1. Cardiovascular System: Blood vessels
Dr. A

2. Blood Vessels – Overall function
Allow blood to distribute its contents (O2, glucose, hormones) and remove wastes (urea, CO2, metabolites)
Acts as radiators that expand to release heat into the environment (hyperthermia) or constrict to retain heat (hypothermia)

3. Arteries and Veins
Arteries – carry blood away from the heart (systolic AND diastolic pressure)
Veins – carry blood towards the heart (diastolic pressure only)
Capillaries – very thin (~ 1 erythrocyte in diameter) to allow for diffusion of materials into and out of blood

4. Blood Vessel Structure – 3 layers
Tunica intima
Tunica media
Lumen
(blood-containing space)
Tunica externa

5. Blood Vessels – 3 Layers
Tunica intima – innermost layer, directly in contact with flowing blood – consists of endothelial cells (continuous with endocardium)
Tunica media – Mid-layer, made of smooth muscle and elastin, controls vessel diameter and thus blood pressure
Tunica externa – outermost layer, made of connective tissue (collagen) but innervated and connected to lymph vessels
Vasa Vasorum – small network of capillaries that nourish the tissues in the outermost parts of larger blood vessels

6. Vasoconstriction and Vasodilation
Vasoconstriction – sympathetic nervous system and/or chemical stimulation flexes the muscle in the tunica media causing vessels to constrict
Retains heat
Increases blood pressure
Vasodilation – same stimulation causes vessels to dilate
Decreases blood pressure
Allows heat loss through skin

7. Arteries
Elastic (conducting) arteries – thick-walled arteries near the heart (aorta and major branches)
High elastin content, but no role in vasoconstriction
“Springiness” assists in blood flow
Muscular Arteries – distal to elastic arteries, conduct blood to body organs, thickest tunica media
Arterioles – smallest arteries, directly connected to capillaries

8. Capillaries Consist only of tunica intima Penetrate all tissues Why?
Notable exceptions
Ligaments and tendons
Cartilage and epithelia ?

9. Capillary Beds
Interweaving network of capillaries that supplies tissue with blood (microcirculation)
Capillary bed is fed blood by the terminal arteriole and blood is collected by the postcapillary venule
2 types of vessels
A continuous channel, formed by the meta-arteriole and thoroughfare channel
True capillaries (exchange vessels)

10. Capillary Bed

11. Capillary Flow Control
Sphincter muscles (precapillary sphincter) regulates flow of blood into each true capillary
Sphincters flexed = blood goes through the shunt and flow to tissues greatly reduced
Sphincters relaxed = blood flows to tissues
This is highly dependent on local chemistry, pressure, and sympathetic nerve input
Try it!

12. Venous System (Veins) Delivers blood to the heart
Venules – smallest diameter veins
Very porous and allow the passage of fluids and WBC’s into interstitial fluid (extracellular fluid)
Veins – compared to arteries, relatively large lumens and thin walls
Tunica externa is heaviest, with large amounts of collagen

13. Venous Flow Blood pressure is much lower in veins than in arteries
Up to 65% of blood volume resides in veins
Pressure to return blood back to heart must be maintained, even though elasticity is less in veins
How?

14. Venous valves Resemble semilunar valves of the heart
Snap shut to prevent the backwards flow of blood
Prolonged increase in venous pressure results in veins with stretched walls and incompetent valves (varicose veins)

15. Physiology of Circulation
Definitions:
Blood Flow – volume of blood circulating in an organ over the course of 1 minute (ml/min)
Blood Pressure – Force created by the heart (ventricles) and vessels to push blood through the body (mm Hg), blood flows from high pressure to low pressure
Resistance – opposition to flow direction, caused by several factors

16. Factors Influencing Resistance
Blood viscosity -
Blood vessel length -
Blood vessel diameter –
How can these vary?

17. Blood Flow vs. resistance
F = ΔP/R
F = flow
ΔP = change in pressure between two points in circulation
R = resistance
Blood flow is inversely proportional to the resistance in the system
Resistance varies inversely with the fourth power of the vessel radius
Resistance = 1/r4

18. Arterial Blood Pressure
Result of two factors
1. How much elastic arteries close to the heart can be stretched (distensibility)
2. The volume of blood forced into them at any time
From where?

19. Arterial Pressure
Systolic Pressure – “pressure peak” resulting from the contraction of the left ventricle
“High” number in blood pressure
Diastolic Pressure – Lowest pressure from aortic valve closing and the elastic arteries (aorta) contracting
“Low” number in blood pressure
Pulse Pressure – the difference between systolic and diastolic pressure

20. Mean Arterial Pressure
Diastole lasts longer than systole, so the equation is
MAP = diastolic pressure + (pulse pressure/3)

21. Capillary Blood Pressure
Blood pressure decreases with distance from the heart
Very low pressure in capillaries and capillary beds (15-35 mm Hg)
Prevents capillary rupture
Prevents fluids from being forced out of capillaries too fast

22. Venous Blood Pressure Very steady, no pulsing
Only ~15 mm of pressure from the capillaries to the vena cavae
This reflects the resistance of blood traveling through capillaries due to friction
Venous blood return benefits from help by other body functions

23. Pressure vs. Distance from Heart

24. Return of Venous Blood to the Heart
Respiratory Pump – Inhalation increases pressure in the thoracic cavity, squeezing blood in veins toward the heart
Muscular Pump – Contracting muscles “squeeze” blood through the veins and to the heart

25. Maintaining Blood Pressure
What factors can affect blood pressure?
Blood volume
Peripheral resistance
Cardiac output
Normal CO is 5 – 5.5L/min – this must be maintained in all situations, so heart and blood vessels will adapt to this (homeostasis)

26. Neural Control of Blood Pressure
MAP is maintained by
Neural control of blood vessel diameter
Altering blood distribution to various organs based on need
E.g. blood moving from digestive system to skeletal muscles during exercise
Vasomotor Center
Baroreceptor
Chemoreceptor

27. Vasomotor Center Oversees blood vessel diameter
Located in the medulla of the brain
Nerves innervate smooth muscles of blood vessels

28. Baroreceptor Reflexes
Receptors located in
carotid arteries (in neck)
Aortic arch
Other arteries in neck and thorax
When nerves are stretched, vasomotor center is inhibited, and smooth muscle in blood vessels relax to decrease blood pressure, CO is reduced
When blood pressure drops, (unstretched baroreceptors), muscle in blood vessels contracts and CO increased

29. Chemoreceptor-Based Reflexes
Increases in CO2 or drop in pH results (via cardioacceleratory center in brain stem) in an increase in CO and in vasoconstriction
Chemoreceptors located in carotids and aortic arch
Chemoreceptors also increase respiratory rate

30. Higher Brain Center Influence on Blood Pressure
Regulated by hypothalamus, hormone release and medullar (and midbrain) control
E.g. fight or flight response

31. Hormonal Control of Blood Pressure
Adrenal medulla hormones (epinephrine & norepinephrine)
Norepinephrine is a vasoconstrictor
Anti-Diuretic Hormone (ADH)
Conserves water, vasoconstrictor

32. Long term Mechanisms Renal Control
Direct Renal Mechanism – High blood pressure leads to a physical increase in the removal of fluid from the blood, ultimately decreasing blood pressure
Indirect Renal Mechanism – Under low blood pressure conditions, kidneys release renin which produces angiotensin II
Vasoconstrictor
Stimulates the release of aldosterone and ADH
Make you feel thirsty!

33. Hypotension Decrease in Blood Pressure Poor nutrition Age Hemorrhage
Shock (acute hypotension)

34. Tissue Perfusion Blood flow through body tissues
To deliver O2 and remove CO2 to tissues
Blood flow to organs is adaptable to changing situations
Exercise
Rest
Mental condition

35. Blood Flow Velocity
Velocity is inversely related to cross sectional area of the vessel
Fastest in largest arteries, slows in capillaries, speeds up again in veins back to heart (Vena cavae)
Right?

36. Differential Blood Flow Speeds
Major Arteries (Aorta) = cm/s
Capillaries = 0.3 cm/s
Why is it good to be slow?
Vena cavae = ~30 cm/s

37. Autoregulation of Local Blood Flow
How might blood flow be differentially regulated to a particular area of the body?
Organs/ tissues control the diameter of blood vessels feeding their own capillary beds
This is called intrinsic regulation

38. Intrinsic Control of Blood Vessels - Metabolic Control
Nitric Oxide Production –
Released from vascular endothelial cells
potent vasodilator, antagonist to nervous vasoconstriction, relaxes precapillary sphincters
Application?
Histamines also cause vasodilation
Where are these from?

39. Angiogenesis
Long term tissue starvation may lead to the development of long-term regulatory responses
More blood vessels to an organ
Blood vessels may increase in diameter
Very adaptable!
Examples?

40. Blood Flow Regulation in Special Areas
Skeletal muscle
Constantly changing in metabolic requirements
Resting muscle vs. actively flexing muscles
Active or exercise hyperemia supports working muscles
Controlled by reduced O2 levels and increased CO2 levels and other metabolic factors
During exercise, local controls override sympathetic vasoconstriction

41. Blood Flow Regulation in Special Areas
The Brain
gets roughly 750ml/minute
Blood flow increases in more active areas of the brain
MAP decrease results in brain vessel dilation while MAP increase results in vasoconstriction
Brain swelling
Fainting

42. Blood Flow Regulation in Special Areas
The Skin
In addition to supplying nutrients, blood flow to the skin also regulates body temp
Increases in body temp are sensed by hypothalamus and result in vasodilation, sweating
Decreases in body temp (or outside environment) cause vasoconstriction
Blood can get trapped in the skin when capillary sphincters shut, causing “rosy cheeks”

43. Blood Flow Regulation The lungs
Less pressure needed as vessel walls are thin and it is a “short trip”
Vasoconstriction and vasodilation according to O2 is opposite other areas of the body
Low O2 = vasoconstriction
High O2 = vasodilation

44. Blood Flow Regulation in Special Areas
The Heart
Increases in heart activity (exercise, stress, etc.) increase the flow of blood to the myocardium by dilating coronary blood vessels
The net increase in blood flow increases the delivery of O2 to the heart