The human gas exchange systemconsists of the nasal passages, thepharynx or throat, the larynx orvoice box, the trachea, the rightand left bronchus and the lungs

Larynx

Trachea(with rings of cartilage)

Left lung

Ribs

Diaphragm(a powerful sheet of muscle

separating the thorax from the abdomen)

Intercostalmuscles

Section throughribs

Rightbronchus

Bronchioles

The Human Gas Exchange SystemThe Human Gas Exchange System

The bronchioles dividemany times forming

respiratory bronchioles,which in turn divide toto form alveolar ducts

that terminate in groupsof sacs – the alveoli

Respiratorybronchioles

Alveolarduct

Alveoli

A single alveolus

Each alveolus is ahollow, thin-walled

sac that is surroundedby a dense network ofcapillaries and is thesite of gas exchange

in the lungs

The Gas Exchange SurfaceThe Gas Exchange Surface

Gases are exchanged across the alveoli by diffusion

According to Fick’s Law...

Rate of diffusion =surface area x difference in concentration

thickness of exchange surface

Maximum rate of diffusion of respiratory gases is achieved by:

• the large surface area presented by the alveoli (there are about 350 millionalveoli in the two lungs presenting an enormous surface area of

approximately 90 square metres – about the area of a tennis court)

• the large differences in concentration of metabolites between the alveoli and the blood capillaries

• the thinness of the diffusion barrier (alveolar and capillary walls providea total thickness of only 0.005 mm)

As deoxygenated blood from the body tissues flows through the network ofcapillaries surrounding each alveolus, oxygen diffuses into the blood and carbondioxide diffuses from the blood into the alveolus; oxygenated blood travels from

the lungs to the left of the heart for delivery to the body tissues

The Gas Exchange SurfaceThe Gas Exchange Surface

The exchange of gases between the lungs and the circulating blood takes placeacross the thin-walled alveoli whose walls consist of squamous epithelium

This photomicrograph shows the thin walls and large surface area displayed by the alveoli in human lung tissue

ALVEOLI

Gas Exchange occurs across the Alveolar EpitheliumGas Exchange occurs across the Alveolar Epithelium

The wall of the alveolus is composed of squamous epithelium together withits basement membrane

squamouscells

basement membrane

This thin wall is surrounded by an extensive network of capillaries whosewalls are also composed of flattened,

squamous epithelium

Gas Exchange occurs across the Alveolar EpitheliumGas Exchange occurs across the Alveolar Epithelium

capillary

The thinness of the capillary and alveolar walls, togetherwith their basement membranes, provides a barrier

between the alveolar air and the blood in the capillaries of approximately 0.3 m

The thinness of this barrier is one of the features thataids rapid diffusion of gases into and out of the blood

Gas Exchange occurs across the Alveolar EpitheliumGas Exchange occurs across the Alveolar Epithelium

capillary and alveolar wallsThis electron micrograph shows

a portion of human lung

Alveolarwall

Capillarywall

Red bloodcell

Nucleus ofcapillary

The short diffusionpath for efficientexchange of gasesbetween the air inthe alveoli and the

blood in the capillariesis provided by the thinsquamous epitheliumlayers of these tissues

O2

CO2

Gas Exchange occurs across the Alveolar EpitheliumGas Exchange occurs across the Alveolar Epithelium