M5: Heart Physiology

Question 1

Cardiac output

Answer 1

CO = HR x SV
amount of blood pumped out of the heart per minute (typically refers to left ventricle, systemic ouput)

Question 2

cardiac reserve

Answer 2

difference between cardiac output at rest and maximum cardiac output

Question 3

2 ways the heart can be regulated

Answer 3

INTRINSIC REGULATION
- results from normal functional characteristics, not neural or hormonal regulation

EXTRINSIC REGULATION
- involves neural and hormonal control

Question 4

3 factors that regulate stroke volume

Answer 4

1. PRELOAD (intrinsic regulator)
   - amount of stretch in ventricular walls before contraction
   - Bigger stretch = bigger contraction force (Frank-Starling law of the heart)
   - stretch puts actin and myosin in optimal position to contract
2. AFTERLOAD (intrinsic regulator)
   - pressure the contracting ventricles must produce to overcome pressure in aorta (usually 80 mm Hg, but depends on blood pressure)
3. CONTRACTILITY (extrinsic regulation)
   - strength of ventricle contraction
   - controlled by INOTROPIC AGENTS
   - POSITIVE INOTROPIC AGENTS: open Ca2+ channels. SNS controls via CARDIAC ACCELERATOR NERVES and norepinephrine
   - NEGATIVE INOTROPIC AGENTS: calcium channel blockers

Question 5

Frank-Starling Law

Answer 5

explains how bigger stretch in heart walls = bigger contraction force of muscles

increased EDV
increases stretch on heart walls
optimises actin myosin overlap
increases force of contraction
increases stroke volume

Question 6

venous return

Answer 6

• amount of blood returning to the heart from systemic circulation
• determines end-diastolic volume (EDV)
• can change with skeletal muscle contraction

Question 7

factors that regulate heart rate

Answer 7

• autonomic nervous system (SNS and PNS)
• hormones
• ions
• age
• sex
• physical fitness
• temperature (temp up HR up, temp down HR down)

Question 8

explain neural and hormonal control of the heart

Answer 8

NERVE STIMULATION
1. PARASYMPATHETIC
- vagus nerve decreases HR
- acetylcholine hyperpolarizes heart (more K+ channels open, takes longer to reach threshold)
2. SYMPATHETIC
- cardiac accelerator nerves increase HR (& contractility)
- norepinephrine released at SA/AV nodes (also coronary vessels and myocardium), opens Ca2+ channels, faster time to threshold

HORMONAL CONTROL
- epinephrine and noepinephrine from adrenal medulla
- occurs in response to exercise, stress, excitement
- slower process but lasts longer (hormones need to reach heart from circulation as opposed to direct nerve signal)

nerve stim and hormonal control like backup systems for each other. If one doesn’t work the other should

Question 9

What signals does the body produce to signal heart regulation?

Answer 9

Blood Pressure
- BARORECEPTORS monitor blood pressure
- located in INTERNAL CAROTID ARTERIES and aorta (CAROTID and AORTIC BODIES)
- sensory information sent to medulla oblongata

Changes in pH, C02 and O2
- CHEMORECEPTORS detect pH and C02 changes in medulla oblongata
- chemoreceptors monitor O2 in carotid and aortic bodies

Changes in ion concentration
- excess or reduces extracellular K+ decreases HR

Body temperature
- temp up = HR up, temp down = HR down

Question 10

process of baroreceptor and chemoreceptor reflux

Answer 10

1) SENSORY NEURONS
- baroreceptors and chemoreceptors in carotid and aortic bodies detect change
2) either
1. PNS response
- vagus nerve innervates SA node to decrease HR
2. SNS response
- nervous: cardiac nerves increase HR and myocardium contractility
- hormonal: innervation of adrenal medulla causes epinephrine and norepinephrine release

Question 11

carotid arteries

Answer 11

blood vessels that provide brain’s blood supply