Blood vessels and hemodynamics Flashcards
Differentiate between arterioles, veins and capillaries
Arterioles = blood away from the heart
Veins = blood towards the heart
Capillaries = directly serve cells
What is the tunica intima?
Innermost layer found in both veins and arteries
Composed of:
- endothelium
- subendothelial layer → thicker in veins compared to arteries
- internal elastic membrane → present ONLY in arteries
Between veins and arteries, which contains an internal elastic membrane within the tunica intima? Why is this important?
Found only in arteries
- Arteries are stretchy at early parts where blood is pushed out ∴ this helps to allow elasticity and stetch with recoil
- This applies for the external elastic membrane found within the tunica media which is also only found within arteries
What is the tunica media?
Middle layer found in both veins and arteries
Composed of;
- smooth muscle and elastic fibers
- external elastc membrane (only found in arteries)
Smooth muscle = helps regulate diameters (for contraction/dilation)
- can also infleunce where the blood is going at any moment in time
What is the tunica externa?
Outermost layer found in both veins and arteries (outer wrapping)
Composed of;
- collagen fibers = anchoring the blood vessels
- vasa vasorum
What are some key characteristics about veins that arteries dont possess?
Veins have;
- thinner walls
- more room to accomodate blood
- larger lumens
- valves = keeps blood from backing up on itself
Recall; veins bring blood back towards the heart
What are the structures contained within a capillary?
- Basement membrane
- Endothelial cells
Very thin barrier to allow for better exchange
What are the 3 types of arterial vessels?
1) Elastic (conducting) arteries
2) Muscular (distributing) arteries
3) Arterioles
Describe elastic arteries
- Thick walled
- Large diameter arteries near heart
- highest proportion of elastin
Functions;
Smooth out pressure fluctuations, this recoil helps maintain pressure and flow of blood
Describe muscular arteries
- Smaller diameter branches
- Delivers blood to specific organs
- more smooth muscle than elastin
This is so that they can open up more when blood is needed and close more when its not needed
There is only a finite amount of blood, so choices must be made to accomodate the greatest needs of blood in the body
Describe arterioles
- Tunica media primarily smooth muscle - this is a single layer in smallest arterioles
Arterioles determine which capillary beds are flushed minute to minute
What are the 3 types of capillaries?
Walls are only a thin tunica intima as the function of capillaries is exchange
1) Continuous capillaries
2) Fenestrated capillaries
3) Sinusoidal capillaries
Describe continuous capillaries
- found in skin, muscles, brain
- endothelial cells are linked by tight junctions providing an uninterrupted lining
- except in CNS - here there are intercellular clefts that allow limited passage of fluids, small solutes (ions)
Least leaky
Describe fenestrated capillaries
- Similar to continuous but endothelial cells are riddled with pores (fenestrations) allowing for increased permeability to fluids/small solutes
- found in small intestine (digestion), endocrine organs (protein hormones), kidney (filtration and reabsorbtion)
Fenestrations = direct openings for larger molecules such as proteins
Descibe sinusoidal capillaries
- highly modified leaky capillaries
- found in liver, bone marrow (where RBCS and WBCs are made so that they can be sent out), lymphoid tissues, endocrine organs
- Have large irregular lumens and are usually fenestrated with fewer tight junctions
- large intercellular clefts for passage of proteins, RBCs
What is microcirculation?
Flow of blood from an arteriole to a venule through a capillary bed
- this flow is regulated by the diameter of the terminal arteriole
What are veins vs venules?
Venules;
Post capillary venules = just endothelium few fibroblasts
Larger venules have a couple of layers of smooth muscle and a thin tunica adventitia
Vein;
3 tunics but walls are thinner and lumers larger
Less smooth muscle in tunica media (compared to corresponding artery) and minimal elastin
Tunica adventitia is heaviest layer
Up to 65% of blood in veins at one time (at rest) = capacitance vessels (blood reservoirs)
What are true capillaries?
Actual exchange vessels
How is blood flow regulated?
Can be regulated independently for various tissues and organs
this is so that more blood can be sent to areas that need it more at the expense of sending less blood to another area that can manage without it at that moment in time
What is blood pressure?
force per unit of area exerted on the wall of a blood vessel by its contained blood (in mm Hg)
- usually refers to systemic arterial blood pressure (120 over 80) in the largest arteries near the heart (avg aortic bp = 90 mm Hg)
How does flow occur? What is the equation used for this?
Flow occurs in response to a pressure gradient
F = (P1-P2)/R
Flow = difference in blood pressure / peripheral resistance
High pressure in heart and low pressure in capillaries allows blood flow (movement of blood) in a fwd direction
What is the sigificance of resistance in blood flow?
It is the major determinant of blood flow because a decrease in blood vessel radius increases resistance to the 4th power
- Arterioles can regulate their diameters allowing for v quick and effective changes in resistance to flow
- measures the total of frictional forces that impede flow -> flow and resistance are inversely related
What can resistance be influences by? How do these elements relate to the value of resistance?
Blood viscosity (n) due to formed elements, plasma proteins
Total blood vessel length (L) - more length = more resistance
Blood vessel radius (r) - can be regulated, fluid not touching walls moves faster
Resistance is proportional to (n)(L)/(r^4)
What is pulse pressure?
Pulse pressure = systolic bp - diastolic bp
- systolic bp = high point (felt when you measure your heart rate on wrist/neck)
indicated vigor of contraction of ventricle - how forcefully your left ventricle is contracting
also provides info on elasticity of aorta and major arteries - if they stretch and recoil realy well, they will effectively push the diastolic value down
What is mean arterial pressure (MAP)?
MAP = diastolic pressure + 1/3 pulse pressure
The pressure that propels blood to tissues during the cardiac cycle
MAP + pulse pressure decrease w distance from the heart (at the end of arterial tree, blood flow is more steady and pulse pressure has disappeared
NOT the mean of systolic and diastolic pressures, ventricular diastole is longer than ventricular systole
Why is the mean capillary blood pressure kept at a low value?
1) capillaries are fragile; high pressures would rupture them
2) most capillaries are extremely permeable for lots of exchange at low bp
What is venous blood pressure?
- Steaddy - changes very little during cardiac cycle
- there is a gradient and veins provide more open pathways so blood can keep moving fwd
Think about walking around vs sitting still to promote more blood movement
What are some factors that aid in venous return?
1) Venous valves + muscular pump = skeletal muscles on either side
2) respiratory pump = breathing causes blood to propel upwards
3) Sympathetic venoconstriction = squeezes blood already in venous system (~65%) and helps propel it in a fwd direction
Why is blood harder to propel back up in the lower levels of your body compared to the upper levels?
higher levels dont have the interference of gravity while lower levels have to work against the force of gravity to proper it upwards
What is pulse?
Pressure waves due to alternating expansion and recoil or elastic arteries
Pushes surface artery against firm tissue
What are pressure points?
Areas of your body where you are able to take a pulse
- Helpful if you have a cut in your upper limb, pushing on the artery or applying pressure in that area can constrict blood flow where the injury is, further reducing blood loss
What are the different functions and areas of the body that short term vs long term regulation of BP work with?
Short term reg. of BP = works with total peripheral resistance (blood vessel diameter) and cardiac output
- for very quick changes and rapid responses
Long term reg. of BP = works with blood volume - adjustments are made at the level of the kidneys
- change in blood volume = long term changes in BP
What are the 2 main goals of short term reg. of BP?
1) alter blood distribution to respond to specific demands
2) maintain adequate MAP (altering blood vessel diameter)
What is usually involved in short term neural and chemical reg. of BP?
Usually reflex arcs; baroreceptors, vasomotor centre of medulla, vascular smooth muscle
What is vasomotor tone?
Constant output espeically to arterioles (NE)
What does the cardiovascular centre respond to input from?
Cardiovascular centre + incluing the vasomotor centre
1) Baroreceptors
2) Chemoreceptors
3) Higher brain centres/hormones
Which neurons regulate blood vessel diameter?
Only sympathetic neurons
Where are baroreceptors found?
Located in carotid sinuses (blood flow to brain), aortic arch (blood flow to rest of systemic circulation) and other large arteries of neck and thorax
What effect does increasing MAP have on receptors?
This stretches receptors, the response is;
- dilation of arterioles (increase diameter)
- venodilation to shift blood to venous reservoirs (dilates veins to increase amount of blood to veins)
- PNS stimulated by afferents to cardiac centre and SNS inhibited (decreasing heart rate and contractile force)
Altogether these decrease MAP and initiates the opposite response
When does a reflex vasoconstriction occur?
When there is a drop in O2, pH or a rise in CO2
- chemoreceptors in aortic arch and large arteries of neck to vasomotor centre initiate the reflex
What are the 4 main hormones involved in ST and LT regulation of BP? Which are vasodilators and which are vasoconstrictors?
Adrenal medulla hormones (vasoconstrictor)
Angiotensin II (vasoconstrictor)
Atrial natriuretic peptide (ANP) (vasodilator)
Antidiuretic hormone (ADH) (vasoconstrictor)
What is the function of the kidneys in relation to blood?
The kidneys adjust blood volume - anything that changes blood volume will change blood pressure
What is the direct action of the kidneys on blood?
If BP increases, filtrate will be made more rapidly
Increased blood volume/pressure causes increased rate of filtrate formation -> insufficient time to reclaim water leads to increased volume of urine
What is the indirect action of the kidneys on blood?
Renin-angiotensin system - if MAP decreases, kidneys release renin leading to the release of angiotensin II a potent vasoconstrictor
Angiotensin II stimulates secretion of alldosterone from the adrenal cortex - this stimulates increased renal absorption of Na+
This increases release of ADH and promotes water reabsoption
