Chapter 22 Respiratory system

what does the respiratory system do

• Provides cells with oxygen 

• Removes carbon dioxide 

  • It accomplishes this through a process called respiration 

What additional functions does the respiratory system perform 

• Speech

• Olfactory 

4 process of respiration

1. Pulmonary ventilation 

2. External respiration

3. Transport of respiratory gasses 

4. Internal respiration 

Pulmonary ventilation (4 process of respiration)

• Ventilation consists of inspiration and expiration

• Inspiration moves air into the lungs from the atmosphere

• Expiration moves air out of the lungs into the atmosphere

External respiration (4 process of respiration)

• Oxygen diffuses from the lungs to the blood

• Carbon dioxide diffuses from the blood to the lungs

Transport of respiratory gasses (4 process of respiration)

• The cardiovascular system transports gasses using blood as the transporting fluid

• Oxygen is transported from the lungs to the tissue cells of the body

• Carbon dioxide is transported from the tissue cells to the lungs

Internal respiration (4 process of respiration)

• Oxygen diffuses from blood to tissue cells

• Carbon dioxide diffuses from tissue cells to blood

Cellular respiration

• The use of oxygen and production of carbon dioxide by cells 

• Is NOT part of the respiratory system 

• It is central to all energy-producing chemical reactions in the body

Upper respiratory system  

• From nose to larynx

Lower respiratory system 

• Larynx and all structures below it 

Upper respiratory Nose and nasal cavity (palate)

• Separates nasal cavity from the oral cavity 

• Has hard and soft portions 

• As air twists and turns around the indented surface (nasal conchae) larger particles get trapped in mucous - thus the nasal cavity acts as a filter

Pharynx 

• Nasopharynx 

• Oropharynx 

• Laryngopharynx

Nasopharynx 

• Is above the food entry point, so only for air passage 

• During swallowing soft palate and uvula move upward to prevent food from entering the nasal cavity 

Oropharynx 

• Passageway of air and food 

• Epithelium is more protective due to increased friction and chemical trauma from food 

• Location of palatine tonsils, which are often removed in children 

Laryngopharynx

• Passageway of air and food 

• Is posterior to larynx and marks divergence of Respiratory and digestive passageways 

  • Food enters the esophagus posteriorly 

  • Air enters the larynx anteriorly 

Lower respiratory system 

• Larynx 

• Trachea 

• Bronchi 

• Lungs 

Larynx characteristics and location (aka voice box)

• Extends about 5cm from the level of the third to sixth cervical vertebrae 

• Superiorly it is attached to the hyoid bone and inferiorly it is continuous with the trachea 

Functions of the larynx

• Provides a patent (open) airway 

• Acts as a switching mechanism to ensure food and air enter the proper channels 

• Voice production (houses the vocal cords)

• Under some conditions it also acts as a sphincter that prevents air passage 

  • Referred to as the Valsalva maneuver 

  • Occurs when during abdominal straining (defecation or heavy lifting)

Structure of the larynx 

• Consists of 9 different cartilage pieces

• Epiglottis (guardian of the airways)

• Vocal folds or true vocal cords 

Epiglottis (guardian of the airways)

• Composed of elastic cartilage 

• Covered in taste buds

• During air flow, the inlet to the larynx is open and the epiglottis covers the opening to the larynx 

• When swallowing larynx is pulled upwards and the epiglottis covers the opening to the larynx 

Vocal folds or true vocal cords 

• These structures vibrate to produce sounds as air rushes up from the lungs 

• The opening between them is called the glottis 

  • Size of glottis and length of vocal folds are controlled by muscles 

  • As these change, the rate of vibration changes which impacts pitch of the the sound that is heard 

  • Loudness of sound determined by force of air passing over vocal folds 

Trachea (or windpipe)

• Descends from the larynx into the mediastinum 

• Ends by dividing into two main bronchi 

• 10-12 cm long and around 2cm in diameter 

• It is very flexible and mobile 

Layers of the trachea

• Mucosa

• Submucosa

• Layer of C-shaped rings of hyaline cartilage

• Adventitia

• Opening in c-shaped cartilage is posterior

  • This allows flexibility for the esophagus to expand when swallowing food 

Mucosa (inner most layer)

• Covered in cilia that help propel debris filled mucus toward the pharynx 

Submucosa

• Contains glands that help produce mucus 

Bronchi zones

1. Conducting zone 

2. Respiratory zone 

Structure of bronchiole walls 

As they get smaller the following changes occur

• Support structure changes

• Epithelium changes 

• Amount of smooth muscle increases 

Support structure changes

• Cartilage rings replace plates of cartilage. Once reach bronchioles, there is no longer any cartilage in the walls

Epithelium changes 

• Mucus producing cells and cilia are sparse once the bronchiole level is reached. This means debris must be removed by macrophages

Amount of smooth muscle increases 

• As passageways become smaller the amount of smooth muscle increases. Bronchioles have a complete layer of smooth muscle. 

Respiratory zone

• Where gas exchange in the lung occurs 

• It is defined by the presence of thin-walled air sacs called alveoli 

• It begins at the terminal bronchioles as they feed into them

Respiratory membrane 

• The walls of the alveoli are extremely thin (a sheet of paper is 15 times thicker than alveoli walls)

• On the outside, alveoli are covered with a cobweb of pulmonary capillaries 

• The thin alveolar walls + capillary walls form a respiratory membrane. 

• It has blood flowing on one side and air flowing on the other 

• Gas exchange occurs easily across this membrane by diffusion 

Alveoli 3 major cell types:

1. Type 1 alveolar cells 

   • form a major part of alveolar walls 

2. Type 2 alveolar cells 

   • Less abundant, but are important because they secrete surfactant that plays an important role in reducing the surface tension of the alveolar fluid 

3. Alveolar macrophages 

   • Consume bacteria, dust and other debris from the air in alveoli

Lungs 

• Occupy the whole thoracic cavity (except the mediastinum)

• Right and left are slightly different shapes

• Left has 2 lobes, Right has 3 lobes 

• They weigh just over 1kg

• Consists mainly of air spaces (alveoli) the rest is stroma

stroma 

• It is mostly elastic tissue (important for breathing)

• As a result lungs are soft, spongy, elastic organs 

Bronchopulmonary segments 

• Each lobe contains pyramid shaped bronchopulmonary segments 

• Each segment is served by ts own artery and vein and receives air from an individual tertiary bronchus 

• Clinically important because disease tends to be confined to individual segments 

Blood supply of lungs 

• Pulmonary circulation 

  • Low pressure 

  • Total blood volume flow through lungs every minute 

Blood supply and innervation of lungs 

• Bronchial circulation 

• Innervation of lungs 

Bronchial circulation 

• Bronchial arteries supply oxygenated blood to the lung tissue

• They arise from the aorta 

• They provide high-pressure, low volume supply of blood to all tissues 

Innervation of lungs 

• Innervated by parasympathetic and sympathetic fibers and visceral sensory fibers 

• These enter the lungs via the pulmonary plexus 

Lung pleura 

• Parietal (covers thoracic wall)

• visceral (covers the lungs pleura)

Pulmonary ventilation phases:

1. Inspiration 

2. Expiration 

Pulmonary ventilation

• It is a mechanical process that happens due to volume changes in the lungs. Volume changes lead to ventilation because they lead to pressure changes and pressure changes lead to flow of glasses to equalize pressure 

What happens during quiet (resting) ventilation

1. Diaphragm contracts and flattens – this increases the height of thoracic cavity 

2. The intercostal muscles also contract – this increase anterior/posterior and lateral dimension of thoracic cavity 

Forced ventilation 

• Forced inspiration 

• Forced expiration

Forced inspiration 

• Additional muscle contributes to increasing the thoracic volume (scalenes, sternocleidomastoid). Also back extends as erector spinae straighten the thoracic curve 

Forced expiration

• Is an active process produced by contracting the abdominal muscles, which decreases thoracic volume 

  • Increases intra-abdominal pressure 

  • Depresses the rib cage 

Physical factors that influence ventilation 

1. Airway resistance 

2. Alveolar surface tension 

3. Lung compliance 

4. Compliance of thoracic wall

Airway resistance 

• This is impacted by the diameter of airways. Airway diameter is not a factor limited ventilation in healthy individuals. However in people with asthma, constriction of airways can make ventilation very difficult 

Alveolar surface tension 

• Water and gas don't like each other 

• When they meet the water pulls away from the gas 

• Because there is a lot of water in coating alveolar walls, this surface tension is always acting to keep the alveoli as small as possible (i.e. to collapse them)

• This would require a lot of energy to overcome and would make breathing very inefficient 

• Surfactant reduces this effect and keeps alveoli open 

Importance of negative intrapleural pressure 

• The lungs have a natural tendency to collapse because:

  1. They are elastic and have a tendency to recoil 

  2. The surface tension of the alveolar fluid – this surface tension is constantly pulling the alveoli to their smallest possible dimension (i.e. collapse)

• The chest wall has a natural tendency to keep lungs from collapsing:

  • The elasticity of the chest wall is placing an outward pull on the thorax which tends to keep the lungs enlarged 

Lung compliance 

• Lungs are extremely stretchy – this stretchiness is referred to as compliance 

• The higher the compliance the easier it is for the lungs to expand 

• This compliance is determined by distensibility of lung tissue and alveolar surface tension (in healthy, fully developed lungs, neither of these limits ventilation

  • Pathology that limits distensibility (chronic inflammation leading to scar tissue (fibrosis) or limits production of surfactant will result in lower compliance and increased work of breathing 

Compliance of thoracic wall

• Anything (deformities, paralyzed muscles etc) that limit thoracic expansion will negatively impact ventilation 

Transpulmonary pressure

• = Ppul - Pip

• So it is the difference in pressure between the lung -pressure and intrapleural pressure 

• It is the pressure that keeps the lungs from collapsing 

• The greater the pressure the larger the lungs are. If the pressure is 0 the lungs will collapse 

Assessing pulmonary ventilation 

1. Volume of air during ventilation – Spirometry

2. Rate at which that air is flowing – Pulmonary function tests 

3. Efficiency of the system (minute ventilation and alveolar ventilation)

Spirometry 

• To assess ventilation function and efficiency spirometry is often used 

• It is typically used as a diagnostic tool, as different lung conditions will produce different results 

• The client blows into the device and computer software assesses a variety of important variables 

• Most useful for evaluating losses in function and for following course of certain diseases 

Pulmonary function test

• Information on rate at which air is moving in and out of the lungs also has important diagnostic and monitoring function 

• Pulmonary function tests (PFT) are similar to spirometry, however the rate of flow of air is also measured 

  • Forced vital capacity (FVC)

  • Forced expiratory volume (FEV)

Assessing respiratory efficiency

• MInute ventilation

• Alveolar ventilation rate (AVR)

  • Both can be determined using results of spirometry test an can provide useful diagnostic and monitoring data 

Alveolar ventilation rate (AVR)

• It takes into account the volumes of air wasted in dead space 

• AVR = breaths/min * (tidal volume - dead space)

MInute ventilation

• The total amount of gas that flows into or out of the respiratory tract in 1 minute. It is usually around 6L/min (can rise to 200 L/min with vigorous exercise)

Factors that influence external respiration 

1. Partial pressure gradients and gas solubilities 

2. Thickness and surface areas of respiratory membrane 

3. Ventilation - perfusion coupling 

Partial pressure and gas solubility 

• In a gas mixture (room air) each gas exerts a certain amount of pressure. 

• This pressure is referred to as partial pressure of the gas 

• Remember that gas will always flow from high to low pressure 

Thickness and surface area of respiratory membrane 

• Thickness 

  • The respiratory membrane is very thin (0.5 – 1 micrometre)

  • This thickness is ideal for diffusion and does not impede gas exchange 

    • If lungs become waterlogged (pneumonia or left heart failure) then thickness increases dramatically and gas exchange is negatively impacted 

• Surface area 

  • The greater the surface area the more gas can diffuse 

  • Alveoli increase surface area (spread flat = 90 m^2)

Ventilation – perfusion coupling 

• There should be a close match between the amount of gas reaching the alveoli (ventilation) and the amount of blood in the pulmonary circulation 

• If these aren't matched the there is either:

  • Too much gas in alveoli for available blood to remove 

  • Too much blood for available gas 

• To ensure this coupling (matching) occurs ventilation and perfusion are controlled by local mechanisms:

  • PO2 controls perfusion by changing arteriole diameter 

  • PCO2 control ventilation by changing bronchiole diameter 

PCO2 influence on ventilation 

• If levels are high then triggers dilation of bronchioles to enable CO2 to be eliminated more easily 

• If levels are low then triggers constriction of bronchioles which will decrease airflow in attempt to balance it with perfusion 

In summary: Balancing ventilation and perfusion 

• Poor ventilation results in lower oxygen and higher carbon dioxide 

  • Pulmonary arterioles constrict and airways dilate 

  • This reduces blood flow and increases air flow to balance ventilation and perfusion 

• Increased ventilation results in higher oxygen and lower carbon dioxide 

  • Pulmonary arterioles dilate and airways constrict 

  • This increases blood flow and decreases air flow to balance ventilation and perfusion 

• The balance is never perfect:

  • This accounts for the slight difference in oxygen partial pressure from alveoli to pulmonary veins (104mmHg vs. 100mmHg)

Oxygen transport 

1. Bound to hemoglobin 

2. Dissolved in plasma (only about 1.5% is transported this way because oxygen is poorly soluble in water)

Hemoglobin 

• RBCs are like bag of hemoglobin

• Hemoglobin consists of red heme pigment bound to the protein globin 

• Each hemoglobin binds 4 molecules of oxygen 

• Hemoglobin – Oxygen combination called oxyhemoglobin 

• Hemoglobin that has released oxygen is called deoxyhemoglobin 

What controls rate of binding of oxygen and hemoglobin 

1. Partial pressure of oxygen (the amount of O2 that is available)

   • Generally the more oxygen that is present the more oxygen will bind to hemoglobin 

2. Temperature 

   • Increases in temperature lower hemoglobin's affinity for oxygen (enhance oxygen unloading from the blood)

3. Blood pH

   • Lower pH lowers hemoglobin's affinity for oxygen (enhance oxygen unloading from blood)

4. Partial pressure of carbon dioxide

   • Increases in this will lower affinity for oxygen (enhance oxygen unloading from the blood) 

5. Blood concentration of BPG

   • Decreased BPG leads to increased oxygen affinity for hemoglobin 

When oxygen transport is not adequate hypoxia occurs

1. Anemic hypoxia 

   • Too few RBCs or RBCs with too little hemoglobin 

2. Ischemic (stagnant) hypoxia

   • Impaired or blocked 

3. Histotoxic hypoxia 

   • Body cells are unable to use oxygen (some poisons like cyanide can cause this)

4. Hypoxemic hypoxia 

   • Reduced arterial PO2 (can be caused by unbalanced ventilation-perfusion coupling, breathing air with very low levels of O2, diseases that cause impaired ventilation)

5. Carbon monoxide poisoning 

   • CO has 200 more times affinity to hemoglobin than oxygen so it outcompetes oxygen for heme binding sites

Carbon dioxide transport 

1. Dissolved in plasma (7-10%)

2. Chemically bound to hemoglobin (just over 20%)

3. As bicarbonate ions in plasma (About 70%)

CO2 transport: bound to hemoglobin 

• When bound to hemoglobin it is called carbaminohemoglobin 

• It doesn't compete with O2 because it binds to globin, not heme 

• Deoxygenated hemoglobin binds more readily with CO2 then does oxygenated hemoglobin – can you think of why this might be the case?

CO2 transport: As bicarbonate ions in plasma  

• Dissolved CO2 quickly enters RBCs

• In the RBCs it combines with water to form carbonic acid 

• This acid ultimately becomes hydrogen ions (H+) and bicarbonate ion (HCO3-)

• Once formed the bicarbonate ions move quickly to the lungs where it combines with H+ to form carbonic acid which then splits in to C)2 and water 

• The CO2 is diffused from the blood in to the alveoli lungs along it partial pressure gradient

Control of breathing

• Higher brain centers 

• Chemoreceptors 

• Other reflexes 

Neural control of breathing 

• A center in the medulla sets basic rhythm of inspiration and expiration 

• Breathing rate of 12-16 breaths/minute 

• This center is impacted by overdose of morphine or alcohol (stops respiration)

• Uncertain where the rhythm comes from, but likely multiple sets of pacemaker cells that cycle activity to generate rhythm 

Factors that influence breathing rate 

1. Chemical factors 

2. Higher brain centers (hypothalamus and cortical controls)

3. Mechanoreceptors 

Chemical factors 

1. CO2 in arterial blood 

2. O2 in arterial blood 

3. H+ in arterial blood

   • These changes are sensed by chemoreceptors in the brain stem (central chemoreceptors) and aortic arch and carotid arteries (peripheral chemoreceptors)

Influence of PCO2 

• PCO2 has the most potent influence on breathing rate.

• PCO2 is also the most tightly controlled 

• Increased PCO2 in the blood is referred to as hypercapnia 

Influence of PO2 on breathing rate 

• Chemoreceptors for PO2 peripheral receptors (aortic arch and carotid sinus)

• Minor drops in O2 sensitize peripheral receptors to CO2 

• Substantial drops in arterial PO2 (to 60mmHg) Directly stimulate increased ventilation 

Influence of pH

• Changes in arterial pH can modify respiratory rate and rhythm even if CO2 and O2 levels are normal 

• Has its effect through the peripheral chemoreceptors

• A drop in pH will stimulate respiratory system controls that will attempt to raise the pH by increasing respiratory rate and and depth to eliminate CO2 from the blood  

Influence of Higher brain centers 

• Hypothalamic controls

  • Act through the limbic system to modify rate and depth of respiration

  • Example: Breath holding that occurs in anger, sudden cold shock or gasping with pain

  • A rise in body temperature acts to increase respiratory rate 

• Critical control

  • Although breathing is generally involuntary, we can also exert conscious control

  • This happen via cortical signals from the cerebral motor cortex that bypass medullary controls 

  • Example: volunta

Mechanoreceptors influence on breathing rate 

• Inflation or hering-breuer reflex

  • Stretch receptors in the pleurae and airways are stimulated by lung inflation

  • Inhibitory signals to the medullary respiratory center end inhalation and allow expiration to occur 

  • Acts more as a protective response than a normal regulatory mechanism 

• Protective reflexes 

  • Chemical or physical irritants of the upper airways include coughing and sneezing 

  • An irritant causes a brief period of apnea (breath holding) usually at the end of an inspiration; followed by a forceful expulsion of ait to remove the offending irritant 

Influence of exercise on the respiratory system 

• Because exercising muscles consume more O2 and produce more CO2 ventilation increases substantially (hyperpnea)

• FYI: The panting athlete needs more oxygen in the muscles, not in the lungs. So breathing O2 by mask won't help – it wont get more O2 to muscles