gas exchange

• lungs are in chest cavity surrounded by the pleural membranes, which enclose an airtight space. This space contains a small quantity of fluid to allow friction-free movement as the lungs are ventilated by the movement of the diaphragm and ribs.
• Ventilation of lungs: replaces alveolar air with air from outside the body. This helps maintain the diffusion gradient.
• Bronchi: branch to form smaller bronchioles.
• Bronchioles are surrounded by smooth muscle, which can contract or relax to adjust the diameter of these tiny airways due to the absence of cartilage. During exercise, the muscles relax to allow a greater flow of air to the aleveoli.
• Cartilage: It is a connective tissue. It keeps airways in trachea and bronchi open and air resistance low, and prevents them from collapsing or bursting as the air pressure changes during breathing.
• cartilage is present in irregular plates AND incomplete rings in the bronchus but only incomplete rings in the trachea
• Epithelium: Air flows down lungs through trachea and bronchi which are lined by cells adapted to remove particles from air before it reaches the lungs. These cells make up a tissue called epithelium
• what are the 2 main types of epithelial cells?

  cilliated and goblet cells
• Ciliated cells: lined with tiny cytoplasmic extensions known as cilia. They are responsible for the continual beating of mucus towards the larynx.
• Goblet cells: found in between ciliated cells in large amounts. The upper part of a goblet cell is swollen with mucin droplets that are secreted by the cell. The mucus secreted by goblet cells traps pathogens which are then moved out with the help of the cilia.
• Mucus: a slimy solution of mucin, which is composed of glycoproteins with many carbohydrate chains that make them sticky and able to trap inhaled particles. It is also made by mucous glands (multicellular) beneath the epithelium. SO2 and NO2 can dissolve in mucus to form an acidic solution that irritates the lining of the airway
• Large number of alveoli are present to increase surface area.
• The wall of alveolus is very thin (single layer of squamous epithelium) to decrease the diffusion distance for efficient gas exchange.
• Outside the alveolus are capillaries also one cell thick. The steep concentration gradient of CO2 and O2 is maintained by blood circulation and breathing
• Elastic fibres allow the alveoli to stretch during inhalation and recoil during expiration. During maximum expansion, surface area for diffusion increases and the air is expelled efficiently during recoil.