Heartbeat

1
Regulation of the Circulatory System

• Heartbeat
• Blood Pressure

2
Heart Beat

• Muscles of heart contract when they depolarize
• Inside the cell is negatively charged outside
  the cell is positively charged
• Voltage-activated Ca2 channels open and Ca2
  enters the cell
• K channels close during depolarization

3
Heartbeat initiates in heart itself

• SA Node (aka pacemaker)
• Mass of specialized tissue near opening of
  superior vena cava in right atrium
• Action potential begins when Ca2 channels open
• Ends of SA node fibers fuse with other atrial
  muscle fibers
• Action potential spreads through both atria and
  they contract
• AV Node
• Right atrium along lower septum
• Delays transmission so atria complete contraction

4

• AV Bundle
• Branches into fibers in both ventricles
• Smallest divisions are Purkinje fibers
• Ordinary cardiac muscle fibers in ventricles
• Ventricles contract

5
Heart Rate

• Average of 70 bpm
• One cardiac cycle takes about 0.8 sec
• Systole contraction
• Diastole relaxation
• Pulse results from expansion and recoil of artery
  as ventricle contracts and relaxes

6
Heart Sounds

• Due to valves closing
• lub
• Caused by closing of atrial ventriclular valves
• Longer, fainter sound
• Beginning of ventricular systole
• dup
• Semilunar valves close
• Shorter, snappy sound
• Beginning of ventricular diastole

7
Regulation of heart rate

• Beats independently but regulated by nervous and
  endocrine systems
• Medulla of brain has cardiac centers
• Sensors in blood vessels walls and chambers of
  heart detect pressure
• Acetylcholine slows heart by increasing
  permeability to K
• Norepinephrine speeds heart rate increasing Ca2
  channel opening
• Stress higher body temp. speeds heart

8
Cardiac Output

• Volume of blood pumped by left ventricle in 1
  minute
• CO stroke volume x heart rate
• Stroke volume volume of blood pumped by
  ventricle during one beat
• Mainly venous return

9
Blood Pressure

• Force exerted by blood against walls of blood
  vessels
• Determined by CO, volume and resistance to blood
  flow
• Blood pressure affected by systole and diastole
• Systolic/Diastolic measured in mmHg
• Measured by a sphygmomanometer
• Healthy blood pressure gt 120/80
• Prehypertensive 120-139 over 80-89
• Hypertensive 140 or 90

10
Variation in Blood Pressure

• Greatest in arteries
• Veins
• Very low pressure
• Large diameter, little muscle
• Muscle movement may constrict veins
• When veins fully distended, no more blood
  accepted
• Pressure in capillaries forces plasma out
• Up to 20 of volume lost from blood
• Fainting may result

11
Regulation of Blood Pressure

• Baroreceptors in arteries and heart wall relay to
  medulla
• Vasoconstriction or vasodilation
• Hormonal control
• Low Blood Pressure kidneys release renin which
  triggers production of angiotensin II
• Vasoconstrictor
• Releases aldosterone increases blood volume
• ADH increases reabsorption of water by kidneys
  blood volume increases
• Atrial Natriuretic peptide (ANP) decreases
  blood pressure increasing sodium excretion