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Cardiac cycle

time between 1 heartbeat and the next


phase of contraction

atrial systole

atria contract together (follows P wave)

ventricle systole

ventricles contract together (follows QRS complex)


phase of relaxation

atrial diastole

atria relax (follows QRS)

ventricular diastole

ventricles relax (follows T wave)

1 cardiac cycle

atria systole, ventricular systole, ventricular diastole

atria systole in the cardiac cycle

atria contract, pressure increases in atria, blood flows through open AV valves into relaxed ventricles, end-diastolic volume of filled ventricle

ventricular systole in the cardiac cycle

ventricles contract, (simultaneous atrial diastole- atria relax) pressure increases in ventricles, AV valves close, isovolumetric contraction, semilunar valves open, ventricular ejection

isovolumetric contraction

pressure in ventricle increases but volume stays the same until pressure is greater than pressure in aorta or pulmonary trunk

ventricular ejection

ventricles continue to contract, blood pumped into aorta/pulmonary trunk

isovolumetric relaxation

pressure in ventricle decreases but volume stays the same until pressure is less than atrial pressure

blood pressure

pressure in systemic circulation (pulmonary pressure is lower)

systolic pressure

due to maximum left ventricular contraction

diastolic pressure

during ventricular relaxation, pressure maintained by smooth muscle in arteries

Heart sounds

due to turbulence as blood flow patterns are altered, there are 4 sounds

1st sound

closing of AV valves

2nd sound

closing of semilunar valves

3rd sound

opening of AV valves

4th sound

contraction of atria


abnormal sounds (valve disorders, septal defects)

Circulation in the fetal heart

2 shunts: by-pass lungs (no oxygen in fetal lungs )

2 shunts

foramen ovale, ductucs arteriosus

foramen ovale

hole between right and left atria, shunts blood from right atrium to left atrium.

fossa ovalis

foramen ovale closes following birth

ductus arteriosus

artery between pulmonary trunk and aorta, shunts blood from pulmonary trunk to aorta.

ligamentum arteriosum

ductus arteriosus closes following birth


stroke volume, heart rate, cardiac output, regulation of heart rate, autonomic nervous system regulation, chemical regulation, other factors

stroke volume (SV)

ml of blood ejected by each ventricle/cardiac cycle


=end diastolic volume(EDV) - end systolic volume(ESV)

heart rate (HR)

heart beats/minute

cardiac output (CO)

volume of blood ejected by each ventricle/minute (equal for right and left ventricles)


=stroke volume x heart rate (eg. 70ml/beat x 75 beats/min = 5.25 liter/min)

CO changes to meet body needs

cardiac reserve

cardiac reserve

difference between maximum cardiac output and resting cardiac output, normally 4-5 times resting CO

Increasing cardiac output

increases stroke volume

3 factors affect SV

preload, contractility, afterload

1. preload

stretch (fullness) of ventricle before contraction >EDV = >preload = stronger contraction, pericardial sac prevents overstretching heart muscle, preload is affected by filling time and venous return

2. contractility

strength of contraction, positive inotropic agents and negative inotropic agents

positive inotropic agents

increase contractility (most increase Ca entering cardiac muscle cells) sympathetic NS, epinephrine, digitalis, Ca ions

negative inotropic agents

decrease contractility, parasympathetic NS, calcium-channel blockers, beta-blockers (block sympathetic response)

3. afterload

pressure in aorta/pulmonary trunk, resists ejection of blood from ventricles, high blood pressure and atherosclerosis decrease SV

increase heart rate

up to maximum heart rate (too high HR decreases filling time = decreases preload

regulation of heart rate

heart rate depends on tissue demands and stroke volume

tissue demands

basal metabolic rate and activity level

stroke volume

as SV decreases HR must increase

autonomic nervous system regulation

cardiovascular center

cardiovascular center

medulla oblongata, inputs, output


emotions (fear, excitement, anxiety) sensory-proprioceptors: physical activity, chemoreceptors: oxygen, carbon dioxide levels, baroreceptors: blood pressure

output- sympathetic (norepinephrine)

cardiac accelerator nerves- bets receptors in SA and AV, nodes- increases depolarization rates

output- parasympathetic (acetylcholine)

vagus nerves- SA and AV nodes, decreases depolarization rates, predominate at rest

chemical regulation

hormones, adrenal medulla (sympathetic) - epinephrine, norepinephrine, increases heart heart rate, thyroid hormone- increases heart heart rate

other factors affecting heart rate

body temperature: hyperthermia (faster HR), hypothermia (slower HR), fitness (decreases HR), electrolyte levels (Na and K decreases HR, Ca increases HR)


abnormal heart rhythm


high resting heart rate


low resting heart rate

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