peripheral circulation, use of O2, venous return

elastic arteries

biggest

aorta, pulmonary trunk

more elastic fibers: stretch as blood is ejected from heart

muscular arteries

medium & smaller arteries

some elastic fibers, more SM cells—dilate or constrict to control flow and BP

arterioles

SM resistance vessels

capillaries

single cell thickness

exchange of gases and nutrients

blood pressure

refers to psi in systemic arteries

SBP=psi in arteries during systole

DBP=psi in arteries during diastole

=CO*TPR (total peripheral resistance)

total peripheral resistance

same concepts apply as w/ airway resistance

important variables: vessel size, blood viscosity, compliance of vessel walls

factors that can cause BP to increase

incr of blood volume, HR, SV, blood viscosity, peripheral resistance

mean arterial pressure/MAP

weighted avg of systolic and diastolic BP (usually closer to DBP than SBP by 2:1)

reflects how much time each phase lasts during cardiac cycle

O2 transport in blood

very small fraction of O2 (less than 1%) can dissolve in blood

remainder of O2 needs hemoglobin

hemoglobin

O2 carrier molecule

protein in RBCs that can bind 4 O2 molecules at a time

main determinant of how much O2 blood can carry

almost always fully loaded (98-100% saturated) with O2 as it leaves pulmonary capillaries

properties of hemoglobin

cooperativity: one O2 molecule binding incr its affinity for other molecules

binding is driven by PO2

high PO2 in pulmonary capillaries→bind O2

low PO2 at tissue level→unload O2

diffusion of O2 and CO2

follows pressure gradients

SaO2 (SpO2)

normal >94%

measured w/ arterial catheter or approximated w/ pulse o i meter

supplemental O2 may be needed if below 90%, required if below 88%

SvO2

reflects O2 saturation of blood returning to heart

normal=60-80%

measured w/ pulmonary artery catheter or central venous catheter

VO2

=CO*A-V O2 diff (arteriovenous O2 difference)

represents oxygen consumption

arteriovenous O2 difference

measures peripheral tissue’s ability to extract O2 for use in aerobic metabolism

O2 consumption at rest

delivery (DO2) is approx. 4x higher than consumption (VO2)

VO2max

maximal O2 consumption during peak work

veins

larger diameter &thinner walls than arteries→low psi/resistance

circulation acts as a reservoir for ~70% of blood volume

valves maintain unidirectional flow

how does blood return to the heart?

venous circulation +…

muscle pump activity

changes in body position

ventilatory pump: cyclic changes in intra-abdominal and intrathoracic psi