3.2 - Human Gas Exchange

• Gas exchange takes place in the human thorax, this is a collection of organs and tissues in the chest cavity.
• The trachea is the airway that leads from the mouth and nose to the bronchi. It is lined with mucus - secreting goblet cells and cilia. The cilia sweep microorganisms and dust away from the lungs.
• Humans have two lungs, both of which are a central part of the respiratory system and are where gas exchange takes place.
• The bronchi (right and left) are at the bottom of the trachea and are similar in structure, but narrower, the bronchi lead to bronchioles.
• Bronchioles are narrow tubes that carry air from the bronchi to the alveoli, as they are so narrow they have no supporting cartilage and can collapse.
• The alveoli are the main site of gas exchange in the lungs, they are tiny sacs with many structural adaptations to enable efficient gas exchange, such as their thin walls and large surface area to volume ratio.
• The capillary network is an extensive network of capillaries that surround the alveoli and are an exchange surface between the lungs and blood. During gas exchange oxygen diffuses from the alveoli and into the capillaries, while carbon diffuses the other way and is exhaled.
• Tracheal rings are made of cartilage and help to support the trachea and ensure it stays open.
• Ciliated epithelium is a specialised tissue found along the trachea down to the bronchi. Each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs.
• Goblet cells are mucus-producing cells that secret mucus which traps dust, bacteria and other microorganisms preventing them from reaching the lungs. Goblet cells can be found scattered throughout the ciliated epithelium in the trachea. The mucus is swept along by the cilia of the ciliated epithelium upwards and is swallowed. The mucus and any other microorganisms are then destroyed by the acid in the stomach.
• Explain how the lining of the trachea, bronchus and bronchioles provide protection against pathogens.

  Cilia beat to move mucus away from the lungs and up towards the mouth. Mucus is produced by goblet cells and acts as a barrier to pathogens from entering the ciliated epithelium. The mucus also traps bacteria and microorganisms, while blood vessels bring macrophages to engulf any pathogens in phagocytosis
• The alveoli has a lining of thin and squamous epithelium, that allows for gas exchange. The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gas.
• Smooth muscle is found throughout the walls of the bronchi and bronchioles, it helps regulate the flow of air into the lungs by dilating when more air us needed and contracting when less air is needed.
• The extensive network of capillaries surrounding the alveolus allows CO2 to diffuse out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from the alveoli and into the capillaries to be carried around the body.
• Capillaries have a diameter of around 3-4 micrometers, which is only wide enough for one red blood cell to travel through at any time. This ensures that there's a sufficient time and opportunity for gas exchange to occur.
• The key structures of mammalian lungs are the trachea, bronchi, bronchioles (smaller structures can be hard to distinguish in a dissected lung).
• The key structures that can be seen from the dissection of bony fish gills are the gill arch and filaments.
• How optical microscopes work;

  • Light is directed through the thin layer of biological material that is supported on a glass slide
  • This light is focused through several lenses so that an image is visible through the eyepiece
  • The magnifying power of the microscope can be increased by rotating the higher power objective lens into place
• The exchange of oxygen and carbon dioxide occurs between the alveoli and the capillaries in the lungs.
• Simple diffusion is the passive movement of substances from an area of high to low concentration.
• • The air in the alveoli contains a high concentration of oxygen. The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration
  • The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide. The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled
• Gas exchange in the lungs requires a concentration gradient.
• Ventilation in the lungs and the continuous flow of blood in the capillaries helps ensure that there is always a higher concentration of oxygen in the alveoli than in the blood.
• • The oxygen in the alveoli diffuses into the red blood cells which are rapidly carried away in the blood and replaced by oxygen-depleted red blood cells.
• Passage of Air

  1. Nose / mouth
  2. Trachea (windpipe)
  3. Bronchi
  4. Bronchioles
  5. Alveoli
• Breathing In - the breathing-in process causes the volume in the chest to increase and the air pressure in the lungs to decrease until it is slightly lower than the atmospheric pressure. As a result, air moves down the pressure gradient and rushes into the lungs.
• Breathing-In Mechanism when at rest - the diaphragm contracts and flattens, increasing chest volume.
• Breathing-In Mechanism when exercising - The diaphragm contracts and the external intercostal muscles contract, causing the ribcage to move upwards and outwards.
• • Breathing-In mechanism summary; External intercostal muscles contract, ribcage moves up and out, diaphragm contracts and flattens, volume of the thorax increases, pressure inside the thorax decreases, air is drawn in.
• Exhalation mechanisms - when at rest breathing out, the volume in the chest decreases and pressure increases, causing air to be forced out.
• Exhalation mechanisms when at rest;

  • External intercostal muscles relax
  • The recoil of elastic fibres surrounding alveoli causes the air to be forced out
  • Diaphragm becomes dome-shaped
• Exhalation Mechanism when exercising:

  • Internal intercostal muscles contract to pull the ribs down and back
  • Abdominal muscles contract to push organs upwards against the diaphragm, increasing the internal pressure
  • This causes forced exhalation
• Tidal Volume (TV) = amount of air breathed in/out per breath
• Exhalation mechanism summary; External intercostal muscles relax, ribcage moves down and in, diaphragm relaxes and becomes dome-shaped, volume of thorax decreases, pressure inside the thorax increases, air is forced out.
• • The breathing rate is the number of breaths taken per minute
• Calculating the Pulmonary Ventilation Rate

  • The volume of air breathed (in OR out) in one minute is known as the pulmonary ventilation rate
  • PVR = tidal volume x breathing rate
• Factors affecting pulmonary ventilation rate include age, sex, exercise level, body size, temperature, humidity, altitude, smoking habits, lung disease, pregnancy, drugs, alcohol consumption, stress levels, emotions, sleep patterns, posture, physical activity, work environment, diet, climate, pollution, allergies, infection, hormones, genetics, etc.
• There are several diseases that can prevent the lungs from functioning properly, these include;

  • Lung cancer
  • COPD (Chronic obstructive pulmonary disease)
• Lung cancer occurs when mutations affect the regulation of mitosis in cells, tumours develop which are a mass of cells, often in the lumen of the airways. The tumour becomes larger and will interfere with the function of lungs by squeezing against blood vessels or entering the lymphatic system.
• COPD includes a range of lung-based diseases. It is often when goblet cells becomes enlarged and produce excess mucus, destroying the cilia in the trachea and preventing them from sweeping mucus away from the lungs, meaning the bronchioles can become blocked.