Chapter 10: The respiratory system

Question 1

The respiratory system

Answer 1

The cells of the body need energy for all their metabolic activities. Most of this energy is derived from chemical reactions, which can only take place in the presence of oxygen (O2). The main waste product of these reactions is carbon dioxide (CO2). The respiratory system provides the route by which the supply of oxygen present in the atmospheric air enters the body, and it provides the route of excretion for carbon dioxide.

Question 2

The organs of the respiratory system

Answer 2

Blood provides the transport system for O2 and CO2 between the lungs and the cells of the body. The exchange of gases between the blood and the lungs is called external respiration and that between the blood and the cells internal respiration. The organs of the respiratory system are:

• nose
• pharynx
• larynx
• trachea
• two bronchi (one bronchus to each lung)
• bronchioles and smaller air passages
• two lungs and their coverings, the pleura
• muscles of breathing – the intercostal muscles and the diaphragm.

Question 3

Nose and Nasal cavity (position and structure)

Answer 3

• The nasal cavity is the main route of air entry and consists of a large irregular cavity divided into two equal passages by a septum. The posterior bony part of the septum is formed by the perpendicular plate of the ethmoid bone and the vomer. Anteriorly, it consists of hyaline cartilage.
• The roof is formed by the cribriform plate of the ethmoid bone and the sphenoid bone, frontal bone, and nasal bones.
• The floor is formed by the roof of the mouth and consists of the hard palate in front and the soft palate behind. The hard palate is composed of the maxilla and palatine bones and the soft palate consists of involuntary muscle.
• The medial wall is formed by the septum.
• The lateral walls are formed by the maxilla, the ethmoid bone, and the inferior conchae.
• The posterior wall is formed by the posterior wall of the pharynx.

Question 4

The lining of the nasal cavity

Answer 4

The nasal cavity is lined with very vascular ciliated columnar epithelium (ciliated mucous membrane, respiratory mucosa) which contains mucus-secreting goblet cells. At the anterior nares, this blends with the skin and posteriorly it extends into the nasal part of the pharynx (the nasopharynx).

Question 5

Openings into the nasal cavity

Answer 5

•The anterior nares, or nostrils, are the openings from the exterior into the nasal cavity. Nasal hairs are found here, coated in sticky mucus.
•The posterior nares are the openings from the nasal cavity into the pharynx.
•The paranasal sinuses are cavities in the bones of the face and the cranium, containing air. There are tiny openings between the paranasal sinuses and the nasal cavity. They are lined with mucous membrane, continuous with that of the nasal cavity. The main sinuses are:
-maxillary sinuses in the lateral walls
-frontal and sphenoidal sinuses in the roof
-ethmoidal sinuses in the upper part of the lateral walls.

Question 6

Respiratory function of the nose

Answer 6

The nose is the first of the respiratory passages through which the inspired air passes. In the nasal cavity, the air is warmed, moistened, and filtered. The three projecting conchae increase the surface area and cause turbulence, spreading inspired air over the whole nasal surface. The large surface area maximizes warming, humidification, and filtering.

Question 7

Warming

Answer 7

The immense vascularity of the mucosa permits rapid warming as the air flows past. This also explains the large blood loss when a nosebleed (epistaxis) occurs.

Question 8

Filtering and cleaning

Answer 8

Hairs at the anterior nares trap larger particles. Smaller particles such as dust and bacteria settle and adhere to the mucus. Mucus protects the underlying epithelium from irritation and prevents drying. The synchronous beating of the cilia wafts the mucus towards the throat where it is swallowed or coughed up (expectorated).

Question 9

Humidification

Answer 9

As air travels over the moist mucosa, it becomes saturated with water vapor. Irritation of the nasal mucosa results in sneezing, a reflex action that forcibly expels an irritant.

Question 10

The sense of small

Answer 10

The nose is the organ of the sense of smell (olfaction). Specialized receptors that detect smell are in the roof of the nose in the area of the cribriform plate of the ethmoid bones and the superior conchae. These receptors are stimulated by airborne odors. The resultant nerve signals are carried by the olfactory nerves to the brain where the sensation of smell is perceived.

Question 11

Pharynx (position)

Answer 11

The pharynx (throat) is a passageway about 12–14 cm long. It extends from the posterior nares and runs behind the mouth and the larynx to the level of the 6th thoracic vertebra, where it becomes the esophagus

Question 12

Structures associated with the pharynx

Answer 12

• Superiorly – the inferior surface of the base of the skull
• Inferiorly – it is continuous with the esophagus
• Anteriorly – the wall is incomplete because of the openings into the nose, mouth, and larynx
• Posteriorly – areolar tissue, involuntary muscle, and the bodies of the first six cervical vertebrae.
• For descriptive purposes, the pharynx is divided into three parts: nasopharynx, oropharynx, and laryngopharynx.

Question 13

The nasopharynx

Answer 13

The upper part of the pharynx, connecting with the nasal cavity above the soft palate.

Question 14

The oropharynx

Answer 14

The part of the pharynx that lies between the soft palate and the hyoid bone.

Question 15

The laryngopharynx

Answer 15

The laryngeal part of the pharynx extends from the oropharynx above and continues as the esophagus below, with the larynx lying anteriorly.

Question 16

Mucous membrane lining

Answer 16

The mucosa varies slightly in the different regions. In the nasopharynx it is continuous with the lining of the nose and consists of the ciliated columnar epithelium; in the oropharynx and laryngopharynx, it is formed by tougher stratified squamous epithelium, which is continuous with the lining of the mouth and esophagus. This lining protects underlying tissues from the abrasive action of foodstuffs passing through during swallowing.

Question 17

Submucosa

Answer 17

The layer of tissue below the epithelium (the submucosa) is rich in mucosa-associated lymphoid tissue, involved in protection against infection. Tonsils are masses of MALT that bulge through the epithelium. Some glandular tissue is also found here.

Question 18

Smooth muscle

Answer 18

• The pharyngeal muscles help to keep the pharynx permanently open so that breathing is not interfered with. Sometimes in sleep, and particularly if sedative drugs or alcohol have been taken, the tone of these muscles is reduced and the opening through the pharynx can become partially or totally obstructed. This contributes to snoring and periodic awakenings, which disturb sleep.
• Constrictor muscles close the pharynx during swallowing, pushing food and fluid into the esophagus.

Question 19

Blood and nerve supply

Answer 19

• Blood is supplied to the pharynx by several branches of the facial artery. The venous return is into the facial and internal jugular veins.
• The nerve supply is from the pharyngeal plexus and includes both parasympathetic and sympathetic nerves. Parasympathetic supply is by the vagus and glossopharyngeal nerves. The sympathetic supply is by nerves from the superior cervical ganglia.

Question 20

Passageway for air and food

Answer 20

The pharynx is involved in both the respiratory and the digestive systems: air passes through the nasal and oral sections and food through the oral and laryngeal sections.

Question 21

Warming and humidifying

Answer 21

By the same methods as in the nose, the air is further warmed and moistened as it passes towards the lungs.

Question 22

Hearing

Answer 22

The auditory tube, extending from the nasopharynx to each middle ear, allows air to enter the middle ear. This leads to air in the middle ear being at the same pressure as the outer ear, protecting the tympanic membrane (eardrum) from any changes in atmospheric pressure.

Question 23

Protection

Answer 23

The lymphatic tissue of the pharyngeal and laryngeal tonsils produces antibodies in response to swallowed or inhaled antigens. The tonsils are larger in children and tend to atrophy in adults.

Question 24

Speech

Answer 24

The pharynx functions in speech; by acting as a resona­ting chamber for sound ascending from the larynx, it helps (together with the sinuses) to give the voice its individual characteristics.

Question 25

Larynx (position)

Answer 25

The larynx or ‘voice box’ links the laryngopharynx and the trachea. It lies in front of the laryngopharynx and the 3rd, 4th, 5th, and 6th cervical vertebrae. Until puberty, there is little difference in the size of the larynx between the sexes. Thereafter, it grows larger in the male, which explains the prominence of the ‘Adam’s apple’ and the generally deeper voice.

Question 26

Structures associated with the larynx

Answer 26

• Superiorly – the hyoid bone and the root of the tongue
• Inferiorly – it is continuous with the trachea
• Anteriorly – the muscles attached to the hyoid bone and the muscles of the neck
• Posteriorly – the laryngopharynx and 3rd–6th cervical vertebrae
• Laterally – the lobes of the thyroid gland.

Question 27

Cartilages

Answer 27

The main cartilages are:

• 1 thyroid cartilage
• 1 cricoid cartilage
• 2 arytenoid cartilages
• 1 epiglottis

Question 28

The thyroid cartilage

Answer 28

• This is the most prominent of the laryngeal cartilages. Made of hyaline cartilage, it lies to the front of the neck. Its anterior wall projects into the soft tissues of the front of the throat, forming the laryngeal prominence or Adam’s apple, which is easily felt and often visible in adult males. The anterior wall is partially divided by the thyroid notch. The cartilage is incomplete posteriorly and is bound with ligaments to the hyoid bone above and the cricoid cartilage below.
• The upper part of the thyroid cartilage is lined with stratified squamous epithelium like the larynx, and the lower part with ciliated columnar epithelium like the trachea. There are many muscles attached to its outer surface.
• The thyroid cartilage forms most of the anterior and lateral walls of the larynx.

Question 29

The cricoid cartilage

Answer 29

This lies below the thyroid cartilage and is also composed of hyaline cartilage. It is shaped like a signet ring, completely encircling the larynx with the narrow part anteriorly and the broad part posteriorly. The broad posterior part articulates with the arytenoid cartilages and with the thyroid cartilage. It is lined with ciliated columnar epithelium and there are muscles and ligaments attached to its outer surface. The lower border of the cricoid cartilage marks the end of the upper respiratory tract.

Question 30

The arytenoid cartilages

Answer 30

These are two roughly pyramid-shaped hyaline cartilages situated on top of the broad part of the cricoid cartilage forming part of the posterior wall of the larynx. They give attachment to the vocal cords and to muscles and are lined with ciliated columnar epithelium.

Question 31

The epiglottis

Answer 31

This is a leaf-shaped fibroelastic cartilage attached on a flexible stalk of cartilage to the inner surface of the anterior wall of the thyroid cartilage immediately below the thyroid notch. It rises obliquely upwards behind the tongue and the body of the hyoid bone. It is covered with stratified squamous epithelium. If the larynx is likened to a box, then the epiglottis acts as the lid; it closes off the larynx during swallowing, protecting the lungs from the accidental inhalation of foreign objects.

Question 32

Blood and nerve supply of the larynx

Answer 32

• Blood is supplied to the larynx by the superior and inferior laryngeal arteries and drained by the thyroid veins, which join the internal jugular vein.
• The parasympathetic nerve supply is from the superior laryngeal and recurrent laryngeal nerves, which are branches of the vagus nerves. The sympathetic nerves are from the superior cervical ganglia, one on each side. These provide the motor nerve supply to the muscles of the larynx and sensory fibers to the lining membrane.

Question 33

Interior of the larynx

Answer 33

The cavity of the larynx extends from its triangle-shaped inlet to the epiglottis, and to the circular outlet at the lower border of the cricoid cartilage, where it is continuous with the lumen of the trachea. The mucous membrane lining the larynx forms two pairs of lateral folds that project inward into its cavity.

Question 34

Production of sound

Answer 34

Sound has the properties of pitch, volume, and resonance.

• Pitch of the voice depends upon the length and tightness of the cords. Shorter cords produce higher-pitched sounds. At puberty, the male vocal cords begin to grow longer, hence the lower pitch of the adult male voice.
• Volume of the voice depends upon the force with which the cords vibrate. The greater the force of expired air, the more strongly the cords vibrate and the louder the sound emitted.
• Resonance, or tone, is dependent upon the shape of the mouth, the position of the tongue and the lips, the facial muscles, and the air in the paranasal sinuses.

Question 35

Speech

Answer 35

This is produced when the sounds produced by the vocal cords are amplified and manipulated by the tongue, cheeks, and lips.

Question 36

Protection of the lower respiratory tract

Answer 36

During swallowing the larynx moves upwards, blocking the opening into it from the pharynx. In addition, the hinged epiglottis closes over the larynx. This ensures that food passes into the esophagus and not into the trachea.

Question 37

Passageway for air

Answer 37

The larynx links the pharynx above with the trachea below.

Question 38

Humidifying, filtering, and warming

Answer 38

These processes continue as inspired air travels through the larynx.

Question 39

Trachea (position)

Answer 39

The trachea or windpipe is a continuation of the larynx and extends downwards to about the level of the 5th thoracic vertebra where it divides at the carina into the right and left primary bronchi, one bronchus going to each lung. It is approximately 10–11 cm long and lies mainly in the median plane in front of the esophagus

Question 40

Structures associated with the trachea

Answer 40

• Superiorly – the larynx
• Inferiorly – the right and left bronchi
• Anteriorly – upper part: the isthmus of the thyroid gland; lower part: the arch of the aorta and the sternum
• Posteriorly – the esophagus separates the trachea from the vertebral column
• Laterally – the lungs and the lobes of the thyroid gland.

Question 41

The structure of the trachea

Answer 41

•The trachea wall is composed of three layers of tissue and is held open by between 16 and 20 incomplete (C-shaped) rings of hyaline cartilage lying one above the other. The rings are incomplete posteriorly where the trachea lies against the esophagus. The cartilages are embedded in a sleeve of smooth muscle and connective tissue, which also forms the posterior wall where the rings are incomplete.
•Three layers of tissue ‘clothe’ the cartilages of the trachea.
-The outer layer contains fibrous and elastic tissue and encloses the cartilages.
-The middle layer consists of cartilages and bands of smooth muscle that wind around the trachea in a helical arrangement. There is some areolar tissue, containing blood and lymph vessels and autonomic nerves. The free ends of the incomplete cartilages are connected by the trachealis muscle, which allows for adjustment of tracheal diameter.
•The lining is the ciliated columnar epithelium, containing mucus-secreting goblet cells.

Question 42

Support and patency

Answer 42

Trachea cartilages hold the trachea permanently open (patent), but the soft tissue bands in between the cartilages allow flexibility so that the head and neck can move freely without obstructing or kinking the trachea. The absence of cartilage poste­riorly permits the esophagus to expand comfortably during swallowing. Contraction or relaxation of the trachealis muscle, which links the free ends of the C-shaped cartilages, helps to regulate the diameter of the trachea.

Question 43

Mucociliary escalator

Answer 43

This is the synchronous and regular beating of the cilia of the mucous membrane lining that wafts mucus with adherent particles upwards towards the larynx where it is either swallowed or coughed up.

Question 44

Cough reflex

Answer 44

Nerve endings in the larynx, trachea, and bronchi are sensitive to irritation, which generates nerve impulses conducted by the vagus nerves to the respiratory center in the brain stem. The reflex motor response is deep inspiration followed by the closure of the glottis, i.e., closure of the vocal cords. The abdominal and respiratory muscles then contract to cause a sudden and rapid increase of pressure in the lungs. Then the glottis opens, expelling air through the mouth, taking mucus and/or foreign material with it.

Question 45

Warming, Humidifying, and filtering

Answer 45

These continue as in the nose, although air is normally saturated and at body temperature when it reaches the trachea.

Question 46

Lungs

Answer 46

There are two lungs, one lying on each side of the midline in the thoracic cavity. They are cone-shaped and have an apex, a base, a tip, a costal surface, and a medial surface.

Question 47

The apex

Answer 47

This is rounded and rises into the root of the neck, about 25 mm above the level of the middle third of the clavicle. It lies close to the first rib and the blood vessels and nerves in the root of the neck.

Question 48

The base

Answer 48

This is concave and semilunar in shape and lies on the upper (thoracic) surface of the diaphragm.

Question 49

The costal surface

Answer 49

This is the broad outer surface of the lung that lies directly against the costal cartilages, the ribs, and the intercostal muscles.

Question 50

The medial surface

Answer 50

• The medial surface of each lung faces the other directly across the space between the lungs, the mediastinum. Each is concave and has a roughly triangular-shaped area, called the hilum, at the level of the 5th, 6th, and 7th thoracic vertebrae. The primary bronchus, the pulmonary artery supplying the lung and the two pulmonary veins draining it, the bronchial artery and veins, and the lymphatic and nerve supply enter and leave the lung at the hilum.
• The mediastinum contains the heart, great vessels, trachea, right and left bronchi, esophagus, lymph nodes, lymph vessels, and nerves.
• The right lung is divided into three distinct lobes: superior, middle, and inferior. The left lung is smaller because the heart occupies space left of the midline. It is divided into only two lobes: superior and inferior. The divisions between the lobes are called fissures.

Question 51

Pleura and pleural cavity

Answer 51

The pleura consists of a closed sac of the serous membrane (one for each lung) which contains a small amount of serous fluid. The lung is pushed into this sac so that it forms two layers: one adheres to the lung and the other to the wall of the thoracic cavity.

Question 52

The visceral pleura

Answer 52

This adherent to the lung, covering each lobe and passing into the fissures that separate them.

Question 53

The parietal pleura

Answer 53

This is adherent to the inside of the chest wall and the thoracic surface of the diaphragm. It is not attached to other structures in the mediastinum and is continuous with the visceral pleura around the edges of the hilum.

Question 54

The pleural cavity

Answer 54

The pleural cavity is a fluid-filled space that surrounds the lungs. It is found in the thorax, separating the lungs from its surrounding structures such as the thoracic cage and intercostal spaces, the mediastinum, and the diaphragm. The pleural cavity is bounded by a double-layered serous membrane called pleura.

Question 55

Interior of the lungs

Answer 55

The lungs are composed of the bronchi and smaller air passages, alveoli, connective tissue, blood vessels, lymph vessels, and nerves, all embedded in an elastic connective tissue matrix. Each lobe is made up of many lobules.

Question 56

Pulmonary blood supply

Answer 56

The pulmonary trunk divides into the right and left pul­monary arteries, carrying deoxygenated blood to each lung. Within the lungs, each pulmonary artery divides into many branches, which eventually end in a dense capillary network around the alveoli.
The alveoli and the capillaries each consist of only one layer of flattened epithelial cells. The exchange of gases between air in the alveoli and blood in the capillaries takes place across these two very fine membranes (together called the respiratory membrane). The pulmonary capillaries merge into a network of pulmonary venules, which in turn form two pulmonary veins carrying oxygenated blood from each lung back to the left atrium of the heart.

Question 57

Bronchi and bronchioles

Answer 57

The two primary bronchi are formed when the trachea divides, at about the level of the 5th thoracic vertebra.

Question 58

The right bronchus

Answer 58

This is wider, shorter, and more vertical than the left bronchus and is, therefore, more likely to become obstructed by an inhaled foreign body. It is approximately 2.5 cm long. After entering the right lung at the hilum, it divides into three branches, one to each lobe. Each branch then subdivides into numerous smaller branches.

Question 59

The left bronchus

Answer 59

This is about 5 cm long and is narrower than the right. After entering the lung at the hilum, it divides into two branches, one to each lobe. Each branch then subdivides into progressively smaller airways within the lung substance.

Question 60

The structure of the bronchial walls

Answer 60

The bronchial walls contain the same three layers of tissue as the trachea and are lined with ciliated columnar epithelium. The bronchi progressively subdivide into bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and finally, alveoli. The wider passages are called conducting airways because their function is to bring air into the lungs, and their walls are too thick to permit gas exchange.

Question 61

Structural changes in the bronchial passages

Answer 61

As the bronchi divide and become progressively smaller, their structure changes to match their function.

Question 62

Cartilage

Answer 62

Since rigid cartilage would interfere with the expansion of lung tissue and the exchange of gases, it is present for support in the larger airways only. The bronchi contain cartilage rings like the trachea, but as the airways divide, these rings become much smaller plates, and at the bronchiolar level there is no cartilage present in the airway walls at all.

Question 63

Smooth muscle

Answer 63

As the cartilage disappears from airway walls, it is replaced by smooth muscle. This allows the diameter of the airways to be increased or decreased through the influence of the autonomic nervous system, regulating airflow within each lung.

Question 64

Epithelial lining

Answer 64

The ciliated epithelium is gradually replaced with non-ciliated epithelium, and goblet cells disappear.

Question 65

Blood and nerve supply (Lymph drainage)

Answer 65

• The arterial supply to the walls of the bronchi and smaller air passages is through branches of the right and left bronchial arteries and the venous return is mainly through the bronchial veins. On the right side, they empty into the azygos vein and on the left into the superior intercostal vein.
• The vagus nerves (parasympathetic) stimulate contraction of smooth muscle in the bronchial tree, causing bronchoconstriction, and sympathetic stimulation causes bronchodilation.
• Lymph is drained from the walls of the air passages in a network of lymph vessels. It passes through lymph nodes situated around the trachea and bronchial tree then into the thoracic duct on the left side and right lymphatic duct on the other.

Question 66

Control of air entry

Answer 66

•The diameter of the respiratory passages is altered by contraction or relaxation of the smooth muscle in their walls, regulating the speed and volume of airflow into and within the lungs. These changes are controlled by the autonomic nerve supply: parasympathetic stimulation causes constriction and sympathetic stimulation causes dilation.
•The following functions continue as in the upper airways:
-warming and humidifying
-support and patency
-removal of particulate matter
-cough reflex.

Question 67

Respiratory bronchioles and alveoli

Answer 67

• The respiratory bronchioles are the narrowest airways of the lungs, 0.5 mm across. The bronchi divide many times before evolving into the bronchioles. The respiratory bronchioles deliver air to the exchange surfaces of the lungs. They are interrupted by alveoli which are thin-walled evaginations.
• Any of the many tiny air sacs of the lungs allow for rapid gaseous exchange.

Question 68

Nerve supply to bronchioles

Answer 68

Parasympathetic stimulation, from the vagus nerve, causes bronchoconstriction. The absence of supporting cartilage means that small airways may be completely closed off by constriction of their smooth muscle. Sympathetic stimulation relaxes bronchiolar smooth muscle (bronchodilation).

Question 69

Defense against infection

Answer 69

At this level, ciliated epithelium, goblet cells and mucus are no longer present, because their presence would impede gas exchange and encourage infection. By the time inspired air reaches the alveoli, it is usually clean. Defense relies on protective cells present within the lung tissue. These include lymphocytes and plasma cells, which produce antibodies, and phagocytes, including alveolar macrophages. These cells are most active in the distal air passages where ciliated epithelium has been replaced by squamous (flattened) cells.

Question 70

Warming and humidifying

Answer 70

These continue as in the upper airways. Inhalation of dry or inadequately humidified air over a period irritates the mucosa and encourages infection.

Question 71

Exchange of gases

Answer 71

This takes place:

• in the lungs: external respiration
• in the tissues: internal respiration.

Question 72

Breathing

Answer 72

• This movement of air into and out of the lungs.

* Breathing supplies oxygen to the alveoli and eliminates carbon dioxide

Question 73

Muscle of breathing

Answer 73

Expansion of the chest during inspiration occurs as a result of muscular activity, partly voluntary and partly involuntary. The main muscles used in normal quiet breathing are the external intercostal muscles and the diaphragm.

Question 74

Intercostal muscles

Answer 74

There are 11 pairs of intercostal muscles occupying the spaces between the 12 pairs of ribs. They are arranged in two layers, the external and internal intercostal muscles

Question 75

The external intercostal muscles

Answer 75

These extend downwards and forwards from the lower border of the rib above to the upper border of the rib below. They are involved in inspiration.

Question 76

The internal intercostal muscles

Answer 76

• These extend downwards and backward from the lower border of the rib above to the upper border of the rib below, crossing the external intercostal muscle fibers at right angles. The internal intercostals are used when expiration becomes active, as in exercise.
• The first rib is fixed. Therefore, when the external intercostal muscles contract, they pull all the other ribs towards the first rib. The ribcage moves as a unit, upwards and outwards, enlarging the thoracic cavity. The inter­costal muscles are stimulated to contract by the inter­costal nerves.

Question 77

Diaphragm

Answer 77

The diaphragm is a thin skeletal muscle that sits at the base of the chest and separates the abdomen from the chest. It contracts and flattens when you inhale. This creates a vacuum effect that pulls air into the lungs. When you exhale, the diaphragm relaxes, and the air is pushed out of the lungs.

Question 78

Accessory muscles of respiration

Answer 78

When extra respiratory effort is required, additional muscles are used. Forced inspiration is assisted by the sternocleidomastoid muscles and the scalene muscles, which link the cervical vertebrae to the first two ribs and increase ribcage expansion. Forced expiration is helped by the activity of the internal intercostal muscles and sometimes the abdominal muscles, which increase the pressure in the thorax by squeezing the abdominal contents.

Question 79

Cycle of breathing

Answer 79

• The average respiratory rate is 12–15 breaths per minute. Each breath consists of three phases: inspiration, expiration, and pause.
• The visceral pleura is adherent to the lungs and the parietal pleura to the inner wall of the thorax and to the diaphragm. Between them is a thin film of pleural fluid.
• Breathing depends upon changes in pressure and volume in the thoracic cavity. It follows the underlying physical principle that increasing the volume of a container decreases the pressure inside it and that decreasing the volume of a container increases the pressure inside it. Since air flows from an area of high pressure to an area of low pressure, changing the pressure inside the lungs determines the direction of airflow.

Question 80

Inspiration

Answer 80

Inspiration refers to inhalation—it is the flow of the respiratory current into an organism. In humans, it is the movement of ambient air through the airways and into the alveoli of the lungs.

Question 81

Expiration

Answer 81

Relaxation of the external intercostal muscles and the diaphragm results in downward and inward movement of the ribcage and elastic recoil of the lungs. As this occurs, the pressure inside the lungs rises and expels air from the respiratory tract. At the end of expiration, the lungs still contain some air and are prevented from complete collapse by the intact pleura. This process is passive as it does not require the expenditure of energy.
•At rest, expiration lasts about 3 seconds, and after expiration, there is a pause before the next cycle begins.

Question 82

Elasticity

Answer 82

Elasticity is the ability of the lung to return to its normal shape after each breath. Loss of elasticity, e.g., in emphysema, of the connective tissue in the lungs necessitates forced expiration and increased effort on inspiration.

Question 83

Compliance

Answer 83

This is the stretchability of the lungs, i.e., the effort required to inflate the alveoli. The healthy lung is very compliant and inflates with very little effort. When compliance is low the effort needed to inflate the lungs is greater than normal, e.g., when an insufficient surfactant is present. Note that compliance and elasticity are opposing forces.

Question 84

Airway resistance

Answer 84

When this is increased, e.g., in bronchoconstriction, the more respiratory effort is required to inflate the lungs.

Question 85

Lung volumes and capacities

Answer 85

In normal quiet breathing, there are about 15 complete respiratory cycles per minute. The lungs and the air passages are never empty and, as the exchange of gases takes place only across the walls of the alveolar ducts and alveoli, the remaining capacity of the respiratory passages is called the anatomical dead space (about 150 mL).

Question 86

Tidal volume

Answer 86

This is the amount of air passing into and out of the lungs during each cycle of breathing (about 500 mL at rest).

Question 87

Inspiratory reserve volume

Answer 87

This is the extra volume of air that can be inhaled into the lungs during maximal inspiration, i.e., over and above normal TV.

Question 88

Inspiratory capacity

Answer 88

This is the amount of air that can be inspired with maximum effort. It consists of the tidal volume (500 ml) plus the inspiratory reserve volume.

Question 89

functional residual capacity

Answer 89

This is the amount of air remaining in the air passages and alveoli at the end of a quiet expiration. Tidal air mixes with this air, causing relatively small changes in the composition of alveolar air. As blood flows continuously through the pulmonary capillaries, this means that the exchange of gases is not interrupted between breaths, preventing moment-to-moment changes in the concentration of blood gases. The functional residual volume also prevents the collapse of the alveoli on expiration.

Question 90

Expiratory reserve volume

Answer 90

This is the largest volume of air that can be expelled from the lungs during maximal expiration.

Question 91

Residual volume

Answer 91

This cannot be directly measured but is the volume of air remaining in the lungs after forced expiration.

Question 92

Vital capacity

Answer 92

This is the maximum volume of air that can be moved into and out of the lungs: VC=Tidal volume + IRV + ERV

Question 93

Total lung capacity

Answer 93

This is the maximum amount of air the lungs can hold. In an adult of average build, it is normally around 6 liters. The sum of the vital capacity and the residual volume. It cannot be directly measured in clinical tests because even after forced expiration, the residual volume of air remains in the lungs.

Question 94

Alveolar ventilation

Answer 94

This is the volume of air that moves into and out of the alveoli per minute. It is equal to the tidal volume minus the anatomical dead space, multiplied by the respiratory rate.

Question 95

Exchange of gases

Answer 95

Although breathing involves the alternating processes of inspiration and expiration, gas exchange at the respiratory membrane and in the tissues is a continuous and ongoing process. Diffusion of oxygen and carbon dioxide depends on pressure differences, e.g., between atmospheric air and the blood, or blood and the tissues.

Question 96

Composition of air

Answer 96

• Atmospheric pressure at sea level is 101.3 kilopascals (kPa) or 760 mmHg. With an increasing height above sea level, atmospheric pressure is progressively reduced and at 5500 m, about two-thirds the height of Mount Everest (8850 m), it is about half that at sea level. Underwater, pressure increases by approximately 1 atmosphere per 10 m below sea level.
• Air is a mixture of gases: nitrogen, oxygen, carbon dioxide, water vapor, and small quantities of inert gases. Each gas in the mixture exerts a part of the total pressure proportional to its concentration, i.e., the partial pressure. This is denoted as, e.g., PO2, PCO2.

Question 97

Alveolar air

Answer 97

The composition of alveolar air remains constant and is different from atmospheric air. It is saturated with water vapor and contains more carbon dioxide and less oxygen. Saturation with water vapor provides 6.3 kPa (47 mmHg) thus reducing the partial pressure of all the other gases present. Gaseous exchange between the alveoli and the bloodstream (external respiration) is a continuous process, as the alveoli are never empty, so it is independent of the respiratory cycle. During each inspiration, only some of the alveolar gases are exchanged.

Question 98

Diffusion of gases

Answer 98

• Exchange of gases occurs when a difference in partial pressure exists across a semipermeable membrane. Gases move by diffusion from the higher concentration to the lower until equilibrium is established (p. 29). Atmospheric nitrogen is not used by the body, so its partial pressure remains unchanged and is the same in inspired and expired air, alveolar air, and blood.
• These principles govern the diffusion of gases in and out of the alveoli across the respiratory membrane (external respiration) and across capillary membranes in the tissues (internal respiration).

Question 99

External respiration

Answer 99

External respiration is the formal term for gas exchange. It describes both the bulk flow of air into and out of the lungs and the transfer of oxygen and carbon dioxide into the bloodstream through diffusion.

Question 100

Internal respiration

Answer 100

Internal respiration is the process of diffusing oxygen from the blood, into the interstitial fluid and into the cells. Waste and carbon dioxide are also diffused in the other direction, from the cells to the blood. Oxygen is released from blood cells in response to the oxygen concentration in the capillaries of blood vessels, which is usually low. This enables the exchange of gases and other solutes during internal respiration between the plasma and the interstitial fluid.

Question 101

Transport of gases in the bloodstream

Answer 101

Oxygen and carbon dioxide are carried in the blood in different ways.

Question 102

Oxygen

Answer 102

•Oxygen is carried in the blood in:
-chemical combination with hemoglobin as oxyhemoglobin (98.5%)
-solution in plasma water (1.5%).
•Oxyhemoglobin is unstable, and under certain conditions readily dissociates releasing oxygen. Factors that increase dissociation include low O2 levels, low pH, and raised temperature. Inactive tissues, there is increased production of carbon dioxide and heat, which leads to increased release of oxygen. In this way, oxygen is available to tissues in greatest need. Whereas oxyhemoglobin is bright red, deoxygenated blood is bluish-purple in color.

Question 103

Carbon dioxide

Answer 103

Carbon dioxide is one of the waste products of metabolism. It is excreted by the lungs and is transported by three mechanisms:
-as bicarbonate ions (HCO3−) in the plasma (70%)
-some is carried in erythrocytes, loosely combined with hemoglobin as carbaminohemoglobin (23%)
-some is dissolved in the plasma (7%).
•Carbon dioxide levels must be finely managed, as either an excess or a deficiency leads to significant disruption of acid-base balance. Sufficient CO2 is essential for the bicarbonate buffering system that protects against a fall in body ph. Excess CO2 on the other hand reduces blood pH because it dissolves in body water to form carbonic acid.

Question 104

Regulation of air and blood flow in the lung

Answer 104

• During quiet breathing, only a small portion of the lung’s total capacity is ventilated with each breath. This means that only a fraction of the total alveolar numbers is being ventilated, usually in the upper lobes, and much of the remaining lung is temporarily collapsed. Airways supplying alveoli that are not being used are constricted, directing airflow into functioning alveoli. In addition, the pulmonary arterioles bringing blood into the ventilated alveoli are dilated, to maximize gas exchange, and blood flow (perfusion) past the non-functioning alveoli is reduced.
• When respiratory requirements are increased, e.g., in exercise, the increased tidal volume expands additional alveoli, and the blood flow is redistributed to perfuse these too. In this way, airflow (ventilation) and blood flow (perfusion) are matched to maximize the opportunity for gas exchange.

Question 105

Control of respiration

Answer 105

Effective control of respiration enables the body to regulate blood gas levels over a wide range of physiological, environmental, and pathological conditions and is normally involuntary. Voluntary control is exerted during activities such as speaking and singing but is overridden if blood CO2 rises (hypercapnia).

Question 106

The respiratory center

Answer 106

• This is formed by groups of nerves in the medulla, the respiratory rhythmicity center, which control the respi­ratory pattern, i.e., the rate and depth of breathing. Regular discharge of inspiratory neurons within this center sets the rate and depth of breathing. The activity of the res­piratory rhythmicity center is adjusted by nerves in the pons (the pneumotach center and the apneustic center), in response to input from other parts of the brain.
• Motor impulses leaving the respiratory center pass in the phrenic and intercostal nerves to the diaphragm and intercostal muscles respectively to stimulate respiration.

Question 107

Chemoreceptors

Answer 107

These are receptors that respond to changes in the partial pressures of oxygen and carbon dioxide in the blood and cerebrospinal fluid. They are located centrally and peripherally.

Question 108

Central chemoreceptors

Answer 108

These are located on the surface of the medulla oblongata and are bathed in cerebrospinal fluid. When arterial PCO2 rises (hypercapnia), even slightly, the central chemoreceptors respond by stimulating the respiratory center, increasing the ventilation of the lungs, and reducing arterial PCO2. The sensitivity of the central chemoreceptors to raised arterial PCO2 is the most important factor in controlling normal blood gas levels. A small reduction in PO2 (hypoxemia) has the same, but the less pronounced effect, but a substantial reduction depresses breathing.

Question 109

Peripheral chemoreceptors

Answer 109

These are situated in the arch of the aorta and in the carotid bodies. They respond to changes in blood CO2 and O2 levels but are much more sensitive to carbon dioxide than oxygen. Even a slight rise in CO2 levels activates these receptors, triggering nerve impulses to the respiratory center via the glossopharyngeal and vague nerves. This stimulates an immediate rise in the rate and depth of respiration. An increase in blood acidity (decreased pH or raised [H+]) also stimulates the peripheral chemoreceptors, resulting in increased ventilation, increased CO2 excretion and increased blood ph. These chemoreceptors also help to regulate blood pressure.

Question 110

Exercise and respiration

Answer 110

Physical exercise increases both the rate and depth of respiration to supply the increased oxygen requirements of the exercising muscles. Exercising muscles produces higher quantities of CO2, which stimulates central and peripheral chemoreceptors. The increased respiratory effort persists even after exercise stops, in order to supply enough oxygen to repay the ‘oxygen debt’. This represents mainly the oxygen needed to get rid of wastes, including lactic acid.

Question 111

Other factors

Answer 111

•Breathing may be modified by the higher centers in the brain by:
-speech, singing
-emotional displays, e.g., crying, laughing, fear
-drugs, e.g., sedatives, alcohol
-sleep.
•Body temperature influences breathing. In fever, respiration is increased due to increased metabolic rate, while in hypothermia respiration and metabolism are depressed. Temporary changes in respiration occur in swallowing, sneezing and coughing.
•The Herring–Breuer reflex prevents over inflation of the lungs. Stretch receptors in the lung, linked to the respiratory center by the vague nerve, inhibit respiration when lung volume approaches maximum.