Flashcards in Systemic Circulation And Cardiovascular Pressure Deck (24)
How is the circulatory system arranged ?
In series and in parallel
-important for resistance, flow and pressure
Which blood vessels have the greatest total cross sectional area ?
Where is most of the blood in circulation?
Venous circulation - about 2/3 - in systemic veins
Known as capacitance vessels
What is the relationship between velocity of flow and total cross sectional area ?
Essentially mirror images of one another
The larger the total cross sectional area the slower the flow of blood
- eg flow is slowest in capillaries
What are the 3 distinctive layers of blood vessel walls ?
-tunica Adventitia- made of lots of collagen fibres - provides structural support
-tunica media- Lots of smooth muscle and elastin - elastin enables vessel to distend but muscle opposes this
-tunica intima-endothelium (squamous epithelium) - provides a smooth surface to reduce resistance
Explain the structure of the elastic arteries ?
=aorta and its largest branches ( brachiocephalic trunk, left common carotid artery, left subclavian artery)
- large diameter 12.5mm= enabling low resistance
- lots of elastin - withstand pressure and smooth put pressure fluctuations
- known as pressure reservoirs
- they can stretch so during ejection they stretch the reduce the pressure rise
Explain the structure of the muscular arteries ?
Deliver blood to specific organs
Smaller diameter to elastic - 0.3-10mm
Have less elastin and more smooth muscle - therefore less distension and more active in vasoconstriction
Important in redirecting blood to regions which need it more
Explain the structure of the arterioles ?
Deliver blood to capillaries
Have smaller diameter than muscular arteries - 10-300 micrometers
Media is almost entirely smooth muscle
Regulates blood flow to capillaries - redirects blood flow
Explain the structure of the continuous capillaries ?
Their endothelial cells have tight junctions between them
The intercellular clefts are just large enough to allow some solutes and fluid to pass through
Explain the structure of fenestrated capillaries ?
Some of the endothelial cells have fenestrations which makes them more permeable to fluid and solutes
Mainly in places where active active absorption (GIT) or filtration (kidney) is occurring
What are true capillaries ?
Actual exchange vessels - only 10-100 per capillary bed
What are metarterioles ?
These are called vascular shunts which bypass the true capillaries
Enable control of blood flow through the capillary bed
What is a precapillary sphincter ?
Ring of smooth muscle which wraps around the capillary at the arterioles end
- relaxed = they allow blood flow through the bed for exchange
- contracted= close off blood flow through the bed
Causes more blood flow through metarterieoles reducing exchange of solutes
Explain the structure of venues ?
Diameter of 8-100 micrometers
Smaller venues have walls which consist of just an endothelium
Larger venules contain a sparse media and Adventitia
What is the purpose of valves in veins ?
Prevent good flowing in the opposite direction
Due to the low pressure in the veins they are important for ensuring blood returns to the heart
Where does the steepest drop in pressure occur in the systemic blood flow ?
Arterioles as they offer the greatest resistance due to their vasoconstrictive abilities
Why is pressure in the aorta pulsatile ?
Due to the contraction and relaxation of the heart
What does the arterial blood pressure reflect ?
The compliance of the elastic arteries near to the heart
The volume of blood forces into arteries at a particular point - as blood volume into arteries varies though th cycle this is a mother consequence of pulsatile flow
What are the typical values of systolic and diastolic pressure in the arterial blood system ?
Systolic = 120mmHg
What are the pressures at the arterial end of the capillaries and the venous end ?
Arterioles = 40mmHg
Venous = 20mmHg
Why is capillary pressure so important ?
Has to be low otherwise capillaries will rupture
Has to be closely controlled to regulate the extent of filtration into interstitial space
What are the 2 factors which ensure blood is pumped back to he heart at the same rate as the heart is pumping blood into systemic arteries ?
-inspiration increase abdominal pressure and compresses abdominal veins, this helps to force blood along the veins
- also it causes thoracic pressure to decrease so thoracic veins expand helping to move blood towards right atrium
- expiration the decrease in abdominal pressure helps blood return from the lower body and the increase in throes in pressure helps push blood into right atrium
-skeletal muscles contract they compress deep veins and propel blood towards heart
What is a sphygmomanometry used for ?
To indirectly measure systemic arterial blood pressure