Special Circulations Flashcards
What are the two blood supplies to the lungs?
Bronchial circulation - part of systemic circulation, meets the metabolic requirements of the lungs.
Pulmonary circulations - Blood supply to alveoli, required for gas exchange.
What is cardiac output at rest?
5 L/min
What is maximum cardiac output?
20-25L/min for non-athelets
What is the pressure during systole in the ventricles?
15-30 mmHg in right ventricle
100-140 mmHg in left ventricle.
What are the diastolic pressures of the ventricles?
Right ventricle = 0-8mm/Hg
Left ventricle = 60 - 90 mm/Hg
These are the same as the pressures on the atria.
Why does pressure in arteries not get very low?
Because of the elastic recoil of artery. This means that the volume always stays similar.
What are some features of the pulmonary circulation?
- Low pressure
- Mean arterial pressure - 12-15mmHg
- Mean capillary pressure 9-12mmHg
- Mean venous pressue - 5mmHg
- Low resistance
- Short, wide vessels.
- Lots of capillaries (Many parallel elements)
- Arterioles habe relatively little smooth muscle
What adaptations does the pulmonary circulation have? Why?
The adaptations are to promote efficient gas exchange.
- Very high density of capillaries in alveolar wall
- large capillary surface area.
- Short diffusion distance
- Very think layer of tissue separating gas phase from plasma.
- Combined endothelium and epithelium thickens is about 0.3um
- Larger surface area and short diffusion distance produce high O2 and CO2 transport capacity.
What is the ventilation perfusion ratio (V/Q ratio)?
For efficient oxygenation, ventilation (air flow) and perfusion (blood flow) of alveoli need to match
Optimal V/Q = 0.8
Maintaining this means diverting blood from alveoli which are not well ventilated.
How does the ventilation perfusion ratio remain well matched?
Hypoxic pulmonary vasoconstriction ensures optimal ventilation / perfusion ratio.
- Most important mechanism regulating pulmonary vascular tine.
- Alveolar hypoxia results in vasoconstriction of pulmonary vessels.
- Ensures that perfusion matches ventilation
- Poorly ventilated alveoli are less well perfused
- Helps optimise gas exchange.
What are the disadvantages of chronic hypoxia vasoconstriction?
It can cause right ventricular failure.
- Chronic hypoxia can occur at altitude or as a consequence of lung disease such as emphysema.
- Chronic increase in vascular resistance - chronic pulmonary hypertension
- High after-load on right ventricles - can lead to right ventricular heart failure.
- RV heart failure rarely occurs on its own.
Why are low pressure pulmonary vessels strongly influenced by gravity?
In the upright position (orthostasis) there is greater hydrostatic pressure in vessel in the lower part of the lung. This increases the transmural pressure within the blood vessles at the base of the lung.
The increase in pressures leads to some filtration of tissue fluid, but will also distend the vessles and increase the flow to those areas.
What effect does exercise have on pulmonary blood flow?
- Increased cardiac output
- Small increase in pulmonary arterial pressure
- Open apical capillaries
- Increased uptake of O2 by lungs
- As blood flow increases capillary transit time is reduced. -
- At rest transit time = 1s.
- This can fall to 0.3s without compromising gas exchange.
How is tissue fluid formed?
- Starling forces determine fluid formation
- Hydrostatic pressure of blood within the capillaries - Pushes fluid out of the capillary.
- This is influenced more by venous pressure in systemic circulation so, hypertension does not usually result in peripheral oedema.
- Oncotic pressure (colloid osmotic pressure) - pressure exerted by large molecules such as plasma proteins - Draws fluid into the capillary.
What effect does low capillary pressure have?
It minimises the formation of lung lymph to prevent oedema.