BIPN 102 Lec5
Effects of gravity on the lung:
Gravity pulls down lung and blood in it and therefore in the top of the lung, when we are
erect, there is a more negative interplueral pressure which causes a larger transpulmonary
pressure which increases the volume at rest at FRC increases volume of alveoli and
bronchioles at top of lung.
. at the bottom, the weight of the lung causes the interplurel
pressure to be less negative,
because the interplurel space is more compressed, causing a
lower transpulmonary pressure so the alveolar are less expanded.
When there is
inspiration, the top of the lung has less compliance.
Blood flow responds to gravity, more blood flow at bottom of lung. At around the third
rib, center of lung, that ideal perfusion and diffusion ratio is matched, 1 due to gravity.
Pulmonary arterials and bronchioles have effect on po2 and co2 balance. There will be
regulatory adjustments of these systems as the po2 and the pco2 will change.
At the bottom of the lung, perfusion is higher than ventilation, the po2 in the alveoli is
going to be less than at sea level. This will cause the pulmonary arterials to constrict and
bronchioles to dilate. Serves to match ventilation with perfusion, blood coming from
bottom of the lung has lower po2 and higher co2.
At the top, ventilation is higher than perfusion, so at the top po2 is greater than 100, and
pco2 is smaller than 40. So the pulmonary arterials will dilate and bronchioles will
The ratio of ventilation and perfusion ~ 1. Only occurs around the middle of the lung,
because of this the blood coming from the heart, is a mixture of blood from the top and
bottom of the lung(more from the bottom). The blood pumped into systemic circulation is
a little lower than po2 100 because of that.
When one exercises, sympathetic sytem activated, cardiac output inc, blood flow inc.
more forceful pumping of bood into pulmonary inc, more co output, bp inc in pulm
artery, more uniform perfusion of lung during exercise, more blood to top of lungs, inc
SA of exchange.
In the pulmonary system at rest, pressure is much lower than systemic circulation
Pulmonary: Cystolic- 25mmHg diastolic – 8mmHg Mean arterial pressure is around
15mmHg in pulmonary circulation. Mean arterial = diastolic pressure+1/3 systolic