size
Signals increasing actin & myosin cross bridges ==> Vasoconstriction
Signals decreasing actin & myosin cross bridges ==> Vasodilation
What is a unique property of smooth muscle cells
- Once activated it can stay contracted without more signals (Very slow response to vasoconstriction/dilation)
- Contain gap junctions to allow ions to go through and allow for Ca2+ induced–Ca2+ release
Myosin-light chain phosphorylation: Continues to stay contracted as the tropomyosin is basically non-existent so it cannot block off the actin; phosphorylating myosin light chain allows for it to attach and form a cross bridge with actin w/o tropomyosin being an issue
Based on the picture how does smooth muscle contract?
When it contracts it contracts in all different directions while it stays attached to dense bodies (anchors)
What are the functions of endothelial cells?
- Line the heart and blood vessels
- Prevent blood cell and platelet adherence (thrombis)
- Control exchange of fluid and nutrients in capillaries
- Secrete vasodilator [nitric oxide] & vasoconstrictor [endothelin]
They are the inner layer
How is nitric oxide produced by endothelial cells? (Steps to doing this)
- Amino acid L-arginine and through the enzyme nitric oxide synthase to create nitric oxide
- Nitric oxide diffuses into smooth muscle
- Nitric oxide activates the enzyme guanylate cyclase to form cGMP
- cGMP causes more relaxation to occur
More cGMP = More relaxation = more vasodilation = reduce resistance = increase blood flow through that arteriole
What does guanylate cyclase convert into cGMP
It takes GTP and converts it into cGMP
What will cause more cGMP and Greater increase in blood flow?
- ↑ production of cGMP (nitroglycerin)
- ↓ breakdown of cGMP (Viagra)
A young person who touches nitroglycerin can cause all their vessels to vasodilate and faint
Note: Decrease production of PDE to breakdown cGMP in order to increase blood flow
How do we control the breakdown of cGMP so there’s not a lot?
Enzyme: Phosphodiasterase (PDE)
- Breaks down cGMP into GMP to reduce the concentration of cGMP
Promote reduction in blood flow
How else can we increase nitric oxide externally?
Dietary supplements; Beet juice which contains nitrate which breaks down into nitrite and then finally forms Nitric Oxide
- Supplements claim to increase fatigue resistance, exercise performance and exercise efficiency
Nitrate to nitrite conversion mostly happens in saliva
Significant outcome measures of dietary supplement studies?
- Decreased SP and DP
- Increase endothelial function
- Decreased platelet aggregation
- Reduce arterial stiffness by increasing NO and more compliant vasculature)
Increased endothelial means NO induced NO (It induced more NO to be released)
Why is a vegetarian diet more effective than a normal diet at increasing Nitrate
A vegetarian diet consisting of a lot of root vegetables which contain plenty of nitrate & this is 4x more effective at increasing nitrate than the normal diet
