1. Hemodynamics, Blood pressure and Microcirculation
Dr. Meg-angela Christi Amores

2. The Circulation Functions: to transport nutrients to the body tissues
to transport waste products away
to conduct hormones from one part of the body to another
rate of blood flow through most tissues is controlled in response to tissue need for nutrients

3. Physical Characteristics
Arteries
transport blood under high pressure to the tissues
have strong vascular walls, high blood flow rate

4. Arterioles last small branches of the arterial system
act as control conduits through which blood is released into the capillaries
capability of vastly altering blood flow in each tissue bed in response to the need of the tissue

5. Physical Characteristics
Capillaries
exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid
very thin and have numerous minute capillary pores permeable to water

6. Venules
collect blood from the capillaries, and they gradually coalesce into progressively larger veins
Veins
conduits for transport of blood from the venules back to the heart
Thin walled, muscular enough to expand or contract

7. Cross-sectional area (cm2)
The circulation
If all the systemic vessels of each type were put side by side, their approximate total cross-sectional areas for the average human being would be as follows:
Note larger cross sectional area of veins than arteries, explains large storage of blood in the venous system
Vessel
Cross-sectional area (cm2)
Aorta
2.5
Small arteries 20
Arterioles 40
Capillaries
2500
Venules
250
Small veins 80
Venae cavae 8

8. velocity of blood flow is inversely proportional to vascular cross-sectional area
velocity averages about 33 cm/sec in the aorta but only 1/1000 as rapidly in the capillaries, about 0.3 mm/sec

9. Basic theory of Circulatory Function
The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need
The cardiac output is controlled mainly by the sum of all the local tissue flows
In general the arterial pressure is controlled independently of either local blood flow control or cardiac output control.

10. Blood flow - the quantity of blood that passes a given point in the circulation in a given period of time
expressed in milliliters per minute or liters per minute
overall blood flow in the total circulation of an adult person at rest is about 5000 ml/min

11. Factors affecting blood flow
pressure difference of the blood between the two ends of the vessel – pressure gradient
the force that pushes the blood through the vessel
the impediment to blood flow through the vessel – vascular resistance
Ohm’s law:
blood flow is directly proportional to the pressure difference but inversely proportional to the resistance

12. P – pressure difference R - resistance
Ohm’s Law:
F = P R
F – blood flow
P – pressure difference
R - resistance

13. Laminar vs. Turbulent blood flow
Laminar flow
blood flows at a steady rate through a long, smooth blood vessel
flows in streamlines
each layer of blood remaining the same distance from the vessel wall
the central most portion of the blood stays in the center of the vessel

14. Laminar vs. Turbulent blood flow
Turbulent flow
blood flowing in all directions in the vessel and continually mixing within the vessel
When the rate of blood flow becomes too great
when it passes by an obstruction in a vessel
when it makes a sharp turn
when it passes over a rough surface
Increased resistance to blood flow

15. Laminar vs. Turbulent blood flow

16. Resistance to blood flow
Resistance - the impediment to blood flow in a vessel, but it cannot be measured by any direct means
Conductance - measure of the blood flow through a vessel for a given pressure difference
Slight changes in the diameter of a vessel cause tremendous changes in conductance
Conductance = Diameter4

17. Resistance to blood flow
Poiseuille’s Law
the rate of blood flow is directly proportional to the fourth power of the radius of the vessel
the diameter of a blood vessel (which is equal to twice the radius) plays by far the greatest role of all factors in determining the rate of blood flow through a vessel
F= π Pr4
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18. In large diameter vessels, with laminar flow, the velocity is different in concentric rings
the blood that is near the wall of the vessel flows extremely slowly, whereas that in the middle of the vessel flows extremely rapidly

19. Effects of pressure Increase in arterial pressure:
increases the force that pushes blood through the vessels
distends the vessels at the same time, which decreases vascular resistance

20. Vascular Distensibility
all blood vessels are distensible
the distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations
most distensible by far of all the vessels are the veins, providing a reservoir function for storing large quantities of extra blood

21. Arterial Pulse Pulsations
Because of vascular distensibility, blood flow is continuous, with systole and diastole
Blood does not flow instantaneously in the peripheral circulation all at once
Pressure pulsations:
Systolic pressure – 120mmHg – pressure at top of each pulse
Diastolic pressure - 80mmHg – at the lowest point of each pulse
Pulse pressure – difference between SP and DP

22. Methods in determining BP
Direct method:
direct catheter measurement from inside the arteries
Most accurate
impractical

23. Methods in determining BP
Auscultatory method
Stethoscope over antecubital area
BP cuff inflated over upper arm
Korotkoff sounds
Mechanism:
When cuff pressure is higher than systolic P, brachial artery remains occluded
As cuff pressure is reduced, blood jets through the artery, hearing tapping sounds from antecubital artery
When cuff pressure is equal diastolic pressure, blood no longer jets through squeezed artery, tapping stops

24. To be continued.....
Next topic: Control of BP

25. Nervous control of BP Autonomic nervous system
Sympathetic NS – most important regulator
Leave SC through Thoracic and L1 and L2 spinal nerves
To the sympathetic chain
Innervates all vessels except capillaries, precapillary sphincters and metarterioles
capability to cause rapid increases in arterial pressure

26. Nervous control of BP 3 major changes:
Almost all arterioles of the systemic circulation are constricted
The veins especially (but the other large vessels of the circulation as well) are strongly constricted.
Finally, the heart itself is directly stimulated by the autonomic nervous system, further enhancing cardiac pumping

27. Nervous control of BP
the most rapid of all our mechanisms for pressure control
often increasing the pressure to two times normal within 5 to 10 seconds

28. Nervous control During exercise During stress
motor areas of the brain become activated to cause exercise, most of the reticular activating system of the brain stem is also activated
increased stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor center
During stress
In extreme fright, the arterial pressure sometimes rises to as high as double
Called alarm reaction

29. Reflex mechanisms for maintaining normal BP
Autonomic nervous system
Mostly Negative feedback reflex mechanisms
Baroreceptor Reflexes
Initiated by stretch receptors in large systemic arteries
Inc arterial pressure stretches baroreceptors
Sends signals to CNS
Negative feedback signals are sent back

30. Baroreceptor reflex Locations:
Internal Carotid artery bifurcation – carotid sinus
Wall of Aortic Arch
Signals enter tractus solitarius of the medulla
Inhibit vasomotor center and excite vagal parasympathetic center
Effects: vasodilatation of arteries and veins
decreased heartrate and heart contraction

31. Baroreceptor reflex During changes in posture
Upon standing, arterial pressure in head falls, causing loss of consciousness
Prevented by the reflex, causing strong sympathetic discharge throughout the body

32. Long term control of BP
Which organ regulates arterial pressure in the long term?

33. Renal-Body Fluid System for Arterial Pressure Control
two primary determinants
The degree of pressure shift of the renal output curve for water and salt
The level of the water and salt intake line
Role of NaCl
When there is excess salt in the extracellular fluid, the osmolality of the fluid increases stimulates the thirst center in the brain
stimulates the hypothalamic-posterior pituitary gland secretory mechanism to secrete increased quantities of antidiuretic hormone

34. MICROCIRCULATION

35. MICROCIRCULATION
Where the most purposeful function of the circulation occurs
transport of nutrients to the tissues and removal of cell excreta
CAPILLARIES
walls of the capillaries are extremely thin, constructed of single-layer, highly permeable endothelial cells

36. Microcirculation
10 billion capillaries with a total surface area estimated to be 500 to 700 square meters (about one-eighth the surface area of a football field)
Nutrient artery – arterioles – metarteriole –precapillary sphincter - capillaries

37. Flow of blood through capillaries
Intermittent – turning on and off every few seconds
Phenomenon of VASOMOTION – intermittent contraction of metarterioles and sphincter
Physiologic significance:
most important factor found thus far to affect the degree of opening and closing of the metarterioles and precapillary sphincters is the concentration of oxygen in the tissue.

38. Transcapillary movement
Diffusion
Lipid-Soluble Substances Can Diffuse Directly Through the Cell Membranes of the Capillary Endothelium
Water-Soluble, Non-Lipid-Soluble Substances Diffuse Only Through Intercellular "Pores" in the Capillary Membrane.

39. Transcapillary movement
Effect of Molecular Size on Passage Through the Pores
width of the capillary intercellular cleft-pores, 6 to 7 nanometers, is about 20 times the diameter of the water molecule, which is the smallest molecule that normally passes through the capillary pores
Effect of Concentration Difference on Net Rate of Diffusion Through the Capillary Membrane.

40. EDEMA

41. Lymphatics What is lymph?
derived from interstitial fluid that flows into the lymphatics
protein concentration in the interstitial fluid of most tissues averages about 2 g/dl
protein concentration in the interstitial fluid of most tissues averages about 2 g/dl,
lymph formed in the intestines has a protein concentration as high as 3 to 4 g/dl

42. Most of the fluid filtering from the arterial ends of blood capillaries flows among the cells and finally is reabsorbed back into the venous ends of the blood capillaries; but on the average, about 1/10 of the fluid instead enters the lymphatic capillaries and returns to the blood through the lymphatic system rather than through the venous capillaries.

43. Lymphatics
one of the major routes for absorption of nutrients from the gastrointestinal tract, especially for absorption of virtually all fats in food
special lymph channels that drain excess fluid directly from the interstitial spaces
all the lymph vessels from the lower part of the body eventually empty into the thoracic duct
total quantity of all this lymph is normally only 2 to 3 liters each day.