Respiratory system

The Respiratory System

 There are two primary components to the

human respiratory system:

 Ventilation

 The movement of air in and out of the lungs

 Gas exchange

 The exchange of oxygen (O2) and carbon

dioxide (CO2) between the lung tissue and

the circulatory system

 Cell respiration creates a constant

demand for oxygen and a need to

remove carbon dioxide

The Respiratory Sytem

 The human respiratory

system is generally

divided into two main

sections:

 The upper respiratory

 Mouth, nose, pharynx etc

 The lower respiratory

 Bronchi, lungs etc

Lung Structure

 Air enters through the nose or mouth and then passes through the

pharynx to the trachea

 The trachea is positioned in front of the oesophagus and is held open by rings of

cartilage

 The air travels down the trachea until it divides into two bronchi (singular:

bronchus) which connect to the left and right lungs

 The bronchi divide into many smaller airways called bronchioles

 Each bronchiole terminates with a cluster of air sacs called alveoli, where

gas exchange with the bloodstream occurs

 A large muscle called the diaphragm sits under the lungs to assist

ventilation

Lung Structure

Lung Structure

Lung Structure

 Ciliated, mucus-secreting epithelial cells lines the trachea and bronchi,

which traps and removes dust and pathogens before they reach the gas

exchange surfaces

Ventilation

 Ventilation of the lungs occurs due to the

action of the respiratory muscles

 Respiratory muscles contract to change the

volume of the thoracic cavity and hence

alter the pressure in the lungs

 Changing volume creates a pressure

differential between the lungs and

atmosphere – with air then moving to

equalise

Ventilation

 The mechanism of breathing occurs

according to the principle of Boyle’s

Law: pressure is inversely proportional to

volume

 When the volume of the thoracic cavity

increases, pressure in the thorax decreases

 When the volume of the thoracic cavity

decreases, pressure in the thorax increases

 Gases will move from a region of high

pressure to a region of lower pressure

(pressure gradient)

Inspiration

 During inspiration, the respiratory

muscles contract and increase the

thoracic volume which decreases the

pressure in the lungs

 The diaphragm flattens and pulls down when

it contracts, and the external intercostal

muscles lift the ribs up

 Air then flows into the lungs in response

to decreased pressure inside the lungs

Expiration

 During expiration, the respiratory muscles

relax and decrease the thoracic volume

which increases the pressure in the lungs

 The diaphragm returns to its dome shape and

pushes up against the lungs, and the external

intercostal muscles let the ribs lower

 Air then flows passively out of the lungs

to equalize with the air pressure

 Other muscles can assist in forced breathing

(inspiration or expiration)

Respiratory Muscles

Gas Exchange

 The numerous alveoli at the terminal bronchioles provides a large surface area

for gas exchange (about 70 m2)

 Each lung contains more than 150 million alveoli

 Gases are exchanged by diffusion between the alveoli and the surrounding

capillaries

 The alveoli provide the wet membranes needed for diffusion of oxygen and

carbon dioxide, but water loss (and the risk of them drying out) is reduced as

they are protected inside the body

 Efficient diffusion is achieved by:

 The short distance and minimal membranes that need to be crossed between the alveoli

and the capillaries

 Concentration gradients that are maintained by the continuous flow of blood through the

capillaries and the continuous movement of air in and out of the alveoli

Gas Exchange

 Summary of key gas exchange

features:

 Alveoli are small and numerous which

generates a large surface area for

diffusion

 The number of membranes that

needs to be crossed is minimal – just

the alveoli and capillary membranes

 The distance between the alveoli and

capillary is small – shorter distance

to diffuse

Gas Exchange

 The blood arriving at the alveoli is

deoxygenated blood that has

come to the lungs via the right

ventricle/pulmonary arteries so it

will have a low concentration of

oxygen and a high concentration

of carbon dioxide

 The air entering the alveoli is going

to have a high concentration of

oxygen and low concentration of

carbon dioxide

 As these gradients are continuously

maintained, efficient gas exchange

can be also be maintained

Alveoli Tissue

 The inner surface of the alveolus is lined by two special types of alveolar

cell called a pneumocyte

 Type I pneumocytes

 Thin, flat cells – minimises distance for diffusion to occur

 Form 95% of alveoli surface

 Tight junctions between the cells prevents tissue fluid entering the alveoli

 Type II pneumocytes

 Secrete pulmonary surfactant which reduces the surface tension within the alveoli

and prevent collapse when it recoils after expanding (the alveoli also contains

elastic connective tissue)

Alveoli Tissue

 Alveoli have wet surfaces, as it is easier for the

oxygen to diffuse if it is dissolved

 The surfactant prevents the collapse and resistance

to inflation that can occur in the alveoli by reducing

surface tension

Oxygen Transport

 Oxygen is transported in red blood cells by the protein haemoglobin

(Hb)

 Haemoglobin can reversibly bind up to four oxygen molecules

 (Hb + 4O2 = HbO8)

 As each O2 molecule binds, it alters the conformation of haemoglobin,

making subsequent binding easier (cooperative binding)

Oxygen Dissociation Curves

 Because binding potential changes with each

additional O2 molecule, the saturation of

haemoglobin is not linear with regards to

oxygen levels (as partial pressure)

 The oxygen dissociation curve for adult

haemoglobin is sigmoidal (S-shaped) due to

cooperative binding

 There is a low saturation of haemoglobin when

oxygen levels are low (haemoglobin releases O2 in

hypoxic tissues)

 There is a high saturation of haemoglobin when

oxygen levels are high (haemoglobin binds O2 in

oxygen-rich tissues)

Oxygen Dissociation Curves

 Foetal haemoglobin has a higher affinity

for oxygen than adult haemoglobin

(dissociation curve is shifted to the left)

 This is important as it means foetal

haemoglobin will load oxygen when

adult haemoglobin is unloading it (i.e. in

the placenta)

Carbon Dioxide Transport

 Carbon dioxide produced by the cells is transported by one of three

mechanisms:

 Some is bound to haemoglobin to form HbCO2 (carbon dioxide binds to the globin not the

heme group and so doesn’t compete with O2 binding)

 A very small fraction gets dissolved in water and is carried in solution (~5% as carbon

dioxide dissolves poorly in water)

 The majority (~75%) diffuses into the erythrocyte and gets converted into carbonic acid

(a reaction catalysed by the enzyme carbonic anhydrase - CAH)

 Carbonic acid (H2CO3) then dissociates to form hydrogen ions (H+) and bicarbonate

(HCO3

–)

 The hydrogen ions make the environment more acidic/less alkaline, causing

haemoglobin to release its oxygen

 The process is reversed in the lungs so the carbon dioxide can be released into the

alveoli and breathed out of the lungs

Carbon Dioxide Transport

Bohr Shift

 Carbon dioxide lowers the pH of the blood

(by forming carbonic acid), which causes

haemoglobin to release its oxygen

 This is known as the Bohr effect – a decrease

in pH shifts the oxygen dissociation curve to

the right

 Cells with increased metabolism (i.e.

respiring tissues) release greater amounts of

carbon dioxide (product of cell respiration)

 Hence haemoglobin is promoted to release

its oxygen at the regions of greatest need