Flashcards in Cardiovascular Physiology 4- Relationship between blood pressure and blood flow Deck (19):
What are the 4 different blood vessels which make up the cardiovascular system?
What are the arteries and what are their properties?
Arteries: Blood vessels that carry blood away from the heart
1) Large diameter
2) Elastic + fibrous tissue= Can withstand high pressure
3) Stiff + Flexible
4) Pressure reservoirs= Low resistance to blood flow
5) Low compliance= Measure of how stretchy they are=Cannot stretch
How do arteries act as pressure reservoirs?
When heart is in systole= Blood flows into aorta= Greater volume of blood flows into arteries from the heart than leaves them to flow into arterioles
Causes artery wall to expand= Pressure energy in stretched walls
Diastole= Stretched walls passively recoil= Exerting pressure on blood= Pushes blood into downstream vessels to ensure flow, Keeps blood pumping through the cardiovascular system
What do arterioles do? What are their properties?
Arterioles= Primary resistance vessels, determine the distribution of cardiac output
They are the smallest arteries
1) Little elastic connective tissue
2) Thick layer of smooth muscle on walls= Can contract and relax as they are chemical sensitive
3) Small radii
4) Can selectively stretch and relax depending on the tissue= Variable resistance
How do arterioles act as major resistance vessels?
Radius is very small= Offers resistance to flow= Since it is so small, blood does not have to have as much pressure in order to push it through
Blood flow is opposed by resistance of the system
Drop in arterial pressure when it reaches arterioles= Decrease in pressure is due to energy lost as a result of the resistance to flow offered by the vessels
Stimulation of sympathetic nervous system= Can constrict or widen the rings
Smooth muscle contraction= Vasoconstriction= Increased resistance and decreased flow through the vessel
Smooth muscle relaxation= Vasodilation
What do capillaries do? What are their properties?
Capillaries: Exchange materials between blood and tissue cells, branch extensively to bring blood within the reach of essentially every cell
Once blood reaches capillaries, the plasma and the cells can exchange material across the thin walls
1) Thin wall
2) Narrow diameter
3) Extensive network
4) Continuos Endothelial cells are joined to one another with leaky junctions
5) Can also have fenestrated capillaries (pores): Have large pores that allow high volumes of fluid to pass rapidly between the plasma and interstitial fluid= Intestines, kidneys
6) Can also have discontinuous ones: Gaps between the cells, wide= blood cells and plasma proteins need to cross the endothelium to enter the blood e.g. liver. Need free exchange
What happens to the speed of blood flow in capillaries?
Flow rate of fluid is constant as it is equal to cardiac output BUT speed can change:
Velocity of flow changes as it is inversely proportional to total cross sectional area of all the vessels in a given area= It is fast when there is a small cross section
A single capillary has small diameter but when put together= Very large total cross sectional area= Very slow speed= Gives time for material to be exchanged
What are veins and what are their properties?
Veins: Carry blood towards the heart
1) Thinner walls compared to arteries
2) High compliance= Stretchy
3) Larger radii
Small change in pressure= Large degree stretch due to volume
What is blood pressure?
The pressure exerted by blood on the walls of the blood vessels
Depends upon volume of blood within the vessel and the compliance of the vessel
What is the main driving force for blood flow?
Mean Arterial pressure
What is diastolic pressure?
DP= Lowest pressure in the circulatory system, associated with the relaxation of the ventricles
Ventricles: falls only to a few mm Hg as the ventricles relaxes
Arteries: High as the vessels are able to capture and store energy in their walls
What is systolic pressure?
The highest pressure in the circulatory system that reflect the pressures created by contraction of the ventricles
What is the formula for MAP?
MAP= DP + 1/3 (SP-DP)
It is closer to diastolic pressure than to systolic pressure as diastole lasts twice as long as systole
What is flow rate and how is it related to pressure gradient?
Flow rate through a vessel= Volume of blood passing through per unit time
Blood flows if a pressure gradient is present= blood flows from areas of high pressure to low pressure
Directly proportional to the pressure gradient= As PG increase, flow rate increases
How is flow related to resistance?
Blood flow is inversely proportional to vascular resistance
Resistance in vessels= Pressure drops as blood flows through the vessels
High resistance= Narrow diameter= Reduce blood flow
What is the equation for blood flow?
Flow= Change in pressure / Resistance
What are the other factors which can be related to resistance to blood flow?
Resistance to blood flow is directly proportional to the viscosity of the blood= The thicker the blood, the more resistant to blood flow
Resistance to blood flow is directly proportional to vessel length= The longer the vessel, the more resistant the blood flow= STRAW
Resistance to blood flow is inversely proportional to vessel radius= The smaller the radius, the more resistance there is to blood flow
What is total peripheral resistance? How does it affect MAP?
Combined resistance in all organs including blood vessels
Determining mean arterial pressure: Balance between blood flow into and out of the arteries
Blood flow into aorta= Determined by cardiac output the left ventricle
Blood flow out of aorta= Peripheral resistance
THEREFORE: MAP= CO x TPR
If cardiac output increases, the heart pumps more blood into arteries per unit time, and if resistance stays the same, flow into arteries is greater than flow out= Blood volume increases= MAP increases