Hemodynamics, Blood pressure
and Microcirculation
Dr. Meg-angela Christi Amores
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
Physical Characteristics
• Arteries
• transport blood under high pressure to the tissues
• have strong vascular walls, high blood flow rate
• 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
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
• 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
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:
Vessel
Cross-sectional area (cm2)
Aorta
2.5
Small arteries
20
Arterioles
40
Capillaries
2500
Venules
250
Small veins
80
Venae cavae
8
• Note larger cross sectional area of veins than arteries,
explains large storage of blood in the venous system
• 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
Basic theory of Circulatory Function
1. The rate of blood flow to each tissue of the
body is almost always precisely controlled in
relation to the tissue need
2. The cardiac output is controlled mainly by
the sum of all the local tissue flows
3. In general the arterial pressure is controlled
independently of either local blood flow
control or cardiac output control.
• 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
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
• Ohm’s Law:
F=
P
R
F – blood flow
P – pressure difference
R - resistance
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
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
Laminar vs. Turbulent blood flow
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
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
8ήl
• 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
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
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
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
Methods in determining BP
• Direct method:
– direct catheter measurement from inside the
arteries
– Most accurate
– impractical
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
• To be continued.....
– Next topic: Control of BP
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
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
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
Nervous control
• During exercise
– 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
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
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
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
Long term control of BP
• Which organ regulates arterial pressure in the
long term?
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
MICROCIRCULATION
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
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
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.
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.
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.
EDEMA
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
• 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.
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.