Introduction

   The process of gas exchange within the body is know as respiration and has three fundamental stages:

  • Pulmonary ventilation – also known as breathing, is the inhalation and exhalation of gas between the air outside and the alveoli of the lungs
  • External respiration – refers to the exchange of gases in the alveoli of the lungs and the blood in pulmonary capillaries, where the capillary blood gains O² and loses CO²
  • Internal respiration – is the gas exchange between blood in systemic capillaries and tissue cells.  This process can also be referred to as cellular respiration.

   Pulmonary ventilation is a result of alternating pressure differences, between the atmosphere and the alveoli of the lungs, due to contraction and relaxation of respiratory muscles.  To enable air to flow into the lungs, the pressure within the alveoli must be lower than that of the atmosphere outside.  This can be achieved by increasing the volume of the lungs.  

   The process of pulmonary ventilation is organized by the CNS via the spinal nerves.  These spinal nerves innervate the intercostals muscles and the diaphragm by fibers of the phrenic nerves which appear from the spinal cord at cervical levels C3, C4 & C5.  The output of these fibers, also known as motor neurons, is controlled by the medullary ryhthmicity area located at the respiratory center in the medulla oblongata in the brain.

   External respiration is the diffusion of O² from the alveoli of the lungs to the blood in capillaries and diffusion of CO² in the opposing direction.  In the lungs deoxygenated blood coming from the right side of the heart is converted into oxygenated returning to the left side of heart.  O² is uploaded through the pulmonary capillaries and CO² is unloaded into the alveolar air.

   Internal respiration is the gas exchange of O² and CO² between systemic capillaries and tissue cells throughout the whole body.  Here oxygenated blood is reduced to deoxygenated blood as O² leaves the bloodstream and enters the cells.  CO² then diffuses in the opposite direction from the cells and into the bloodstream.

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   The respiratory center is located in the medulla oblongata and pons section of the brain.  It consists of a group of neurons which transmit nerve impulses to the respiratory muscles allowing them to alter the size of the thorax.  Within the respiratory center are sensory neurons known as chemoreceptors which monitor the levels of CO², H+ and O².

   There are two types of chemoreceptors; Central chemoreceptors and Peripheral chemoreceptors.  Central chemoreceptors are found located in the medulla oblongata in the CNS and react to changes found in H+ concentrations or PCO² in the spinal fluid.  Peripheral chemoreceptors are ...

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