Biomedical Engineering Reference
In-Depth Information
14.1 BIOMEDICAL MASS TRANSPORT
Mass transport in the human body is a vital process that affects how the lungs function
in transferring air and its components to the bloodstream, how the capillaries function in
transferring nutrients and gases to surrounding body tissues, and how the kidneys function
in transferring metabolic waste products and excess water from the blood into the urine.
Mass transfer processes also occur in artificial devices such as artificial kidneys (dialysis)
and artificial ventilators and respirators. Mass transfer in the body also affects transport
across cell membranes, which controls processes in millions of cells affecting every area
of the body. The majority of mass transfer occurs across small membranes of thin surfaces
in order to shorten the distance over which substances must travel from point A to point B.
This is true of cells in the body, which are quite thin, as well as artificial devices whose
components are manufactured to be very thin.
14.1.1 Analysis of Respiration and Gas Transport
The human lungs control gas exchange from our environment into the bloodstream by
means of pressure and concentration gradients. When we breath, air enters the lungs through
a large entrance, the trachea, and eventually branches into smaller and smaller segments until
reaching the smallest elements of the lungs, the alveoli. Each of these thin elements is in close
proximity to blood in pulmonary capillaries, which are the smallest and thinnest of the blood
vessels. With each of the alveoli in close proximity to a pulmonary capillary, the distance for
gas exchange is very short, which thus shortens the time by which complete gas exchange
occurs. A diagram of the lungs and its branching system is shown in Figure 14.1.
The two human lungs contain approximately 300 to 500 million alveoli, having a total
surface area of about 75 m
2
in adults, the size of a tennis court. The branching of the air-
ways into the alveoli represents millions of tiny sacs, which not only represents thinner
membranes to speed gas exchange but also more surface area to speed gas exchange. The
alveolar network is shown in Figure 14.2.
The lungs may be separated (for the purposes of mass transfer/gas exchange) as a dead
space and an alveolar space. The dead space consists of the trachea, the bronchioles, and the
bronchi, which are all large segments of the airway where there is air flow but no gas
exchange with the bloodstream. The alveolar space is where the actual gas exchange occurs.
However, both zones heat the inspired air to body temperature (37
C), as well as humidify-
ing the air. Thus, expired air is heated to body temperature and is usually fully saturated
with water vapor. In fact, it is possible to lose up to a half pound per day merely from
losing water from the body via respiration. Figure 14.3 shows the relationship between
the dead space and the alveolar space.
The amount of air that one inhales and exhales (without exertion) is called the
tidal
volume
, which is 500 ml (about a fluid pint) per breath. The typical breathing rate is
12 breaths per minute at rest. The tidal volume represents only that portion of the lung
volume where relatively easy breathing (in and out) occurs. Figure 14.4 shows all of the
lung volumes and how the tidal volume compares to other (forced) breathing volumes.
Ambient air is approximately 79 percent nitrogen and 21 percent oxygen on a dry basis (not
including any water vapor/humidity in the air). With a single breath, oxygen is transferred
