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Learning Resources - Respiratory System- Revision Notes

Cells need energy for all metabolic activities

Most energy is derived from chemical reactions

Can only take place in presence of oxygen

Main waste product is carbon dioxide

Respiratory system supplies the route by which oxygen enters the body – also provides the route for excretion of carbon dioxide

Condition of atmospheric air entering body varies according to the external environment





As inhaled air moves through the air passages to reach lungs it is cooled or warmed by body temperature

Moistened to become saturated with water vapour and ‘cleaned’ as particles of dust stick to the mucus lining membrane

Blood transports oxygen and carbon dioxide between lungs and cells of body

Oxygen from lungs to cells

Carbon dioxide from cells to lungs

Exchange of gases between:

Blood and lungs = external respiration

Blood and cells = internal respiration


Nose and Nasal cavity

Main route of air entry

Consists of a large irregular cavity divided into two equal passages by a septum

Posterior-bony part – formed by the perpendicular plate of the ethmoid bone and the vomer

Anteriorly – consists of hyaline cartilage

Roof – formed by the cribriform plate of the ethmoid bone and sphenoid bone, frontal bone and nasal bones

Floor – formed by the roof of mouth – consists of hard palate and soft palate

Consists of involuntary muscle

Composed of maxilla and palatine bones

Medial wall – formed by the septum

Lateral walls – formed by maxilla, ethmoid bone and inferior conchae

Posterior wall – formed by posterior wall of the pharynx


Lining of the Nose

Lined with vascular ciliated columnar epithelium (ciliated mucous membrane)

Contains mucus-secreting goblet cells

At anterior nares (nostrils) membrane blends with the skin

Posteriorly extends into nasal part of pharynx


Openings into the Nasal Cavity

Anterior nares are openings from the exterior into nasal cavity

Nasal hairs coated in sticky mucus are present

Posterior nares are openings from nasal cavity to the pharynx

Paranasal sinuses are cavities in bones of the face and cranium – contain air

Tiny openings between Paranasal sinuses and nasal cavity

Lined with mucous membrane

Continuous with nasal cavity

Main sinuses:

Maxillary sinuses – lateral walls

Frontal and sphenoidal sinuses – in roof

Ethmoidal sinuses – upper part of lateral walls

Sinuses function in speech and lighten the skull

Nasolacrimal ducts extend from lateral walls of nose to conjunctival sacs of eye (drain tears)


Respiratory Function of the Nose

Begins process by which air is warmed, moistened and filtered

Projecting conchae increase surface area and cause turbulence, spreading inspired air over the whole nasal surface

Maximises warming, humidification and filtering

Warming: due to immense vascularity of the mucosa

Filtering and cleaning: occurs as hairs at anterior nares trap large particles. Smaller particles (dust and microbes) settle and adhere to mucus

Protects underlying epithelium from irritation and prevents drying

Synchronous beating of the cilia wafts the mucus towards the throat where it is swallowed or coughed up (expectorated)

Humidification: as air travels over the moist mucosa it becomes saturated with water vapour

Irritation of nasal mucosa results in sneezing – reflex action that forcibly expels an irritant


Olfactory Function

Nerve endings detect smell

Located in roof of nose in area of cribriform plate of ethmoid bones and superior conchae

Stimulated by airborne odours

Resultant nerve impulses – conveyed by olfactory nerves to the brain where sensation of smell is perceived




Tube 12-14cm – extends from base of skull to level of 6th cervical vertebra

Lies behind the nose, mouth and larynx – wider at upper end


Structures associated with the Pharynx

Superiorly – inferior surface of the base of the skull

Inferiorly – continuous with oesophagus

Anteriorly – wall – incomplete due to openings into nose, mouth and larynx

Posteriorly – areola tissue, involuntary muscle and bodies of first 6 cervical vertebrae


Nasal part

Lies behind nose above the level of soft palate

On lateral walls – two openings of auditory tubes – one leading to each middle ear

Posterior wall – pharyngeal tonsils (adenoids) – contain lymphoid tissue


Oral part

Lies behind mouth – extends from below level of soft palate to level of upper part of the body of the 3rd cervical vertebrae

Lateral walls of pharynx blend with the soft palate – forms two folds on each side

Collection of lymphoid tissue (palatine tonsil)

During swallowing – nasal and oral parts are separated by soft palate and uvula


Laryngeal part

Extends from oropharynx above

Continues as oesophagus below from 3rd – 6th cervical vertebrae



Mucous membrane lining:

In Nasopharynx:

Continuous with lining in nose

Ciliated columnar epithelium

In oropharynx and laryngopharynx:

Formed by stratified squamous epithelium

Continuous with lining of mouth and oesophagus

Lining protects underlying tissues from abrasive action of foodstuffs passing through

Fibrous tissue:

Intermediate layer

Thicker in Nasopharynx

Little muscle

Becomes thinner towards lower end where muscle layer becomes thicker

Smooth muscle:

Several involuntary constrictor muscles – important in swallowing

Upper end of oesophagus is closed by lower constrictor muscle, except during swallowing


Blood and Nerve Supply

Blood supplied by several branches of facial artery

Venous return is into facial and internal jugular veins

Nerve supply – from pharyngeal plexus – formed by parasympathetic and sympathetic nerves

Parasympathetic supply is by vagus and glossopharyngeal nerves

Sympathetic supply is by nerves from the superior cervical ganglia



Passage way for air and food – organ involved in respiratory and digestive systems

Air passes through the nasal and oral sections

Food passes through the oral and laryngeal sections

Further warming and humidifying

Taste – olfactory nerve endings in epithelium of oral and pharyngeal parts


Protection – lymphatic tissue produce antibodies





Larynx extends from the root of the longue and hyoid bone to the trachea

Lies in front of the laryngopharynx at the level of the 3rd to 6th cervical vertebrae


Structure associated with the Larynx

Superiorly – hyoid bone and root of tongue

Inferiorly – continuous with trachea

Anteriorly – muscles attached to hyoid bone and muscles of neck

Posteriorly – the laryngopharynx and 3rd – 6th cervical vertebrae

Laterally – lobes of thyroid gland




Composed of several irregularly shaped cartilages attached to each other by ligaments and membranes

1 thyroid cartilage

1 cricoid cartilage Hyaline cartilage

2 Arytenoid cartilages

1 epiglottis Elastic fibrocartilage


Thyroid cartilage

Consists of two flat pieces of hyaline cartilage (laminae), fused anteriorly, forming the laryngeal prominence

Laminae are separated forming a V-shaped notch – thyroid notch

Incomplete posteriorly – border of each lamina is extended to form two processes called the superior and inferior cornu

Upper part – lined with stratified squamous epithelium

Lower part – lines with ciliated columnar epithelium

Many muscles attached to outer surface

Forms most of anterior and lateral walls of larynx

Cricoid cartilage

Lies below thyroid cartilage

Composed of hyaline cartilage

Shaped like a signet ring – encircling larynx with narrow part anteriorly – broad part posteriorly

Articulates with arytenoid cartilages and inferior cornu

Lined with ciliated columnar epithelium, muscles and ligaments attached to outer surface

Lower border marks end of upper respiratory tract.

Arytenoid cartilages

Pyramid-shaped hyaline cartilages

Sit on top of the broad part of the cricoid cartilage forming part of the posterior part of the larynx.

Give attachment to the vocal cords and to muscles

Lined with ciliated columnar epithelium


Leaf-shaped fibroblastic cartilage attached to the inner surface of the anterior wall of the thyroid cartilage immediately below the thyroid notch.

Rises obliquely upwards behind the tongue and the body of the hyoid bone

Covered with stratified squamous epithelium

Closes off the larynx during swallowing – protecting the lungs from accidental inhalation of foreign objects.


Ligaments and Membranes

There are several ligaments that attach the cartilages together and to the hyoid bone


Blood and Nerve Supply

Blood is supplied to the larynx by the superior and inferior laryngeal arteries and drained by the thyroid veins – join the internal jugular vein

Parasympathetic nerve supply is from the superior laryngeal and recurrent laryngeal nerves, which are branches of the vagus nerves.

Sympathetic nerves are from the cervical ganglia, one on each side

Provide motor nerve supply to the muscles of the larynx and sensory fibres to the lining membrane


Interior of the Larynx

Vocal cords are two pale folds of mucous membrane with cord-like free edges

Extend from the inner wall of the thyroid prominence anteriorly to the arytenoid cartilages posteriorly

When muscles controlling the vocal cords are relaxed, the vocal cords open and the passageway for air coming up through the larynx is clear

Vocal cords – abducted

Pitch of the sound produced by vibrating vocal cords in this position is low

When the muscles controlling the vocal cords contract, cords are stretched out tightly across the larynx (are adducted/closed)

When stretched and vibrated by passing air pitch produced is high

Pitch is therefore determined by tension applied to the vocal cords by appropriate sets of muscles

When not in use vocal cords are adducted

Space between the vocal cords is the glottis



Production of sound

Sound has properties:




Pitch of voice depends on length and tightness of the cords

Puberty – male vocal cords grow longer causing a lower pitch

Volume of voice depends on force at which cords vibrate – greater the force of expired air – more cords vibrate and louder the sound produced

Resonance/tone depends on the:

Shape of mouth

Position of tongue and lips

Facial muscles and air in Paranasal sinuses


Occurs during expiration when sounds produced by vocal cords are manipulated by:




Protection of the Lower Respiratory Tract

During swallowing (deglutition) larynx moves upwards, occluding the opening into it from pharynx – epiglottis closes over the larynx

Ensures that food passes into the oesophagus rather than the lower respiratory passages

Passageway for Air

Between pharynx and trachea

Humidifying, Filtering and Warming

Processes continue as inspired air travels through the larynx




Continuation of larynx and extends downwards to level of 5th thoracic vertebrae

Divides (bifurcates) at carina into right and left primary bronchi – one to each lung

Approximately 10 – 11cm long

Lies mainly in median plane in front of oesophagus


Structures associated with the Trachea

Superiorly – larynx

Inferiorly – right and left bronchi

Anteriorly – upper part, isthmus of thyroid gland; lower part, arch of aorta and sternum

Posteriorly – oesophagus separates trachea from vertebral column

Laterally – lungs and lobes of thyroid gland




Composed of three layers of tissue

Held open by 16 – 20 incomplete (posteriorly) C-shaped rings of hyaline cartilage

Connective tissue and involuntary muscle join cartilages together – form posterior wall

Soft tissue posterior wall is in contact with oesophagus

Three layers of tissue cover cartilages of trachea:

Outer layer – fibrous and elastic tissue – encloses the cartilages

Middle layer – cartilages and bands of smooth muscle – wind around trachea in a helical arrangement; some areolar tissue containing blood and lymph vessels and autonomic nerves

Inner lining – ciliated columnar epithelium, containing mucus-secreting goblet cells


Blood and Nerve Supply, Lymph Drainage

Atrial supply – mainly inferior thyroid and bronchial arteries

Venous return – by inferior thyroid veins into the brachiocephalic veins

Parasympathetic nerve supply – recurrent laryngeal nerves and other branches of vagi

Sympathetic supply – nerves from sympathetic ganglia

Parasympathetic stimulation constricts the trachea – sympathetic stimulation dilates it

Lymph from respiratory passages drains through lymph nodes

Situated around trachea and in carina – area where divides into two bronchi



Support and patency

Arrangement of cartilage and elastic tissue prevents kinking and obstruction of airway when head and neck moves

Absence of cartilage posteriorly allows trachea to dilate and constrict in response to nerve stimulation, and for indentation as oesophagus distends during swallowing

Cartilage prevent collapse of trachea when internal pressure is less than intrathoracic pressure – at end of forced expiration

Mucociliary escalator

Synchronous and regular beating of cilia of mucous membrane lining that wafts mucus with adherent particles upwards towards larynx where swallowed or coughed up

Cough reflex

Nerve endings in larynx, trachea and bronchi – sensitive to irritation – generates nerve impulses conducted by vagus nerves to the respiratory centre in brain stem

Reflex motor response is deep inspiration followed by closure of the glottis

Abdominal and respiratory muscles then contract and suddenly the air is releases under pressure expelling mucous and/or foreign material from mouth

Warming, Humidifying and Filtering

Continue in nose – air normally saturated and at body temperature at trachea



Position and Gross Structure

Two lungs – one each side if the midline in thoracic cavity


They have:

An apex

A base

A costal surface

A medial surface


Rounded and rises into root of neck

25mm above level of middle third of the clavicle

Lies close to first rib and blood vessels and nerves in root of neck


Concave and semilunar

Lies on thoracic surface of diaphragm

Costal surface


Lies against costal cartilages, ribs and intercostal muscles

Medial surface

Concave – roughly shaped triangular-shaped (hilum)

At level of 5th – 7th thoracic vertebrae

Structures forming root of lung enter and leave hilum – include primary bronchi, pulmonary artery supplying lung and two pulmonary veins draining it, the bronchial artery and veins and lymphatic and nerve supply

Area between lung – mediastinum

Occupied by:


Great vessels


Right and left bronchi


Lymph nodes

Lymph vessels


Right lung – divided into three lobes




Left lung is smaller – heart occupies space of midline – divided into two lobes




Pleura and Pleural Cavity

Pleura consists of closed sac or serous membrane (one for each lung) – contains a small amount of serous fluid

Lung is invaginated (pushed into) sac – forming two layers:

One adheres to the lung

One adheres to wall of thoracic cavity

Visceral pleura – adheres to lung covering each lobes – passing into fissures that separate them

Parietal pleura – adherent to inside of chest wall and thoracic surface of diaphragm – remains detached from adjacent structures in the mediastinum and is continuous with the visceral pleura around edges of hilum

Pleural cavity – only a potential space. In health – two layers are separated by a thin film of serous fluid – allows layers to glide over each other, preventing friction during breathing. Serous fluid is secreted by epithelial cells of membrane

Two layers of pleura, with serous fluid in between, behave in same way as two pieces of glass separated by a layer of water – glide over each other easily but are difficult to pull apart because of surface tension between the membranes and fluid. If either layer of punctured – underlying lung collapses owing to its inherent property of elastic recoil.


Interior of the Lungs

Composed of:


Smaller air passages


Connective tissue

Blood vessels

Lymph vessels


All embedded in an elastic connective tissue matrix

Each lobe if made up of a large number of lobules


Pulmonary Blood Supply

Pulmonary trunk divides into right and left pulmonary arteries – transport deoxygenated blood to each lung

Each pulmonary artery divides into many branches - eventually end in a dense capillary network around walls of alveoli

Walls of alveoli and capillaries consist of one layer of flattened epithelial cells

Exchange of gases between air in the alveoli and blood in the capillaries takes place across these two very fine membranes (together – respiratory membrane)

Pulmonary capillaries join up, forming two pulmonary veins in each lung – leave lungs at hilum and carry oxygenated blood to the left atrium of the heart

Blood capillaries and blood vessels in the lungs are supported by connective tissue


Bronchi and Bronchioles

Two primary bronchi are formed when the when trachea divides about level of 5th thoracic vertebra

Right bronchus

Wider, shorter and more vertical than left bronchus – more likely to become obstructed by an inhaled foreign body

Approximately 2.5cm long

After entering right lung at the hilum it divides into three branches – one to each lobe

Each branch subdivides into numerous smaller branches

Left bronchus

5cm long

Narrower than right bronchus

After entering left lung at hilum it divides into two branches – one to each lobe

Each branch subdivides into progressively smaller tubes within the lung substance



Bronchi are composed of the same tissues as the trachea and are lined with ciliated columnar epithelium

Progressively subdivide into:


Terminal bronchioles

Respiratory bronchioles

Alveolar ducts


Towards distal end on bronchi – cartilages become irregular in shape and are absent at bronchiolar level

In absence of cartilage – smooth muscle in walls of bronchioles becomes thicker – responsive to autonomic nerve stimulation and irritation

Ciliates columnar mucous membrane changes gradually to non-ciliated cuboidal-shaped cells in the distal bronchioles.

The wider passages are conducting airways – function – to bring air into the lungs and walls are too thick to permit gas exchange


Blood and Nerve Supply, Lymph Drainage

Arterial supply to walls of bronchi and smaller air passages is through branched of right and left bronchial arteries

Venous return is mainly though bronchial veins

On right side – empty into azygos vein

On left side – empty into superior intercostal vein

Vagus nerves (parasympathetic) stimulate contraction of smooth muscle in bronchial tree causing bronchoconstriction

Sympathetic stimulation causes bronchodilatation

Lymph – drained from walls of air passages in a network of lymph vessels

Passes through lymph nodes situated around trachea and bronchial tree then into thoracic duct on left side and right lymphatic duct on the other side



Control of air entry

Diameter of respiratory passages in altered by contraction or relaxation of involuntary muscles in their walls – regulating volume of air entering the lungs

Controlled by autonomic nerve supply

Parasympathetic stimulation causes constriction

Sympathetic stimulation causes dilatation




Support and patency

Removal of particulate matter

Cough reflex


Respiratory Bronchioles and Alveoli


Within each lobe – lung tissue is divided by sheets of connective tissue into lobules

Each lobule is supplied with air by a terminal bronchiole – further subdivides into respiratory bronchioles, alveolar ducts and large numbers of alveoli

Approximately 150 million alveoli in adult lung

In alveoli – gas exchange occurs

As airways progressively divide and become smaller their walls gradually become thinner until muscle and connective tissue disappear leaving a single layer of epithelial cells in alveolar ducts and alveoli

Distal respiratory passages are supported by a loose network of elastic connective tissue in which macrophages, fibroblasts, nerves, blood and lymph vessels surrounded by a dense network of capillaries

Exchange of gases in lung (external respiration) takes place across a membrane made up of alveolar wall and the capillary wall fused firmly together (respiratory membrane)

Lying between squamous cells are septal cells that secrete surfactant, a phospholipid fluid which prevents the alveoli from drying out

Surfactant reduces surface tension and prevents alveolar walls collapsing during expiration

Secretion of surfactant into distal air passages and alveoli begins about 35th week of fetal life

Nerve supply to Bronchioles

Parasympathetic fibres from the vagus nerve cause bronchoconstriction

Absence of supporting cartilage means the small airways may be completely closed off by constriction of their smooth muscle

Sympathetic stimulation relaxes bronchiolar smooth muscle



External respiration

Defence against microbes





Exchange of gases between body cells and the environment

Involves two main processes

Breathing (pulmonary ventilation) – movement of air into and out of lungs

Exchange of gases – takes place:

In lungs: external respiration

In the tissues: internal respiration



Supplies oxygen to the alveoli

Eliminates carbon dioxide


Muscles of Breathing

Expansion of chest during inspiration occurs as a result of muscular activity, party voluntary and party involuntary

Main muscles used in normal, quiet breathing are:

Intercostal muscles


During deep breathing assisted by muscles of





Intercostal Muscles

11 pairs that occupy spaces between 12 pairs of ribs

Arranged in two layers

External intercostal muscles – muscle fibres

Extend downwards and forwards from the lower border of rib above to the upper border of rib below

Internal intercostal muscles – muscle fibres

Extend downwards and backwards from the lower border of rib above to upper border of rib below, crossing the external intercostal muscle fibres at right angles

First rib is fixed, so when intercostal muscles contract they pull all the other ribs towards the first rib

Because of shape and sizes of ribs they move outwards when pulled upwards, enlarging the thoracic cavity

Intercostal muscles are stimulated to contract by the intercostal nerves



Dome-shaped muscular structure separating the thoracic and abdominal cavities

Forms the floor of thoracic cavity and the roof of the abdominal cavity

Consists of a central tendon from which muscle fibres radiate to be attached to the lower ribs and sternum and to the vertebral column by two crura

When muscle of diaphragm is relaxed – central tendon is pulled downwards to level of 9th thoracic vertebra, enlarging the thoracic cavity in length

Decreases pressure in thoracic cavity

Increases pressure in abdominal and pelvic cavities

Supplied by phrenic nerves

Intercostal muscles and diaphragm contract simultaneously, enlarging the thoracic cavity in all directions

Back to front

Side to side

Top to bottom


Cycle of Breathing

Average respiratory rate = 12 to 15 breaths per min

Each breath consists of three phases:






When capacity of thoracic cavity is increased by simultaneous contraction of intercostal muscles and diaphragm – parietal pleura moves with walls of thorax and diaphragm

Reduces pressure in pleural cavity to a level lower than atmospheric pressure

Visceral pleura follows the parietal pleura pulling lungs

Expands the lungs

Pressure within alveoli and in air passages falls

Air is drawn into the lungs as a result and aims to equalise atmospheric and alveolar air pressures

Process of inspiration is active – needs energy for muscle contraction

Negative pressure created in thoracic cavity aids venous return to heart and is known as respiratory pump

At rest inspiration lasts around 2 seconds



Relaxation of intercostal muscles and diaphragm results in downward and inward movement of the rib cage and elastic recoil of the lungs

Pressure inside lungs exceeds that in the atmosphere and so air is expelled from the respiratory tract

Lungs still contain some air

Prevented from collapse by the intact pleura

Process is passive – does not require energy

At rest expiration lasts about 3 seconds

There is a pause after respiration before the next cycle begins


Physiological Variables affecting Breathing


Ability of lungs to return to normal shape after each breath

Loss of elasticity and connective tissue in lungs necessitates forced expiration and increased effort on inspiration


Measure of distensibility of lungs – effort required to inflate alveoli

Healthy lung is compliant and inflates with little effort

When compliance is low more effort is required – in some diseases where elasticity is reduced or when insufficient surfactant is present

Compliance and elasticity are opposing forces

Airway resistance

When increased – in bronchoconstriction – more respiratory effort is required to inflate lungs


Lung Volumes and Capacities

Normal quiet breathing around 15 complete respiratory cycles per minute

Lungs and air passages are never empty as exchange of gases only occurs across the walls of alveolar ducts and alveoli

Remaining capacity of respiratory passages is the anatomical dead space 150ml

Tidal volume (TV) is amount of air passing into and out of lungs during each cycle of breathing 500ml at rest

Inspiratory reserve volume (IRV) extra volume of air that can be inhaled into the lungs during maximal inspiration – over and above normal TV

Inspiratory capacity (IC) amount of air that can be inspired with maximum effort- consists of TV and IRV

Functional residual capacity (FRC) amount of air remaining in the passages and alveoli at the end of quiet expiration

Expiratory reserve volume (ERV) largest volume of air expelled from lungs during maximal expiration

Residual volume (RV) cannot be directly measured – volume of remaining air in the lungs after forced expiration

Vital capacity (VC) maximum volume of air which can be moved into and out of lungs


Alveolar ventilation volume of air that moves into and out of the alveoli per minute – equal to tidal volume minus the anatomical dead space multiplied by the respiratory rate:

Alveolar ventilation = (TV – anatomical dead space) X respiratory rate

= (500-150)ml X 15 resps per min

= 5.25 litres per min

Lung function tests are carried out to determine respiratory function

Based on parameters outlined above

Results of these tests can help in diagnosis and monitoring of respiratory disorders


Exchange of Gases

Gas exchange at the respiratory membrane and in tissues is continuous

Diffusion of oxygen and carbon dioxide depends on pressure differences – between atmospheric air and the blood – blood and tissues


Composition of Air

Atmospheric pressure at sea level is 101.3 kilopascals (kPa) or 760 mmHg

With increasing height above sea level, atmospheric pressure is reduced

At 5500m pressure is half that at sea level

Under water pressure increases by around 1 atmosphere per 10m below sea level

Air is a mixture of gases



Carbon dioxide

Water vapour

Small quantities of inert gases

Each of these gases in the mixture exerts a part of the total pressure proportional to its concentration

Parietal pressure




Alveolar Air

Composition remains fairly constant

Different from atmospheric air

Saturated with water vapour

Contains more carbon dioxide and less oxygen

Saturation with water vapour provided 47mmHg – reducing parietal pressure of all other gases present

Gaseous exchange between the alveoli and bloodstream (external respiration) is a continuous process – alveoli are never empty

Independent of respiratory cycle

During each inspiration cycle only some of the alveolar gases are exchanged


Expired Air

Mixture of alveolar air and atmospheric air in dead space


Diffusion of Gases

Exchange of gases occurs when a difference in partial pressure exists across a semipermeable membrane

Gases move by diffusion from a higher concentration to a lower concentration until equilibrium is established

Atmospheric nitrogen is not used by the body so its partial pressure remains unchanged – same in inspired and expired air, alveolar air and in the blood


External Respiration

Exchange of gases by diffusion between alveoli and blood in the alveolar capillaries across respiratory membrane

Each alveolar wall is one cell thick and surrounded by a network of tiny capillaries – these walls are also one cell thick

Total area of respiratory membrane for gas exchange in the lungs is equivalent to the area of a tennis court

Venous blood arriving at the lungs has travelled from all tissues of the body, contains high levels of carbon dioxide – low levels of oxygen

Carbon dioxide diffuses from venous blood down its concentration gradient into the alveoli until equilibrium with alveolar air is reached

Oxygen diffuses from the alveoli into the blood

Slow flow of blood through the capillaries increases time available for gas exchange to occur

When blood leaves alveolar capillaries oxygen and carbon dioxide concentrations are in equilibrium with those of alveolar air


Internal Respiration

Exchange of gases by diffusion between blood in the capillaries and body cells

Gaseous exchange does not occur across walls of arteries carrying blood from the heart to the tissues because walls are too thick

PO­2 of blood arriving at the capillary bed is therefore the same as blood leaving the lungs

Blood arriving at the tissues has been cleanse of CO2 and saturated with O2 during passage through the lungs

Therefore higher PO2 and lower PCO2­ than the tissues

Creates concentration gradients between capillary blood and the tissues

Gaseous exchange occurs

O2­ diffuses from the bloodstream through the capillary wall into the tissues

CO2 diffuses from the cells into the extracellular fluid then into the bloodstream towards the venous end of the capillary


Transport of Gases in the Bloodstream

Transport of oxygen and carbon dioxide is essential for internal respiration to occur


Oxygen is carried in the blood in:

Chemical combination with haemoglobin – oxyhaemoglobin 98.5%

Solution in plasma water 1.5%

Oxyhaemoglobin is an unstable compound that under certain conditions readily dissociates releasing oxygen

Factors that increase dissociation include

Low levels of O2

Low pH

Raised temperature

In active tissues there is increased production of carbon dioxide and heat which leads to increased release of oxygen

Oxygen is available to tissues in greatest need

When oxygen leaves the erythrocyte, the deoxygenated haemoglobin turns purplish in colour


Carbon Dioxide

Waste product of metabolism

Excreted by lungs

Transported by three mechanisms

As bicarbonate ions (HCO3-­­) in plasma 70%

Carries in erythrocytes, loosely combined with haemoglobin – carbaminohaemoglobin 23%

Some dissolved in plasma 7%


Control of Respiration

Normally involuntary

Voluntary control is exerted during activities such as speaking and singing but is overridden if blood CO2 rises (hypercapnia)


Respiratory Centre

Formed by groups of nerves in medulla – respiratory rhythmicity centre

Control respiratory pattern

Rate and depth of breathing

Regular discharge of inspiratory neurones within this centre set the rate and depth of breathing

Activity of this centre is adjusted by nerves in the pons (pneumotaxic centre and apneustic centre) in response to input from other parts of the brain

Motor impulses leaving centre pass in the phrenic and intercostal nerves to the diaphragm and intercostal muscles respectively



Receptors that respond to changes in partial pressures of oxygen and carbon dioxide in the blood and cerebrospinal fluid – located centrally and peripherally

Central chemoreceptors

Located on surface of medulla oblongata

Bathed in cerebrospinal fluid

When arterial PCO2 rises (hypercapnia) the central chemoreceptors respond by stimulating the respiratory centre, increasing ventilation of the lungs and reducing PCO2

Sensitivity of central chemoreceptors to raised PCO2 is important factor in controlling normal blood gas levels

Small reduction in PO2 (hypoxaemia)same but less pronounced effect – substantial reduction depresses breathing

eripheral chemoreceptors

Situated in arch of aorta and carotid bodies

More sensitive to small rises in arterial PCO2 than to small decreases in PO2 levels

Nerve impulses generated in peripheral chemoreceptors – conveyed by glossopharyngeal and vagus nerves to medulla and stimulate respiratory centre

Rate and depth of breathing is increased

Increase in blood acidity (decreased pH or raised [H+]) stimulates the peripheral chemoreceptors resulting in

Increased ventilation

Increased CO2 excretion

Increased blood pH

Other Factors that Influence Respiration



Hering-Breuer reflex

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