Physical Principles

Both respiration and breathing are essentially physical processes. Air and/or various gases are moved from one place to another. Their movement is because of differences in their relative pressures from one space to another.

a. Pressure Gradient. Consider a situation in which there are two separate but connected spaces. If the concentration or pressure of that substance is greater in one space than the other, then there is a pressure gradient for that substance. As a result, the substance will move from the area of higher pressure to the area of lower pressure.

b. Boyle's Law. Assume that we have a container and we can change the volume of the container without allowing a gas to escape. Boyle's law tells us that if we increase the volume, the pressure inside will decrease. Likewise, if we decrease the volume, the pressure inside will increase.

c. Pascal's Law. If a closed container is filled with a fluid, a pressure applied to the fluid will produce an equal pressure at each and every point on the inner surface.

d. Surface Area. Most phenomena in breathing and respiration take place at one surface or another. As surface area increases, more gases can be exchanged or treated.


The human trunk (Figure 7-1) can be considered a hollow cylinder. A muscular membrane, the thoracic diaphragm, extends across this hollow and divides the trunk into upper and lower cavities.



Figure 7-1. Schematic frontal section of the human trunk.


Figure 7-1. Schematic frontal section of the human trunk.

a. Thoracic Cavity. The thoracic cavity is the space of the trunk above the diaphragm. It is open to the outside by way of the neck and head. Since the wall of the thorax is reinforced by special muscles, bones, and cartilages, we can consider the thorax to be a "solid-walled container" filled with gas.

b. Abdominopelvic Cavity. The abdominopelvic cavity is the rest of the trunk cavity below the diaphragm. The abdominopelvic cavity is a closed system. Its walls are "elastic" since they are made up of musculature. The abdominopelvic cavity is filled with a fluid continuum. This fluid continuum consists primarily of water contained in the soft tissues of the abdomen and the pelvis.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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