Lecture 25 : Specialised Circulation Flashcards
What is generic circulation?
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
What is generic circulation blood flow regulated by?
- Myogenic control (resistance vessels)
- Neural control
- Metabolic requirements
What are the 2 main specialised regional blood flows?
- Fetal circulation
- Pulmonary blood flow
What are the roles of the placenta?
- Intestine (nutrient uptake)
- Kidney - remove waste
- Lungs - Uptake of O2
- Receives large fraction of CO2 - low resistance circuit
What is the distribution of blood flow in a fetus?
- Dependent of local requirements
- Organs not yet active e.g. lungs are by-passed
Where is fetal blood oxgenated?
Site of placenta via the maternal blood supply
- Relatively hypoxic = 80% saturated
Why are the lungs by-passed in a fetus?
- Extreme pulmonary vasoconstriction
- High resistance to blood flow
- Airways are collapsed
What is the function of the ductus arteriosus?
It couples the pulmonary artery directly through to the aorta so that the lungs are by-passed
What is the foramen ovale?
Hole in the heart - blood goes straight to left side of heart, without needing to go through pulmonary circuit
What is the function of the ductus venosus?
Causes the liver and kidneys to be by-passed
In which part of fetal circulation is SO2 highest?
Placental arterial blood - low compared to adult
Describe the inferior blood flow to the heart through fetal circulation:
- 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
Describe the superior blood flow to the heart through fetal circulation:
- Venous blood from superior vena cava crosses pathways with inferior blood flow
- Travels from right atrium to right ventricle
Diagram on onenote
What is the role of the folds in the vena cava?
“Guide” blood flow
through:
1. Right atrium
2. Foramen Ovale
3. Left atrium
What type of flow is fetal circulation?
Laminar
What is critical as soon as the baby is born?
Lungs
- Stimulus for breathing = increased CO2
What inspiratory motions begin after birth?
Generate -ve thoracic pressure
-> draws blood out of placenta
-> inflates lungs (VE)
-> decrease pulmonary vascular resistance
-> increases systemic resistance
-> blood flows into lungs
How does the direction of blood flow through the ductus arteriosus change at birth?
- Favours blood flow through pulmonary circulation
- Reduction in prostaglandins causes ductus arteriosus to close
How does the closure of the ductus arteriosus affect the foramen ovale?
- Reduction in right atrial pressure
- Increase in left atrial pressure
- Left-right pressure gradient causes folds of foramen ovale to close
What does the main pulmonary trunk originate from and what does it divide into?
Originates: Right ventricle
Branches: Left/right main axial artery to supply blood to left and right lung
How do each of the axial branches branch?
Can branch up to 20 times and each time they get smaller to ensure adequate perfusion of alveoli for gas exchange
How much of cardiac output does pulmonary circulation receive?
100%
Compare the resistance of systemic vs pulmonary circulation:
Pulmonary has 10-15 fold lower resistance than systemic
Compare the arterial pressure of systemic vs pulmonary circulation:
Systemic: 120/80 mmHg
Pulmonary: 25/10 mmHg
What are the 3 characteristics of the pulmonary circulation?
- Low resistance:
- Lots of parallel vessels
- Larger diameter vessels (r)
- Shorter vessels (l) - High compliance:
- Vessels distend easily = accomodate increased CO without large increase in pressure - Low arterial pressure
What happens if pulmonary pressure increases?
Can lead to:
1. Impaired ejection of blood (increased afterload)
2. Right heart failure
3. Pulmonary edema
Name the 4 regulators of pulmonary blood flow:
- Gravity
- Hypoxia (low O2)
- Endothelial control
- Sympathetic nervous system
Describe the effect of gravity on the lower region of the lung when standing:
- Increase hydrostatic pressure in vessels
- Vessels distend easily
- Resistance decreases
- Increase blood flow
Describe the effect of gravity on the upper region of the lung when standing:
- Reduced intravascular pressure
- Vessels partially collapse
- Increase resistance
- Decrease blood flow
What happens to blood flow to upper regions of lungs during exercise?
Blood flow increases
How is hypoxia a regulator of pulmonary blood flow?
- 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
How does endothelial control regulate pulmonary blood flow?
Endothelium of pulmonary vessels release vasoactive peptides to regulated vascular tone
1. Nitric oxide - vasodilator
2. Endothelin-1 - vasoconstrictor
How is the sympathetic nervous system a regulator of pulmonary blood flow?
Via α-adrenoreceptors and β-adrenoreceptors
- They negate each other therefore, SNS has minor role in regulating pulmonary blood flow
What is the role of α-adrenoreceptors?
Located in large conduit vessels
* Vasoconstriction
- Increase resistance
- Increase pulmonary pressure
What is the role of β-adrenoreceptors?
Located in smaller resistance vessels
* Vasodilation
- Decrease resistance
- Decrease pulmonary pressure
