THE RESPIRATORY SYSTEM Chapter 12 Memorise Understand Importance * Basic structures and function * Basic functions: thermoregulation, Medium level: 8% of GAMSAT Biology gas exchange, etc. questions released by ACER are related to * Protection against disease, particulate matter content in this chapter (in our estimation). * Breathing mechanisms: diaphragm, rib cage, * Note that approximately of the differential pressure; Henry’s Law 75% questions in GAMSAT Biology are related * Resiliency and surface tension effects to just 7 chapters: 1, 2, 3, 4, 7, 12, and 15. * The carbonic acid-bicarbonate buffer Introduction The respiratory system permits the exchange of gases with the organism's environment. This critical process occurs in the microscopic space between alveoli and capillaries. It is here where molecules of oxygen and carbon dioxide passively diffuse between the gaseous external environment and the blood. As you have done before, get familiar with the basics and then the practice questions will have you exercising your reasoning and graph-analysis skills which you can apply to many other GAMSAT-question types and topics. Multimedia Resources at GAMSAT-Prep.com Open Discussion Boards Flashcards Special Guest THE BIOLOGICAL SCIENCES BIO-227 12.1 Overview There are two forms of respiration: cel- • the filtration of incoming particles lular respiration which refers to the oxidation • to help control the water content and of organic molecules (see BIO 4.4 - 4.10) temperature (= thermoregulation) of the and mechanical respiration where the gases incoming air related to cellular respiration are exchanged to assist in speech production, the between the atmosphere and the circulatory • sense of smell, and the regulation of pH. system (O2 in and CO2 out). The respiratory system is composed of The respiratory system, which is con- the lungs and a series of airways that connect cerned with mechanical respiration, has the the lungs to the external environment, deliver following principal functions: air to the lungs and perform gas exchange. • providing a conducting system for the exchange of gases 12.2 The Upper Respiratory Tract The respiratory system can be divided space behind the nose, contains a ciliated into an upper and lower respiratory tract mucous membrane (= a form of respiratory which are separated by the pharynx. The epithelium) to entrap smaller particles and upper respiratory tract is composed of the prevent infection (this arrangement is com- nose, the nasal cavity, the sinuses, and the mon throughout the respiratory tract; for cilia Medium-level Importance nasopharynx. This portion of the respiratory see the Generalised Eukaryotic Cell, BIO system warms, moistens and filters the air 1.2). The nasal cavity adjusts the humid- before it reaches the lower respiratory sys- ity and temperature of incoming air. The tem. The nose (nares) has receptors for the nasopharynx helps to equilibrate pressure sense of smell. It is guarded by hair to entrap between the environment and the middle ear coarse particles. The nasal cavity, the hollow via the eustachian tube (BIO 6.2.3). 12.3 The Lower Respiratory Tract The lower respiratory tract is com- into smaller airways (→ 2o bronchi → 3o bron- posed of the larynx which contains the vocal chi → bronchioles → terminal bronchioles). cords, the trachea which divides into left and The terminal bronchioles are the most distal right main bronchi which continue to divide part of the conducting portion of the respira- BIO-228 CHAPTER 12: THE RESPIRATORY SYSTEM GAMSAT MASTERS SERIES Figure IV.A.12.1: Illustra- Medium-level Importance tion representing the lower respiratory tract including the Figure IV.A.12.2: Chest x-ray of an adult male smoker. dividing bronchial tree and Notice the coin-shaped shadow in the right lung which grape-shaped alveoli with presented with coughing blood. Further tests confirmed blood supply. Note that “right” the presence of a right lung cancer. Cancer-causing refers to the patient’s perspective which means the left side from your chemicals (carcinogens) can irritate any of the cells lin- perspective. ing the lower respiratory tract. tory system. Starting from respiratory bron- between the alveolar airspace and the capil- chioles → alveolar ducts → alveolar sacs until lary lumen. The blood-gas barrier is composed the level of the alveolus, these are considered of three layers: type I pneumocyte cells, fused the respiratory portion of respiratory system, basal laminae and the endothelium of capil- where gas exchange takes place. laries. Alveolar macrophages are phagocytes which help to engulf particles which reach the It is in these microscopic air sacs called alveolus. A surfactant is secreted into alveoli alveoli that O2 diffuses through the alveolar by special lung cells (pneumocytes type II). walls and enters the blood in nearby capillaries The surfactant reduces surface tension and (where the concentration or partial pressure of prevents the fragile alveoli from collapsing. O2 is lowest and CO2 is highest) and CO2 dif- fuses from the blood through the walls to enter Sneezing and coughing, which are the alveoli (where the partial pressure of CO2 reflexes mediated by the medulla, can expel is lowest and O2 is highest). Gas exchange particles from the upper and lower respiratory occurs by diffusion across the blood-gas barrier tract, respectively. THE BIOLOGICAL SCIENCES BIO-229 The lungs are separated into left and reduce friction while breathing. The lungs con- right and are enclosed by the diaphragm and tain the air passages, nerves, alveoli, blood the thoracic cage. It is covered by a mem- and lymphatic vessels of the lower respiratory brane (= pleura) which secretes a lubricant to tract. 12.4 Breathing: Structures and Mechanisms Inspiration is active and occurs accord- leaving it relatively negative; iv) the relatively ing to the following main events: i) nerve positive atmospheric pressure forces air into impulses from the phrenic nerve cause the the respiratory tract thus inflating the lungs. muscular diaphragm to contract; as the dome shaped diaphragm moves downward, the Expiration is passive and occurs ac- thoracic cavity increases; ii) simultaneously, cording to the following main events: i) the di- the intercostal (= between ribs) muscles and/ aphragm and the accessory respiratory mus- or certain neck muscles may contract further cles relax and the chest wall pushed inward; increasing the thoracic cavity (the muscles ii) the elastic tissues of the lung, thoracic mentioned here are called accessory respira- cage, and the abdominal organs recoil to their tory muscles and under normal circumstances original position; iii) this recoil increases the the action of the diaphragm is much more pressure within the lungs (making the pres- important); iii) as the size of the thoracic cavity sure relatively positive) thus forcing air out of increases, its internal pressure decreases the lungs and passageways. Medium-level Importance Figure IV.A.12.3: Breathing. BIO-230 CHAPTER 12: THE RESPIRATORY SYSTEM GAMSAT MASTERS SERIES 12.4.1 Control of Breathing Though voluntary breathing is possible anhydrase (about 75%). The reaction is (!), normally breathing is involuntary, summarised as follows: rhythmic, and controlled by the respiratory - + centre in the medulla of the brain stem. The CO2 + H2O ↔ H2CO3 ↔ HCO3 + H respiratory centre is sensitive to pH of the carbonic bicarbonate cerebrospinal fluid (CSF). An increase in acid blood CO2 or consequently, decrease in pH of the CSF, acts on the respiratory centre and According to Henry’s Law, the concen- stimulates breathing, returning the arterial tration of a gas dissolved in solution is directly pCO2 (partial pressure of carbon dioxide) proportional to its partial pressure. From the back to normal. The increase in blood CO2 preceding you can see why the respiratory and the decrease in pH are two interrelated system, through the regulation of the partial Medium-level Importance events since CO2 can be picked up by pressure of CO2 in blood, also helps in main- hemoglobin forming carbaminohemoglobin taining pH homeostasis (= a buffer). More (about 20%, BIO 7.5.1), but it can also be generally, the carbonic acid-bicarbonate buf- converted into carbonic acid by dissolving in fer is the most important buffer for maintain- blood plasma (about 5%) or by conversion ing acid-base balance in the blood and helps in red blood cells by the enzyme carbonic to maintain pH around 7.4. 12.4.2 Henry’s Law, Pop and The Bends Higher gas pressure and lower tem- pleasant, because of the loss of carbonic perature cause more gas to dissolve in a acid due to the release of carbon dioxide liquid. When a carbonated drink (soda/pop) bubbles/fizz. is manufactured, water is chilled, optimally to just above freezing, in order to permit the So pop is stored in a way to seal pres- maximum amount of carbon dioxide to dis- sure, preventing gas escape and maintain- solve. Then CO2 is pumped in at high pres- ing the supersaturation of CO2 in the solvent. sure, the pressure is maintained by closing It is pressure and temperature that drive the the container (can or bottle), which forces outgassing process. the carbon dioxide to dissolve into the liq- uid, creating carbonic acid (Le Chatelier’s Diving underwater exposes the body to principle; CHM 9.9) and giving ‘pop’ its tang. increasing pressure (PHY 6.1). A diving cyl- Flat soda tastes strange, or at least less inder (scuba tank) is used to store and trans- THE BIOLOGICAL SCIENCES BIO-231 port high