Lecture 7 - Matching lung ventilation and tissue perfusion Flashcards
What is ventilation
movement of gases in and out of the lungs. occurs through the airways
What is Perfusion?
blood flow through any organ
eg. lungs perfusion occurs through blood vessels
What are the regional differences in lung ventilation in different parts of the lung?
Apical alveoli - 4 x larger than basal alveoli when upright
due to effect of gravity
basal regions have lot better ventilation than apical regions
basal alveoli - expand more than apical alveoli
How is pulmonary circulation brought about?
oxygenation of the venous blood
deox blood from right ventricles pumped into the pulmonary arteries to the lungs - then through the pulmonary veins to the left atrium to the left ventricle to the aorta and to the body.
What are the structures that supply the lung tissue
Arteries - bronchial artery from thoracic aorta
bronchial vein - to SVC
2% of cardiac output
How does the pulmonary circulation compare to the systemic?
lung receives full cardiac output (3-5 L/min)
but at a lower pressure to systemic circulation
aorta - 100 pulmonary artery - 15-30
How does the pulmonary artery re-oxygenate the blood in the lungs?
PA carries deoxygenated blood from right ventricle and splits into right and left branches to supply the respective side lung .
then these split further following the airway divisions.
What are the successive branches of the PA called while running through the lung parenchyma
they are called extra -alveolar vessels while running through the lung parenchyma
Describe branching after terminal bronchioles
blood vessels form capillary beds (starting with smaller arterioles) where gas exchange starts
each alveoli - approx 1000 capillaries
280 billion capillaries
forms rich network of capillaries - large surface area
very efficient exchange system
describe venous return from lungs to left atrium
ox blood flows into the pulmonary venules which unite to form large 4 veins emptying into the left atrium
What happens in emphysema
Widespread destruction of vascular beds
regional destruction of vascular beds
poor gas exchange and hypoxia
overdilation of alveoli - become damaged
Describe the regional differences in lung perfusion
Extra alveolar vessels: - running through lung parenchyma - diameter is affected by the lung volume (via pull of the lung parenchyma) Alveolar vessels - diameter dependent on many factors
What is hydrostatic pressure
force exerted by weight of a fluid (blood/water) due to gravity
how is lung perfusion at the top of the lung?
At the top: blood vessels collapse, halting flow
pressure at top is close to atmospheric pressure
lung perfusion in the middle of the lung.
In the middle: blood flow increases, capillary and arterial pressure exceeds alveolar pressure.
venule pressure lower than alveolar pressure.
Lung perfusion in the lower part of the lung
Alveolar pressure is lower than in capillaries veins and arteries.
Hence there is more perfusion.
circulation is dependent on arterial -venous pressure difference
P arterial
hydrostatic/blood pressure at the arterial end of the capillary
P venous
hydrostatic/blood pressure at the venous end of the capillary
Zone 1: at the top of the lung
Alveolar dead space
good ventilation but no perfusion
Poor blood flow - lung apex above right ventricle
Apices of the lungs - very small area
alveolar pressure > P arterial > P venous
blood flow determined by difference in P alveolar and P arterial
Zone 2: Recruitment zone
middle of lung
Lower down the lung
higher arterial pressure as more blood and higher hydrostatic pressure
recruitment of more alveolar unit especially in systole
P arterial > P alveolar > P venous
Blood flow determined by difference in P arterial and P alveolar
During haemorrhage what happens to lung perfusion?
pathological expansion of zone 1 : bleeding. there is a drop in P arterial and there is a rise in alveolar pressure . positive pressure ventilation
Zone 3: distention zone
lung bases
hydrostatic forces raise p arterial and p venous above p alveolar
p arterial> p venous > p alveolar
continuous blood flow
blood flow determined by difference p arterial and p venous
ventilation - perfusion (V/Q) ratio
When ventilation and perfusion is equal the ratio is 1
Apex = 3.3 as there is good ventilation but no perfusion
base = 0.6 ( both ventilation and perfusion is high so there is less than a ratio)
Anatomical dead space
conducting airways, no gas exchange
approx 2ml/kg or 150 ml in 70 kg man
Alveolar dead space
unperfused or poorly perfused alveoli
Physiological dead space
part of each breath that does not take part in gas exchange
Anatomical + Alveolar dead space
What is a pulmonary embolism
blood clot from leg vein travels to the heart and is lodged inside a blood vessel in the lungs, blocking blood supply
fatal emergency
Blood clot inside a lung blood vessel blocking the blood supply.
blocks large or intermediate pulmonary arteries
severe hypoxia
What does a pulmonary embolism do the alveolar dead space
enlarges alveolar dead space
What happens when there is a shunt in blood
deoxygenated blood reaching left side of the heart bypassing the lungs or failing to get oxygenated in the lungs
eg. bronchial veins, pneumothorax, age related changes: low V/q ratio
- there is a minimal effect of oxygen supplementation on a large shunt
What happens in pneumothorax
abnormal collection of air or gas in the pleural space.
treated by immediate needle decompression
chest drain connected to an underwater sealed system
What happens to V/Q ratio during shunting
no ventilation
good perfusion - as there is a presence of blood flow
it is 0 or very low
Which abnormality of oxygenation are you most likely to see in pneumonia?
nothing
