THE CARDIOVASCULAR SYSTEM: BLOOD
Overview of Blood
Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular
matrix. The cellular elements—referred to as the formed elements—include red blood cells (RBCs), white blood
cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among
connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and
enables them to circulate throughout the body within the cardiovascular system.
Functions of Blood
The primary function of blood is to deliver oxygen and nutrients to and remove wastes from body cells, but that is only
the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of
homeostasis.
Transportation
Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the
liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air you
breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body.
Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream,
which carries them to distant target cells. Blood also picks up cellular wastes and byproducts, and transports them to various
organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste
products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.
Defense
Many types of WBCs protect the body from external threats, such as disease-causing bacteria that have entered the
bloodstream in a wound. Other WBCs seek out and destroy internal threats, such as cells with mutated DNA that could
multiply to become cancerous, or body cells infected with viruses.
When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, the fluid
portion of the blood, interact to block the ruptured areas of the blood vessels involved. This protects the body from further
blood loss.
Maintenance of Homeostasis
Recall that body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your
rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from
your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be
dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood
is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.
Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers,
which thereby help to regulate the pH of body tissues. Blood also helps to regulate the water content of body cells.
Composition of Blood
You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of
the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances
are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including
the quantities and types of formed elements.
One such test, called a hematocrit, measures the percentage of RBCs, clinically known as erythrocytes, in a blood
sample. It is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements
, suspended within the blood sample to separate from the lightweight, liquid plasma (Figure 18.2). Because the heaviest
elements in blood are the erythrocytes, these settle at the very bottom of the hematocrit tube. Located above the erythrocytes
is a pale, thin layer composed of the remaining formed elements of blood. These are the WBCs, clinically known as
leukocytes, and the platelets, cell fragments also called thrombocytes. This layer is referred to as the buffy coat because of
its color; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally
a pale, straw-colored fluid, which constitutes the remainder of the sample.
The volume of erythrocytes after centrifugation is also commonly referred to as packed cell volume (PCV). In normal
blood, about 45 percent of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, however,
about 36–50 percent, according to gender and other factors. Normal hematocrit values for females range from 37 to 47,
with a mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean of 47. The percentage of other formed
elements, the WBCs and platelets, is extremely small so it is not normally considered with the hematocrit. So the mean
plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 (or 100 minus 41), and
for males, it is approximately 53 (or 100 minus 47).
Figure 18.2 Composition of Blood The cellular elements of blood include a vast number of erythrocytes and
comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A
sample of blood spun in a centrifuge reveals that plasma is the lightest component. It floats at the top of the tube
separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the
percentage of the total sample that is comprised of erythrocytes. Depressed and elevated hematocrit levels are shown
for comparison.
Characteristics of Blood
When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up
oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dusky red. This is because
hemoglobin is a pigment that changes color, depending upon the degree of oxygen saturation.
Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water.
Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins
and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider
the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow
than the less viscous water. The same principle applies to blood.
The normal temperature of blood is slightly higher than normal body temperature—about 38 °C (or 100.4 °F),
compared to 37 °C (or 98.6 °F) for an internal body temperature reading, although daily variations of 0.5 °C are normal.
Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and
resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher
temperature.
The pH of blood averages about 7.4; however, it can range from 7.35 to 7.45 in a healthy person. Blood is therefore
somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous
buffers that actually help to regulate pH.
Blood constitutes approximately 8 percent of adult body weight. Adult males typically average about 5 to 6 liters of
blood. Females average 4–5 liters.
Blood Plasma
Like other fluids in the body, plasma is composed primarily of water: In fact, it is about 92 percent water. Dissolved
or suspended within this water is a mixture of substances, most of which are proteins. There are literally hundreds of
substances dissolved or suspended in the plasma, although many of them are found only in very small quantities.
Overview of Blood
Recall that blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular
matrix. The cellular elements—referred to as the formed elements—include red blood cells (RBCs), white blood
cells (WBCs), and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among
connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and
enables them to circulate throughout the body within the cardiovascular system.
Functions of Blood
The primary function of blood is to deliver oxygen and nutrients to and remove wastes from body cells, but that is only
the beginning of the story. The specific functions of blood also include defense, distribution of heat, and maintenance of
homeostasis.
Transportation
Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the
liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen from the air you
breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body.
Moreover, endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream,
which carries them to distant target cells. Blood also picks up cellular wastes and byproducts, and transports them to various
organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste
products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.
Defense
Many types of WBCs protect the body from external threats, such as disease-causing bacteria that have entered the
bloodstream in a wound. Other WBCs seek out and destroy internal threats, such as cells with mutated DNA that could
multiply to become cancerous, or body cells infected with viruses.
When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma, the fluid
portion of the blood, interact to block the ruptured areas of the blood vessels involved. This protects the body from further
blood loss.
Maintenance of Homeostasis
Recall that body temperature is regulated via a classic negative-feedback loop. If you were exercising on a warm day, your
rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from
your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be
dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood
is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.
Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers,
which thereby help to regulate the pH of body tissues. Blood also helps to regulate the water content of body cells.
Composition of Blood
You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of
the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances
are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including
the quantities and types of formed elements.
One such test, called a hematocrit, measures the percentage of RBCs, clinically known as erythrocytes, in a blood
sample. It is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements
, suspended within the blood sample to separate from the lightweight, liquid plasma (Figure 18.2). Because the heaviest
elements in blood are the erythrocytes, these settle at the very bottom of the hematocrit tube. Located above the erythrocytes
is a pale, thin layer composed of the remaining formed elements of blood. These are the WBCs, clinically known as
leukocytes, and the platelets, cell fragments also called thrombocytes. This layer is referred to as the buffy coat because of
its color; it normally constitutes less than 1 percent of a blood sample. Above the buffy coat is the blood plasma, normally
a pale, straw-colored fluid, which constitutes the remainder of the sample.
The volume of erythrocytes after centrifugation is also commonly referred to as packed cell volume (PCV). In normal
blood, about 45 percent of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, however,
about 36–50 percent, according to gender and other factors. Normal hematocrit values for females range from 37 to 47,
with a mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean of 47. The percentage of other formed
elements, the WBCs and platelets, is extremely small so it is not normally considered with the hematocrit. So the mean
plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59 (or 100 minus 41), and
for males, it is approximately 53 (or 100 minus 47).
Figure 18.2 Composition of Blood The cellular elements of blood include a vast number of erythrocytes and
comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A
sample of blood spun in a centrifuge reveals that plasma is the lightest component. It floats at the top of the tube
separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the
percentage of the total sample that is comprised of erythrocytes. Depressed and elevated hematocrit levels are shown
for comparison.
Characteristics of Blood
When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up
oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dusky red. This is because
hemoglobin is a pigment that changes color, depending upon the degree of oxygen saturation.
Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water.
Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins
and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow. Consider
the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow
than the less viscous water. The same principle applies to blood.
The normal temperature of blood is slightly higher than normal body temperature—about 38 °C (or 100.4 °F),
compared to 37 °C (or 98.6 °F) for an internal body temperature reading, although daily variations of 0.5 °C are normal.
Although the surface of blood vessels is relatively smooth, as blood flows through them, it experiences some friction and
resistance, especially as vessels age and lose their elasticity, thereby producing heat. This accounts for its slightly higher
temperature.
The pH of blood averages about 7.4; however, it can range from 7.35 to 7.45 in a healthy person. Blood is therefore
somewhat more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous
buffers that actually help to regulate pH.
Blood constitutes approximately 8 percent of adult body weight. Adult males typically average about 5 to 6 liters of
blood. Females average 4–5 liters.
Blood Plasma
Like other fluids in the body, plasma is composed primarily of water: In fact, it is about 92 percent water. Dissolved
or suspended within this water is a mixture of substances, most of which are proteins. There are literally hundreds of
substances dissolved or suspended in the plasma, although many of them are found only in very small quantities.
