1. Peripheral Circulation and Regulation
Lect 17, Chapter 21
Peripheral Circulation and Regulation

2. Peripheral Circulatory System
Systemic vessels
Transport blood through most all body parts from left ventricle and back to right atrium
Pulmonary vessels
Transport blood from right ventricle through lungs and back to left atrium
Blood vessels and heart regulated to ensure blood pressure is high enough for blood flow to meet metabolic needs of tissues
Skeletal muscle pump

3. Blood Vessel Structure
Arteries
Elastic, muscular, arterioles
Capillaries
Blood flows from arterioles to capillaries
Most of exchange between blood and interstitial spaces occurs across the walls
Blood flows from capillaries to venous system
Veins
Venules, small veins, medium or large veins

4. Capillaries Capillary wall consists mostly of endothelial cells
Types classified by diameter/permeability
Continuous
Do not have fenestrae many tissues
Fenestrated
Have pores (20-100nm) found in endocrines
Sinusoidal
Large diameter with large fenestrae
In the liver, bone marrow, spleen

6. Capillary Network
Blood flows from arterioles through metarterioles, then through capillary network
Venules drain network
Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow

7. Structure of Arteries and Veins
Three layers except for capillaries and venules
Tunica intima
Endothelium
Tunica media
Vasoconstriction
Vasodilation
Tunica adventitia
Merges with connective tissue surrounding blood vessels

8. Structure of Arteries Elastic or conducting arteries
Largest diameters, pressure high and fluctuates
Muscular or medium arteries
Smooth muscle allows vessels to regulate blood supply by constricting or dilating
Arterioles
Transport blood from small arteries to capillaries

9. Structure of Veins Venules and small veins Medium and large veins
Tubes of endothelium on delicate basement membrane
Medium and large veins
Valves
Allow blood to flow toward heart but not in opposite direction
Atriovenous anastomoses
Allow blood to flow from arterioles to small veins without passing through capillaries

10. Blood Vessel Comparison

11. Aging of the Arteries Arteriosclerosis Atherosclerosis
General term for degeneration changes in arteries making them less elastic
Atherosclerosis
Deposition of plaque on walls

12. Pulmonary Circulation
Moves blood to and from the lungs
Pulmonary trunk
Arises from right ventricle
Pulmonary arteries
Branches of pulmonary trunk which project to lungs
Pulmonary veins
Exit each lung and enter left atrium

13. Fig. 21-8 Cardiovascular physiology

14. Systemic Circulation: Arteries
Aorta
From which all arteries are derived either directly or indirectly
Parts
Ascending, descending, thoracic, abdominal
Coronary arteries
Supply the heart

16. Dynamics of Blood Circulation
Interrelationships between
Pressure
Flow
Resistance
Control mechanisms that regulate blood pressure
Blood flow through vessels

17. 2. Physics and Physiology of Circulation
Human's circulatory system is a closed circulatory system. Thus the blood pressure and the flow resistance become dependent on each other.

18. What is meant by 760 mmHg? 1 atmosphere is 760 mm of mercury (760 torr) and is the barometric pressure at the sea level and at 0˚C, and decreases in response to the altitude. At Detroit the barometric pressure is about 740 torr.
1 A.P. = 14.7 lb/in2 = 1,033 gm/cm 2
1 lb/in2 = gm/cm2
Tire pressure of 30 lb/in2 = 2.04 A.P.

19. a. Pressure and blood flow
Pressure is the driving force of the blood flow.
When blood vessels are connected, the blood flows from the higher pressure site to the lower pressure site and the rate of flow is proportional to the pressure difference.
FLOW RATE = PRESSUREDIFFERENCE/RESISTANCE
The overall pressure difference is between the ascending aorta and the entrance to the right atrium - the circulatory pressure (about 100 mmHg or torr). (Fig and 31 of Seeley) Note the overall cross-sectional areas.

22. POISEUILLE’S LAW
FLOW RATE = PRESSURE DIFFERENCE/Resistance
=(PRESSURE DIFFERENCE) / (8VL/  R4 )
Where V is the viscosity of the blood, L the length of the blood vessel and R the radius of the blood vessel..
How can you obtain a faster flow rate?
Apply a large entrance pressure -- P
Keep the diameter large -- R
Use low viscosity fluid -- V
Use a short tube -- L

23. Critical closing pressure and Laplace's law.
Blood vessels collapse below a critical closing pressure.
At this point, the blood flow stops.
Thus, if the blood pressure is reduced by shock or loss of blood, it may lead to a collapsed blood vessel.

24. Laplace's law states:
F = D * P
where, F is the force applied to the vessel, D diameter of the vessel and P the blood pressure.
If the blood pressure remains the same, but a disease caused to enlarge the diameter of the blood vessel, the force to the blood vessel will increase and could even burst the blood vessel, if its wall is not strong enough!!

25. Laminar and Turbulent Flow
Laminar flow
Streamlined
Outermost layer moving slowest and center moving fastest
Turbulent flow
Interrupted
Rate of flow exceeds critical velocity
Fluid passes a constriction, sharp turn, rough surface

26. Blood Pressure Measure of force exerted by blood against the wall
Blood moves through vessels because of blood pressure
Measured by listening for Korotkoff sounds produced by turbulent flow in arteries as pressure released from blood pressure cuff

27. Blood Pressure Measurement

29. Physiology of Systemic Circulation
Determined by
Anatomy of circulatory system
Dynamics of blood flow
Regulatory mechanisms that control heart and blood vessels
Blood volume
Most in the veins
Smaller volumes in arteries and capillaries

30. b Resistance
Resistance to circulation is dominated at the site of the arterial system, inclusive of capillaries. The peripheral resistance, and resistance of the venous system is low.
Resistance is dependent on the viscosity of blood, the diameter and length of small blood vessels.

31. i. Vascular resistance
It is not difficult to imagine that vascular resistance becomes large when the blood attempts to pass through thin capillary blood vessels.
It is the resistance between the blood matters and the wall of the capillaries.
The size of capillaries may also be regulated with the contraction and relaxation of muscle tissues, arterioles and constriction of precapillary sphincter muscles.

32. ii. Viscosity
We have already seen that the viscosity of blood is about five times higher than that of water.
Under pathological conditions blood viscosity may change due to the change of erythrocytes or plasma protein contents.

33. iii. Turbulence
When a solution passes through a smooth tubing, the flow will be fastest at the center of the tube and, due to the adhesion to the wall, the slowest near at the wall forming multiple layers of liquid with different velocities - LAMINAR flow.
Turbulence of flow may be created when the LAMINAR flow is disturbed by adding a restrictive substance in the flow, bending or changing he diameter of the vessels.
When there is a sudden increase in the diameter of the vessel, turbulence will be created causing the flow rate to decrease and decrease the pressure.
Turbulent blood flow becomes the source of sound, especially in the heart and can be heard with the stethoscope leading observation of Korotkoff sounds.

34. c. Circulatory pressures
The change of pressure in the circulatory system is shown in Fig. 21-9, 10

36. i. Arterial blood pressure
Note that the systolic and diastolic pressures (pulse pressure) of the ventricle are distinctive in the arteries. The pressure drops gradually as the blood passes through arterioles, then drops more in the pressure prior to the capillary sphincter, and in the veins.

38. ii. Capillary pressures and capillary dynamics
Up to 40 mmHg of pressure difference exist between the entrance and exit of a capillary and is an important difference.
The exchanges of blood substances across the capillary epithelia are via (a) passing through the cellular gaps, (b) solubilizing through the membranes (c) fenestrae, and (d) through vesicles. Fig
Also note that in the brain, the matters cross through the capillary epithelia only through mediated transport.

40. At least, several driving forces must be considered:
(a) Blood pressure difference between the lumen of capillary and interstitial fluid,
(b) The concentration gradient
(c) Osmotic pressure.
Recall that the osmotic pressure drives transport of water from the region of a low concentration of solute to that of a higher concentration across a semi-permeable membrane.
Thus, at the entrance to the capillary, high blood pressure pushes water out from the capillary, but a higher osmotic pressure in the capillary due to its high content of plasma proteins, brings the water back into the capillary. The balance of the two is to push water out of the capillary.
(Fig )

41. Fluid Exchange Across Capillary Walls

43. The blood pressure decreases at the other side of the capillary, but the osmostic pressure (proteins inside the capillaries vs. proteins in the tissue fluid) remains about the same. The balance will be that to bring water back into the capillary.
Water and solubilized substances, such as oxygen, glucose, nutrients, may get into the interstitial fluids along with their concentration gradients at the start of the capillary.
90% of water (the rest goes to the lymphatic circulatory system) and the waste products will come back to the capillary at the other end.

44. Vein Characteristics and Effect of Gravity on Blood Pressure
Venous return to heart increases due to increase in blood volume, venous tone, and arteriole dilation
Effect of Gravity
In a standing position, hydrostatic pressure caused by gravity increases blood pressure below the heart and decreases pressure above the heart

45. iii. Venous pressure
The venous pressure is low and it gets lower as the blood vessels become larger.
It is only 2 torr at the right atrium, or about 16 torr in the vein.
Blood flow through the venous of skeletal muscles may be maintained through muscular compression and the valves.
The calves are the second heart.

47. Control of Blood Flow by Tissues
Local control
In most tissues, blood flow is proportional to metabolic needs of tissues
Nervous System
Responsible for routing blood flow and maintaining blood pressure
Hormonal Control
Sympathetic action potentials stimulate epinephrine and norepinephrine

48. 3. Cardiovascular regulation (Fig. 21-14)
The homeostatic mechanism to maintain the steady flow of blood relies on (a) cardiac output, (b) peripheral resistance, and (c) blood pressure.
They are under the control of neuronal and endocrine systems.
Throughout the body, most of the cells are relatively close to the capillaries. Within 134 um.
This is important, since after leaving capillaries, nutrients and oxygen must reach the target sites mostly by diffusion.
In addition to the neuronal and endocrine factors, the regulation of cardiovascular function may be maintained through "local factors"
Local factors imply the shift of blood flows within the capillary bed to direct the flow to the needed site - auto-regulation.

51. a. Autoregulation of blood flow
Precapillary sphincter muscle responds to the tissue oxygen or carbon dioxide level and regulates the direction of blood flow.
Release of histamine, bacterial toxin and prostaglandins cause a relaxation of precapillary sphincters and vasodilation occurs at the injury site - vasodilators vs. vasoconstrictors. (Fig of Seeley)

52. Local Control of Blood Flow by Tissues
Blood flow can increase 7-8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism
Vasodilator substances produced as metabolism increases
Vasomotion is periodic contraction and relaxation of precapillary sphincters

53. b. Neuronal control of blood pressure and blood flow FIG. 21
b. Neuronal control of blood pressure and blood flow FIG of Seeley.
The nervous system regulates the cardiac output and peripheral resistance.
We have already learnt that the cardioacceleratory center in the medulla oblongata to increase cardiac output through sympathetic innervation and the cardioinhibitory center to reduce cardiac output through parasympathetic innervation.
The regulation of peripheral resistance at the region of the arterioles is primarily dependent on the vasomotor center of the medulla oblongata. Through the sympathetic motor neurons, it provides tonic stimulation demonstrating vasomotor tone that partially constricts the blood vessels. This peripheral resistance results in a lower cardiac output.
Another region of the vasomotor center is initiatory to the above and results in vasodilation, thus an increased cardiac output.
The cardiovascular center of the medulla oblongata responds to the signals from the baroreceptors and chemoreceptors in response to the blood pressure change, pH and dissolved gases.

54. Nervous Regulation of Blood Vessels

55. Short-Term Regulation of Blood Pressure
Baroreceptor reflexes
Change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure
Chemoreceptor reflexes
Sensory receptors sensitive to oxygen, carbon dioxide, and pH levels of blood
Central nervous system ischemic response
Results from high carbon dioxide or low pH levels in medulla and increases peripheral resistance

56. Baroreceptor Reflex Control

58. Chemoreceptor Reflex Control

61. Long-Term Regulation of Blood Pressure
Renin-angiotensin-aldosterone mechanism
Vasopressin (ADH) mechanism
Atrial natriuretic mechanism
Fluid shift mechanism
Stress-relaxation response

62. Renin-Angiotensin-Aldosterone Mechanism

64. Vasopressin (ADH) Mechanism

66. iii. Erythropoietin
Loss of blood pressure or the oxygen content, releases erythropoietin from the kidneys.
Increased erythrocytes.

67. iv. Atrial natriuretic peptide
Released when blood pressure is increased.
When the atrial wall is stretched this hormone is made.
Reduces blood volume and pressure by (a) loss of Na+ and water in the kidneys, (b) increasing water losses at the kidneys by blocking ADH and aldosterone, (c) blocking the release of epinephrine and norepinephrine, (d) peripheral vasodilation.

68. Shock Inadequate blood flow throughout body Three stages
Compensated: Blood pressure decreases only a moderate amount and mechanisms able to reestablish normal blood pressure and flow
Progressive: Compensatory mechanisms inadequate and positive feedback cycle develops; cycle proceeds to next stage or medical treatment reestablishes adequate blood flow to tissues
Irreversible: Leads to death, regardless of medical treatment

69. Fig. 21-35 Fetal circulation