Blood Vessels and HemodynamicsDr. Michael P. Gillespie

Cardiovascular System

Cardiovascular SystemTransports and delivers blood to the body

to deliver oxygen, nutrients, and hormones as well as carries away wastes.

Blood vessels form a closed system of tubes, which carries blood away from the heart, transports it to the tissues of the body, and then returns it to the heart.

HemodynamicsHemo – blood.Dynamics – power.

Main Types Of Blood VesselsArteries – carry blood away from the heart.Arterioles – very small arteries.Capillaries – tiny vessels which allow

exchange of substances between the blood and body tissues.

Venules – very small veins.Veins – carry blood back to the heart.

Vaso VasorumLarger blood vessels require oxygen and

nutrients just like other tissues of the body.Vaso vasorum (vasculature of the vessels)

are located within the walls of larger vessels and supply them.

Tunics (Coats) Of ArteriesTunica interna (intima) – contains a lining

of endothelium which makes contact with the lumen and blood.

Tunica media – thickest layer and has high compliance (stretches).

Tunica externa – outer coat, elastic and collagen fibers.

Changes In Vascular DiameterVasoconstriction – a decrease in the

diameter of the lumen of a blood vessel.Sympathetic stimulation causes the smooth

vessels of the vessels to contract, squeezing the vessel wall and narrowing the lumen.

Occurs when an artery or an arteriole is damaged, producing vascular spasm and limiting the blood flow to reduce blood loss.

Changes In Vascular DiameterVasodilation – an increase in the diameter

of the lumen of a blood vessel.Occurs when sympathetic stimulation

decreases or when nitric oxide, K+, H+, and lactic acid are present.

Elastic ArteriesElastic arteries propel blood forward

while the ventricles are relaxing.Blood is ejected from the heart and

stretches the walls of the elastic arteries.

The stretch of the arteries stores mechanical energy and act as a pressure reservoir.

The vessels recoil and convert stored (potential) energy in the vessel into kinetic energy of the blood.

Muscular ArteriesMedium sized arteries are muscular

arteries.They contain more smooth muscle and

fewer elastic fibers than elastic arteries.They are capable of greater

vasoconstriction and vasodilation.They are called distributing arteries

because they distribute blood to various parts of the body.

ArteriolesA very small (almost microscopic) artery

that delivers blood to capillaries.Arterioles regulate resistance.Vasoconstriction of arteriole walls

increases resistance to capillaries and vasodilation of arteriole walls decreases resistance.

Resistance regulates blood flow to the capillaries.

CapillariesMicroscopic vessels that connect

arterioles to venules.The flow of blood from arterioles to

venules is microcirculation.Tissues with high metabolic

requirements, such as muscles, liver, kidneys, and nervous system, have more capillaries.

Tissues with lower metabolic requirements, such as tendons and ligaments, contain fewer capillaries.

CapillariesCapillaries are absent in a few tissues, such

as covering and lining epithelia, the cornea of the lens of the eyes, and cartilage.

Exchange vessels – exchange nutrients between blood and tissue cells through the interstitial fluid.

CapillariesSingle layer of endothelial cells.Branch extensively to increase surface area

for exchange.Usually only a small part of the capillary

network is active; However, when a tissue is active (i.e. Contracting muscle) the entire network fills with blood.

MetarterioleA metarteriole (met = beyond) – is a

vessel that emerges from an arteriole and supplies a group of 10 – 100 capillaries (capillary bed).

The proximal end of the metarteriole is surrounded by smooth muscle fibers, which regulate blood flow through the capillary bed.

The distal end of the metarteriole has a thoroughfare channel, which bypasses the capillary bed.

True CapillariesTrue capillaries emerge from arterioles

or metarterioles.Precapillary sphincter – ring of

smooth muscle that controls blood flow into a true capillary.

Vasomotion – intermittent contraction and relaxation of precapillary sphincters and metarteriole smooth muscle (5-10 times per minute).

Types Of CapillariesContinuous capillaries -

Continuous tube interrupted only by intercellular clefts.

Found in smooth muscle, connective tissue, and lungs.

Types Of CapillariesFenestrated capillaries (fenestr = window) -

The plasma membranes have fenestrations (small pores).

Located in the kidneys, villi of the SI, choroid plexus of the ventricles of the brain, cilary processes of the eyes, and endocrine glands.

Types Of CapillariesSinusioids –

Wider and more winding than other capillaries.

Unusually large fenestrations which allow protein and blood to pass from the tissues into the bloodstream.

Found in the liver, spleen, anterior pituitary, and parathyroid glands.

VenulesSmall veins formed when several capillaries

unite.The walls of the smallest venules (closest

to the capillaries) are very porous and serve as a site of emigration for white blood cells.

VeinsVeins contain the same three coats as

arteries.The lumen of a vein is larger than that of a

comparable artery.

VeinsMany veins also contain valves (especially

in the lower limbs).The valves are thin folds of the tunica

interna. The cusps point toward the heart.The valves prevent backflow of returning

blood in the lower pressure venous system.

Vascular (Venous) SinusA vascular (venous) sinus is a vein with a

thin endothelial wall that has no smooth muscle to alter its diameter.

Examples:Dural venous sinuses (supported by dura

mater).Coronary sinus of the heart.

Varicose VeinsLeaky valves can cause veins to become

dilated and twisted in appearance.This is most common in the esophagus

and veins of the lower limb, although it can occur in any veins.

Hemorrhoids are varicose veins in the anal canal.

Deeper veins are not as susceptible because surrounding skeletal muscles prevent their walls from stretching.

AnastomosesThe union of the branches of two or

more arteries supplying the same body region is called an anastomosis.

Anastomoses between arteries provide alternate routes for blood to reach a tissue or an organ.

The alternate route of blood flow is known as collateral circulation.

Arteries that do not anastomose are known as end arteries.

Blood DistributionThe largest portion of your blood volume

at rest is in the veins (60%).Systemic capillaries hold about 5%.The veins and venules function as a

blood reservoir.Blood can be diverted quickly if the

need arises through venoconstriction.The veins of the abdominal organs and

skin serve as principal blood reservoirs.

Capillary ExchangeThe mission of the cardiovascular

system is to keep blood flowing through the capillaries to allow for exchange of nutrients and waste products with the interstitial fluid.

Substances enter and leaved the capillaries through three basic mechanisms:Diffusion.Transcytosis.Bulk flow.

DiffusionSubstances diffuse down their

concentration gradients (from areas of high concentration to low).

All plasma solutes except proteins pass easily across most capillary walls.

Water soluble substances such as glucose and amino acids pass easily through either fenestrations or intercellular clefts.

DiffusionLipid-soluble materials (O2, CO2, & steroid

hormones) pass through the lipid bilayer.Liver capillaries have large gaps which do

allow proteins to pass through. Hepatocytes synthesize proteins such as fibrinogen (clotting) and albumin (osmotic pressure), which diffuse into the blood.

Brain capillaries have tight junctions, which allow only a few substances to enter and leave. This forms the blood-brain barrier.

TranscytosisSubstances within the blood plasma are

enclosed in tiny pinocytic vesicles that enter endothelial cells by endocytosis.

They move across the membrane and exit the other side by exocytosis.

TranscytosisThis method of transport is utilized for

large, lipid-insoluble molecules that cannot cross the capillary walls in any other way.

Insulin enters the blood this way and some maternal antibodies enter the fetal circulation this way.

Bulk Flow: Filtration & ReabsorptionBulk flow is a process by which large

numbers of ions, molecules, or particles in a fluid move together in the same direction.

It occurs from an area of high pressure to an area of low pressure at a rate faster than diffusion would produce alone.

Regulates relative volumes of fluids rather than concentrations of solutes.

Bulk Flow: Filtration & ReabsorptionContinues as long as pressure variances

exist.Pressure driven movement of fluid and

solutes from blood capillaries to interstitial fluid is termed filtration.

Pressure driven movement of fluid and solutes from interstitial fluid into blood capillaries is called reabsorption.

Pressures Involved In Filtration And AbsorptionBlood hydrostatic pressure (BHP) – pressure

from the pumping action of the heart promotes filtration.

Interstitial fluid osmotic pressure filters blood promotes filtration.

Pressures Involved In Filtration And AbsorptionBlood colloid osmotic pressure (BCOP)

promotes reabsorption.Interstitial fluid hydrostatic pressure

promotes reabsorption.Net filtration pressure is the balance of

these pressures (NFP).

Starling’s Law Of The CapillariesThe volume of fluid and solutes reabsorbed

normally is almost as large as the volume filtered.

EdemaIf filtration greatly exceeds reabsorption,

the result is edema (swelling), an abnormal increase in interstitial fluid volume.

Excess FiltrationIncreased capillary blood pressure.Increased permeability of capillaries which

allows plasma proteins to escape. Chemical, bacterial, thermal, or mechanical agents can damage capillary walls.

Inadequate ReabsorptionDecreased concentration of plasma

proteins associated with liver disease, burns, malnutrition, and kidney disease.

HemodynamicsHemodynamics refer to the factors that

affect blood flow.Blood flow is the volume of blood that flows

through any tissue in a given period of time.

Cardiac output (CO) is the total blood flow.Cardiac output (CO) – heart rate (HR) *

stroke volume (SV).

Factors That Determine Distribution Of Cardiac OutputPressure difference drives blood flow

through a tissue.Blood flows from regions of higher to lower

pressure.Resistance to blood flow in specific blood

vessels.The higher the resistance, the smaller the

blood flow.

Blood PressureContraction of the ventricles generates

blood pressure (BP).Systolic blood pressure is the highest

pressure attained in the arteries during systole.

Blood PressureDiastolic blood pressure is the lowest

arterial pressure during diastole.Mean arterial blood pressure (MABP) is

the average pressure in the arteries.MABP = diastolic BP + 1/3 (systolic BP –

diastolic BP).Blood pressure also depends on the

total volume of blood in the cardiovascular system.

ResistanceVascular resistance is the opposition to

blood flow due to friction between blood and the walls of blood vessels.

Vascular resistance depends upon:The size of the blood vessel lumen.Blood viscosity.Total blood vessel length.

Systemic vascular resistance (SVR) is the total peripheral resistance from all factors combined.

Venous ReturnVenous return to the heart is caused by the

following:Pressure generated from contractions of the

heart’s left ventricle.Skeletal muscle pump.Respiratory pump.

Velocity Of Blood FlowThe speed or velocity of blood flow is

inversely related to the cross-sectional area.

Each time an artery branches, the cross sectional area increased and the velocity decreases.

Each time a venule merges to form a vein, the cross sectional area decreases and the velocity increases.

SyncopeSyncope, or fainting, is a sudden,

temporary loss of consciousness that is not due to head trauma.

SyncopeIt is commonly due to cerebral ischemia.Causes:

Vasodepressor syncope – sudden emotional stress.

Situational syncope – pressure stress associated with urination, defecation, or severe coughing.

Drug-induced syncope – antihypertensives, diuretics, vasodilators, & tranquilizers.

Orthostatic hypotension – an excessive decrease in blood pressure that occurs upon standing up.

Control Of Blood Pressure & Blood FlowNegative feedback systems control blood

pressure by adjusting the following factors:Heart rate.Stroke volume.Systemic vascular resistance.Blood volume.

Cardiovascular CenterThe cardiovascular (CV) center of the

medulla oblongata regulates heart rate and stroke volume.

Sympathetic nerves reach the heart via the cardiac accelerator nerves. Sympathetic stimulation increases the heart rate and contractility.

Cardiovascular CenterParasympathetic stimulation decreases the

heart rate and is conveyed by the vagus nerves (cranial nerve X).

The CV center sends impulses to smooth muscle in blood vessel walls via vasomotor nerves. They moderate the rate of vasoconstriction (vasomotor tone).

Neural Regulation Of Blood PressureBaroreceptor Reflexes – baroreceptors

are pressure-sensitive receptors located in the aorta, internal carotid arteries, and other large arteries of the neck and chest.Carotid sinus reflex – carotid sinuses are

small widenings of the right and left internal carotid arteries. Pressure stretches the wall of the carotid sinus. Signals are sent to the CV center via the glossopharyngeal nerves (CN IX).

Aortic reflex – signals are sent to the CV center via the vagus nerves (CN X).

Carotid Sinus Massage & Carotid Sinus SyncopeCarotid sinus massage involves massaging

the neck over the carotid sinus, to slow the heart rate in a person who has paroxysmal superventricular tachycardia (originates in the atria).

Carotid sinus syncope – fainting due to excessive pressure on the carotid sinus from hyperextension of the head or tight collars.

Chemoreceptor ReflexesChemoreceptors monitor the chemical

composition of the blood.They are located close to the baroreceptors

in carotid bodies and aortic bodies.

Chemoreceptor ReflexesThey detect changes in blood level of O2,

CO2, and H+.Hypoxia, acidosis, or hypercapnia

stimulates the chemoreceptors to send impulses to the cv center producing sympathetic stimulation and vasoconstriction.

Hormonal Regulation Of Blood PressureRenin-angiotensin-aldosterone (RAA)

system raises blood pressure.Angiotensin II is a vasoconstrictor and

stimulates aldosterone which increases absorption of Na+ ions by the kidneys.

Epinephrine and norepinephrine raise blood pressure.Increase cardiac output by increasing heart

rate.Cause vasoconstriction of arterioles in the

skin and abdomen and vasodilatation of arterioles in cardiac and skeletal muscles.

Hormonal Regulation Of Blood PressureAntidiuretic hormone (ADH) raises blood

pressure.Causes vasoconstriction.

Atrial natriuretic peptide (ANP) lowers blood pressure.Causes vasodilatation and promotes loss of

salt and water in the urine.

Autoregulation Of Blood PressurePhysical changes.

Warming promotes vasodilation and cooling causes vasoconstriction.

Myogenic response – smooth muscle contracts more forcefully when it is stretched and relaxes when stretching lessens.

Autoregulation Of Blood PressureVasodilating and vasoconstricting

chemicals.Vasodilating chemicals include K+, H+, lactic

acid, ATP, and nitric oxide (NO). Kinins and histamine are released from tissue trauma and cause vasodilation.

Vasoconstricting chemicals include thromboxane A2, superoxide radicals, serotonin (from platelets), and endothelins.

Checking Circulation