WEEK 1 CR PHYSIO--CHAPTER 9 The heart as a pump

Question 1

Cardiac Muscle

• **SIMILARITIES— to skeletal muscle:
  1. Both types of fibers have a striated appearance.

2. Both rely on the opening of fast Na+ channels to
   initiate the action potential for contraction.
3. Contractions are based on sliding protein
   filaments (actin and myosin).
4. Cardiac muscle is most similar to Type I fibers.

Answer 1

KNOW THESE 4

Question 2

DIFFERENCES from skeletal muscle:
1. Cardiac muscle is involuntary while skeletal is voluntary.

2. Cardiac muscle fibers contain more and larger nuclei because the heart requires more energy (ATP) production.
3. Cardiac fibers are interconnected by intercalated discs.
4. The heart is divided into two functional synctia which allows the atria to contract before the ventricles.
5. Cardiac muscle has slow Ca2+ channels that must open to achieve action potential.
6. Theses slow channels cause a plateau in the action potential.
7. Duration of contraction is ̴10 times longer for cardiac muscle than skeletal muscle.

Answer 2

KNOW THESE 7

Question 3

Functions of Intercalated Discs
1• Serve to fuse the cells together to form
___ _______allowing for rapid diffusion of ions.

2• This allows the action potential to travel rapidly from one
cardiac ________ to the next.

Answer 3

gap junctions

myocyte

Question 4

Action Potential (AP) 
1.  At rest (resting potential), the intracellular surface is ̴-80 to -90 mV and if threshold is reached at -70 mV, it continues to +20 to + 30 mV at the peak of the AP.

2. For cardiac muscle, once the AP spikes at ~ +20 to +30mV, it begins to _________but then plateaus before it
   completely repolarizes.

3• The _________ is the reason why cardiac contraction is significantly longer than skeletal muscle contraction.

Answer 4

repolarize

plateau

Question 5

Phases of Action Potential—based on ion movement

Phase 0: Opening of fast Na+ channels = rapid depolarization.

Phase 1: At peak, repolarization begins = short downward interval between peak and beginning of plateau. This brief
repolarization is from Na+ channels closing and K+ leaving the cell.

Phase 2: Opening of slow Ca2+ channels and continued Na+ influx = plateau. (sometimes called Ca2+ - Na+ channels)
*** Also, when slow Ca2+ channels open, ↓ outflux of K+ = plateau

Phase 3: Slow Ca2+ channels close so ↑ in K+ outflow = rapid repolarization

Phase 4: Resting membrane potential.

Answer 5

KNOW THESE STEPS

Question 6

Cardiac Refractory Periods 
• \_\_\_\_\_\_\_\_\_\_ Refractory Period (ARP)- the 
interval during the AP when a “normal" 
electrical impulse cannot re-excite the 
already excited area. 

•___________Refractory Period (RRP)—a
stronger than normal stimulus can
re-excite the area.

Answer 6

Absolute

Relative

Question 7

Excitation-Contraction- Coupling

• Unlike skeletal myofibrils, cardiac myocytes rely on
extracellular______ for contraction to occur.

This influx of Ca2+ activates ________ receptors on the SR
membrane which triggers the SR to release its Ca2+.

Answer 7

Ca2+

ryanodine

Question 8

The Na+ entering through this pump is transported back out of the cell via Na+-K+ exchange pump (__Na+ out for__K+ in)

Answer 8

3

2

Question 9

The cardiac cycle is often defined as ___ wave to___

wave.

Answer 9

P

P

Question 10

Electrocardiogram (ECG or EKC)

1. P wave –depolarization of the _____; atrial contraction follows which causes the slight increase on the atrial pressure curve (“a” wave).
2. QRS complex—depolarization of the _______; ventricular contraction follows causing the significant rise on the ventricular pressure curve.
3. T wave—_______ of ventricles occurs before ventricular contraction is complete; followed by relaxation of ventricles with a significant decrease on the ventricular pressure curve.

Answer 10

atria

ventricles

repolarization

Question 11

The Atria as “Primer” Pumps

Blood returning to the heart from the great veins (superior and inferior vena cava; right and left pulmonary veins) first goes to the atria but ____ passes right through the open A-V valves into the ventricles.

The atria are called primer pumps because the contraction of these upper chambers only provides the last ___ to fill the ventricles.

Therefore, if the atria fail to contract properly, there is enough blood in the ventricles to pump to the peripheral organs to meet the demands under resting conditions.

Answer 11

80%

20%

Question 12

The aorta and systemic arteries work at a higher pressure

compared to the_______arteries because they have to send blood to all the cells in the body.

Answer 12

pulmonary

Question 13

Ventricular Volumes:
1. End-Diastolic Volume (EDV)—at the end of ventricular
diastole (relaxation), the volume of blood in each ventricle
is ̴ 120 ml.

2. End-Systolic Volume (ESV)—the volume of blood remaining in the ventricles after contraction (systole) is
   ̴ 50 ml.

Answer 13

3. Stroke Volume (SV)—the volume of blood ejected from the ventricles during systole is ̴ 70 ml.
   EDV - ESV = SV
   120 - 50 = 70 ml
4. Ejection Fraction (EF)—the fraction (%) of the EDV
   that is ejected during systole. At rest, normally
   EF is ̴ 60% of EDV: 120 X .60 = 72 ml.

Question 14

______—the tension (ventricular pressure) on the muscle fibers at the onset of contraction

The _________ is the pressure in the left ventricle to overcome the pressure in the aorta the ventricle can eject blood into the aorta.

Answer 14

Preload

afterload

Question 15

Energy Required for Cardiac Contraction
1 • Cardiac muscle is highly ______—high number of
mitochondria.

2 • Mainly metabolizes FA (80%) and smaller amounts of glucose and lactate (10%) to make _____

3 • O2 consumption of the heart is directly related to the measure of external work (EW).

4 • Potential energy (PE) represents the additional work required for a contraction to move toward completely emptying the_____ ventricle.

5 • If the ventricle is abnormally dilated (as in heart failure), then the amount of energy required for contraction is _________ than normal even though the ventricle is failing and not filling.

Answer 15

oxidative

ATP.

left

greater

Question 16

In a normal heart, 20-25% of chemical energy (ATP) is converted into work energy and 75- 80% to____. Therefore, maximum efficiency of the heart is considered to be ___.

Answer 16

heat

25%

Question 17

Regulation of heart pumping

** At rest, the average person pumps ___-____ liters of
blood out of the heart per minute which is
= cardiac output (Q).
( ̴ 3.8 L = 1 gallon)
HR X SV = Q 72 bpm X 70 ml = 5040 ml or
5.04 L/min (1.3 gal)

Answer 17

4-6

Question 18

INTRINSIC Regulation = Frank-Starling Mechanism
1 • Venous Return (VR)—the amount of blood flowing into the right _____ from the great veins.

2 • VR regulates how much blood will be pumped out of the left_______per beat (SV).

3 • If VR ↑, the volume of blood in the ventricle ↑ so there is an ↑ in ventricular diastolic pressure—places a stretch on the cardiac fibers.

Answer 18

atrium

ventricle

Question 19

EXTRINSIC Regulation
2 Types

1. Sympathetic Stimulation:

A. increase HR (can ↑ from 70 bpm at rest to
over 200 bpm at max exercise).

B. Normally at rest, there is some sympathetic stimulation to the heart so Q is 30% higher than it would be without any sympathetic stimulation.

Answer 19

2. Parasympathetic (Vagal) Stimulation:

A. The vagus nerve serves to slow the heart rate (and can ↓ force of contraction so ↓ SV). This nerve primarily effects the atria (slows HR) and not so much the ventricles (↓ SV).

In heart disease, a ↓ HR with even a slight ↓ SV can sig. ↓ Q.

B. For trained endurance athletes, ↑ vagal stimulation slows HR to as low as 30 bpm without ↓ SV—in fact, they have a
stronger than normal resting contraction
so have a higher SV at rest.