vascular sys student notes

Types of Capillaries

1) Continuous

2) Fenestrated

3) Sinusoids

In addition, to provide more selective exchange of materials, there may be additional barriers (e.g.:

1) [from last term]

2) [later this term]

Acute Control of Local Blood Flow

• Increases in tissue metabolism lead to increases in blood flow.

• Decreases in oxygen availability to tissues increases tissue blood flow.

• Two major theories for local blood flow are:1) The vasodilator theory2) Oxygen demand theory

Tissue Metabolism Blood Flow

Effect of Tissue Metabolic Rate on Tissue Blood Flow

Figure 17-1; Guyton and Hall

Effect of Tissue Oxygen Concentration on Blood Flow

Tissue Oxygen Concentration Blood Flow


Vasodilator Theory for Blood Flow Control

• Vasodilators: Adenosine, CO2, Lactic acid, ADP compounds, Histamine, K ions, H ions

TISSUE METABOLISM

BLOOD FLOW

ARTERIOLE RESISTANCE

RELEASE OFVASODILATORS

TISSUE METABOLISMOR

OXYGEN DELIVERY TO TISSUES

ARTERIOLE RESISTANCE

TISSUEOXYGEN

CONCENTRATION

BLOOD FLOW

Oxygen Demand Theoryfor Blood Flow Control

Local Control of Blood Flow

• Each tissue controls its own blood flow in proportion to its needs.

• Tissue needs include:1) delivery of oxygen to tissues2) delivery of nutrients such as glucose, amino

acids, etc.3) removal of carbon dioxide hydrogen and other

metabolites from the tissues4) transport various hormones and other

substances to different tissues

• Flow is closely related to metabolic rate of tissues.

Control of Blood Vessels - I

Extrinsic Factors:

Autonomic Control

1) Sympathetic nerves:a) adrenergic stimulation of smooth muscles by

(mostly) norepinephrine causes vasoconstriction.even when calm, this system is maintaining alevel of “tone” – when this fails, you “faint”from the resulting drop in blood pressure.

Stronger sympathetic reaction will vasoconstrictvessels in the digestive tract, kidneys and skin.

b) [forget adrenergic stim. – relatively unimportant]

c) Arterioles in muscles receive “unusual” Sympathetic Cholinergic fibers – only under strongest sympathetic stimulation, they releaseAch, dilating these vessels!

Control of Blood Vessels - II

2) Parasympathetic Stimulation:Always Cholinergic, and always produce vasodilation.

Limited to digestive tract, external genitals (e.g.,penile erection), and salivary glands.

Hormonal Control

1) Systemic (widespread):a) Angiotensin II – vasoconstrictor, physiologically

important when there is a loss of blood.b) ADH (Vasopressin) – direct vasoconstriction only

occurs at pharmacological levels.2) Localized:

a) Histamines – vasodilator, at sites of inflammation and allergies.

b) Bradykinins – vasodilator, secreted by sweat glands, probably helps evaporative cooling.

Humoral Regulation of Blood Flow

• VasoconstrictorsNorepinephrine and epinephrineAngiotensinVasopressinEndothelin

• Vasodilator agentsBradykininSerotoninHistamineProstaglandinsNitric oxide

Control of Blood Vessels - III

Intrinsic Factors:

1) Metabolic FactorsVasodilation:

Diminished Oxygen, pH (increased acidity – as from lactic acid release)

e.g., Reactive Hyperemia – your arm orfinger turning red following the release ofa tourniquet

Increased Carbon Dioxide, Adenosine, K+. Other factors (e.g., NO, with a variety of uses)

2) Myogenic Factors – stretch response (a characteristic of vascular smooth muscle) creating Autoregulation.

Autoregulation

e.g., kidneys, brain (belowBlood 200 mmHg)FlowRate

Autoregulated range

Arteriole or Precapillary Pressure

Blood Flow Autoregulation Theories

• Metabolic theory suggests that as arterial pressure is decreased, oxygen or nutrient delivery is decreased resulting in release of a vasodilator.

• Myogenic theory proposes that as arterial pressure falls the arterioles have an intrinsic property to dilate in response to decreases in wall tension. I.e., it is the effect of unitary smooth muscle – stretch it, and it contracts in response.

Blood Flow Autoregulation Theories

• Metabolic theory suggests that as arterial pressure is decreased, oxygen or nutrient delivery is decreased resulting in release of a vasodilator.

• Myogenic theory proposes that as arterial pressure falls the arterioles have an intrinsic property to dilate in response to decreases in wall tension. I.e., it is the effect of unitary smooth muscle – stretch it, and it contracts in response.

• Certain tissues have other mechanisms for blood flow control the kidneys have a feedback system between the tubules and arterioles and the brain blood flow is controlled by carbon dioxide and hydrogen ion conc.

Venous Pressure in the Body

•Gravity in the upright human body can cause a vertical gradient of venous pressures (pressures are analogous to water pressures, likewise due to the weight of water or blood).

•Veins’ valves (in legs) have to help pump blood upward (against gravity) back to the heart.

Starling Forces• Normal Capillary hydrostatic pressure is

approximately 17 mmHg.

• Interstitial fluid pressure in most tissues is negative 3. Encapsulated organs have positive interstitial pressures (+5 to +10 mmHg).

• Negative interstitial fluid pressure is caused by pumping of lymphatic system.

• Colloid osmotic pressure is caused by presence of large proteins.

Structure of Capillary Wall

• Composed of unicellular layer of endothelial cells surrounded by a basement membrane.

• Diameter of capillaries is 4 to 9 microns.

• Solute and water move across capillary wall via intercellular cleft (space between cells) or by plasmalemma vesicles.


Effect of Molecular Size on Passage Through Capillary Pores

• The width of capillary intercellular slit pores is 6 to 7

nanometers.

• The permeability of the capillary pores for different substances varies according to their molecular diameters.

• The capillaries in different tissues have extreme differences in their permeabilities.


Variations in Tissue Blood Flow

Brain 14 700 50Heart 4 200 70Bronchi 2 100 25Kidneys 22 1100 360Liver 27 1350 95 Portal (21) (1050) Arterial (6) (300)Muscle (inactive state) 15 750 4Bone 5 250 3Skin (cool weather) 6 300 3 Thyroid gland 1 50 160 Adrenal glands 0 .525 300 Other tissues 3.5 175 1.3

Total 100.0 5000 ---

Percent ml/minml/min/100 gm

Blood Pressure Profile in the Circulatory System

Systemic Pulmonary

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• High pressures in the arterial tree• Low pressures in the venous side of the circulation• Large pressure drop across the arteriolar-capillary junction

Blood Flow• Blood flow is the quantity of blood that passes a given

point in the circulation in a given period of time.

• Unit of blood flow is usually expressed as milliliters (ml) or Liters (L) per minute.

• Overall flow in the circulation of an adult is 5 liters/min which is the cardiac output.

Blood Vessel

Characteristics of Blood Flow• Blood usually flows in streamlines with each layer of

blood remaining the same distance from the wall, this type of flow is called laminar flow.

– When laminar flow occurs, the velocity of blood in the center of the vessel is greater than that toward the outer edge creating a parabolic profile.

Blood Vessel

Laminar flow

Laminar Vs. Turbulent Blood Flow

Turbulent flow

Causes of turbulent blood flow:• high velocities• sharp turns in the circulation• rough surfaces in the circulation• rapid narrowing of blood vessels

• Laminar flow is silent, whereas turbulent flow tend to cause murmurs.

• Murmurs or bruits are important in diagnosing vessels stenosis, vessel shunts, and cardiac valvular lesions.

Damping of Pulse Pressuresin the Peripheral Arteries

• The intensity of pulsations becomes progressively less in the smaller arteries.

• The degree of damping is proportional to the resistance of small vessels and arterioles and the compliance of the larger vessels.


Abnormal Pressure Pulse Contours



• Arteriosclerosis–decreases compliance of arterial tree, thus leading to increase in pulse:The matter of arteriosclerosis is its effect on vessels’ compliance, reducing it and effectively “stiffening” the vessels’ walls. Normal vessel walls expand elastically to accommodate the stroke volume of each heartbeat and absorb and store the resulting increased pressure’s energy – preventing what would otherwise be a large pressure pulse. Arteriosclerotic vessels cannot elastically expand as much, and more of the pressure generated by the forceful ejection of blood (its stroke volume) results in a much larger (higher) pressure pulse.Diastolic pressure, however, is only minimally affected.


• Arteriosclerosis• Aortic Stenosis—less stroke volume:

Such narrowing, or inability to fully open, of the aortic semilunar valve simply reduces the amount of stroke volume that the heart can eject into the aorta (past this occlusion, a point of high resistance to blood flow). Less output means less pressure pulse and a lower pulse contour.


• Arteriosclerosis• Aortic Stenosis• Patent ductus arteriosus–associated with low diastolic

pressure and high systolic pressure, net result is very high pulse pressure:this fetal blood vessel has remained open instead of its normal closure after birth, and more than half the blood ejected into the aorta is diverted, instead, through this shunt into the (lower pressure) Pulmonary Trunk, letting the diastolic pressure drop significantly before each subsequent contraction. The systolic pressure is high, however, because the body compensates by stronger stimulations of the heart to compensate.


• Arteriosclerosis• Aortic Stenosis• Patent ductus arteriosus• Aortic regurgitation–a backward flow of blood through the

aortic valve. Low diastolic and high systolic pressure leads to high pulse pressure:this valve is defective – it fails to properly close and seal at the end of ventricular contraction, and therefore pressurized blood in the aorta readily flows backward into the left ventricle again. Systolic pressure is high, to compensate, but diastolic pressure falls even farther (it could drop to almost zero) because there is no effective valve; and thus, there is no incisura because there is no closed valve to abruptly halt the reversed pulse of blood flow.

Hydro- (or “Hemo-”) dynamics

Ohm’s Law: Current =

Poiseuille’s Law:Flow = ( Pressure / Resistance )

Resistance = ( [length * viscosity] / r4 )

[pwah-suh’-yez]

Voltage

Resistance

“Laminar Flow” through vessels

Relative Velocity in a vessel = radial distance from wall

Area of high friction – turbulence Area of low friction (blood vs. endothelium) (smooth laminar flow)

equal

Small vessel has a higher proportionof high friction blood flow than a largervessel (i.e., ratio of to ).

Hydro- (or “Hemo-”) dynamics

Flow = ( Pressure / Resistance )

Resistance = ( [length * viscosity] / r4 )

Because of the radius to the FOURTH power:

A vessel 4 mm in diameter has 16 x less resistance, or permits 16 x more flow (given the same pressure)

than a vessel that is 2 mm in diameter (half the diameter)– i.e., a 2:1 difference in diameter results in a 16:1

difference in flow!

Pressure is a form of energy !

And it follows Laws ofThermodynamics!

Pressure does not just “come back” –it must be replaced by new energy converted from other energy (i.e.,

chemical, during muscle contraction:

e.g., the heart, or the muscles aroundthe valve-equipped veins in the legs.

Regarding these vessels’ sizes:

• An individual vessel’s cross-sectional area:This affects the rate of the loss of pressure,

due to resistance of blood passing through narrower vessels.

• Total (for all vessels, of a given size) cross-sectional area:This affects the velocity of the blood: faster

when small, slower when large, as follows…

Velocity vs. Cross-sectional Area

Flow rate = 80 cm³/min.

15 cm.15 cm.

The volume of a cylinder is cross-sectional area (r²) times length:The volume of the left cylinder is x 2² x 15, 3.1416 x 4 x 15, 188.5 cm³.The volume of the right cylinder is x 4² x 15, 3.1416 x 16 x 15, 754cm³.

How long does it take for a fluid to flow through each segment? Thesame time to fill a hypothetical container of the same dimensions.

At 80 cm³/min., the left cylinder would fill in 188.5/80, or 2.36 minutes,while the right cylinder would fill in 754/80, or 9.46 minutes. So,

Velocities: left cylinder: 15 cm./2.36 min. = 6.4 cm/min.right cylinder: 15 cm./9.46 min. = 1.6 cm/min.

Fluid flows through the left cylinder FOUR times faster than the right.

4 cm. 8 cm.

Material Exchange in Capillaries

The Purpose of the Circulatory is largely realized in theCapillaries – it is the place where exchange takes place!!!

Exchange takes place by:

1) Diffusion: Hydrophobic substances, which can pass through

hydrophobic cell membranes. Gases.

2) Cytopempsis (active transport, via vessicles): Many substances, but mostly in special areas

(digestive tract, kidney, liver).

3) Filtration or Bulk Flow (Starling Model): Most hydrophilic substances except the largest

proteins that cannot fit through capillary “pores”.

Filtration, or Starling Forces

Arterial End Venous End

HydrostaticHydrostatic

Osmotic Osmotic

Net Flow:Outward

Net Flow:Inward

Summary of Starling Forces

Arterial

FILTRATION

Blood Bulk FlowVessel

Negative FILTRATION or “Reabsorption”

Tissue CellsVenous

Summary of Starling Forces

Arterial End Venous End

HydrostaticHydrostatic

Osmotic Osmotic

FILTRATION

REABSORPTION

Remember, arrows are directed with respect to the capillary: 1) Forces outward from the capillary (arrows to the right, in the

example below) are positive. 2) Forces inward (arrows to the left in this example) are neg. Then, just add them up algebraically ….

Arterial end of the capillary, for example…

Capillary (hydrostatic) pressure, always outward because it pushes fluid out of the capillary, is positive. E.g., this is 28 in this example.

Plasma colloid osmotic pressure, always inward because it draws fluid from the interstitium into the capillary, is negative. E.g., this is 29 in this example.

Interstitial tissue pressure, always inward because it pushes fluid from the interstitium to the capillary, is always algebraically added as a negative, regardless of whether it is actually negative (in most tissues, as in this example as –3) or positive as in encapsulated organs.

Interstitial colloid osmotic pressure, always outward because it draws fluid from the capillary to the interstitium, is positive. E.g., 8.

Now, algebraically add these up: Capillary pressure:

28 Interstitial pressure:

– (–3) [this is +3]

Plasma osmotic pressure:

–29 Interstitial osmotic pressure:

8 Total of Starling forces at this arterial end:

+10

ARTERIAL End’s Net Starling Force (pos. is outward, neg. is inward):

Cap. Hydro. Press. = +28 ( + because it’s an outward force)

Plasma Coll. Osm. Press. = –29 ( – because it’s inward)

Interstitial Tiss. Press. = – (– 3) = +3 ( – because it’s inward)

Interstitial Colloid Osm. Press. = +8 ( + because it’s outward)

Sum total = net Starling Force = +10 mm Hg (an outward force)

VENOUS End’s Net Starling Force (pos. is outward, neg. is inward):

Cap. Hydro. Press. =

Plasma Coll. Osm. Press. =

Interstitial Tiss. Press. =

Interstitial Colloid Osm. Press. =

Sum total = net Starling Force =

FILTRATION vs. REABSORPTION

Pressure (mm Hg.)

45 Normal

18

Areas ( = flows):Filtration > Reabsorption

Arterial end Venous end

Hydrostatic P.

Osmotic P.Filtration

Reabsorption


Pressure (mm Hg.)

45 Normal

18



Pressure (mm Hg.)

45 Pre-cap Constr.

18



Pressure (mm Hg.)

45 Pre-cap Dilation

18



Pressure (mm Hg.)

45 Incr. Ven. Press.

18



Pressure (mm Hg.)

45 Decr. Osm. Press.

18


Alternative to the Starling Model

Pressure (mm Hg.) Precapillary

45 Dilation

18


Alternative to the Starling Model

Pressure (mm Hg.) Precapillary

45 Constriction

18


The changes that occur at birth are the most dramaticand profound changes that will occur at any timeduring your life:

Pay particular attention to FOUR structures:

1) The Lungs.

2) The Ductus arteriosus.

3) The Foramen ovale.

4) The Ductus venosus.

as well as (obviously) the Placenta and the UmbilicalCord containing two arteries and one vein.

The placement of the Placenta in the Systemic Circuit (parallel with the body’s circulation) presents a substantial additional load on the left ventricle – in effect, the equivalent of two bodies. The right ventricle, on the other hand, is otherwise underutilized (for reasons to be discussed).

Retrospectively, it might have been wiser to evolve so that the placenta is parallel to the lungs (to reduce the work of the left ventricle and to better use the substantially underutilized right ventricle) – but evolution found an alternative way.

Thus, a noteworthy adaptation in mammalian reproduction is the utilization of the right ventricle to help the left with its doubled burden – the Ductus arteriosus (and to a lesser extend, the Foremen ovale) accomplishes this.

Keep in mind that at birth, certain events trigger the vascular changes:

1) The baby takes its first breath, inflating the lungs –Prior to this, the not-yet-inflated lung containscompacted pulmonary vessels that present high resistance to the flow of blood. When the lungs inflate, the pulmonary vessels dilate dramatically and reduce their resistance to 1/10 of what it was.

2) The Umbilical Cord (and its vessels) is clamped,stopping flow inward from the umbilical vein, and occluding outflow through the two umbilical arteries.

These, through a series of “domino” effects, will induceall the other necessary changes.

Before Birth,

1) Because of the high resistance through the compactedlung’s blood vessels, only 1/10 of the Right Ventricle’soutput flows to the lungs; 9/10 of its output passes fromthe Pulmonary Trunk through the open Ductus arteriosusinto the Aorta, which offers relatively less resistance tothe blood flow; the Right Ventricle has greater totaloutput than the Left Ventricle, and is often larger at birth.

2) The pulmonary resistance increases right-side pressure,even in the Atria; this keeps the Foramen ovale’s valveopen, and blood flows right to left. Meanwhile, theUmbilical Vein flows into the Ductus venosus, whichdrains into the Inferior Vena Cava; here, most of thatumbilical blood stays on one side of the IVC and is mostof the blood that flows through the Foramen ovale to theleft side.

At Birth,

1) a) Umbilical Vein blood flow stops – the Ductus venosus eventuallycloses; the lessened IVC flow reduces right Atrium pressure.

b) The lungs inflate, and its Pulmonary vessels dilate – thesevessels’ resistance drops to 1/10 of what it was (and is even less than the systemic resistance). This lowered resistance also contributes to lessening right Atrium (and Ventricle) pressure.

2) a) The increased pulmonary return into the left Atrium increases its pressure.

b) Occlusion of the Umbilical Arteries increases systemicresistance and left Ventricle (and Atrium) pressures.

Because right Atrium pressure drops (#1a&b) while left Atrium pressure rises (#2a&b), blood flows momentarily flows from the left to the right Atrium through the Foramen ovale, whose valve then quickly closes it –eventually, it seals permanently and becomes the Fossa ovalis.

At Birth (continued),

3) Lowered right Ventricle pressure (#1b, above) and greater left ventricle pressure (#2b) makes Aorta pressure higher thanPulmonary Trunk pressure – this makes blood reverse flow in the Ductus arteriosus.

4) Because the lungs are now functioning, blood from the left side of the heart is now better oxygenated than it ever was when the fetuswas getting oxygen from the placenta – the oxygen saturation ishigher (see Hb’s oxygen dissociation graph of HbA and HbF).

The reversed flow (#3) of highly oxygenated blood (#4) through theDuctus arteriosus stimulates its muscles and they contract and closethis shunt. It eventually seals and becomes the Ligamentumarteriosum.

With the closing of both the Foramen ovale and also the Ductusarteriosus, the separation of the Systemic and Pulmonary Circuits iscomplete along with assurance that systemic blood will be completelyoxygenated, instead of diluted with deoxygenated blood by mixing thesecircuits’ bloods as it was in the fetus – the newborn baby “pinks up”.

vascular sys student notes

Documents

blood flow blood flowis

brain blood flow

local blood flow

blood flow inproportion

tissue blood flow brain

local control of blood

type of flow

calledlaminar flow