Thursday, September 23, 2010

6.19 Respiration - Air and Gas We Breath

Physician's Notebooks 6  - - See Homepage

19. Respiratory and Lung Function and Disease (Update 24 Jan. 2018)

The purpose of respiration is to get oxygen into the red blood cell from the inhaled air and get rid of the blood cell's carbon dioxide. In the process, other substances in the air, including toxic pollutants get breathed and absorbed and other substances in the body are partly excreted via exhalation, chiefly water as vapor and carbon dioxide which also regulates acidity in the body. Then, too, breathing excretes volatile toxins like ethyl alcohol. And germs from air enter or leave the body and may infect and even kill (Anthrax letters).

Air and Gases We May Breathe
Air we breathe at sea-level is weighing down on us at atmospheric pressure that causes a standard column of quicksilver mercury (Hg) to rise 760 mm Hg, or 760 torr (aka 1 atmosphere, the unit of air pressure named after the scientist Evangelista Torricelli). It is a mix of 21% oxygen (O2), 78% nitrogen (N2), and other trace gases, chief of which is carbon dioxide (CO2).
   At sea-level, the 21% O2 in air exerts a partial (in the total air mix) pressure (PO2) of c.160 mm Hg at opening of mouth and nostrils. But after the air is breathed in and reaches the air exchange sacs (alveoli) deep in lungs, the effective PAO2 is c. 105 mm Hg due to mix with already breathed air and other alveolar pressure loss. Thus, a PAO2 105 mm Hg is max for breathed air at level of air exchange with pulmonary arterial blood and it translates to a max PaO2 (partial pressure of oxygen in the pulmonary arterial blood) of 95 mm Hg at sea level in a healthy person. However, as one goes up into heights in mountains or aircraft, or goes down into depths scuba diving or as worker underwater, the percent and partial pressure of oxygen in breathed air and therefore in the blood will change based on the change in partial pressure of oxygen in the breathed air.
   Air comes into lungs and oxygen gets used but what about the atmospheric nitrogen gas? It is dissolved in the blood and gets exhaled unchanged at the same rate it is inhaled. But when diver goes deep in ocean and then too suddenly ascends (goes upwards), the less soluble nitrogen in fluid blood, under sudden lowered pressure, may come bubbling out as gas in the circulation, and the diver gets “the bends”, which is muscle pain and brain and spinal cord damage from tiny bubbles of gas each occluding a small end artery. The prevention of that is for the diver preliminarily to breathe pure oxygen or an artificial mix of air with the more soluble gas Helium instead of the nitrogen. (Best prevented by no deep diving or doing it more slowly)
   Illness may also be caused by bad breathed air; the most famous being carbon monoxide (CO) poisoning. The CO combines with hemoglobin (Hb-CO) in blood 200 times more than with oxygen and blocks hemoglobin's union with oxygen and causes lack of O2 delivered to body tissue. Since CO is odorless and colorless you must be alert to CO toxicity risk. (Closed bedroom with space heater, sitting inside closed auto)  In nonsmoker, the blood Hb-CO must rise to 10% before symptoms start. If you have a finger-pinch pulse oximeter, in bad CO poisoning the Oxygen (O2) percent will fall below 90%. The starting symptom is headache in an enclosed space with a source of CO (Inside an automobile or in space-heated room). Immediately refresh breathing air by opening window but better to get outside. A small oxygen canister at home can be lifesaving.
   Another place to be alert to atmosphere toxicity is if caught near or in flash fire. Immediately clap hand over mouth and nostrils and if time allows, cover mouth and nose with wet cloth to block inhalation of toxic gas and get outside. Illness from lack of oxygen can occur if jet plane at 35,000 feet develops a hole that allows its pressurized air to leak out, exposing passenger to low air pressure.
   The breathed air also contains many solid particles (high in city dweller low in rural, unless farm worker in silo, and highest in a smoker), which may be cancer-causing, and potentially damage lung and contribute to wear and tear, and cause terminal lung disease in old age and are responsible for person you see going around with oxygen cylinder. More than 99% of these particles (the larger ones) are caught by the mucous in upper respiratory tract but those smaller than 10 micrometers diameter get down to smallest alveoli  sacs in lung. It emphasizes the importance of protecting lungs from particulates. Whenever you are in a possible risk circumstance (Making love in the hay, sleeping in dusty room over years at home or at high risk job), use good filtering nose and face surgical mask.
   Everyone should know that breathed air contains infectious particles: flu virus, maybe dangerous spores from a terrorist letter, and the bacteria pneumococci that eventually will kill us all by causing pneumonia if we live long enough. So practicing keeping one's mouth shut and breathing by nostrils will offer some protection. Also getting immunizing shots against pneumococci (pneumovax).
   By protecting lungs during life you will make old bones. Every bit helps from good nose and face mask, to avoiding smoking and smoke, and living in clean air environment. 
Neural Control of Respiration (Optional read for those interested in the subject)
2327 Respiratory Centers of the Brain.jpg
Respiratory centers in the brainstem and their influence
In the above Figure are the respiratory centers  - 4 main ones - in the brainstem, from upper dorsal pons to lower dorsal and ventral medulla. The Dorsal Respiratory Group (DRG) in the medulla receives signals - arterial blood gas pCO2, acid-base marker pH and blood gas paO2 from the carotid arteries sensing bodies -  via cranial nerves IX and X into the nucleus tractus solatarius which is part of the DRG. These signals are transmitted to ventral respiratory group (VRG) in medulla and Pons Respiratory Group (PRG) which control the muscles of respiration. The most important nucleus is in theVRG in the medulla and if that gets damaged the victim can only live on mechanical ventilator.
       The central nervous system (CNS) control: Breathing is the first function at birth and the last function to stop just after death. (The death rattle is the final breath expiration) Except for brief voluntary pauses it must be continual. Therefore the CNS controls of breathing must be autonomic (free of exclusive voluntary control). It has been found that the central control and generator of breathing is a series of neurons located in the brainstem and upper spinal cord, from the mid pons, the medulla and the cervical-thoracic spinal cord grey matter. So damage to these areas, which most often occurs in poisonings like carbon monoxide or accidents to the upper spinal cord, often results in troubles with breathing. This also includes diseases like amyotrophic lateral sclerosis (Lou-Gehrig Disease) and is the reason so many people die in respiratory failure on ventilators. The lesson we should learn from this is to be very, very careful about our air environment and about accidents especially around head and neck.  
   Finally, useful information about the CNS's sensory regulation of both the respiratory and cardiovascular systems. It resides in the carotid bodies, sensory groups of cells at the bifurcation where the common carotid divides into external and internal branches (and so it samples the artery blood going directly to brain). Also there is a sensory input from other sensors in the brain itself. By these means our bodies are constantly sampling the of arterial oxygen (PaO2), carbon dioxide (PCO2) and acidity (pH).  The normal values at sea level in good health are PaO2 80 - 100 mm Hg, pCO2 35 - 45 mm Hg and pH 7.35 - 7.45.  The first thing that happens with troubled respiration is difficulty excreting CO2 in lung expiration and this leads quickly to PCO2 > 40 and pH< 7.35 and automatically speeds up one's breathing rate and increases its depth. As condition worsens, the PaO2 falls below 80 and the body starts emergency even more rapid breathing and ups the heart rate. In a condition at the top of Mt Everest (Highest human point in the world act c. 30,000 feet) the PCO2 in climbers without supplemental oxygen, falls to 10 (1/4 of normal) because of the very rapid, deep breathing stimulated by the very low oxygen in the breathed air. Oppositely, a low PCO2 or increase in pH and PaO2 beyond the normal range stimulates the neural system to slow breathing and to depress its depth. So this gives an idea of the self regulating controls of respiration that you may extend to many aspects of your life.
  End of Chapter. To read  next now, click 6.(20-22) Respiratory Tracts, Lungs/Asthma/COPD

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