Cardiac output

Question 1

cardiac output

Answer 1

volume of blood each ventricle pumps as a function of time, expressed in litres per minute

in steady state, cardiac output flowing through systemic and pulmonary circulation is the same

Question 2

cardiac output equation

Answer 2

CO = HR x SV

bpm
volume ejected per ventricle per beat, L/beat

Question 3

typical cardiac output

Answer 3

CO = 72 bpm x 0.07 L/beat = 5.0 L/min

Question 4

effect of blood loss on stroke volume and heart rate

Answer 4

stoke volume decreases, heart rate increases

Question 5

inherent heart rate

Answer 5

100 bpm without nervous or hormonal influences on the SA node
inherent autonomic discharge rate

HR usually higher or lower than this, as SA node is affected by nerves and hormones

Question 6

effects of nerves on SA node

Answer 6

parasympathetic and sympathetic postganglionic neurons end on the SA node.

activity in parasympathetic neurons (travel within vagus nerve) decreases heart rate

activity in sympathetic neurons increases heart rate

Question 7

resting heart rate

Answer 7

much more parasympathetic activity to the heart than sympathetic - 70 bpm

Question 8

sympathetic stimulation

Answer 8

increases slope of pacemaker potential by increasing F-type channel permeability - main current is Na+, so faster depolarisation. causes SA node cells to reach threshold more quickly and the heart rate to increase

Question 9

parasympathetic stimulation

Answer 9

decreases slope of pacemaker potential due to reduction of inward current
threshold reached more slowly, heart rate decreases
hyperpolarises plasma membranes of SA node cells by increasing permeability to K+. pacemaker potential starts from more negative value (closer to K+)

Question 10

hormones affecting heart rate

Answer 10

norepinephrine and epinephrine

Question 11

epinephrine

Answer 11

main hormone secreted by adrenal medulla
increases heart rate
acts on same beta adrenergic receptors in the SA node as the norepinephrine released from neurons

Question 12

what else is the heart rate sensitive to?

Answer 12

changes in temperature, plasma electrolyte concentrations, other hormones, and adenosine (metabolite produced by cardiac cells)

Question 13

3 factors affecting changes in force during ejection of stroke volume

Answer 13

changes in EDV (preload)
changes in magnitude of sympathetic nervous system input to the ventricles
changes in afterload (arterial pressure against which ventricles pump)

Question 14

effect of EDV on stroke volume

Answer 14

increased EDV increases stroke volume

illustrated as a ventricular function curve

Question 15

Frank-Starling mechanism

Answer 15

length-tension relationship

EDV is a major determinant of how stretched the ventricular sarcomeres are just before contraction - greater EDV, greater stretch and more forceful contraction

Question 16

difference between length-tension relationship between skeletal and cardiac muscle

Answer 16

normal point for cardiac muscle at rest is not its optimal length for contraction - greater filling causes additional stretching and increases the force of contraction

Question 17

mechanisms linking changes in muscle length to changes in muscle force

Answer 17

change the overlap of thick and thin filaments
stretching cells toward their optimum length decreases spacing between thick and thin filaments and increases sensitivity of troponin for binding Ca2+ and release from SR

Question 18

venous return

Answer 18

flow of blood from the veins into the heart

Question 19

role of Frank-Starling mechanism in maintaining equality of left and right cardiac outputs

Answer 19

increased venous return increases cardiac output by increasing edv and stroke volume

ensures blood doesn’t accumulate in pulmonary circulation