What are the functional zones of the pulmonary system?
1. Conducting Zone
2. Respiratory Zone
Conducting Zone ("Airways")
conducts air from the atmosphere down into the alveoli
the trachea, the main stem bronchi & their divisions/branches into secondary/tertiary bronchi, & finally the bronchioles (microscopic part of the airways)
alveoli (air sacks) themselves where exchange of O2 & CO2 takes place
a very dense capillary network surrounds and penetrates between each alveoli
sometimes it's described like each capillary is sitting in a thin layer of blood b/c the capillary network is so dense
What is the combined surface area of where air is in contact with the blood in the body?
50–100 m2 (size of a tennis court, all w/in thoracic cavity)
this is the surface area for gas exchange
the flow of blood through the lungs
branches of the pulmonary arteries follow the branching of the airways as far as the terminal bronchioles then split into capillary beds surrounding alveoli
capillary beds fuse, resulting in alveoli being essentially surrounded by a flowing sheet of blood
surface area for gas exchange ~100 m2
capillaries converge ultimately to form venules → 4 pulmonary veins leading to the L Atrium
Path to the Air-blood Barrier
respiratory bronchiole (smallest bronchiole) opens into an alveolar duct
an alveolar duct supplies air to individual alveolies
alveoli is ~partial sphere, w/ alveolar septa that separate one alveoli from another
1st: alveolar (squamous) epithelium is the most INNER lining of an alveolus; made up of mostly type I & some type II pneumocytes
2nd: basement membrane
4th: another basement membrane
5th: capillary endothelium
non-polar gasses in the air are diffusing through a VERY thin layer, less than 0.5 μm thick, from the inner alveolus to the blood stream
Type I Alveolar Cells
simple squamous epithelial cells that make up 97% of the alveolar surfaces
most of the cell is flattened into a thin sheet which forms part of the blood-air barrier
their nucleus & organelles are clustered together in the ‘thick’ part of the cell
Type II Alveolar (Septal) Cells
cuboidal cells w/ round nuclei found among the type I cells at the ‘corners’ of alveoli
have mitotic capacity
are responsible for regeneration/repair of the alveolar surface
secret pulmonary surfactant packaged in multilamellar bodies (cytosomes) to reduce the surface tension at the air-blood interface
a mixture of lipids, proteins, and a little carbohydrate
lipids are mainly phospholipids (dipalmitoyl phosphatidyl-choline
What is the total circulation time through the pulmonary system at rest?
pulmonary capillary bed contains ~75 mL of blood & the average erythrocyte takes about 0.75 seconds to pass through the alveolar capillary bed – it's within this time that gasses are exchanged
(most blood, 60%, is below your waist)
What is the blood flow in the pulmonary circulation?
the same as that of the systemic circulation, ~5 L/min or 83 mL/s, but in a very different manner
while the systemic circulatory system is a high pressure system, the pulmonary system is LOW pressure
lungs have open vessels & low resistance – why we need only a fraction of the pressure (25/7 mmHg) the L ventricle generates to push blood through pulmonary system
What is the average drop of blood pressure from the right ventricle to the left atrium?
7 mmHg, the Perfusion Pressure
the driving force/amount of energy necessary to push blood through the pulmonary system
the driving force for blood flow in pulmonary system only ~8% of the systemic driving force for same flow (systemic = 93 mmHg)
What is the pulmonary resistance in comparison to that of the systemic resistance?
less than 10%
R = ΔP / Q
Rsyst = 93 mmHg / 83 mL/s = 1.1 PRU
Rpulm = 7 mmHg / 83 mL/s = 0.08 PRU
can push the SAME amount of blood through w/ much LESS pressure
Pulmonary Vessel Characteristics
have larger diameters than comparable systemic vessels (arteries & arterioles stay fairly large)
are shorter & branch more often
pulmonary system has MORE arterioles which don't direct flow like those found in the systemic circulation (other factors do that)
pulmonary arterioles have a lower resting muscle tone therefore lower resistance
arterioles don't dampen out the pulse in the lungs – even capillaries have a pulse
Pulmonary Arterioles Overall
much less muscular
more RAPIDLY dilate (don't have same muscle tone as systemic arterioles)
Pulmonary Vessel Walls
are thin & have less muscle than systemic vessels – results in high compliance (expand easily)
have a relatively low pulse pressure (25 - 8 = 17 mmHg)
vessels can dilate accordingly/easily in response to moderate increases in pulmonary arterial pressure (eg. during exercise)
Is there more blood in the pulmonary system standing up or lying down?
the large compliance of pulmonary vessel walls allows vessels to expand & accept large amounts of blood that shift from the lower limbs to lungs when a person changes from standing to recumbent (lying down)
What happens to pulmonary vascular resistance as mean pulmonary arterial pressure rises?
vessels dilate easily in response to greater flow (high pressure) b/c of their high elastic/low-ish smooth muscle composition
What happens when there is an increases cardiac output, such as during exercise?
pulmonary arterial or venous pressure increases
in dilated vessels, resistance decreases
→ an increase in blood flow
as mean pulmonary arterial pressure increases, SO does pulmonary blood flow
What are the 2 ways in which the pulmonary system can accommodate increasing flow but NOT increasing resistance or pressure (eg. during physical activity)?
previously collapsed vessels OPEN UP
in an upright resting individual not all of the capillaries are being perfused (esp. in upper part, apex, of lung)
vessels that weren't previously being perfused now WILL BE (spec. APICAL part of lung)