Physiology of Respiration Workbook Feb 2005

Contents 1. Anatomy ... 5 2. Physiology ... 15 3. Carbon dioxide ... 41

3

Introduction This workbook does not claim to cover either the whole field of respiratory physiology or all aspects of carbon dioxide monitoring. However, we hope that it can serve as a practical aid to those involved in respiratory care. The book is divided into three main parts; 1. a simple summary of respiratory anatomy 2. a somewhat more detailed description of respiratory physiology 3. CO2 monitoring.

Terminology The terms requiring explanation are printed in bold and defined at the end of each section. You will also find these terms in the Glossary on pages 58-61.

4

1. Anatomy The respiratory organs ... 6 The upper airway The nasal cavity ... 7 The throat ... 7 The lower airways The larynx ... 8 The trachea ... 8 The bronchial tree ... 9 The lungs ... 9 The mechanics of breathing Spontaneous breathing ... 10 The work of breathing ... 10 Pressure changes during spontaneous and controlled ventilation ... 11 Static lung volumes ... 11 Respiratory minute volume ... 11 Pneumothorax Closed pneumothorax ... 12 Tension pneumothorax ... 12 Open pneumothorax ... 13 Summary ... 14

5

The respiratory organs

The upper airway

The lower airways

Alveolus

6

The respiratory organs can be divided into the upper airway, the lower airways, and the lungs. The primary role of the respiratory system is to provide the body with oxygen (”fuel”) and to remove carbon dioxide (”waste gas”). These respiratory gases are transported via the upper and lower airways to and from the lungs, where gas exchange takes place. The lungs’ most important function is gas exchange: oxygen moves from the inspired air to the de-oxygenated venous blood, and carbon dioxide moves in the opposite direction from the venous blood to the air in the lungs. This movement is carried out by passive diffusion, which means that oxygen crosses passively through a membrane from a greater concentration in the lungs to a lower concentration in venous blood. Carbon dioxide crosses through the membrane in the opposite direction by the same principle.

In order for enough oxygen (» 200 ml/min at rest) to be taken up by the body in this way the membrane must be very thin (» 0.2 m) and its surface area large (» 50-100 m2). This large membrane area is accommodated within the chest in the form of a large number of small units, alveoli (» 300 million alveoli in each lung).

The upper airway The nasal cavity

Pharynx

The upper airway comprises the nasal cavity and the pharynx.

The nasal cavity

As well as being an organ of smell, the nose has the important functions of cleaning, warming, and humidifying the • inhaled air. The inside of the nose nearest the nostrils contains hairs which clear the air from larger particles. In the nasal cavity there are a great number of superficial thin-walled blood vessels, which radiate heat and thereby warm the inhaled air. The nasal cavity is kept moist by glandular secretions, which also humidify the air. The inspired air, which passes through the nose, is thus fully humidified and has a temperature of 32oC, no matter what the outside temperature. If the inhaled air does not pass through the nose, (e.g., when breathing through the mouth) partial drying of the mucous membranes of the lower airways occurs, making them more prone to infection. During treatment on a ventilator the importance of correct humidification and warming of the inspired gas has to be considered, as the gas is supplied through an endotracheal tube and not through the nose. •

Breathing

Swallowing

The throat (pharynx) The throat provides a transport function for both food and air. When swallowing, food is prevented from entering the nasal cavity by a closing upward movement of the soft palate in the roof of the mouth. Soft palate Epiglottis

Glossary Endotracheal tube: a tube placed either orally or nasally down between the vocal cords into the trachea 7

The lower airways The lower airways comprise the larynx, the trachea, and the bronchial tree. The bronchial tree branches off into the lungs.

The larynx After passing through the nasal cavity and the pharynx the inhaled air reaches the larynx. The larynx is partly covered by the epiglottis, which during swallowing completely covers the upper opening of the larynx. The vocal cords also close during swallowing. The extrapulmonary airway is at its narrowest at the vocal cords, and further narrowing at this point can give rise to considerable respiratory distress, e.g, during or after intubation the vocal cords can become swollen causing respiratory obstruction or hoarseness (after extubation).

The trachea

Larynx

Trachea

After passing the vocal cords the air stream enters the trachea, which in the adult is 1012 cm long and 20-25 mm in diameter. The trachea is kept open by horseshoe-shaped rings of cartilage (opening facing backwards). The mucous membrane in the trachea is covered with microscopic hairs (cilia). The cilia transport mucus and inhaled foreign material continuously upward toward the laryngeal opening, where it is either coughed up or swallowed into the esophagus, behind the trachea.

Glossary Intubation: inserting the endotracheal tube Extubation: removing the endotracheal tube

8

The lower airways The bronchial tree

Primary bronchus

Stem bronchus

Lobar bronchus

The lungs (pulmones)

The bronchial tree comprises the primary bronchi, stem bronchi, lobar bronchi, and bronchioles The trachea divides within the chest into two primary bronchi, one to the right and one to the left lung. The right primary bronchus lies more vertically than the left, and consequently any accidentally inhaled foreign body is more likely to enter the right lung. The primary bronchi branch off into lesser and lesser bronchi (lobar bronchi, segmental bronchi, and bronchioles) until after 20-30 divisions they terminate in the alveoli. The bronchi and the bronchioles are, as is the trachea, covered with cilia. In contrast to the larger bronchi, the bronchioles lack cartilage plates in their walls and consequently can close under certain circumstances. Air reaching the alveoli gives up its oxygen to the red blood cells in the neighboring capillaries, and at the same time takes up carbon dioxide (gas exchange).

The lungs are divided into lobes; the left lung has two and the right lung three. The lobes are divided into segments, and the segments into lobules, which are the lungs’ smallest units and correspond to the lung tissue supplied by one bronchiole. Each lung is enveloped in its pleural membranes, an inner and an outer, in intimate contact with each other. The inner membrane closely envelopes the lungs’ surface while the outer membrane covers the inside of the chest wall and the large respiratory muscle, the diaphragm. In the potential space between the pleural membranes, the pleural cavity, there is a constant negative pressure causing the lungs to expand on inspiration as the chest expands. Air is “sucked” in this way into the lungs. The pleural cavity

The lungs

Upper lobe

• •

•

Middle lobe Lower lobe

Pulmonary pleura Pleural cavity Parietal pleura

• • •

9

The mechanics of breathing Spontaneous breathing Inspiration

Inspiration

External intercostal muscles

Diaphragm

Inspiration is an active movement, i.e., the elasticity of the lungs and the chest wall must be overcome. The most important respiratory muscle is the diaphragm, a flat domed sheet of muscle attached to the lower ribs. On inspiration, the diaphragm moves downwards as it flattens out. It moves about 1 cm during a normal breath, but can move up to 10 cm. Other muscles which contribute to inspiration are those which join the outside surface of the ribs, the external intercostal muscles, which lift the chest wall upwards and outwards.

Expiration Expiration

Expiration is normally a passive movement, i.e., air is driven out of the lungs by the elastic recoil of the lungs and chest wall as they return to their original position after inspiration. If expiration is obstructed, e.g., by an obstruction in the airway, the abdominal muscles and muscles joining the inside surface of the ribs (internal intercostal muscles) help to draw the chest wall downwards and inwards and expel the air.

The work of breathing The work required for breathing while resting is normally very little, but can, under pathological conditions, demand a considerable share of the body’s oxygen requirements. During controlled ventilation the ventilator carries out this work.

10

The mechanics of breathing Spontaneous breathing

Pressure changes with spontaneous and controlled ventilation

Pressure(kPa) Intrapulmonary pressure

Spontaneous breathing

0 -1 Insp

Intrapleural pressure Exp

Insp

Exp

Time (s)

In spontaneous breathing, the pressure difference between the intrapulmonary and intrapleural pressures is normally 0.8 kPa (8 cm H2O) during resting conditions.

Controlled ventilation

Controlled ventilation

Pressure (kPa)

With the aid of a ventilator or with manually controlled ventilation a positive pressure is applied to the air or gas mixture delivered to the patient. The pressure in the lungs during inspiration is higher than atmospheric pressure, and both intrapulmonary and intrapleural pressures are on average higher than during spontaneous breathing.

+1

Intrapulmonary pressure

0 -1 Insp.

Intrapleural pressure Exp.

Exp.

Insp.

Time (s)

Static lung volumes The accepted terminology for the different lung volumes is:

Static lung volumes

VT = tidal volume (volume of a breath) IRV = inspiratory reserve volume (the volume that can be inhaled above VT) VC = vital capacity (max.volume of a breath) IC = inspiratory capacity (IRV + VT) RV = residual volume (the volume remaining in the lungs after maximum expiration) FRC = functional residual capacity (ERV + RV) ERV = expiratory reserve volume (the maximum volume that can be exhaled starting from FRC) TLC = total lung capacity (volume of a maximum breath VC + RV)

Volume (l) 6

IRV IC VC TLC

5 4 3

VT ERV

2 1 0

FRC

RV

Time (s)

Respiratory minute volume . .

The minute volume (VI = insp., VE = exp.) is determined by the tidal volume (VT) and the respiratory frequency (f). . . VE = V x f or V = VE T T f

Glossary Intrapleural: within the pleural cavity Intrapulmonary: in the lungs

11

Pneumothorax Closed pneumothorax Closed pneumothorax

Spontaneous and controlled ventilation

Tension pneumothorax

Spontaneous and controlled ventilation

If the lung is damaged so that air is allowed to enter the pleural cavity, the lung partly deflates because of its own elastic recoil and its volume diminishes. This condition is called closed pneumothorax, as the chest wall is intact. The lung still functions although function is somewhat reduced compared to the normal state, as expansion is restricted.

Tension pneumothorax If air leaks out into the pleural cavity during inspiration and remains there during expiration, the pressure in the pleural cavity will rise. Lung tissue can function as a valve. This condition is called tension pneumothorax. The increase in pressure impedes ventilation of the lung, not only on the affected side, but also possibly in the healthy lung, due to the fact that the mediastinum is pressed over towards the healthy side. Greater pressure gradients are required to maintain ventilation, increasing the pressure in the pleural cavity with each new breath, and can, if not treated, lead to circulatory arrest.

Glossary Mediastinum: the organs in the middle of the chest, between the lungs: the heart, great vessels, and esophagus

12

Pneumothorax Open pneumothorax If the chest wall is opened for surgery or as a result of an accident, the patient is said to have an open pneumothorax.

Spontaneous breathing Open pneumothorax Spontaneous breathing

During inspiration there is no negative pressure in the pleural cavity on the side of the collapsed lung. The negative pressure in the trachea (produced by the healthy lung) causes air to move from the damaged lung to the healthy lung during inspiration, resulting in ineffective to and fro ventilation. (The opposite occurs during expiration.)

Controlled ventilation Open pneumothorax Controlled ventilation

Treatment by positive pressure controlled ventilation enables adequate ventilation of the collapsed lung.

13

Summary The respiratory organs can be divided into: • the upper airway, • the lower airways • the lungs. The upper airway comprises: • the nasal cavity • the pharynx. The lower airways are made up of: • the larynx, • the trachea • the bronchial tree. The lungs are divided into lobes. The left lung has 2 and the right lung 3. Gas exchange takes place in the lungs’ smallest units, the alveoli. Inspiration is an active movement and expiration normally a passive movement during spontaneous breathing. The diaphragm is the most important respiratory muscle. There is normally a negative pressure in the pleural cavity. If air leaks into the pleural cavity the normal pressure relationships are disturbed, and the lung deflates because of its own elastic recoil (pneumothorax).

14

2. Physiology Respiration ... 16 Ventilation The regulation of breathing ... 18 Distribution of inspired air ... 20 Airway closure ... 22 Compliance ... 23 Resistance ... 25 Restrictive and obstructive lung disease ... 27 Passage over the alveolar membrane ... 29 Circulation - perfusion Ventilation - perfusion ... 30 Hypoxia ... 31 The oxy-hemoglobin’s dissociation curve ... 32 Acid - base balance ... 33 Cellular respiration ... 37 Summary of gas exchange gas transport ... 38

15

Respiration

O2 CO2

16

Respiration Respiration is the uptake by the body of oxygen and the elimination of carbon dioxide. Respiration can be divided into:

Ventilation Transport of air in and out of the alveoli: a) convection b) diffusion of gas in gas.

Passage over the alveolar membrane Gas exchange of O2 and CO2 between air and blood (diffusion of gas in tissue).

Circulation - perfusion Transport of O2 from the pulmonary capillaries to the body’s tissues.

Cellular respiration Transport of O2 to and CO2 from the individual cell (diffusion of gas in tissue).

Glossary CO2: carbon dioxide Convection: mass movement of gas Diffusion: movement of a substance through a fluid or a gas from an area of high concentration of the substance to an area of low concentration O2: oxygen Perfusion: flow of a fluid through blood vessels

17

Ventilation The regulation of breathing The volume and frequency of ventilation is determined by impulses from the respiratory center in the medulla oblongata. These impulses are governed by information from different receptors in the body: central receptors close to the respiratory center and peripheral receptors in the carotid arteries. The respiratory center and central receptors

18

Peripheral receptors

Ventilation The normal regulation of breathing The blood

Signal to the respiratory center

Muscular activity

Central

Low pH

Hyperventilation

Peripheral

High pH

Hypoventilation

Receptors

PaCO2

The regulation of breathing in a patient with chronic lung disease Signal to the The blood PaO2

respiratory center

Muscular activity

Low PaO2

Hyperventilation

High PaO2

Hypoventilation

Receptors Peripheral

The impulses from the central receptors depend mainly on the carbon dioxide level in the blood, expressed as PaCO2 (the partial pressure of carbon dioxide). PaCO2 affects the carbon dioxide level and thereby the pH value in the fluid surrounding the brain and spinal cord (cerebrospinal fluid CSF). The pH value of the cerebrospinal fluid has a direct effect on the respiratory center in such a way that a low pH (high CO2 level) stimulates breathing, and a high pH (low CO2 level) impedes breathing. Also the peripheral receptors are affected by the pH value of the blood in such a way that a low pH stimulates breathing. In patients with chronic lung disease the sensitivity of the respiratory center to raised P aCO2 (low pH in CSF) diminishes over a long period of time. The impulses are then governed instead by the oxygen level in the blood, expressed as PaO2 (the partial pressure of O2) via the peripheral receptors. When PaO2 is lowered to about 8 kPa the respiratory center is stimulated.

Glossary PaCO2: the partial pressure of carbon dioxide in arterial blood PaO2: the partial pressure of oxygen in arterial blood pH: a measure of the hydrogen ion solution, i.e., the degree of acidity or alkalinity. Chemical substances capable of donating hydrogen ions (H+) are acids. Chemical substances accepting hydrogen ions are alkalis, or bases.

The hydrogen ion concentration of a solution is a measure of its acidity and is expressed as the pH value. pH is the negative logarithm of the hydrogen ion concentration. Acidosis means low pH. Alkalosis means high pH Receptor: a sensory nerve terminal which responds to stimuli of various kinds

19

Ventilation Distribution of inspired air Physiological dead space

Anatomical dead space

Alveolar ventilation One way of describing the efficacy of ventilation is to theoretically divide ventilation into two parts, depending on the PaCO2 and the expired CO2 concentration. One part is the volume in which perfect gas exchange has taken place (alveolar ventilation) and the other is the volume in which no gas exchange has taken place (physiological dead space).

Causes of insufficient alveolar ventilation • depression of the respiratory center • paralysis of respiratory muscles: muscular disease, muscle relaxants • large dead space: for example, emphysema, pulmonary embolus • improperly set ventilator

Dead space

Alveolar dead space

Pulmonary embolus

Physiological dead space The physiological dead space is the sum of the anatomical dead space + the alveolar dead space. The physiological dead space is about 2 ml/kg body weight or 80 ml/m2 body surface. (In an intubated patient it is about 50 ml/m2 body surface.) The physiological dead space can be calculated from the formula: VD = VT x (1 –

20

PE-CO2 PaCO2

)

Ventilation Anatomical dead space Part of the ventilatory volume does not take part in gas exchange (anatomical dead space or airway dead space). The anatomical dead space depends on: • the height and weight of the individual • position • age • lung volume • tidal volume • intubation/tracheotomy (reduces anatomical dead space)

Alveolar dead space The alveolar dead space is the volume of gas reaching badly- or non-perfused alveoli. The alveolar dead space is minimal in spontaneously breathing healthy individuals. During positive pressure ventilation alveolar dead space increases markedly (even in healthy individuals) and can amount to 1/4 of the alveolar ventilation. This is because of the undesirable effect of increased intrathoracic pressure (ventilation perfusion imbalance). Anesthetic agents and preexisting lung disease can further increase this effect. A large alveolar dead space is seen in emphysema, and pulmonary embolus.

Glossary Emphysema: pathological increase in the size of the alveoli throughout the lungs, caused by a destruction of the walls of the alveoli. (Also used to describe the pathological presence of air in tissues, for example, subcutaneous air originating from the airway) Intrathoracic pressure: the pressure within the chest P-ECO2: mean partial pressure of carbon dioxide in the expired air Pulmonary embolus: blockage of the arteries to the lungs, usually by blood clots, but also by air, bacteria, and foreign material transported in the blood stream Tracheotomy: surgically opening the trachea VD: dead space

21