UNEC Dr Ossai's Respiratory physiology [1, 1 ed.] 9789785746761

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INTRODUCTION TO RESPIRATORY PHYSIOLOGY

PULMONARY VENTILATION

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TESTS OF PULMONARY FUNCTION

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PULMONARY CIRCULATION

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EXCHANGE & TRANSPORT OF RESPIRATORY GASES

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HYPOXIA AND OXYGEN THERAPY

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REGULATION OF RESPIRATION

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CHAPTER 8 ACCLIMATIZATION TO HIGH ALTITUDE

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PREFACE

On one auspicious day at work, a student walked into my office, the Exam Office, and made a rather striking comment. She said my lectures in class were quite easy to understand and interesting, but the note I gave them was more difficult to understand. It was a most valuable feedback, and I decided to do something about it as quickly as possible. This book in your hand is born out of that decision, to make the written text of physiology lectures as simple and interesting as a good lecture in class. Furthermore, most popular and available physiology textbooks are for reference purposes — they go far beyond the scope of knowledge required for most undergraduate students, and they are adapted for international readership. Most average students of physiology in our environment find the technical English language and medical jargons presented in those books somewhat confusing. This book removes that barrier, presenting the fundamentals in a very simplified yet elaborate manner. I have no doubt that this book will greatly make a positive difference in the lives of students, especially at the undergraduate level.

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ACKNOWLEDGEMENT

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DEDICATION

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INTRODUCTION TO RESPIRATORY PHYSIOLOGY LEARNING OBJECTIVES After studying this chapter, you should be able to:

Know the purpose of respiration and differentiate between internal and external respiration. List the passages through which air passes from the exterior to the alveoli, and describe their basic functional structure Mention and describe the non-respiratory functions of the lungs. Know the respiratory protective reflexes and their respective components. Definition of Terms

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Tracheobronchial tree: Made up of the trachea, and the bronchi and bronchioles which branch off of it. It is so called because it resembles the appearance of a tree with branches, the trachea being the stem.

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Ventilation: The processes whereby fresh air moves in and around an enclosed space, in this case, the lungs.

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Alveoli: Aka air sacs or alveolar sacs. They are the functional units of the lungs where gaseous exchange occurs.

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Pneumocytes: They are the cells that line the alveoli, and there are two types. 07

Key words: Inspiration, expiration, external environment, ventilation, internal environment, medulla oblongata

Overview Energy is the currency of life. What this means is that all the physiological processes that take place in your cells depend on energy in one way or another, especially for transportation, synthesis, and breakdown of various chemical substances. This energy is locked up in the food we eat (represented by glucose), and is mainly released when oxygen (O2) acts on it. This invariably means that for us to be alive, the body needs a constant supply of oxygen, which is achieved by breathing in (inhalation or inspiration) air rich in oxygen and breathing out (exhalation or expiration) air rich in carbon dioxide. C6H12O6 (glucose) + 6O2 (oxygen) (water) + Energy (ATP)

6CO2 (carbon dioxide) + 6H2O

From the equation above, we can see that CO2 and H2O (in form of water vapour) are the by-products of the reaction, that's why they are expired from the lungs; but the energy remains, to be used in the body. So, it is clear at this point that the goal (purpose) of respiration is the production or liberation of energy. Respiration has two meanings: (1) the utilization of oxygen in the liberation (release) of energy from organic molecules in the cells, termed internal or cellular respiration. This is the part we just explained (2) the exchange of oxygen and carbon dioxide between an organism and the external environment termed external respiration. The first part is mostly the concern of Biochemists. The second part is what we will focus on in Respiratory Physiology.

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Components of the Respiratory System The respiratory system is made up of: 1.

A gas exchange organ (the lungs) responsible for taking O2 from the external environment into the internal environment, and CO2 from the internal environment to the external environment.

2. A pump responsible for ventilating the lungs, which consists of:

i.

The chest wall, which is mainly made up of the ribs and the respiratory muscles attached to them, which not only protects the lungs, but helps them expand during ventilation.

ii. Respiratory centres in the brain, that control the muscles. iii. Tracts or nerves that connect the brain to the muscles. Therefore, our discussion is going to centre on the anatomy of the lungs, pulmonary ventilation, pulmonary exchange of gases, transport of respiratory gases, and regulation of respiration.

Functional Anatomy of the Respiratory System As we already know, structure determines function; hence, by the unique way in which the respiratory system is designed, you could tell that its primary purpose is for gas conduction and gas exchange, which is represented by the conducting zone and the respiratory zone respectively as shown in the fig. 1.2. Broadly speaking, the respiratory tract is divided into:

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1. Upper respiratory tract (URT), consisting of the mouth, nostrils, pharynx and larynx. They are majorly concerned with warming and humidifying the air that passes through them. They have other non-respiratory functions as discussed later. 2. Lower respiratory tract (LRT), consisting of the trachea, bronchi, bronchioles, alveolar ducts, and alveoli (air sacs). The early part of the LRT is concerned with conduction of the warm and humidified air, while the distal part is concerned with the exchange of gas

The trachea, which is surrounded by cartilage, divides into two bronchi which enter the two lungs. Each of those two bronchi keep dividing and dividing into many smaller branches in the lungs, and in the process, gradually losing their surrounding cartilage. It gets to a point where they completely lose their cartilages, and they are now known as bronchioles. The division continues, and gradually, alveoli start appearing on the bronchioles, and they are now known as respiratory

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bronchioles. The last bronchioles without alveoli are known as terminal bronchioles. The tracheobronchial tree has 23-25 divisions or generations before it gets to the alveoli. The 1st 16 generations of bronchi and bronchioles are collectively known as the conducting zone because they contain no alveoli. The remaining 7-9 generations are collectively known as the respiratory zone because they contain alveoli. The alveoli are actually the functional units of the lungs, and they are lined by two types of cells (type 1 and type 2 pneumocytes).

Key Insight:

As the cartilage is progressively lost in the

tracheobronchial tree, they are replaced by smooth muscle; hence, the bronchioles are entirely made up of smooth muscles which are under the control of the autonomic nervous system, making them contract or relax, leading to bronchoconstriction or bronchodilation!

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Non-Respiratory Functions of the Respiratory System Apart from the primary function of exchange of respiratory gases between the internal and external environment, the respiratory system also has several other secondary functions as we shall now explore: 1. Olfaction: This has to do with the ability to detect odours. The nostrils are lined by mucous membranes which contain olfactory receptors that perform this function. 2. Phonation: This is the ability to produce sound through the vibration of the vocal cords located in the larynx (aka voice box). 3. Particle filtration: The nostrils also secrete a lot of sticky mucus and contain so many tiny hairs that help to trap and filter dust particles, preventing them from finding their way into the lower respiratory tract. 4. Regulation of acid-base balance: Lungs play a r o l e i n maintenance of acidbase balance of the body by regulating the carbon dioxide content in blood. What do we really mean by this? Acids are substances that give out hydrogen ions, and bases are substances that accept hydrogen ions. However, a buffer is a substance that can both accept and give out hydrogen ions depending on the situation. So, what happens is that as carbon dioxide is produced in tissues, it enters the blood and combines with water to yield carbonic acid. Because carbonic acid is a weak and unstable acid, it immediately dissociates into hydrogen ion and bicarbonate. Co2 + H2O

H2CO3

H+ + HCO3–

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Entire reaction is reversed in lungs when carbon dioxide is removed from blood into the alveoli of lungs. H+ + HCO3–

H2CO3

CO2 + H2O

What really happens is that whenever the pH of the blood reduces (increased acidity), either due to increased production of carbon dioxide in the tissues, or from any other source, chemoreceptors located in some big arteries of the body sense it and send the signal to the brain. The brain then causes increased ventilation, resulting in more carbon dioxide being exhaled through the lungs and bringing the pH back to normal.

Key Insight:

Increased acidity means increased hydrogen ion.

Increased ventilation means more trapping of that hydrogen ion to bicarbonate to form carbon dioxide and water, both of which are exhaled! 5. Maintenance of water balance: As is already apparent, water (in form of water vapour) is part of the by-products that is exhaled during breathing. Thus, the more the water in the body, the more it is exhaled through the lungs. 6

Regulation of temperature: Along with water vapour, heat is also lost through the respiratory tract when we exhale.

7. Defence functions of the lungs: The lungs is exposed to the outside world through the nostrils, hence, the lungs is equipped with specialized macrophages called alveolar macrophages that destroy offending organisms that may try to gain entry into the body. 8. Endocrine functions: The lungs secretes angiotensin converting enzyme (ACE) which converts angiotensin I to angiotensin II, which then stimulates the production of 13

aldosterone, a mineralocorticoid hormone that helps in regulating salt and water content. 9. Metabolic functions: The lungs synthesize a host of chemical substances that act locally within the lungs itself, but when they are produced in excessive quantities, they could diffuse into the bloodstream and have systemic effects. Such substances include histamine, serotonin, prostaglandins, bradykinins etc.

Respiratory Protective Reflexes Reflexes are mainly there to protect us automatically without our conscious efforts. And they comprise five general components – receptor, afferent nerve, control centre, efferent nerve, and effector. The three reflexes related to the respiratory system are the cough, sneezing and swallowing reflexes. COUGH REFLEX

Coughing is a very common phenomenon that can either happen spontaneously or voluntarily, and it involves a very explosive expiration through the mouth. The main aim of the cough reflex is to remove any foreign particle or agent that may have escaped the trapping and filtering action of the nasal mucus and hairs. Hence, cough is produced mainly by irritant agents. It is also produced by several disorders such as cardiac disorders (congestive heart failure), pulmonary disorders (chronic obstructive pulmonary disease – COPD). ¾ Receptors: The receptors for the cough reflex are found in several locations in the respiratory tract, but mainly in the trachea and bronchi, especially at the carina (the point at which the trachea divides into the bronchi). They are less present in the more distal airways, and totally absent beyond the respiratory bronchioles. 14

¾ Afferent pathway: Afferent nerve fibres pass via the internal laryngeal nerve, which is a branch of the superior laryngeal nerve, which is a branch of the vagus nerve. Other nerves are less involved, but they include trigeminal, glossopharyngeal and phrenic nerves.

¾ Control centre: The centre for cough reflex is in the medulla oblongata.

¾ Efferent pathway: Efferent nerve fibres arising from the medullary centre pass through the vagus, phrenic and spinal motor nerves.

¾ Effectors: The efferent nerves activate the primary and accessory respiratory muscles. Mechanism: The phrenic nerve which supplies the diaphragm (chief inspiratory muscle) is activated by the reflex pathway as already described. This causes a deep inspiration. The glottis then closes (effect of the recurrent laryngeal nerve). This is immediately followed by the contraction of the abdominal muscles leading to a forced expiration against an initially closed glottis. This increases the intrapleural pressure above 100 mmHg. Then, the glottis opens suddenly with an explosive outflow of air at a high velocity. Velocity of the airflow may reach 960 km/hr. It causes expulsion of irritant substances out of the respiratory tract. SNEEZING REFLEX

Sneezing reflex is very similar to the cough reflex as both achieve similar purposes. However, in the case of the sneezing reflex it is provoked by irritant agents entering the nasal mucosa, and the 15

explosive expiration is through the nose instead of the mouth like the cough reflex. However, it can also be triggered by a breeze of cold air, and exposure to sudden bright light. ¾ Receptors: The receptors for the sneezing reflex are the olfactory receptors in the nostrils. ¾ Afferent pathway: Afferent nerve fibres pass via the trigeminal and olfactory nerves. ¾ Control centre: The centre for cough reflex is in the medulla oblongata. ¾ Efferent pathway: Efferent nerve fibres arising from the medullary centre pass via trigeminal, facial, glossopharyngeal, vagus and intercostal nerves. ¾ Effectors: The efferent nerves activate the pharyngeal, tracheal and respiratory muscles. Mechanism: The irritation of the nasal mucosa triggers the release of histamines, which in turn irritates the trigeminal and olfactory nerves. The efferent nerves cause a deep inspiration, followed by forceful expiratory effort with opened glottis resulting in expulsion of irritant agents out of respiratory tract.

Ponder: Sneeze reflex and cough reflex have similar purpose. But they still have a slight difference in purpose. What is that? SWALLOWING (DEGLUTITION) REFLEX

Swallowing reflex is a respiratory protective reflex that prevents entrance of food particles into the air passage during swallowing. It will be discussed in full under the digestive system.

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Answer to Ponder Ponder: Sneeze reflex and cough reflex have similar purpose, but they still have a slight difference in purpose. What is that? Answer: The simple difference between the two is that, while sneeze reflex helps to protect the upper respiratory tract, the cough reflex helps to protect the lower respiratory tract.

1)

2)

3)

4)

5)

MULTIPLE CHOICE QUESTIONS Which of the following describes the correct order of structures in the respiratory passages? A. Pharynx, larynx, trachea, bronchioles, bronchi B. Larynx, trachea, pharynx , bronchioles, bronchi C. Pharynx, trachea, larynx, , bronchioles, bronchi D. Pharynx, larynx, trachea, bronchi, bronchioles The following are functions of the conducting zone except: A. Exchange of respiratory gases B. Warming of air C. Secretion of mucus D. Transport of air One of the following respiratory structures has the smallest diameter: A. Bronchiole B. Larynx C. Pharynx D. Trachea Which of the following is not part of the physiologic unit of the lung? A. Alveoli B. Alveolar sac C. Alveolar duct D. Segmental bronchiole What is the goal of respiration:

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6)

7)

8)

9)

10)

A. Extraction of oxygen from atmosphere B. Expulsion of Co2 from the body C. Exchange of gases between blood stream and tissue cells D. Generation of energy in the form of ATP Which of the following is considered the functional unit of the lung? A. Alveolar sac B. Alveoli C. Hemithorax D. Segmental bronchi During quiet inspiration: A. Internal intercostal muscles and diaphragm are normally active B. External intercostal muscles and diaphragm are normally active C. External and internal intercostal muscles are normally active D. Internal intercostal muscles, external intercostal muscles and diaphragm are normally active One of the following functions in filtering and keeping the mucus and dirt away from the lungs: A. Cilia B. Bronchioles C. Hairs in the lungs D. All of the above Type II pneumocytes A. Make up most of the area of the alveolar epithelium B. Are the main source of lung surfactant C. Are squamous and ciliated D. Constitute 95% of alveolar cells Concerning larynx: A. It houses the vocal cords B. It prevents the invading pathogens into the trachea C. It is made of cartilage and connects the pharynx to the trachea D. All of the above

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11)

The upper respiratory tract consists of the following EXCEPT: A. Larynx B. Bronchi C. Pharynx D. Mouth 12) The tracheobronchial tree has how many generations? A. 16 B. 48 C. 32 D. 23 13) The following are non-respiratory functions of the respiratory tract EXCEPT: A. Vocalization B. Regulation of acid-base balance C. Secretion of melatonin D. Olfaction

1)

ANSWERS TO MULTIPLE CHOICE QUESTIONS D- The correct order of structures of the respiratory tract is; nose, pharynx, larynx, trachea, bronchi and lungs.

2)

A- The walls of conducting zone are not involved in exchange of gases because of the thickness of their walls.

3)

A- The bronchioles are air passages