Biomedical Engineering Reference
In-Depth Information
glands) provides protection for the body. The endocrine system (ductless glands such as the
thyroid and adrenals) secretes hormones that regulate many chemical actions within cells.
The lymphatic system (glands, lymph nodes, lymph, lymphatic vessels) returns excess fluid
and protein to the blood and helps defend the body against infection and tissue damage.
The digestive system (stomach, intestines, and other structures) ingests food and water,
breaks food down into small molecules that can be absorbed and used by cells, and removes
solid wastes. The urinary system (kidneys, ureters, urinary bladder, and urethra) maintains
the fluid volume of the body, eliminates metabolic wastes, and helps regulate blood pressure
and acid-base and water-salt balances. The reproductive system (ovaries, testes, reproductive
cells, and accessory glands and ducts) produces eggs or sperm and provides a mechanism
for the production and nourishment of offspring. The circulatory system (heart, blood, and
blood vessels) serves as a distribution system for the body. The respiratory system (airways
and lungs) delivers oxygen to the blood from the air and carries away carbon dioxide. The
nervous system (brain, spinal cord, peripheral nerves, and sensory organs) regulates most
of the body's activities by detecting and responding to internal and external stimuli. The skel-
etal system (bones and cartilage) provides protection and support as well as sites for muscle
attachments, the production of blood cells, and calcium and phosphorus storage. The muscu-
lar system (skeletal muscle) moves the body and its internal parts, maintains posture, and
produces heat. Although biomedical engineers have made major contributions to under-
standing, maintaining, and/or replacing components in each of the 11 major organ systems,
only the last 5 listed will be examined in greater detail.
3.4.1 Circulatory System
The circulatory system (Figure 3.17) delivers nutrients and hormones throughout the body,
removes waste products from tissues, and provides a mechanism for regulating temperature
and removing the heat generated by the metabolic activities of the body's internal organs.
Every living cell in the body is no more than 10-100
m
m from a capillary (small blood vessels
with walls only one cell thick that are 8
m in diameter, approximately the same size as a
red blood cell). This close proximity allows oxygen, carbon dioxide, and most other small
solutes to diffuse from the cells into the capillary or from the capillary into the cells, with the
direction of diffusion determined by concentration and partial pressure gradients.
Accounting for about 8
m
/- 1 percent of total body weight, averaging 5,200 ml, blood is a
complex, heterogeneous suspension of formed elements—the
þ
blood cells
,or
hematocytes
—
suspended in a continuous, straw-colored fluid called
plasma
. Nominally, the composite
/- 0.007 g/cm
3
, and it is six times as viscous as water.
The hematocutes include three basic types of cells: red blood cells (erythrocytes, totaling
nearly 95 percent of the formed elements), white blood cells (leukocytes, averaging less than
.15 percent of all hematocytes), and platelets (thrombocytes, on the order of 5 percent of all
blood cells). Hematocytes are all derived in the active (“red”) bone marrow (about 1,500 g)
of adults from undifferentiated stem cells called
fluid has a mass density of 1.057
þ
hemocytoblasts
, and all reach ultimate
maturity via a process called
.
The primary function of erythrocytes is to aid in the transport of blood gases—about 30
to 34 percent (by weight) of each cell consisting of the oxygen- and carbon dioxide-carrying
protein hemoglobin (64,000 MW
hematocytopoiesis
68,000) and a small portion of the cell containing the
