Although the metabolic system is crucial to make everything work, the cir- culatory system carries the blood, which in turn transports oxygen, car- bon dioxide, nutrients, and waste products. This system is comparable to any closed hydraulic system in an airplane, which includes a fluid (blood), a pump (heart), tubing (blood vessels), and the part requiring support of the system (an organ). This system is of most interest to pilots because any change in oxygen levels to the cells immediately changes the performance of many organs, especially the brain; therefore, anatomy will first be described and then the physiology will be explained in more detail.
The heart
The heart is the pump in this human hydraulic system, which is a closed system with blood flowing from the heart, into arteries, then through capil- laries that are spread around the tissues and individual cells (Fig. 2-1). From there, the blood continues into the venous system, through the lungs, and back to the heart.
This pump is divided into four chambers that essentially take blood from a major vein and pump it into arteries. The two smaller chambers on top (the atria) take in blood from the veins and “squeeze” it into the larger ventricles below. The ventricles then pump blood to the lungs and the rest of the body.
The heart is a muscle and responds just like any other muscle in the body.
Every muscle fiber or cell must contract at the same time, which squeezes the blood through the one-way heart valves. The blood moves forward under pressure but not backward, so there is a moment immediately after contrac- tion when blood pressure goes down, but not to zero.
Flowing blood is not under the same pressure throughout the system, which is one of the differences compared to an aircraft hydraulic system. This
“sinus wave” pressure is why we can feel a pulse. When the heart contracts, the pressure goes up, and we feel that rise. When the heart muscle relaxes, getting ready for the next contraction, there is a decrease in pressure. The heart contracts again before the pressure gets too low, as a result of muscles in the arterial walls.
Like any other muscle, the heart needs a blood supply, and this is supplied via the coronary arteries. These arteries become blocked in heart disease, lead- ing to heart attacks or poor blood perfusion (ischemia) of the muscle cells. The heart muscle requires oxygen for energy; the heart can fail or become less effective if oxygen does not reach the heart muscle. Total blockage of blood to Figure 2-1
Figure 2-1 The heart is the pump of the circulatory system.
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24 Basic human anatomy
the heart muscle leads to death of some of the tissues devoid of blood. Dead heart tissue is called a myocardial infarct (myo = muscle, cardia = heart, infarct = dead), or MI. This myocardial infarction, therefore, is another name forheart attack.
If the heart-muscle fibers do not all contract simultaneously, there are changes in the rhythm and contractility. Fibrillation occurs when the muscle fibers are contracting at different times and not in synchrony, looking like a can of live worms. Rhythm problems also can develop that compromise the efficiency of the heart’s ability to move blood. Any of these situations leads to heart failure.
The amount of blood that flows out of the heart per minute is dependent on the heart rate (pulse), the volume of blood ejected from the heart, and the force of the contraction. Much of this is controlled by the autonomic nervous system that senses the need for more blood to a specific area of the body. An increase in carbon dioxide and other metabolites also makes the heart go faster to compensate for metabolic needs.
The flow of blood will become somewhat turbulent if it is interfered with by either a constriction of a blood vessel or a heart valve that is too small or leaks. This turbulence can be heard with a stethoscope and is called a murmur.
A murmur is not a disease; it’s an audible sign of potential pathology such as a calcified heart valve or an artery that is narrowed from arteriosclerosis.
That cause can impair the performance of blood flow, weaken the heart, and decrease the perfusion of tissues in the organs. Like any hydraulic system, the end result is no better than the weakest part of that system. Therefore finding the cause of the murmur becomes the challenge.
The lungs
The lung organs exchange oxygen and carbon dioxide from the ambient air drawn into the air sacs (alveoli) with the same gases in the blood (Fig. 2-2).
Air enters the lungs through the mouth or nose, into the pharynx, trachea, and then to the main bronchus. This then splits into two bronchi, which further split into thousands of smaller bronchioles and end up in tiny alveoli (tiny rubberlike balloons).
Capillaries surround these alveoli, and because both walls are so thin, gases can pass back and forth (diffuse), out of and into the air or into the bloodstream. This diffusion and gas exchange at the cell level will be further explained in the section on physiology of respiration.
Air is pulled into the lungs by decreasing the pressure within the chest cavity (Fig. 2-3). This is accomplished by the diaphragm, a muscular, dome-shaped covering of the lower part of the rib cage, forming a closed container with one opening, the bronchus. When this muscle contracts, it flattens out, creating lower pressure and bringing in air to fill the alveoli. This inhalation is an active event (a muscle contracting), whereas exhaling is passive when the muscular diaphragm relaxes and returns to its dome shape. The muscles between the ribs also supplement this activity.
Figure 2-2
Figure 2-2 The lung resembles an upside-down tree.
Figure 2-3
Figure 2-3 When the diaphragm descends and the chest cavity expands, air inflates the lungs (left). Air is forced out of the lungs when the diaphragm and lungs relax (right).
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26 Basic human anatomy
This process of breathing is also an autonomic function, controlled by the CNS and feedback data from various organs telling the breathing rate to change. With exercise, both breathing and heart rate are selectively increased to meet demands without the person having to think about it. We can also easily and actively control our breathing rate, whereas it is difficult to actively control heart rate.
The vascular system
The link between the heart, lungs, brain, and other parts of the body is the blood vessels (Fig. 2-4), and they maintain an uninterrupted blood supply to
Figure 2-4
Figure 2-4 Essential oxygen and nutrients are trans- ported throughout the body by the circulatory system.
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.