Gas exchange in humans- bio

• Gas exchange is the exchange of gases between an organism and the environment
• The human gas exchange system:

  1. External nostril
  2. Nasal passage
  3. Pharynx
  4. Larynx
  5. Cartilage
  6. Trachea
  7. Bronchi
  8. Bronchiole
  9. Alveoli
• The nose:

  • The walls of the nostrils have a fringe of hairs
  • The nostrils lead into two nasal passages, which are lined with  moist mucous membrane
• Advantages of breathing through the nose:

  • As air passes through the nasal passages, it is warmed and moistened
  • Dust and foreign particles in the air are trapped by the hairs in the nostrils as well as by the mucus on the mucous membrane
  • Harmful chemicals may be detected by small sensory cells in the mucous membrane
• From the nose to the trachea:

  • From the pharynx, air passes into your larynx and then into your trachea through an opening known as the glottis
• Trachea:

  • The trachea is supported by C-shaped rings of cartilage
  • The cartilage keeps the lumen of the trachea open
  • The membrane next to the lumen is the epithelium
• Epithelium cells:

  1. Goblet cells (secrete mucus to trap dust particles and bacteria)
  2. Ciliated cells (have hair-like structures called cilia on their surfaces, cilia sweep the dust-trapped mucus up the trachea)
• Bronchi and bronchioles:

  • The trachea divides into two tubes called bronchi
  • Each bronchi carries air into the lung. 
  • The bronchi are similar in structure to the trachea. 
  • Bronchioles are very fine tubes. 
  • Each bronchiole ends in a cluster of air sacs or alveoli
• How are the lungs adapted for efficient gas exchange?

  1. The numerous alveoli in the lungs provide a large surface area.
  2. The wall of the alveolus is only one cell thick. This provides a short diffusion distance for gases, ensuring a faster rate of diffusion.
  3. A thin film of moisture covers the surface of the alveolus. This allows oxygen to dissolve in it.
  4. The walls of the alveoli are richly supplied with blood capillaries. The flow of blood maintains the concentration gradient required for gas exchange.
• Gas exchange in the lungs occurs by diffusion
• Blood entering the lungs has a lower concentration of oxygen and a higher concentration of carbon dioxide than the inhaled atmospheric air entering the alveoli
• A concentration gradient for oxygen and carbon dioxide is present between blood and alveolar air
• Oxygen dissolves in the thin film of moisture on the wall of the alveolus
• The dissolved oxygen then diffuses into the blood capillaries
• Oxygen combines with the haemoglobin in the red blood cells to form oxyhemoglobin
• Carbon dioxide diffuses out of the blood capillaries into the alveolar cavity
• The oxygen and carbon dioxide concentration gradients between the alveolar air and the blood are maintained by

  • a continuous flow of blood through the blood capillaries
  • movement of air in and out of the alveoli as a result of breathing
• Transport of oxygen around the body:

  • in the lungs where the oxygen concentration is high, oxygen combined with haemoglobin.
  • This forms oxyhaemoglobin
  • When blood passes through an organ or tissue where the oxygen concentration is low, oxyhaemoglobin will release its oxygen to the respiring cells
• Inspiration or Inhalation: Taking in of air
• Expiration or Exhalation: Giving out of air
• The chest wall (thoracic wall) is supported by the ribs
• Ribs are attached dorsally to the vertebral column and ventrally to the sternum
• Humans have 12 pairs of ribs, but only the first 10 pairs are attached to the sternum
• Two sets of muscles, the external and internal intercostal muscles, are found between the ribs
• External and internal intercostal muscles are antagonistic
• When external intercostal muscles contract, internal intercostal muscles relax
• The thorax is separated from the abdomen by the diaphragm
• The diaphragm is made of muscle and elastic tissue
• When diaphragm muscles contract, the diaphragm flattens downwards
• When diaphragm muscles relax, the diaphragm arches upwards
• Intercostal muscles and the diaphragm work together to change volume and pressure in the thorax, resulting in ventilation of the lungs
• Inspiration or inhalation: When you inspire:

  • Your diaphragm muscle contracts and your diaphragm flattens.
  • Your external intercostal muscles contract, While your internal intercostal muscles relax.
  • Your ribs move upwards and outwards. Your sternum also moves upwards and forward.
  • The volume of your thoracic cavity increases.
  • Your lungs expand. This increases the volume in your lungs and decreases the air pressure inside them. Atmospheric pressure is now higher than the pressure within your lungs.
  • Air from the external environment is forced into the lungs.
• The stimulus for breathing is a high concentration of carbon dioxide in the blood or alveolar air, and not a lack of oxygen
• During vigorous muscular activity, the rate of aerobic respiration increases
• More carbon dioxide is released into the bloodstream
• The rise in carbon dioxide in the blood stimulates the respiratory centre in the brain
• Impulses transmitted from the brain to the respiratory organs bring about an increase in the rate of breathing
• This increase in breathing rate is to ensure that the excess carbon dioxide produced can be removed faster
• Breathing movements do not occur when there is too little carbon dioxide in the lungs