lecture 22

Question 1

Blood Flow Velocity & CSA

Answer 1

Key Rule:
👉 Velocity ∝ 1 / total CSA

What this means:

Small total CSA (arteries) → 🔥 FAST flow
Large total CSA (capillaries) → 🐢 SLOW flow

Why slow in capillaries?
→ Allows time for gas & nutrient exchange

Question 2

Structure:

………… muscle fibers (……… than skeletal)

Type … (…………………….)
Connected by ……………..
discs

Cells contract as one unit (……………..)
❌ No …………. cells (limited repair)

Function/Mechanics:

Answer 2

Structure:

Short muscle fibers (shorter than skeletal)
Type I (fatigue-resistant)
Connected by intercalated discs
Cells contract as one unit (syncytium)
❌ No satellite cells (limited repair)

Function/Mechanics:

Involuntary contraction
Calcium-induced calcium release (CICR)

Question 3

Myocardium Electrical Activity

Answer 3

👉 Self-exciting + electrically connected

Autorhythmic → generates its own signals
Gap junctions → rapid signal spread
→ All cells contract togethe

Question 4

🟥 Front:
How does the heart generate and conduct its own electrical signal?

Answer 4

🟩 Back:

Autorhythmic (self-generates signals)
Gap junctions → rapid spread
Pathway:
👉 SA node (pacemaker)
→ AV node (delay)
→ Bundle of His
→ Purkinje fibers
→ Coordinated contraction

Question 5

🟥 Front:
What is the function of the SA node?

Answer 5

🟩 Back:

Specialized cardiac muscle fibers
❌ Do NOT contribute to contraction
✔️ Generate automatic electrical impulses (pacemaker)
→ Initiate the heart’s rhythm

Question 6

Why is the SA node the pacemaker of the heart?

Answer 6

🟩 Back:

Faster depolarization rate than other cardiac cells
More leaky to Na⁺ (influx) than K⁺ (efflux)
→ Reaches threshold quickest
→ Generates signals faster than AV node & His bundle
✔️ Therefore sets the heart rate (pacemaker)

Question 7

🟥 Front:
Describe the electrical conduction pathway of the heart and why the AV node delay is important

Answer 7

🟩 Back:

SA node → AV node → AV bundle (His) → Purkinje fibers
AV node delay → allows atria to fully empty into ventricles
Purkinje fibers → spread impulse through ventricles → contractio

Question 8

🟥 Front:
How is heart rate controlled extrinsically?

Answer 8

🟩 Back:
Parasympathetic (↓ HR):

From medulla oblongata
Vagus nerve → releases ACh
Slows SA & AV node
Vagal tone → resting HR ~60–80 bpm

Sympathetic (↑ HR):

↑ SA node depolarization
↑ Heart rate

Endocrine:

Epinephrine & norepinephrine (adrenal glands)
↑ HR during stress

Receptors:

Chemoreceptors & metaboreceptors → ↑ HR
Baroreceptors → ↓ HR (brake / homeostasis)

Question 9

🟥 Front:
What happens during the cardiac cycle (diastole vs systole)?

Answer 9

🟩 Back:
Cardiac cycle: repeating contraction + relaxation of the heart (one heartbeat)

Diastole (relaxation):

Ventricles fill with blood
~70% passive filling + 30% atrial contraction
Longer phase (≈2x systole)

Systole (contraction):

Ventricles contract
Blood is ejected into aorta & pulmonary artery

Question 10

🟥 Front:
What happens during systole, and how does it relate to pressure and ejection?

Answer 10

🟩 Back:

~100 ms after atrial contraction, ventricles contract
Ventricles eject blood into pulmonary artery + aorta
At rest, systole ejects about 2/3 of ventricular blood
Ejection happens when ventricular pressure > aorta/pulmonary artery pressure
→ (this is called afterload)

Timing:

Resting HR (~75 bpm):
Systole ≈ 0.3 s
Diastole ≈ 0.5 s
Heavy exercise (~180 bpm):
Systole ≈ 0.2 s
Diastole ≈ 0.13 s