20. Circulation Of Blood Around The Body Flashcards
What pressure must arteries and arterioles withstand?
Carry blood away from the heart so must withstand high pressure
What is the function of the aorta?
Expands during contraction phase of cardiac cycle and contracts elastically (elastic recoil) between contractions WINDKESSEL EFFECT
- Reduces pressure fluctuations between beats
- Help to propel blood into the aorta and prevent back flow
What is the function of the arteries and arterioles?
They adjust their diameter (and therefore their resistance to flow) and so change the amount f blood flowing through them
- Called resistance vessels because they control blood flow to specific tissues
What does the tunica adventita consist of?
- Connective tissue
- External elastic lamina
What does the tunica intima consist of?
- Internal elastic lamina, connective tissue
- Endothelium
What does the tunica media consist of?
smooth muscle
How do arteries and arterioles maintain high pressure?
They have many collagen and elastic fibres in the tunica adventita which strengthen the vessel wall
What enables the aorta to expand and recoil between contractions
Elastin fibres
What enables the arteries and arterioles to adjust their diameter (and therefore the resistance to flow)?
Smooth muscle in tunica media contracts to case vasoconstriction (narrowing of artery diameter)
What do the endothelial cells in the tunica intima do?
They produce factors that cause vasodilation or vasoconstriction (widening or narrowing of artery diameter)
What is the purpose of veins?
They drain blood from capillary beds under very low pressure (not sufficient to propel blood to the heart)
Why are venules and veins called capitance vessels?
Because of their ability to stretch and store blood
What is the structure of an artery?
Same as a vein but with a valve in the centre
How do veins expand/contract?
- Vein walls are expandable because they have little muscles or less collagen in the adventitia
- Smooth muscle contracts to help return blood to the heart under low pressure
- The smooth muscle layer is thin but there is a large blood volume in the veins which means that a small contraction pushes lots of blood towards the heart
How is backflow in veins prevented?
One way valves
How does venous return work?
- Gravity affects venous return
- Larger veins have smooth muscle which contracts to assist venous return
- Skeletal muscles contract in surrounding muscles
What is deep vein thrombosis?
- Formation of a blood clot (thrombus) in a deep vein
- In a person sitting upright, gravity causes blood to accumulate in the veins of the lower body
- Decreased flow rate of blood due to immobility can lead to venous thrombosis
What is the function of capillaries?
- They receive high pressure blood from the arterioles and allow exchange of materials between blood and interstitial fluid
- Gas exchange: O2 delivered to tissues and CO2 removed
- Nutrient delivery: glucose, ions, water etc to the tissues
- Waste removal: metabolites removed from tissue
- Production of tissue fluid: extracellular fluid
How does the structure of capillaries enable their funciton?
- They are very narrow
- Single layer of endothelial cells called the endothelium which improves diffusion and osmosis
- Have tiny holes called fenestrations
- Allows water, some ions, solutes like glucose and very small proteins to leak out
What happens to pressure and velocity when blood enters the large area of the capillary beds?
Pressure and velocity are lost
Why do arterioles have such extensive branching?
This hugely increases the overall cross sectional area
- Extensive branching reduces pressure and flow
- Enough pressure remains for water and solutes to be squeezed through capillaries and into interstitial spaces
What are starling’s forces?
Two opposing forecasts that maintain water balance in the capillaries.
What are the two starling’s forces?
- Blood pressure (hydrostatic): tends to force water and small molecules out
- Osmotic pressure: tends t draw water back into capillaries. Osmotic pressure is due to large protein molecules that cannot leave the capillaries
What is oedema?
An imbalance between blood pressure, osmotic pressure and lymphatic draining
