INTRODUCTION

1


INTRODUCTION




Chapter objectives


After studying this chapter you should be able to:




Introduction


The aim of this book is to provide an understanding of the respiratory system: its structure, function, and the diseases and conditions that may affect it. In attempting to do this we are adopting the philosophy of the new curriculum in medicine, which involves bringing to bear on a particular topic all the sciences relevant to that topic. To include in one book all that a student should know about the anatomy, histology, physiology, pharmacology and medicine of the respiratory system would result in a gigantic and intimidating tome. Equally unsatisfactorily, all these subjects could be treated superficially. We have adopted the policy of basing an understanding of the respiratory system on a full description of its physiology and anatomy, with specific topics of particular clinical importance being expanded upon in terms of clinical sciences.


For students to learn effectively, the material they must master should be broken down into manageable portions with a coherent theme: these are the chapters of this book, with each theme being based on a particular function of the respiratory system.


Students must also know what is expected of them, and each chapter is preceded by a list of aims and objective – things you should be able to do when you have mastered the material of that chapter. To provide experience of that bane of student life, examinations, each chapter contains questions of the type you might be asked at an undergraduate level.



What is respiration?


That depends on the context in which you use the word. Biochemists use it to describe the energy-producing chemical processes that take place in tissues, cells or even parts of cells. In this book we will use the physiologist’s definition, which is ‘An interchange of gases between an organism and its environment’. To all intents and purposes, for human beings this means ‘breathing’ (Latin, spiro, ‘I breathe’). The movement of air into and out of the lungs, which most people call breathing, is called by physiologists ventilation. Breathing is brought about by specific structures of the body, including (but not exclusively) the lungs. A description of these structures at a macroscopic (anatomical) and microscopic (histological) level helps us to understand the processes of the respiratory system and the disruption of these processes and structures (pathology) that brings about disease.


The part of our environment involved in this ‘interchange of gases’ mentioned above is of course the air around us, and our need for air must have been obvious to even our most distant ancestors. This need is recorded in some very ancient writings. For example, Anaximenes of Miletus (c. 570 bc) observed that air or pneuma (Greek, ‘breath’) was essential to life.


What was not clear to the ancients was what the air was used for. Aristotle, drawing on theories dating from the 5th century bc which noted the rapid and repeated movements of the heart, relegated the function of the lungs to a sort of radiator, and stated with his usual authority:



Galen (130–199 ad), probably more by an accident of metaphor rather than on any scientific evidence, came close to describing the true nature of respiration when he compared it to a lamp burning in a gourd:



If Galen had had the benefit of modern gas analysing facilities he would have found even closer parallels between breathing and burning, with oxygen (O2) being consumed and carbon dioxide (CO2) being produced in both cases.


The ‘bottom line’ of an account of the complicated process of respiration begins with a flow of



These two flows are the first and final results of the complex metabolism of the body, and this book describes the respiratory system that facilitates these flows.



The need for respiration


One definition of the success of a species of organism, in evolutionary terms, is how well it can maintain constant the composition of the fluid surrounding its individual cells (its internal environment) despite changes in its external environment (surroundings getting dryer, colder, warmer etc.). This process is called homeostasis and requires energy. Most of the energy generated by our tissues is the result of oxidation of food substrates, and this is the reason we need a flow of OXYGEN IN. Neophytes in physiology often emphasize the role of the respiratory system in providing this oxygen, and certainly an uninterrupted supply is important, particularly for the nervous system, but of more immediate importance is the removal of CO2. The word oxygen means ‘acid producer’ (Greek, oxy; acid; gen, to produce), and the major product of our oxidative metabolism is the acid gas CO2. The accumulation of CO2 would result in acidification of the body fluids. The importance of removing this CO2 can be demonstrated by rebreathing from a plastic bag for a few minutes. The unpleasant sensation that forces you to stop this rather dangerous experiment is due to over stimulation of the reflex that controls breathing to get rid of this gas. You will see later (Chapter 8) that CO2 produces its acidic effect by reacting with water to form carbonic acid.


Ventilation of the lungs would not fulfil the needs of the cells of our bodies if the results of this ventilation did not diffuse into the blood, which is then carried close to the cells of the body by the circulation.



Diffusion in respiration and the circulation


The flows of O2 and CO2 into and out of the body take place as a result of one very basic physical phenomenon: diffusion, which results in the movement of molecules in liquids and gases from regions of high concentration to regions of lower concentration. Because they are small, microscopic organisms such as the humble amoeba in its pond can rely on this phenomenon alone to carry O2 to and remove CO2 from its single cell. Multicellular creatures are too large to rely on diffusion alone: the distances gases would have to diffuse are too great, and the movement of gas therefore too slow to maintain life.


Although in human beings the same passive mechanism of diffusion alone supplies and removes these gases from our bodies (there is no active chemical transport), the phenomenon of diffusion is maximized by complicated respiratory and circulatory systems which accomplish what the pond water does for the amoebae in providing a supply of and a sink for these gases. The lungs promote diffusion by having an enormous surface area, which is very thin, through which diffusion can take place easily. A surface of over 90 m2 is enclosed in a lung volume of less than 10 L. This functional 90 m2 is often reduced in disease, by thickening of the membrane, excess fluid in the lungs, or by a reduction in the supply of air or blood. The circulation of the blood forms the transport link between the diffusion site of the lungs and the diffusion site of the capillaries within the tissues. The distances involved in this link are enormous in molecular terms, and diffusion would be totally useless to transport gas over the metre or so between the lungs and distal tissues of our bodies. This transport is accomplished in seconds by the circulation (Fig. 1.1).




Timing in the circulation and respiration


The processes of breathing and the beating of the heart are both cyclic events. One involves the inhalation of air and then its exhalation; the other involves filling of the heart with blood and then its ejection into the circulation. The time courses of these two cycles are very different: at rest you may take 12 breaths in the minute the heart beats 60 times, ejecting 5 L of blood through the lungs (see Fig. 1.2).


Related posts:

  1. CHEMICAL CONTROL OF BREATHING
  2. ELASTIC PROPERTIES OF THE RESPIRATORY SYSTEM
  3. LUNG FUNCTION TESTS: MEASURING DISABILITY
  4. CARRIAGE OF GASES BY THE BLOOD AND ACID/BASE BALANCE

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Jun 18, 2016 | Posted by in RESPIRATORY | Comments Off on INTRODUCTION

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