2.12A. Coronary circulation and its control.

Question 1

I. Basic anatomy of heart vessels

Answer 1

Entire blood supply to the myocardium derives from right and left coronary arteries
- Originate at the root of aorta
- RCA -> right ventricle and atrium
- LCA -> left ventricle and atrium
- Venous blood drains into the right atrium through coronary sinus or directly into the cardiac chambers through thebesian veins

Question 2

I. Basic anatomy of heart vessels

Answer 2

Entire blood supply to the myocardium derives from right and left coronary arteries
- Originate at the root of aorta
- RCA -> right ventricle and atrium
- LCA -> left ventricle and atrium
- Venous blood drains into the right atrium through coronary sinus or directly into the cardiac chambers through thebesian veins

Question 3

II. Coronary circulation
1. What is the mechanism of coronary circulation?

Answer 3

Blood flow in the coronary circulation is controlled almost entirely by local metabolites, with SYM innervation only playing a minor role.

Blood vessels are compressed during the contractive force of systole, causing ischemia (periodic occlusions). After the occlusion, reactive hyperemia occurs and increases blood flow + oxygen delivery to compensate

Question 4

II. Coronary circulation
2. What are the Most important local metabolite factor?

Answer 4

hypoxia + adenosine

Question 5

III. Coronary blood flow
1. Definition of coronary blood flow (Qc)

Answer 5

Coronary blood flow (Qc)
- Amount of blood the heart gets in a minute

Question 6

III. Coronary blood flow
2. What is the value of Coronary blood flow?

Answer 6

• Qc = 200 – 250 mL blood/ min (5% of CO = 5,6L/min)

Question 7

III. Coronary blood flow
3. What is AVDO2?

Answer 7

• AVDO2 = how efficiently the heart extracts O2 from the blood (difference between arterial and venous O2 content)
• AVDO2 = 110mL O2/L blood

Question 8

III. Coronary blood flow
4. What is the value of AVDO2?

Answer 8

AVDO2 = 110mL O2/L blood

Question 9

III. Coronary blood flow
5. What happen to the blood flow if we get more O2?

Answer 9

increase blood flow (Qc)

Question 10

III. Coronary blood flow
6. What is AVDO2 during exercise?

Answer 10

AVDO2 during exercise
=> 900 – 1200 mL/min (extraction already at maximum at rest -> need to ↑ blood flow to heart)

Question 11

III. Coronary blood flow
7. What is the driving force of coronary blood flow?

Answer 11

Perfusion pressure (created by the heart) is the driving force of the coronary circulation
(compress blood vessel
-> diameter decreases
-> blood resistance increases)

Question 11

III. Coronary blood flow
7. What is the driving force of coronary blood flow?

Answer 11

Perfusion pressure (created by the heart) is the driving force of the coronary circulation
(compress blood vessel
-> diameter decreases
-> blood resistance increases)

Question 12

IV. Coronary blood flow during cardiac cycle
1. Describe coronary blood flow in LCA during cardiac cycle

Answer 12

1st phase of systole:
- Tension in wall increases = AO valve closed
- Condition bad for LCA -> Increased pressure in myocardium compresses the coronary blood vessels
- Increase follows PA -> Increase in Qc = increase in perfusion pressure during systole
- Diastole -> wall tension decreases (heart relaxed)
- Resistance decreases
-> huge increase in blood flow in coronary circulatory

Question 13

IV. Coronary blood flow during cardiac cycle
2. Describe coronary blood flow in RCA during cardiac cycle

Answer 13

• Smaller valves compared to LCA (lower wall tension)
• Pressure in LV not so high, coronary blood flow does not drop to 0

Question 14

IV. Coronary blood flow during cardiac cycle
2. Describe coronary blood flow during Diastole of cardiac cycle

Answer 14

• Heart can rest
• Fed by blood supply
• 80% of coronary circulation occurs during diastole
  -> HR increased (less optimal for QC)
  -> duration of diastole decreased

Question 15

V. Regulation of blood flow
1. What are the 3 types of regulations of coronary circulation?

Answer 15

myogenic, metabolic, neural controls
(During exercise, blood flow can increase to max 5x and this is due to increased heart work)

Question 16

V. Regulation of blood flow
2A. Describe Myogenic mechanism

Answer 16

Autoregulation (Bayliss effect)
- little change in Qc as perfusion pressure changes (from 60 to 160mmHg)
- Heart itself will regulate its own blood flow
- Pa↑ -> vasoconstriction (less perfusion) -> R↑ (constant blood flow)
- Pa↓ -> vasodilation (more blood flow) -> R↓

Question 17

V. Regulation of blood flow
2B. What is the Bayliss effect?

Answer 17

• Pressure ↑ -> wall tension↑ -> non-selective cation channels, depolarization, activate VDCC -> Ca2+↑ -> vasoconstriction
• Pressure ↓ -> wall tension↓ -> same effects -> vasodilation

Question 18

V. Regulation of blood flow - Metabolic regulation
3A. What are the Factors that cause vasodilation?

Answer 18

1. CO↑ (exercise) -> metabolism ↑ + O2-consumption↑
2. Metabolic changes -> K+-channel activators (HCN, VDCC)
3. Metabolism↑ -> NO (produced by endothelial cells) -> vascular SM relaxation -> vasodilation

Question 19

V. Regulation of blood flow - Metabolic regulation
3B. How can increasing CO↑ (exercise) participate in metabolic control>

Answer 19

CO↑ (exercise) -> metabolism ↑ + O2-consumption↑
-> Since ↑O2-consumption, we have different metabolic effects:
1. pO2↓, pCO2↑
2. [H+]↑ (pH↓)
4. [K+]↑
5. [adenosine]↑ - ATP metabolism
=> all lead to vasodilation (R↓ => Qc↑)

Question 20

V. Regulation of blood flow - Metabolic regulation
3C. How can Metabolic changes participate in metabolic control?

Answer 20

Metabolic changes -> K+-channel activators (HCN, VDCC)
-> SM hyperpolarization
-> Ca2+-channel activation
-> [Ca2+]IC↓
-> vasodilation (relaxation)

• ↓[Ca2+] also comes from the increased cAMP due to adenosine

Question 21

V. Regulation of blood flow - Metabolic regulation
3C. How can Metabolism↑ participate in metabolic control?

Answer 21

Metabolism↑ -> NO (produced by endothelial cells) -> vascular SM relaxation -> vasodilation
- This is a positive feedback mechanism, because vasodilation -> ↑QC -> shear stress on endothelial cells↑ -> NO release↑ -> vasodilation again

Question 22

V. Regulation of blood flow - Neural regulation
1. What are the sympathetic effects in neural regulation?

Answer 22

1.Direct effects:
- α1-AR -> vasoconstriction
- β2-AR -> vasodilation

2. indirect effect:
   - Sympathetic stimulation leads to ↑HR = ↑CO -> metabolism↑ -> vasodilation -> ↑QC

Question 23

VI. Pathophysiology of coronary blood flow
-> Describe

Answer 23

1. Abrupt occlusion -> necrosis (myocardial infarct)
2. Gradual occlusion (slow narrowing of vessels)
   -> new blood vessel formation occurs
   => Is a better form of occlusion