Gas Exchange in Mammals Aim – to understand the structure and function of the lungs. Objectives- by the end of this lesson you should be able to Name and.

Gas Exchange in Mammals

Aim – to understand the structure and function of the lungs.

Objectives- by the end of this lesson you should be able to

• Name and identify parts of the gas exchange system.

• Explain how the alveoli are adapted for efficient gas exchange.

• Describe the structure and function of ciliated epithelium, goblet cells, cartilage, smooth muscle and elastic fibres

• Explain how the lungs are ventilated.

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 trachea orwindpipe is about10 cm long and is

supported byC-shaped ringsof cartilage to

prevent the tubefrom collapsing

during breathing

Trachea

The tracheasubdivides to giverise to the right

and left bronchus –these tubes are

also strengthenedby cartilage

The two bronchisubdivide to form

an extensivenetwork of

bronchioles thatdeliver air to

the gas exchangesurfaces – the

alveoli

Right and Leftbronchus

Bronchioles

Air enters the bodythrough the nasal

passages and mouth,and passes via the

pharynx and larynxto the trachea

Air is delivered tothe alveoli as the trachea branchesinto bronchi and

bronchioles

The TracheaThe Trachea

This photomicrograph of a transverse section through the trachea showsthe C-shaped ring of cartilage

Magnify

C-shapedcartilage ring

This magnified view of the wall of the trachea shows the cartilage cells together with the cells that line the lumen of the trachea – ciliated epithelium

Cartilagecells

Ciliatedepithelium

Lumen oftrachea

This highly magnified view of the lining of the trachea shows the cilia andmucus-secreting goblet cells that make up the epithelium

Goblet cell that secretesmucus to trap dust andother foreign material

that may enter the respiratory system

The wafting of these ciliaremoves the mucus and

trapped foreign materialfrom the respiratory

system

Move the cursor over the area of lung (yellow circle) to show the alveoli...

The Gas Exchange SurfaceThe Gas Exchange Surface

Section of lung A Single alveolusThorax

The Gas Exchange SurfaceThe Gas Exchange Surface

Each alveolus is a hollow, thin-walled sac that is surrounded by a

dense network of capillaries and is the site of gas exchange

in the lungs

The bronchioles divide many times forming

respiratory bronchioles, which in turn divide to

form alveolar ducts that terminate in groupsof sacs – the alveoli

Alveolarduct

Respiratorybronchiole

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

Alveolar TissueAlveolar TissueA photomicrograph of a section of alveolar tissueshowing the delicate nature of the lungs and the

'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases.

A photomicrograph of a section of alveolar tissueshowing the delicate nature of the lungs and the

'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases.

Breathing in (inspiration) and breathing out (expiration) are

mechanical processes involving the ribs, intercostal muscles

and the diaphragm

Two sets of antagonistic muscles are located between

the ribs; these are the externaland internal intercostal muscles

External intercostalmuscles

Internal intercostalmuscles

The intercostal muscles are antagonistic in the

sense that contraction of the external muscles raises

the rib cage, whereas contraction of the internal muscles

lowers the rib cage

The diaphragm is a powerful sheet of

muscle thatseparates the

thorax from theabdomen; it is

dome-shaped when relaxed and flattens

on contraction

Diaphragm

The Mechanics of BreathingThe Mechanics of Breathing

During inspiration, the external intercostal muscles

contract and raise the rib cage upwards and

outwards; the diaphragm also contracts and flattens

The volume of the thorax increases, lowering the air pressure in the chest cavity

to less than that of the atmosphere outside

A pressure gradient is created between the atmosphere and the lungs, and air rushes in via the trachea to equalise the pressure

difference

Air moves from a higher to a lower pressure region andinflates the lungs as inspiration takes place

Inspiration - Breathing InInspiration - Breathing In

During an expiration, the external intercostal muscles relax and lower the rib cage; the diaphragm relaxes and becomes dome-shaped

The volume of the thorax decreases, raising the air pressure in the chest cavity to above that of the atmosphere outside

A pressure gradient is created between the lungs and the atmosphere, and air rushes out via the trachea to equalise the pressure difference

Air moves from a higher to a lower pressure region and deflates the lungs as expiration takes place

The mechanism described is that for breathing at rest

At rest, inspiration is an active process involving contraction of the muscles of breathing

Expiration is a purely passive process involving relaxation of the muscles of breathing together with elastic recoil of the lungs

During forced breathing, as in exercise, expiration becomes an active process

Expiration - Breathing OutExpiration - Breathing Out

Expiration is assisted by the elastic recoil of the lungs following the stretching of elastic fibres during the process of inspiration

ExpirationInspiration

External intercostal muscles contract andraise the ribs upwards and outwards

External intercostal muscles relax andthe ribs move downwards and inwards

The diaphragmmuscle contracts

and flattens

The diaphragm muscle relaxes and

becomes dome-shaped

The volume of the thorax increasesThe air pressure in the thoracic cavityfalls below that of the atmospheric air

Air rushes into the lungs along a pressure gradient

The volume of the thorax decreasesThe air pressure in the thoracic cavityrises above that of the atmospheric air

Air rushes out of the lungs along a pressure gradient

SummarySummary