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Chapter 22 The Respiratory System

1 Anatomy of respiration

1 Respiration

The term "respiration" has three different meanings.

1 Ventilation of the lungs (breathing)

2 The exchange of gases

Exchange of gases between the air and the blood in the lungs, and between the blood and tissues in the rest of the body.

3 The use of oxygen in cellular metabolism

This is also called cellular respiration and deals with the extraction of energy through the oxidation of organic compounds. The mitochondria are the organelles responsible for cellular respiration.

2 Respiratory system

The respiratory system is an organ system that takes in air and expels it from the body. The respiratory system supplies oxygen through this process and expels carbon dioxide. This is the most simplistic view of the respiratory system and there is a broad range of functions.

1 Functions

1 Provides oxygen and carbon dioxide exchange between blood and air.

2 Speech and other vocalizations.

3 Sense of smell.

4 Controlling pH by eliminating carbon dioxide.

5 Synthesis of angiotensin II.

6 Promote the flow of lymph and venous blood.

7 Expelling abdominal contents.

2 Organs

1 Nose

2 Pharynx

3 Larynx

4 Trachea

5 Bronchi

6 Lungs

3 Divisions and Tracts

1 Conducting division

The conducting division serves only for air flow and includes those organs from the nostrils to the bronchioles.

2 Respiratory division

The respiratory division consists of the gas exchange tissues including the alveoli and other related tissues at the distal end of the respiratory tract.

3 Upper respiratory tract

The respiratory organs of the head and neck.

4 Lower respiratory tract

The respiratory organs of the thorax including the trachea through the lungs.

3 The nose

1 Functions

1 Warms inhaled air

2 Cleanses inhaled air

3 Humidifies inhaled air

4 Detects odors

5 Resonating chamber for the voice

2 Anatomy

1 Facial part

The facial parts of the nose are formed by bone and cartilage. The superior part is supported by the nasal bones and part of the maxillae.

2 Nostrils

Also called anterior or external nares. These are the most anterior openings of the nose.

3 Choanae

Also called the posterior or internal nares. These are the posterior openings of the nose.

4 Nasal cavity

The inner chamber of the nose. The roof of the nasal cavity is formed by the sphenoid bone and the palate forms the floor of the nasal cavity.

1 Nasal fossae

The nasal cavity is divided into left and right halves called nasal fossae.

2 Nasal septum

The dividing wall between the fossae is called the nasal septum. The septum is composed of bones and cartilage. The ethmoid bone forms part of the septum.

3 Vestibule

The vestibule is the start of the nasal cavity, just inside the nostril.

1 Guard hairs

Also called vibrissae, these hairs block insects and debris from entering the nose.

4 Nasal conchae

Posterior to the vestibule, the nasal cavity opens into a large chamber, but most of it is occupied by folds of tissue called the nasal conchae.

5 Meatus

The meatus is the narrow passageway beneath the conchae. This is a relatively small passageway for air and ensures that air is adequately conditioned (heated, humidified, and cleaned) before it continues along the respiratory tract.

6 Olfactory mucosa

Sensory cells occupying a small patch of epithelium on the roof of the fossae.

1 Goblet cells

Secrete mucus.

2 Ciliated cells

In the nose, the ciliated cells drive the mucus towards the pharynx. Dust, pollen and other particles stick to the mucus and are removed from the respiratory tract.

7 Erectile tissue

Found in the inferior nasal conchae, the erectile tissue restricts airflow on one side and then the other sequentially. This gives the mucosa time to recover from the drying effects of the airflow. Airflow shifts from right to left a couple of times per hour.

4 The pharynx

Muscular funnel extending from the chaonae to the larynx. About 13 cm in length.

1 Three regions

1 Nasopharynx

Lies just posterior to the choanae and dorsal to the soft palate. The nasopharynx only passes air.

1 Auditory tubes

The nasopharynx receives the auditory tubes (eustachian tubes) from the middle ears.

2 Oropharynx

Starts before the soft palate and the root of the tongue and extends inferiorly as far as the hyoid bone. The oropharynx and the laryngopharynx pass air, food, and drink.

3 Laryngopharynx

Starts with the union of the nasopharynx and oropharynx at the level of the hyoid bone and extends inferiorly and dorsal to the larynx. The laryngopharynx ends at the inferior end of the larynx, where the esophagus begins.

5 The larynx

The larynx is a cartilaginous chamber. The primary function of the larynx is to keep food and drink out of the airway, but has evolved into the "voicebox" used for sound production.

1 Epiglottis

The superior opening of the larynx is guarded by a flap of tissue called the epiglottis. During swallowing, muscles pull the larynx up towards the epiglottis and the tongue pushes the epiglottis downwards to meet it. As a result, food and drink are directed into the esophagus.

2 Glottis

The inferior opening of the larynx, and the vocal cords that are located there, is called the glottis.

1 Vocal cords

The vocal cords are folds of the larynx that produce sound when air passes between them. Intrinsic muscles of the trachea control the vocal cords.

6 The trachea

The trachea is a rigid tube lying anterior to the esophagus.

1 Cartilage

These are 16 to 20 C-shaped rings of hyaline cartilage that support the structures of the trachea. The open part of the C faces posteriorly.

2 Smooth muscle

The open part of the C-shaped cartilage is spanned by smooth muscle. These muscles contract or relax to adjust the tracheal airflow. The posterior opening of the C also allows expansion of the esophagus during swallowing.

3 Mucociliary escalator

The inner lining of the trachea is pseudostratified columnar epithelium with mucus-secreting goblet cells and ciliated cells. Mucus traps inhaled particles and the cilia drive the mucus towards the pharynx, where it is swallowed.

4 Carina

The inferior end of the trachea forks into the right and left pulmonary bronchi. The lowermost tracheal cartilage has a ridge called the carina that directs the airflow to the right and left.

7 The lungs and bronchial tree

1 Lungs

Each lung is basically conical in shape.

1 Base

The concave base rests on the diaphragm.

2 Apex

The apex is the peak, which projects slightly above the clavicle.

3 Costal surface

The surfaces of the lungs that are pressed against the ribs.

4 Mediastinal surface

The surface that is on the medial side of the lungs.

5 Hilum

The mediastinal surface has an opening, through which the lungs receive the primary bronchi, blood vessels, lymphatic vessels, and nerves.

6 Cardiac impression

7 Lobes

1 Right lung

Three lobes.

2 Left lung

Two lobes. The heart takes up some of the space that the left lung would occupy.

2 The bronchial tree

1 Primary bronchi

Two primary bronchi arise from the trachea at the level of the angle of the sternum (between the body and the manubrium of the sternum) and enter each lung at the hilum. The right bronchus is larger and straighter than the left (no heart on this side) and so is the most common site of inhaled objects. The primary bronchi are supported by C-shaped cartilage like the trachea.

1 Secondary bronchi

In the lungs, the primary bronchi separate into one secondary bronchus for each lobe of the lung. The secondary bronchi are also called lobar bronchi for this reason.

1 Tertiary bronchi

Each secondary bronchus separates into tertiary bronchi: 10 in the right and 8 in the left lung.

1 Bronchioles

Continuations of the airway that lack supportive cartilage and are small in diameter: 1 mm or less.

1 Terminal bronchioles

Each bronchiole divides into 50-80 terminal bronchiole, which are the final branches of the conducting division. They have no mucous glands but do have cilia that will remove mucus that passes this deeply into the respiratory tract.

2 Respiratory bronchioles

Each terminal bronchiole gives rise to two or more respiratory bronchioles, which have alveoli budding off of their walls. Respiratory bronchioles are the beginning of the respiratory division i.e. they are able to exchange gases across their walls.

3 Alveolar ducts

Each respiratory bronchiole divides into 2-10 thin-walled alveolar ducts, which have elongated alveoli in their walls.

4 Alveolar sacs

The alveolar ducts end in grapelike clusters of alveoli arranged around a central space called the alveolar sac.

2 Alveoli

1 Respiratory membrane

Each alveolus is surrounded by a basket of blood capillaries. The barrier between the alveolar air and blood is called the respiratory membrane. The walls are very thin.

8 The pleurae

1 Visceral pleura

The surface of the lung is covered by a serous membrane called the visceral pleura.

2 Parietal pleura

At the hilum, the visceral pleura folds back on itself to form the parietal pleura, which adheres to the mediastinum, inner surface of the ribcage, and superior surface of the diaphragm.

3 Pleural cavity

A space is formed between the visceral pleura and parietal pleura called the pleural cavity.

4 Pleural fluid

The pleural cavity is a potential space. Most of the time, the visceral and parietal pleura are separated only by a film of pleural fluid.

5 Functions of pleura and pleural fluid

1 Reduction of friction

2 Creation of pressure gradient

3 Compartmentalization

2 Pulmonary ventilation

1 Terminology

This is the same thing as our concept of breathing: a repetitive cycle of inspiration and expiration.

1 Inspiration

Inhaling.

2 Expiration

Exhaling.

3 Respiratory cycle

One complete cycle of inspiration and expiration.

4 Quiet respiration

The way we breath at rest.

5 Forced respiration

Unusually deep or rapid breathing as in the state of exercise.

2 The respiratory muscles

The lungs do not ventilate themselves. The only muscle in lungs is smooth muscle in the walls of the bronchi and bronchioles, which adjust the diameter of the airways. Skeletal muscles of the trunk are responsible for respiration.

1 Diaphragm

This muscle is the prime mover of respiration. It accounts for 2/3 of pulmonary airflow. It is a muscular dome that separates the thoracic cavity from the abdominal cavity.

1 Relaxed diaphragm

When relaxed, the diaphragm bulges upward, pressing against the base of the lungs. When the diaphragm is relaxed, the lungs are at their minimum volume.

2 Contracted diaphragm

When contracted, the diaphragm flattens, dropping about 1.5 cm in relaxed respiration and up to 7 cm in forced respiration.

Contraction of the diaphragm enlarges the thoracic cavity and lungs. The expansion of the thoracic cavity results in the inflow of air.

2 Synergistic respiratory muscles

Several muscles assist the diaphragm in respiration. The intercostal muscles are the chief synergistic muscles of respiration. Their primary function is to stiffen the thoracic cage during respiration and maintain its structure as the diaphragm contracts (otherwise the thoracic cage would collapse).

1 External intercostals

The external intercostals contract during inspiration. They draw the ribs upward and outward (away from the vertebral column). As a result, the thoracic cavity is enlarged in volume.

2 Internal intercostals

The internal intercostal muscles contract during forced expiration. They move the ribs in a downward direction, thereby decreasing the volume of the thoracic cavity.

3 Accessory muscles

The accessory muscles assist the diaphragm and intercostals, especially during forced inspiration.

1 Erector spinae

Deep inspiration is assisted by the erector spinae, which arches the back and increases the volume of the thoracic cavity.

2 Sternocleidomastoid

The sternocleidomastoid contracts and lifts the clavicle and sternum during inspiration.

3 Pectoralis major

Elevate the upper ribs during inspiration.

4 Serratus anterior

Elevates the upper ribs during inspiration.

5 Rectus abdominis

The rectus abdominis contracts during forced expiration.

3 Neural control of breathing

There is no internal autorhythmic pacemaker in the lungs as in the heart: the lungs must be controlled by external pacemakers. The rhythms associated with respiration depend on repetitive stimuli from the brain: if the nerves to the thoracic muscles are severed or the spinal cord damaged high on the neck, breathing stops.

1 Two main reasons for brain control

1 Respiration controlled by skeletal muscle

Skeletal muscle fibers cannot contract without neuronal stimulation. Every muscle fiber must be innervated.

2 Complex muscular coordination

Breathing requires complex coordination of many muscles. Central nervous system control is required for complex muscular activities.

2 Control at two levels of the brain

Breathing is controlled at the cerebrum and is therefore conscious but is also controlled in an automatic unconscious level by respiratory centers in the medulla and pons.

1 Automatic, unconscious control

1 Brainstem respiratory centers

1 Dorsal respiratory group

In the medulla.

2 Ventral respiratory group

In the medulla.

3 Pneumotaxic center

In the pons.

2 Central and peripheral input to the respiratory centers

The rhythm of respiration must adapt to changing circumstances: rest, exercise etc. The respiratory centers in the medulla and pons receive inputs from the nervous system and therefore sense the body's varying physiological needs.

1 Central chemoreceptors

Brainstem neurons that respond to changing pH of the cerebrospinal fluid. Carbon dioxide concentrations in the CSF directly influences pH (more carbon dioxide, more acidic).

2 Peripheral chemoreceptors

Located in the carotid and aortic bodies of the large arteries above the heart. They respond directly to oxygen and carbon dioxide concentrations, and also to pH (which again reflects carbon dioxide concentrations).

3 Stretch receptors

Found in smooth muscle of the bronchi and bronchioles, and in the visceral pleura surrounding the lungs. They respond to inflation of the lungs: excessive inflation signals a stop signal for inspiration.

4 Irritant receptors

Nerve endings in the epithelial cells of the airway. They respond to smoke, dust, pollen, chemicals, cold air and excessive mucus. Signals from these receptors cause contraction of respiratory and bronchiole muscles, resulting in shallower breathing, breath-holding or coughing. All of the responses are attempts to limit contact with the irritant.

2 Voluntary control of breathing

Voluntary control over breathing is centered in the motor cortex of the frontal lobes of the cerebrum.

4 Pressure, resistance and airflow

1 Pressure and airflow

The flow of a fluid (air is a fluid) is directly proportional to the pressure difference between two points. This means that the greater the pressure differences, the greater the flow of a fluid. It applies to the weather (big differences between high and low pressure regions creates big winds), and respiration (decreasing the pressure in the thoracic cavity increases the flow into the lungs).

The flow of fluid is inversely proportional to resistance. The more resistance in the structures that are moving fluids, the slower the flow. If our respiratory tract is congested, it's harder to breathe.

1 Atmospheric pressure

The pressure exerted by the weight of the atmosphere around us. Same idea as water pressure - the deeper you go, the greater the pressure - but we've adapted to the pressure of the atmosphere and don't notice it until it changes (reduced pressure in airplanes etc).

2 Intrapulmonary pressure

The internal gas pressure of the lungs.

3 Boyle’s law

Pressure is inversely proportional to volume: Squeeze on a balloon and the pressure increases, increase the volume of the balloon without adding more gas (not sure how you would do this except by taking it to the top of a mountain) and the pressure drops.

4 High pressure to low pressure direction of air flow

The ability to move gases into and out of the lungs depends upon the flow of air due to pressure differences. Air will always flow (if there is an open passageway) from high pressure to low pressure.

2 Inspiration

1 Volume and pressure changes

Overall, expansion of the thoracic cage results in decrease in lung pressure below atmospheric pressure resulting in inflow of air. The steps involved in inspiration are:

1 Increase in thoracic cage volume

2 Drop in intrapleural pressure

3 Transfer of pressure drop to lungs

4 Results in drop in intrapulmonary pressure

2 Charles’s law

Volume is proportional to temperature: when air is inhaled, it generally heats up and thus the volume increases.

3 Expiration

1 Relaxed expiration

Relaxed expiration is a passive process and results from the elastic properties of the structures of the thorax, for example, the costal cartilages and the walls of the bronchi and bronchioles. These structures spring back into their resting position after they have been stretched during inspiration.

2 Forced expiration

During vigorous activity, expiration is assisted by contraction of the internal intercostal muscles, which depress the ribs. Contraction of the abdominal muscles increases intra-abdominal pressure, which forces the diaphragm upwards, which increases pressure on the thoracic cavity.

4 Resistance to airflow

1 Diameter of bronchioles

The bronchioles are the primary means of controlling resistance.

1 Bronchodilation

An increase in the diameter of the bronchi or bronchioles. Epinephrine and norepinephrine stimulate bronchodilation and increase airflow.

2 Bronchoconstriction

A decrease in the diameter of the bronchi or bronchioles. Histamine, acetylcholine, cold air and chemical irritants stimulate bronchoconstriction.

2 Pulmonary compliance

Pulmonary compliance is the ease with which the lungs expand. The lungs will expand less in people with poor compliance, even though the change in thoracic cage volume during inhalation is the same. Stiffening of the lungs due to scar tissue (resulting from tuberculosis, smoking etc) results in poor compliance.

3 Surface tension of the alveoli and distal bronchioles

A film of water covering the alveolar membranes is necessary for gas exchange. However, the water has surface tension (the phenomenon that allows insects to stand - no float - on top of water) and thus tends to stick the alveolar walls together. Surfactants in the lungs disrupt hydrogen bonds in water and reduce this tendency. A lack of surfactant increases resistance.

5 Measurement of ventilation

1 Spirometer

A device that measures the volume of expelled air.

2 Respiratory volumes

1 Tidal volume

The volume of air inhaled or exhaled in one breath during quiet breathing.

2 Inspiratory reserve volume

The volume of air in excess of tidal volume that can be inhaled with maximum effort.

3 Expiratory reserve volume

The volume of air in excess of tidal volume that can be exhaled with maximum effort.

4 Residual volume

The volume of air remaining in the lungs after maximum expiration. It can never be voluntarily exhaled.

3 Respiratory capacities

These volumes can be obtained by adding two or more or the respiratory volumes.

1 Vital capacity

The volume of air that can be inhaled and then exhaled with maximum effort.

2 Inspiratory capacity

The maximum volume of air that can be inhaled after a normal tidal (quiet) expiration.

3 Functional residual capacity

The volume of air remaining in the lungs after a normal tidal (quiet) expiration.

4 Total lung capacity

The maximum volume of air the lungs can contain.

3 Gas Exchange and Transport

1 Composition of air

Air is composed of: nitrogen gas about 78%; oxygen gas about 21%; water vapor about 0.5%; carbon dioxide gas about 0.04%.

1 Dalton’s law

The total pressure of a mixture of gases like air is the sum of the individual pressures of the gases: each gas contributes a partial pressure to the total pressure that is directly proportional to its percentage in the composition. For example, in an atmospheric pressure of 760 mmHg, oxygen contributes about 159 mmHg or 21% of 760 mmHg.

2 Alveolar gas exchange

Gases from the air must dissolve in water before being exchanged at the alveolar membrane. Gases do not directly enter the blood.

1 Henry’s law

The amount of a specific gas that dissolves in water (or blood) is directly proportional to its solubility and partial pressure in a mixture like air.

Henry's law explains why we give 100% oxygen to patients: we increase the partial pressure of oxygen from 21% (air) to 100% and about five times as much oxygen enters the blood. Also explains why mountain climbers pass out.

2 Variable affecting alveolar gas exchange

1 Pressure gradient of the gases

Higher partial pressure of oxygen in air compared with blood results in the flow of oxygen from air into blood.

Higher partial pressure of carbon dioxide in blood compared with air in lungs results in carbon dioxide flowing out of blood into air of lungs.

2 Solubility of the gases

Carbon dioxide is very soluble in water (and blood). Oxygen is less soluble. The combination of solubility and partial pressure means that the exchange of oxygen and carbon dioxide at the alveolar membrane proceeds at about the same rate.

3 Membrane thickness

The thinner the membrane, the less obstruction to diffusion across it. Anything that thickens the membrane - chronic inflammation for example - slows the diffusion and reduces gas exchange.

4 Membrane area

Big lungs, good gas exchange.

5 Ventilation-perfusion coupling

The circulatory system must be coordinated with gas exchange at the alveoli.

3 Gas transport

Gas transport is the process of moving gases from the alveoli to the systemic tissues and returning them from these tissues to the lungs.

1 Oxygen

1 Hemoglobin

Most oxygen is carried by hemoglobin. Only about 1.5% of oxygen is dissolved in plasma. Each hemoglobin molecule carries four oxygen molecules. This is again because oxygen is poorly soluble in water. We can only deliver sufficient oxygen if it is concentrated on the hemoglobin molecules in red blood cells.

2 Oxyhemoglobin

If one or more oxygen molecules is bound to hemoglobin, it is referred to as oxyhemoglobin.

3 Deoxyhemoglobin

Hemoglobin with no oxygen.

2 Carbon dioxide

When carbon dioxide dissolves in water it quickly forms carbonic acid. Most carbon dioxide is carried in blood plasma as carbonic acid.

About 5% of carbon dioxide binds to the amino acid groups of hemoglobin.

About 5% of carbon dioxide remains as such dissolved in plasma.

4 Systemic gas exchange

1 Carbon dioxide loading

Loading refers to the movement of carbon dioxide into the blood. When referring to systemic gas exchange, this means the gas exchange to and from the blood in actively respiring tissues, for example contracting skeletal muscle. In this tissue, the process of generating ATP in the mitochondria generates one molecule of carbon dioxide for every molecule of oxygen used up in the process. As a result, there is a higher concentration of carbon dioxide in the tissue compared with the blood, and carbon dioxide flows from the tissue to the blood.

2 Oxygen unloading

The carbon dioxide released into the blood from respiring tissue turns into carbonic acid in the blood, lowering the pH. Hemoglobin releases oxygen in an acidic environment. Furthermore, respiring tissue is using up local supplies of oxygen and therefore is a gradient of oxygen, with high concentrations in the blood compared to tissues. Oxygen, therefore flows into respiring tissue.

5 Alveolar gas exchange revisited

The reactions of carbon dioxide and oxygen are opposite in the alveoli.

1 Carbon dioxide unloading

Blood arriving in the lungs from the venous system is high in carbon dioxide. The concentration of carbon dioxide is higher in the blood than the alveolar air so the carbon dioxide leaves the blood, enters the alveolar air and is exhaled.

2 Oxygen loading

The loss of carbon dioxide from the blood causes the pH to rise through the decrease in carbonic acid. As a result, hemoglobin increases its affinity for oxygen. Alveolar air is higher oxygen concentration compared with the blood (the blood has returned from respiring tissue where oxygen has been depleted). As a result, oxygen flows into the blood and attached to hemoglobin.

6 Adjustments to the metabolic needs of individual tissues

Hemoglobin unloads oxygen at different rates, depending upon the environment of the hemoglobin. Several factors determine hemoglobin unloading rates:

1 Ambient PO2

The amount of oxygen present in the respiring tissue: if there is a high rate of metabolism, there will be a low ambient oxygen partial pressure and hemoglobin will unload oxygen at a higher rate.

2 Temperature

Elevated temperature promotes oxygen unloading from hemoglobin. Makes sense: active tissues are warmer than resting tissues.

3 pH (The Bohr effect)

Active tissues are producing more carbon dioxide which forms carbonic acid, lowering the pH. Hemoglobin unloads more oxygen when the pH is low.

7 Blood gases and the respiratory rhythm

1 Hydrogen ions

Pulmonary ventilation is adjusted to maintain a relatively neutral pH of the brain.

Carbon dioxide in the cerebrospinal fluid drops pH.

Drop in pH (increase in hydrogen ion concentration) primarily due to increases in carbon dioxide (carbonic acid).

2 Carbon dioxide

Relatively minor effect due specifically to dissolved carbon dioxide.

3 Oxygen

Relatively little effect due to oxygen concentration in blood.

4 Respiration and exercise

This, and subsequent sections, for your information only.

4 Respiratory disorders

1 Oxygen imbalances

1 Hypoxemic hypoxia

2 Ischemic hypoxia

3 Anemic hypoxia

4 Histotoxic hypoxia

2 Chronic obstructive pulmonary disease

3 Smoking and lung cancer

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