Chapter 16

The Cardiovascular System: Blood Vessels and Circulation

Blood Vessels Arteries

Carries blood away from the heart to body tissues

2 large arteries aorta and pulmonary trunk

Arteries smaller and smaller arteries arterioles

Arterioles capillaries

Blood Vessels Veins

Convey blood from the tissues back to the heart

Veins smaller and smaller veins venules

Venules capillaries

Blood Vessels Capillaries

Microscopic vessels that connect arterioles to venules

“Exchange Vessels” permit exchange of nutrients and wastes between body’s cells and blood

Blood Vessel StructureArteries

Three layersInner layer: endothelium, a basement

membrane, internal elastic laminaMiddle layer: smooth muscle and elastic

tissueOuter layer: elastic and collagen fibers

Blood Vessel StructureVeins

Similar structure to arteriesThinner middle and inner layers, outer layer is the thickest

The lumen is wider than arterySome veins, inner layer folds inward valves to prevent backflow

Capillary Information• Capillaries – layer of endothelium surrounded by basement membrane

• Connected from arterioles venules in networks–Sometimes direct route from arteriole to venule

• Filling controlled by small arterioles & pre-capillary sphincters

Vessel FunctionsMuscular arteries & arterioles regulate flow

Sympathetic activity to smooth muscle vasoconstriction (narrowing)

Decreased sympathetic activity or NO causes relaxation or dilation

Vessel FunctionsArterioles adjust flow into capillariesSystemic veins & venules serve as blood reservoirs (~64% total blood volume)

Capillary ExchangeSlow flow through capillariesCapillary Exchange – movement of substances into and out of capillaries

Capillary Blood Pressure – pressure of blood against the walls of capillaries

Capillary Exchange• Osmosis (protein concentration)

–Reabsorption of fluid from outside to inside• Balance determines fluid in circulation–Excess fluid returned via lymphatic system–Local signals can adjust capillary flow

Venous ReturnVolume of blood flowing back to the heart through the systemic veins

Happens due to pressure generated in 3 ways:Contractions of the heartThe skeletal muscle pumpRespiratory pump

Venous ReturnContractions of the heart –

BP generated by ventricular systoleMeasured in mm of Hg (mercury)Small pressure difference between venules and right atrium is sufficient

Venous ReturnSkeletal muscle pump –

Standing at rest – venous valves in leg are open, blood flows up to heart

Contraction of leg muscles compresses vein pushes blood through valve closer to heart, called milking

Venous ReturnSkeletal muscle pump cont.

Same time as milking, valve farther from heart closes as some blood is pushed against it

After muscle relaxation, pressure drops in compression and open valve closes, and now farther valve opens since BP is higher in foot than leg, vein fills with blood from foot

Venous ReturnRespiratory pump –

During inhalationDiaphragm moves down, causes a drop in

pressure in the thoracic cavity and rise in abdominal(ab) cavity

Ab veins compressed, greater volume of blood from compressed ab veins decompressed thoracic veins rt atrium

During exhalation, pressures reverse

Blood Flow• From high pressure low pressure

–Greater gradient = greater flow• BP – pressure exerted by blood on the walls of a blood vessel

Blood PressureBP is highest in aorta and large systemic arteries

BP depends on total volume of bloodNormal volume of an adult ~5 L

Blood PressureBP rises to ~110 mmHg (systole) and drops to ~70 mmHg (diastole)

BP drops as blood enters veins and reaches 0 mmHg as blood returns right atrium

Blood Flow• Resistance – opposition to blood flow due to friction between blood and walls of vessels

• Increase in resistance = increase in BP

• Decrease in resistance = decrease in BP

• Resistance depends on 3 things

Resistance• Size of lumen

–Smaller lumen greater resistance• Blood viscosity

–Higher viscosity greater resistance• Total vessel length

–Longer length greater resistance

Regulation of Blood Pressure & Flow

CV Center (in the medulla oblongata)Regulate heart rate & stroke volumeControls neural & hormonal negative feedback systems that regulate BP and flow to tissues

Inputs (CV center)Higher centers:

Cerebral cortexLimbic systemHypothalamus

Inputs (CV center)Sensory receptor input:

Proprioceptors Monitor movements of joints and

musclesStart HR change as activity starts

Baroreceptors Monitor pressure changes

Inputs (CV center)Baroreceptors (cont.)

pressure parasympathetic stimulation of the heart sympathetic stimulation

ChemoreceptorsMonitor chemical changesLow O2, high H+, excess CO2 sympathetic stimulation vasoconstriction BP

Output (CV center)ANS to heart

Sympathetic HR & force of contraction

Sympathetic HR & force of contraction

Vasomotor to arterioles vasomotor tone (sets

resting level of vascular resistance)To veins move blood BP

Hormone RegulationRenin – Angiotensin – Aldosterone (RAA) systemAngiotensin II BP (vasocontriction) aldosterone Na+ & water by kidneys

Epinephrine and Norepinephrine CO ( HR & force of contraction)

Hormone RegulationADH = Vasopressin

Constriction BPANP

Vasodilation & loss of salt & water in urine BP and blood volume

PulsePulse strongest in arteries closest to heartRadial artery (at wrist), Carotid artery, Popliteal artery, Dorsal artery, Brachial artery

Tachycardia = rapid rest rate (>100 bpm)

Bradycardia= slow rest rate (<50 bpm)

Blood Pressure (measurement)Use sphygmomanometer

Usually on brachial artery in left armRaise pressure above systolic-

To stop blood flow

Blood Pressure (measurement)Lower pressure in cuff until flow just startsSystolic Pressure

Lower until sound suddenly gets faint (stops)Diastolic pressure

Normal values <120 mmHg for systolic & < 80 mmHg for diastolicEx. 115/75

Circulatory RoutesTwo parts: Systemic & PulmonarySystemic circulation- throughout body

Oxygenated blood deoxygenatedAll systemic arteries branch from aorta

Systemic CirculationAll systemic veins empty into Superior Vena Cava, Inferior Vena Cava or the Coronary SinusCarry deoxygenated blood to heart

Pulmonary CirculationRight ventricle pulmonary trunkR & L pulmonary arteries

Carry deoxygenated blood R & L lungs

Gas exchange occurs

Pulmonary Circulation R & L pulmonary veins

Carry oxygenated blood L atrium

Hepatic Portal CirculationPortal vein transports blood from one capillary bed to another

There are no valves Splenic & superior mesenteric veins

Fetal CirculationSpecialized for exchange of materials with maternal blood and bypass of lungs (placenta)

Umbilical artery: pathway for blood fetus mother

Fetal CirculationUmbilical vein brings O2 blood from placenta liver and Ductus Venosus Inferior Vena Cava R Atrium

Fetal CirculationForamen ovale - hole in atrial septum allows mixing of blood in heart (eventually becomes fossa ovalis)

At BirthUmbilical arteries medial umbilical ligaments

Umbilical vein ligamentum teresDuctus venosus ligamentum venosum

At Birth

Placenta is “afterbirth”Foramen ovalis closes fossa ovale

Ductus arteriosus ligamentum arteriosum

AgingStiffening of aortaLoss of cardiac muscle strength

Reduced CO & increased systolic pressure

Coronary artery disease (CAD)Congestive heart failureAtherosclerosis