Lecture 25 : Specialised Circulation

Question 1

What is generic circulation?

Answer 1

Each organ and tissue receives a blood supply that flows through a circuit
- Blood leaves heart to each organ
- Deliver 02/remove CO2
- Blood returns to heart

Question 2

What is generic circulation blood flow regulated by?

Answer 2

1. Myogenic control (resistance vessels)
2. Neural control
3. Metabolic requirements

Question 3

What are the 2 main specialised regional blood flows?

Answer 3

1. Fetal circulation
2. Pulmonary blood flow

Question 4

What are the roles of the placenta?

Answer 4

1. Intestine (nutrient uptake)
2. Kidney - remove waste
3. Lungs - Uptake of O2
4. Receives large fraction of CO2 - low resistance circuit

Question 5

What is the distribution of blood flow in a fetus?

Answer 5

• Dependent of local requirements
• Organs not yet active e.g. lungs are by-passed

Question 6

Where is fetal blood oxgenated?

Answer 6

Site of placenta via the maternal blood supply
- Relatively hypoxic = 80% saturated

Question 7

Why are the lungs by-passed in a fetus?

Answer 7

• Extreme pulmonary vasoconstriction
• High resistance to blood flow
• Airways are collapsed

Question 8

What is the function of the ductus arteriosus?

Answer 8

It couples the pulmonary artery directly through to the aorta so that the lungs are by-passed

Question 9

What is the foramen ovale?

Answer 9

Hole in the heart - blood goes straight to left side of heart, without needing to go through pulmonary circuit

Question 10

What is the function of the ductus venosus?

Answer 10

Causes the liver and kidneys to be by-passed

Question 11

In which part of fetal circulation is SO2 highest?

Answer 11

Placental arterial blood - low compared to adult

Question 12

Describe the inferior blood flow to the heart through fetal circulation:

Answer 12

• Blood travels up from placenta
• By-pass liver to inferior vena cava
  – Contaminated with circulating placental venous blood
  – SO2 = 67%
• Travels from right atrium to left atrium

Question 13

Describe the superior blood flow to the heart through fetal circulation:

Answer 13

• Venous blood from superior vena cava crosses pathways with inferior blood flow
• Travels from right atrium to right ventricle

Diagram on onenote

Question 14

What is the role of the folds in the vena cava?

Answer 14

“Guide” blood flow
through:
1. Right atrium
2. Foramen Ovale
3. Left atrium

Question 15

What type of flow is fetal circulation?

Answer 15

Laminar

Question 16

What is critical as soon as the baby is born?

Answer 16

Lungs
- Stimulus for breathing = increased CO2

Question 17

What inspiratory motions begin after birth?

Answer 17

Generate -ve thoracic pressure
-> draws blood out of placenta
-> inflates lungs (VE)
-> decrease pulmonary vascular resistance
-> increases systemic resistance
-> blood flows into lungs

Question 18

How does the direction of blood flow through the ductus arteriosus change at birth?

Answer 18

• Favours blood flow through pulmonary circulation
• Reduction in prostaglandins causes ductus arteriosus to close

Question 19

How does the closure of the ductus arteriosus affect the foramen ovale?

Answer 19

• Reduction in right atrial pressure
• Increase in left atrial pressure
• Left-right pressure gradient causes folds of foramen ovale to close

Question 20

What does the main pulmonary trunk originate from and what does it divide into?

Answer 20

Originates: Right ventricle
Branches: Left/right main axial artery to supply blood to left and right lung

Question 21

How do each of the axial branches branch?

Answer 21

Can branch up to 20 times and each time they get smaller to ensure adequate perfusion of alveoli for gas exchange

Question 22

How much of cardiac output does pulmonary circulation receive?

Answer 22

100%

Question 23

Compare the resistance of systemic vs pulmonary circulation:

Answer 23

Pulmonary has 10-15 fold lower resistance than systemic

Question 24

Compare the arterial pressure of systemic vs pulmonary circulation:

Answer 24

Systemic: 120/80 mmHg
Pulmonary: 25/10 mmHg

Question 25

What are the 3 characteristics of the pulmonary circulation?

Answer 25

1. Low resistance: - Lots of parallel vessels - Larger diameter vessels (r) - Shorter vessels (l) 2. High compliance: - Vessels distend easily = accomodate increased CO without large increase in pressure 3. Low arterial pressure

Question 26

What happens if pulmonary pressure increases?

Answer 26

Can lead to: 1. Impaired ejection of blood (increased afterload) 2. Right heart failure 3. Pulmonary edema

Question 27

Name the 4 regulators of pulmonary blood flow:

Answer 27

1. Gravity 2. Hypoxia (low O2) 3. Endothelial control 4. Sympathetic nervous system

Question 28

Describe the effect of gravity on the lower region of the lung when standing:

Answer 28

* Increase hydrostatic pressure in vessels * Vessels distend easily * Resistance decreases * Increase blood flow

Question 29

Describe the effect of gravity on the upper region of the lung when standing:

Answer 29

* Reduced intravascular pressure * Vessels partially collapse * Increase resistance * Decrease blood flow

Question 30

What happens to blood flow to upper regions of lungs during exercise?

Answer 30

Blood flow increases

Question 31

How is hypoxia a regulator of pulmonary blood flow?

Answer 31

* Pulmonary vasculature constricts in response to hypoxia - hypoxic pulmonary vasoconstriction, HPV * HPV optimizes ventilation-perfusion for gas exchange - Decreases blood flow - Directs blood away from poorly ventilated alveoli

Question 32

How does endothelial control regulate pulmonary blood flow?

Answer 32

Endothelium of pulmonary vessels release vasoactive peptides to regulated vascular tone 1. Nitric oxide - vasodilator 2. Endothelin-1 - vasoconstrictor

Question 33

How is the sympathetic nervous system a regulator of pulmonary blood flow?

Answer 33

Via α-adrenoreceptors and β-adrenoreceptors - They negate each other therefore, SNS has minor role in regulating pulmonary blood flow

Question 34

What is the role of α-adrenoreceptors?

Answer 34

Located in large conduit vessels * Vasoconstriction - Increase resistance - Increase pulmonary pressure

Question 35

What is the role of β-adrenoreceptors?

Answer 35

Located in smaller resistance vessels * Vasodilation - Decrease resistance - Decrease pulmonary pressure