Respiration
For cells to perform their vital functions, energy is required. Energy is produced by a breakdown of organic compounds, mainly carbohydrates. Oxygen is required for this task and carbon dioxide and water are also produced in addition to the energy output. This process is known as Respiration. Respiration occurs in all living organisms and occurs in every single cell within that organism because it is the only way to obtain usable energy.
The process of respiration is summarised by this equation:
Although this equation looks a simple process, it most certainly is not. For this to happen, many other things must have already taken place. However, this equation is accurate enough to show the main things required in respiration.
Oxygen is taken from the air by a process known as breathing. Air is sucked into the lungs by a decrease in pressure in the thorax. This comes about by a large muscle called the diaphragm positioned at the bottom of the thorax moving down and the intercostal muscles moving out. This creates a larger area in the thorax and hence the change in pressure.
Air travels through the nose or mouth and enters the throat. Next the air travels into the windpipe through a hole called the glottis. The glottis is covered with a flap of skin strengthened with gristle called the epiglottis. The epiglottis is to stop food from entering the windpipe. The windpipe, or trachea, is a straight tube about 12cm long. It is kept open at all times by rings of gristle. These rings of gristle are designed as horseshoes to allow the trachea to move in all directions whilst always maintaining an open airway. The trachea is covered with a mucus which is designed to catch small bits of bacteria which have escaped the nasal cavity and stop them damaging the lungs.
Once the trachea enters the chest, it splits into two smaller tubes called bronchi with one bronchi leading to each lung. Within each lung the bronchi split into many branches which also split into many branches forming the bronchiole tree. Each branch is called a bronchiole. Bronchioles get smaller towards their ends and their walls are strengthened with rings of gristle to maintain easy passage for air. The end of each Bronchiole leads to a group of small air sacs called alveoli.
Each alveolus is surrounded by a network of capillaries. Each capillary is in very close contact with an alveolus so oxygen and carbon dioxide can easily travel between the two areas. Gaseous exchange takes place in the alveoli. Alveolar surface area is kept to a maximum as much as possible to facilitate gaseous exchange. For this reason, within one air sac, there are many horseshoe like alveoli ready and waiting to exchange carbon dioxide for oxygen. In a condition known as emphysema, these horseshoes stretch out minimising surface area and making the sufferer short of breath.
Blood flowing through the lungs' vast capillary network has a higher concentration of carbon dioxide and a lower concentration of oxygen than the 'fresh' air just breathed into a particular alveolus. This creates a diffusion gradient which causes the two different gases to diffuse in opposite directions. Therefore as blood passes an alveolus, oxygen diffuses into it and carbon dioxide diffuses out so that the gas concentrations in the alveolus and the blood vessel are equal.
When the oxygen diffuses out of the alveolus, is also diffuses through the membranes of red blood cells and forms a compound with the pigment haemoglobin called oxyhaemoglobin. Each red blood cell can carry up to 250 million molecules of haemoglobin which allows up to 1000 million molecules of oxygen to be carried by each red blood cell. The red blood cell's lack of a nucleus facilitates this by providing more space to 'carry' haemoglobin molecules. Red blood cells are also shaped like a biconcave disc which provides the most surface area for oxygen transport. The compound between the two molecules is a loose one, quickly initiated in the capillaries surrounding the alveolus and quickly broken in the tissues.
When the oxygen, which is attached to the haemoglobin at this stage, reaches a cell that requires oxygen, there is once again a diffusion gradient and the oxygen diffuses out of the blood and into the tissues whilst carbon dioxide diffuses into the blood. The carbon dioxide forms another compound with the haemoglobin called carboxyhaemoglobin and returns to the lungs via the heart to once more be exchanged for oxygen.
Energy in the tissues is produced by the breakdown of glucose. Energy from glucose is not released instantly. If this was so, a slice of pizza would more than likely raise a person's body temperature by 10ºC instantly thus killing the person. There are many stages to the oxidisation of glucose. The initial stage of respiration occurs in the cytoplasm and is called glycolysis. Glucose is broken down into an acid called pyruvic acid. The next stage of energy production depends on the availability of oxygen. Without oxygen, lactic acid is produced and this is called anaerobic respiration lactic acid is a mild poison and must be removed from the tissues as soon as possible. When an athlete runs a race, lactic acid builds up in the muscles, if he does not pant heavily after the race and supply oxygen to the muscles his legs will ache terribly. The amount of oxygen needed to neutralise the lactic acid is called the oxygen debt. If oxygen is present, the pyruvic acid enters the mitochondrion and is broken down into carbon dioxide and water and energy, in the form of adenosine triphospate or ATP, is released. This compound is not in itself energy, but is able to be transferred to tissues such as muscles and make them contract. Raw glucose is unable to do that whereas ATP can.
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