gas exchange in humans

• Fick's law states that the rate of diffusion of a substance across unit area is proportional to the concentration gradient.
• Respiration is a biochemical process by which energy is released from glucose or other energy sources.
• Ventilation is a sequence of breathing movements that moves gases to and from the internal gas exchange surface.
• Gas exchange occurs when oxygen passes from the atmosphere into the blood and carbon dioxide passes out by diffusion, these movements are independent of each other.
• Diffusion is the process where substances move from a high concentration to a low concentration.
• We need to breathe because we need oxygen for aerobic respiration, the rate at which cells require energy (ATP) varies and carbon dioxide needs to be removed.
• Active muscle cells, cells that produce useful substances and brain cells need lots of energy.
• Types of muscle cells include Cardiac, Skeletal, and Smooth.
• Carbon dioxide produces an acid solution as it gathers in the blood, lowering the ph of blood and cells.
• The ph of blood is 7.4 and variation upsets the ionic balance.
• The surface area of lungs is 70m2 and the human body has a low surface area to volume ratio (SA:V).
• The lungs are elastic and are moved by structures around them.
• Mechanism of breathing in humans is tidal, flowing in and out by the same route.
• A steep concentration gradient is maintained by blood flow on one side and ventilation on the other.
• The flow of air in and out of the alveoli is ventilation, there are two stages inspiration and expiration.
• The lungs are not muscular and cannot ventilate themselves so the whole thorax moves and changes size due to the action of two sets of muscles - the intercostal muscles and the diaphragm.
• Inspiration involves the diaphragm contracting and flattening, external intercostal muscles contracting, internal intercostal muscles relaxing, and the rib cage moving upwards and outwards, causing an increase in cavity volume and a decrease in pulmonary pressure.
• Expiration involves the diaphragm relaxing, external intercostal muscles relaxing, and internal intercostal muscles contracting, causing the rib cage to move downward and inward, causing a decrease in cavity pressure and increasing pressure, forcing air out of the lungs.
• Forced expiration involves the abdominal muscles contracting, pushing the diaphragm upwards, and internal intercoastal muscles contracting, pulling the ribs downwards, giving a larger and faster expiration, used in exercise.
• Efficient gas exchange surfaces have large surface areas, short diffusion pathway and a large concentration difference on either side of a surface.
• Alveoli are covered with many blood capillaries.
• The alveolar epithelial cells are flattened with a thin layer of cytoplasm in between their cell surface membranes.
• The capillary walls only contain very thin cells.
• Red blood cells touch the walls of the capillaries minimising the distance gases have to diffuse.
• The walls of alveoli are fully permeable allowing oxygen and water to pass into the cells.
• The inner surface of the alveolus is covered with a thin layer of water.
• The rate of diffusion of gas in water is lower than in air, slowing the rate of diffusion.
• The mechanisms of breathing movements are transmitted to the lungs via the pleural sac surrounding each lung.
• The outer membrane of the pleural sac is attached to the thorax and the inner membrane is attached to the lungs.