functions in the rapid internal transport of substances
Your body’s differentiated cells, which perform
specialized tasks, cannot fend for themselves. Different
types must interact in coordinated ways to maintain the
composition, volume, and temperature of a tissue fluid
surrounding them, the interstitial fluid. A circulating
connective tissue-blood-interacts with tissue fluid,
making continual deliveries and pickups that help keep
conditions tolerable for enzymes and other molecules
heart, generates pressure that keeps blood flowing. Like
many anirnals, you have a closed circulatory system-
blood is confined within the continuously connected
walls of the heart and blood vessels.
capillary beds. At such beds, the flow fans out through
vast numbers of senall-diameter blood vessels called
heart’s right half pumps oxygen-poor blood to the
lungs, where blood picks up oxygen and gives up car-
bon dioxide. The freshly oxygenated blood then flows
to the heart’s left half. This is the pulmonary circuit. By
contrast, in a systemic circuit, the Itft half of the heart
pumps oxygenated blood to all tissues where oxygen is
an elaborate network of drainage vessels picks up
excess interstitial fluid and reclaimable solutes, then
returns them to the circulatory system. This network is
Blood, a connective tissue, has multiple functions. It
transports oxygen, nutrients, and other solutes to cells.
The volume of blood depends on body size
average-size adult humans is about 6 to 8 percent
body weight. That amounts to about four or five quarts.
red blood cells, vehite blood cells, and platelets are its
accounts for 50 to 60 percent of the total blood volume.
Plasena Plasma, which is mostly water, functions as a
transport medium for blood cells and platelets. It also
serves as a solvent for ions and molecules, including
hundreds of different kinds of plasma proteins. Some of
the plasma proteins transport lipids and fat-soluble
Erythrocytes, or red blood ceils, are
biconcave disks, like doughnixts with a squashed-in
count is the number of cells of a given type in a micro-
I liter of blood. The average number of red blood cells is
5.4 million in males and 4.8 million in females.
( arise from stem cetls in bone marrow.
Each platelet only lasts five to nine days, but
hundreds of thousands are always circulating in blood.
if the volume of blood were to decrease by more than
30 percent, then circulatory shock would follow
defensive response called agglutination, proteins called
antibodies that are circulating in plasma act against the
foreign cells and cause them to clump.
Molecular variations in one kind of self marker on red
blood cells are analyzed in ABO blood typing.
cular system, a heart pumps blood into large-diameter
arteries. From there, blood flows into small, muscular
arterioies, which branch into the even smaller diameter
capillaries introduced earlier. Blood llows contintrously
from capiliaries into sma!! venules, then into large-
diameter veins that return blood to the heart.
The pulmonary circuit, a short loop, rapidly
oxygenates blood. It leads from the heart’s right half to
capillary beds in both lungs, then returns to the heart’s
left half. The systemic circuit is a longer loop. It starts at
the heart’s left half and the aorta (the main artery carry-
ing oxygenated blood away from the heart), branches to
all organs and tissues with metabolicaily active cells,
then converges into major veins that deliver oxygen-
poor blood to the heart’s right half.
2.5 billion times during a seventy-year life span, and
you know it must be a truly durable pump.
Each half of the heart has two chambers-an atriun
The sequence of contraction and relaxation is a cardiac
About 1 percent of the cardiac muscle
cells don’t contract. Instead, they function as a cardiac
conduction system. These specialized cells initiate and
Propagate waves of excitation abottl seventy-five times
another eell body cluster, the AV node. This is the only
electrical bridge between atria and ventricies (which
connective tissue insuiates everys.vhere else). After the
AV node, conducting cells are arranged as a bundle i=
the partition between the heart’s two halves. The cells
then branch, and the branches detiver the excitatory
wave up the ventricle walls. The ventricles contract in
response with a forvisting movement, upward from the
heart’s apex, that ejects blood into the great arteries.
The SA node fires action potentials faster than the
rest of the system and serves as the cardiac pacemaker.
Blood pressure, the fluid pressure generated by heart
contractions, is highest in contracting ventricles. During
the time it takes for a given volume of blood to leave
and reenter the heart, pressure is still high in arteries,
then drops along the cirettit, and is lowest tn the relaxed
the average restinn value stars fairly constant o-
few weeks, even months about 120/80 mm Hg. ‘
an increase in blood