The Respiratory System

Question 1

Describe the structure of the nose and naval cavity

Answer 1

• the nose contains a large irregular-shaped cavity (the nasal cavity)
• the cavity is formed by a hollow in facial bones and the roof of the mouth
• the surface area of the mucous membrane lining the nasal cavity is increased by the presence of conchae (shelf-like projections from the lateral walls)
• the nasal cavity has 4 openings -> 2 eternal nares (nostrils) trough which air enters the air passages, 2 posterior nares which open into the pharynx

Question 2

Describe the function of the nose and naval cavity

Answer 2

• when the air enters the nose it is warmed, cleaned and filtered as it passes over the mucous membrane
  - -> this is composed of a highly vascular cilitiated columnar epithelium containing goblet cells
  - -> the vascular content of the epithelium warms and humidifies the air
  - -> larger particles of dirt and dust are trapped by the cilia,
  - -> while smaller particles are trapped by the mucus and wafted into the pharynx where they are removed by coughing or swallowing

Question 3

Describe the structure and function of the pharynx

Answer 3

• can be divided into 3 sections
• nasopharynx
  - -> lies behind the nasal cavity and extends down to the level of the soft palate of the mouth
  - -> it contains the openings to the middle ear (auditory tubes) which equalise air pressure between the middle ear and the external environment
  - -> it also contains the pharyngeal tonsils made of lymphoid tissue
• oropharynx
  - -> situated at the level of the mouth and contains two fold of lymphoid tissue (palatine tonsils)
  - -> it is separated from the oral cavity by the pillars of faucets and the uvula
• laryngopharynx
  - -> it is the lowest section of the pharynx and is continuous with both the oesophagus into which food is directed and the larynx through which air passes
• the pharynx is composed of mucous membrane and muscle
• it provides a passage for air and food and acts as a resonance chamber for sound
• the presence of lymph tissue provides a local source of antibodies for protection against infection

Question 4

Describe the structure and function of the larynx

Answer 4

• it is a short passageway that connects the laryngopharynx to the trachea
• it contains the vocal cords which produce sound
  - -> two folds of mucous membrane situated at the entrance of the trachea
  - -> air passing over them can cause them to vibrate producing sound waves
• within the walls of the larynx are 4 sections of cartilage
  - -> thyroid cartilage - also known as the Adam’s apple which is larger in males than females
  - -> cricoid cartilage
  - -> arytenoid cartilage - influences the length and tension of the vocal cords so altering the sound produced
  - -> epiglottis - a flap of elastic cartilage attached to the trachea which during swallowing closes the entrance to the trachea to prevent food or fluids from entering the respiratory passages

Question 5

Describe the structure and function of the trachea

Answer 5

• the trachea is a tube approx 120mm long situated in front of the oesophagus
• it is composed of 16-20 incomplete rings of cartilage stacked on top of the others
  - -> the cartilage prevents closure of the trachea
  - -> and allows slight expansion of the oesophagus during swallowing
• at the point where the trachea divides Ito form the two bronchi there is a ridge of highly sensitive tissue called the carina
  - -> this area is associated with the cough reflex, which prevents the entry of foreign material into the lungs

Question 6

Describe the structure and function of the bronchi

Answer 6

• the trachea bifurcates into two air passages called bronchi which lead to the lungs
• the right bronchus is shorter and wider than the left and leaves the trachea at a more vertical angle due to the position of the heart on th left side
  - -> as a result any inhaled particles are more likely to lodge in the right bronchus than the left
• the structure of the bronchi is similar to that of the trachea with incomplete rings of cartilage keeping the airways wide open (maintaining their patency)

Question 7

Describe the structure and function of the bronchioles

Answer 7

• the bronchi divide into progressively smaller passages
  - bronchioles
  - terminal bronchioles
  - respiratory bronchioles
  - alveoli ducts
• as these passages become smaller the rings of cartilage become replaced with smooth muscle
• the muscle is under autonomic nervous control allowing the diameters of the bronchioles to be altered to regulate the amount of air admitted into the lungs

Question 8

Describe the structure and function of the lungs

Answer 8

• the lungs are two cone-shaped structures that lie in the thoracic cavity
• they are separated by the mediastinum which contains the heart
• the lungs extend from above the clavicles superiorly, down to the diaphragm which divides the thoracic cavity from the abdominal cavity
• laterally the lungs extend to the rib cage
• the are enclosed in two layers of pleural membrane
  - -> the outermost membrane is attached to the wall of the thoracic cavity
  - -> the inner membrane covers the lungs
  - -> between the two layers is a potential space containing a lubricating fluid which allows limited movement of the lungs without friction

Question 9

Describe the macrostructure of the lungs

Answer 9

• the right lung is divided into three distinct lobes but the left only has two due to the space taken up by the heart
• the concave inferior section of each lung is known as the base
• the superior section as the apex
• and the area close to the mediastinum as the hiliium
• the bronchi, blood, lymphatic vessels and nerves enter and leave the lungs through the hiliium

Question 10

Describe the microstructure of the lungs

Answer 10

• the lungs are divided into lobules that contain collections of alveoli
• there are around 750 million alveoli in the lungs
• surrounding the alveoli is a network of capillaries
• both alveolar and capillary walls are composed of a single layer of epithelium through which oxygen and carbon dioxide can pass by diffusion
  - -> this is called gaseous exchange
  - -> it involves oxygen passing form the alveoli across into the capillaries to be transported to body cells and carbon dioxide passing in the opposite direction to be exhaled
• within the alveolar walls are cells that secrete a fluid to keep the alveoli cells moist
  - -> this fluid contains surfactant, it reduces surface tension and prevents the alveoli walls collapsing and sticking together

Question 11

What are the intercostal muscles?

Answer 11

• there are 11 pairs
• the 2 layers of each muscle are called the external and internal intercostal muscles
• the first rib is fixed to the skeleton so that when these muscles are stimulated to contract, each pulls the rib towards the one above
• due to the shape of the ribs this action results in movement outwards as well as upwards so the thoracic cavity increases in volume, drawing air into the lungs

Question 12

What is the diaphragm?

Answer 12

• it is large dome-shaped muscle that separates the thoracic and abdominal cavities
• it is attached to the lower ribs
• when stimulated to contract it becomes flattened which increases the volume of the thoracic cavity so drawing more air into the lungs

Question 13

Describe the blood supply to the respiratory system

Answer 13

• the right and left pulmonary arteries bring deoxygenated blood to the respiratory structures
• oxygenated blood is retuned to the heart via four pulmonary veins
• oxygenated blood needed for pulmonary cell function is provided by the right and left bronchial arteries that branch directly from the aorta
• most of this blood returns via the pulmonary veins but some drains into bronchial veins

Question 14

What are the 3 main functions of the respiratory system?

Answer 14

• ventilation -> movement of air into and out of the lungs
• external respiration -> gas exchange between the alveoli and the blood
• internal respiration -> gas exchange between the blood and body cells

Question 15

Describe the process of inspiration

Answer 15

• the diaphragm and intercostal muscles contract
  - -> this causes the rib cage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thoracic cavity
• the pleural membranes are attached to the inside of the thoracic cavity and the outside of the lungs so as the thoracic cavity expands so do the lungs
• as the volume of the lungs increases the pressure inside of them decreases to below atmospheric pressure
• this causes air to flow into the lungs

Question 16

Describe the process of expiration

Answer 16

• the diaphragm and intercostal muscles relax
  - -> this causes the rib cage to move downwards and inwards and the diaphragm to become curved again, decreasing the volume of the thoracic cavity
• the pleural membranes are attached to the inside of the thoracic cavity and the outside of the lungs so as the thoracic cavity reduces in volume so do the lungs
• as the volume of the lungs decreases the pressure inside of them increases to above atmospheric pressure
• this causes air to flow out of the lungs

Question 17

Describe the chemical control of ventilation

Answer 17

• central chemoreceptors on the surface of the medulla oblongata detect slight increase in CO2 levels resulting in nerve impulses being sent to the respiratory centre
  - -> this responds rapidly by altering the rate and depth of respiration
• peripheral chemoreceptors are found in the arch of the aorta and in the carotid bodies
  - -> here they respond rapidly to increases in CO2 levels by stein impulses to the respiratory centre
  - -> they also respond to changes in blood pH or in H+ and stimulate the respiratory centre to alter respiration to return pH to normal values
• both central and peripheral chemoreceptors detect decreases in oxygen levels but respond more slowly than to CO2

Question 18

What is pO2 and pCo2?

Answer 18

• partial pressure of oxygen is a measure of oxygen concentration, the greater the the concentration of dissolved oxygen in cells the higher the partial pressure
• similarly pCO2 is a measure of the concentration of CO2

Question 19

Describe the process of gaseous exchange

Answer 19

• gas is exchanged between the alveoli and the pulmonary capillaries via diffusion
• the pO2 of O2 is lower in the alveoli in comparison to the external environment which allows for diffusion of O2 into the alveoli
• O2 then diffuses out of the alveoli across the alveolar epithelium and the capillary endothelium and into haemoglobin in the blood
• the pCO2 of CO2 is higher in the capillaries than in the alveoli which allows which allows for diffusion into the alveoli from the blood, crossing the capillary endothelium and then the capillary epithelium where it is exhaled during expiration

Question 20

What happens to oxygen once it enters blood capillaries?

Answer 20

• oxygen enters blood capillaries at the alveoli in the lungs
• alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhemoglobin
• when cells respire they use up oxygen, this lowers the pO2
• RBC’s deliver oxyhaemoglobin to respiring tissues where it unloads its oxygen
• the haemoglobin then returns to the lungs to pick up more oxygen

Question 21

What happens to carbon dioxide once it has entered the capillaries?

Answer 21

• CO2 diffuses from the tissues into the capillaries
• it is taken up by red blood cells where it reacts with water to form carbonic acid
• this reaction is catalysed by the enzyme carbonic anhydrase
• carbonic acid is unstable and it dissociates to bicarbonate and hydrogen ions
• the bicarbonate diffuses out of the red blood cell into the plasma

Question 23

How does the respiratory system change during pregnancy?

Answer 23

• pregnancy increases the cell’s need for oxygen by up to 20%, there is a corresponding increase in the amount of carbon dioxide being removed, these demands are met by an increase in tidal volume (the amount of air moving into and out of the lungs during one ventilation)
• increased levels of progesterone lower the sensitivity of the peripheral and central chemoreceptors for CO2
  - -> this means that respiratory drive is stimulated at lower carbon dioxide levels so pregnant women breathe more deeply
  - -> as progesterone increases during the pregnancy the increased responsiveness to pCO2 results in an increased tidal volume and therefore minute volume
  - -> so hyperventilation is normal in pregnancy
• pregnancy also causes displacement of the rib cage, the diaphragm is elevated, the transverse diameter and circumference increases and the ribs flare out
  - -> changes are mediated by progesterone and relaxin which increase rib cage elasticity by relaxing ligaments
• progesterone also facilitates bronchial and tracheal smooth muscle relaxation, therefore reducing airway resistance
  - -> this improves air flow
• increased capillary engorgement due to larger blood volume and vasodilation result in hyperaemia and oedema of the upper respiratory mucosa which can cause nasal congestion
  - -> it can also lead to airway obstruction which can make intubation more difficult
• changes in respiratory function result in increased pO2 and decreased pCO2
  - -> this ensures adequate oxygenation to the fetus but also maintains an exaggerated CO2 gradient from fetus to mother to facilitate the transfer of CO2 from the fetus to the mother across the placenta ad subsequent expiration of CO2 from the maternal lungs