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

Vessel Cross-sectional area (cm2)Aorta 2.5

Small arteries 20

Arterioles 40

Capillaries 2500

Venules 250

Small veins 80

Venae cavae 8

• 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

• 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 flowP – 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 conductanceConductance = 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.