all tissues of the body. It is a closed system of arteries and veins. Basically, arteries distribute the blood from the heart to various organs, getting smaller (arterioles) as they branch out. The collecting vessels that return blood to the heart are veins; smaller veins are called venules. Very small vessels at the tissue-cell level are called capillaries.
Pulmonary arteries carry oxygen-poor blood from the heart to the lungs. Pul- monary veins carry oxygen-rich (reoxygenated) blood from the lungs to the heart. Oxygen-rich blood is then pumped through the arteries of the circula- tory system to the tissues and individual cells. Arteries are unique among blood vessels in that they have muscle and elastic cells within their walls, allowing them to dilate or constrict by muscle contraction or the inherent elasticity.
This is an effective way of increasing or decreasing (shunting) blood flow to various parts of the body. For example, during exercise, the leg muscles need more blood, so arteries to these muscles automatically dilate to their full size.
Arteries to other parts, such as the digestive tract, are constricted by the wall muscles, shunting blood away from that area and into the legs. The same process happens in control of body temperatures.
The elasticity of the artery wall helps keep the blood pressure more constant during the period when the heart muscle relaxes between beats (contrac- tions). Like blowing up a balloon, the pressure within the artery resulting from the elasticity enhances and prolongs the blood pressure that is gener- ated by the heart during the relaxed phase.
Veins are simple tubes without muscles or elastic tissues. Some veins in the arms and legs have one-way valves that prevent blood from flowing backward during the pause or relaxed phase after it has been pumped forward during a heart contraction. This is necessary because blood pressure in the veins is very low, less than 10 millimeters of mercury (mm Hg), as compared to pres- sure within the major arteries of about 120 mm Hg at the height of a heart contraction. This low venous pressure is inadequate to move blood back up to the heart and needs the help of our “muscle pump”.
Contracting arm and leg muscles that surround veins compress the vessel’s wall and squeeze the blood back to the heart in only one direction because of the one-way valves. This is often called the “muscle pump.” Such action does not take place when you have been sitting for a long period of time and blood has pooled in the extremities (stagnant hypoxia). Veins carry blood that is deficient in oxygen and high in carbon dioxide.
Tissues and cells get their oxygen and nutrient supply from blood flowing next to the cells in the smaller capillaries. These very thin-walled tubes allow dif- fusion of gases through the walls from the blood to the cell and back again.
28 Basic human anatomy
from another gas mixture (the atmosphere) to a liquid (blood). It is more than just dissolving a gas in a liquid.
Blood is essentially cells and a liquid called serum. About 55 percent of blood is serum, and about 90 percent of serum is water. One of the blood cells is a red blood cell, which physically carries oxygen molecules attached to a chemical on the cell called hemoglobin. The comparison of partial pressures and the percentage of hemoglobin saturation is called the dissociation curve (Fig. 2-5). Inadequate oxygen to the cells (hypoxia) will occur when the satu- ration goes below 72 percent, where the curve drops off quickly with decreas- ing oxygen partial pressure.
The red blood cells transport oxygen through the use of the hemoglobin. Serum can carry only a small amount of oxygen in solution. The difference between the body’s needs and what the blood can carry is made up by the hemoglobin’s ability to carry oxygen. Any change in the hemoglobin, the number of red blood cells, or the diffusion of gases will cause some form of hypoxia (see Chapter 5);
therefore, respiration involves taking in air, transferring it to blood, and then transporting it via arteries to the cells. The whole process is reversed for remov- ing waste gases via the veins.
Red blood cells are unique in how they carry oxygen. The bright-red color of arterial blood results from the combination of oxygen with hemoglobin. The darker color of venous blood is an indication of hemoglobin that has minimal oxygen. The amount of oxygen carried (or saturated) by the red blood cells is normally (and needs to be) about 95–98 percent.
Figure 2-5
Figure 2-5 Arterial and venous saturation points for the normal sea-level conditions are shown.
This relationship between oxygen available and its partial pressure (see Chap- ter 5) is seen in the oxygen dissociation curve and indicates that oxygen satu- ration of the blood falls rapidly when the partial pressure of oxygen goes below 60 mm Hg. The curve describes how venous blood carries less oxygen than arterial blood. The lower availability of oxygen is when hypoxia develops.
Physiology of respiration
Respiration is the process of exchanging gases in an environment with the gases within the tissues and cells of a living organism (Fig. 2-6). This process can be divided into two activities: external respiration and internal respi- ration. External respiration occurs in the lungs, where air is inhaled and exhaled and gases are transferred (diffused) through the lungs and into the bloodstream. The internal process refers to the transport of gases to and from body cells and tissues by the blood and red blood cells.
The function of respiration is to get oxygen into the body and to the cells and to take carbon dioxide from the cells and remove it from the body. Body tem- perature is also controlled somewhat by the transfer of heat that takes place during this same circulatory process (Chapter 10).
By reviewing the actions of the lungs, chest wall, and diaphragm, one can see how external respiration takes place. It becomes more complex when considering how the gases get into the bloodstream. The alveoli within the
Figure 2-6
Figure 2-6 Gas exchange within the body.
Respiration 29
30 Basic human anatomy
lungs are very thin walled (one cell thick) and are surrounded by the small capillaries, also with walls one cell thick. These capillaries are so small that only one red blood cell can pass through, one behind the other; therefore, the gas molecules and the blood cell are separated by only two permeable walls each one cell thick.
Gases move across these membranes by the process of diffusion, which lets a gas move from an area of high pressure to an area of lower pressure (Fig. 2-7). The oxygen, when it reaches the alveoli, has a partial pressure of about 95–100 mm Hg (Fig. 2-8). Within venous blood, the partial pressure Figure 2-7
Figure 2-7 Tissue-capillary diffusion of oxygen and carbon dioxide (in mm Hg).
Figure 2-8
Figure 2-8 Alveolar-capillary diffusion of oxygen and carbon dioxide.
is 40 mm Hg. Diffusion takes place as long as there is a differential. Carbon dioxide diffuses in the same manner. The partial pressure of carbon diox- ide in venous blood is 47 mm Hg; in the arteries, it’s 41 mm Hg; and in the alveoli, it’s 40 mm Hg. The same process of diffusion occurs at the tissue and cell level within the body.
The body has a variety of receptors and feedback mechanisms that can detect when oxygen supply and carbon dioxide levels are incompatible with metabolism. This control and regulation determines how quickly and deeply we breathe, how hard and fast the heart pumps, where blood is shunted, and how metabolic processes are controlled. A description of this process is beyond the scope of this text, but it should be recognized that this regulatory system determines our level of competence or impairment in flight.
Obviously, the body is far more complex than what is described here; how- ever, in order to understand how the physiology of flight can affect perfor- mance, you must have a basic understanding of how the body works. Then some of what you hear and read about in terms of impairment and asso- ciated medical standards will make sense; the regulations and company insurance policies will also make better sense. Misinformation abounds in the press, ads, and many anecdotal stories regarding why these standards, policies, and regulations exist. Only people who understand and respect flight physiology can judge their significance in safe flight performance.
Furthermore, anything that interferes with a normally functioning body and mind—your health—can also determine if you can meet FAA medical certification standards.
Respiration 31
3
The atmosphere
The air, like the sea, can be lethal to the human animal stripped of his artificial protections. Adapted to breathing air at sea level density and pressure, he/she loses consciousness and dies at great altitudes where the pressure of the atmosphere is too low to force life-giving oxygen through his lungs and into his blood stream. In the realm of the “special senses,” man is found deficient. God may not have built man to fly: yet today, borne aloft by the power of thousands of horses, he soars through the heavens as one returned to an ancient domain, the land of fantasy now made real.
Douglas H. Robinson, M.D., The Dangerous Sky, 1973
THERE ARE THREE DISTINCT PARTS TO FLIGHT: the plane (or “aerospace vehicle”), the pilot, and the environment in which the pilot lives and flies (the atmosphere). All three are interrelated, but the atmosphere affects both the plane and the pilot. The airplane might be right at home in thin air; the pilot is not. An aircraft engine might be more efficient at altitude; the human body is not.
Therefore, it is imperative that all aircrew have a complete understanding of the makeup of the atmosphere from a physiological perspective. Knowledge of the pilot’s working environment will help in recognizing the potential dan- gers and subtle changes in himself and his crewmates and maintain a high index of suspicion and potential problems in performance. Only through awareness of the atmosphere’s impact on a pilot will he/she have a true respect for his vulnerability. The safe pilot knows all about where he/she is working.
This chapter will explain the atmosphere. This insight will be a base of needed information for later subjects that impact most physiological events. Plan on using this chapter as a frequent reference. Subjects covered will include the composition of the atmosphere with elements that have a direct effect on our health and performance, as we all know but often minimize. This composi- tion is affected by pressure and temperature, and one needs to know when and where.
Additionally, a variety of independent layers are within the atmosphere (troposphere, stratosphere, etc.), much like layers of water and oil in a standing bottle. The composition of air within these distinct atmospheric layers is different. These layers also can be divided physiologically to identify where the dangers to humans lie. All of these factors are directly related to known gas laws, especially how our body is affected.
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