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Blood
Article by: Isidore Gersh, Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia,
Pennsylvania. Hartwig Cleve, Institute for Anthropology and Human Genetics, University of Munich, Munich, Germany.
Oscar D. Ratnoff, Department of Medicine, Division of Hematology and Oncology, Case Western Research University, Cleveland, Ohio.
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A fluid connective tissue that circulates in the vascular
system (blood vessels) of the body. Blood circulates through
the body via vascular blood vessels (arteries, veins, and capil-
laries). Blood consists of plasma (the fluid component) and cells
floating (suspended) within it (Fig. 1). The chief components
of plasma are proteins (albumin and globulins), anions (mainly
chloride and bicarbonate), and cations (mainly sodium, with
smaller concentrations of potassium, calcium, and magnesium).
The cells are derived from extravascular sites and then enter
the circulatory system. They frequently leave the blood vessels
to enter the extravascular spaces, where some of them may
be transformed into connective tissue cells. The fluid part of
the blood is in equilibrium with the tissue fluids of the body.
The circulating blood carries nutrients and oxygen to the body
cells; therefore, it is an important means of maintaining the
homeostasis of the body. In addition, blood carries hormones
from their sites of origin throughout the body, and is thus the
transmitter of the chemical integrators of the body. Blood
plasma also circulates immune bodies and contains several
of the components essential for the formation of blood clots.
Finally, blood transports waste products to excretory organs
for elimination from the body. Because of its basic composi-
tion (cells surrounded by a matrix), development, and ability
to modify into other forms of connective tissues, blood can be
regarded as a special form of connective tissue.
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Content
• Formed elements
• Erythrocytes
• Leukocytes
• Platelets
• Plasma
• Albumin
• Immunoglobulins
• Other proteins
• Other constituents
• Blood coagulation
• Thrombin
• Platelets
• Hereditary defects
• Heparin
Key Concepts
• Blood is a fluid tissue that circulates through the vascular system of living animals.
It consists of plasma and cells.
• Plasma is made up of proteins such as albumin and globulins, anions such
as chloride and bicarbonate, and cations such as sodium.
• Erythrocytes (red blood cells) contain hemoglobin, which allows blood
to transport oxygen.
• Leukocytes (white blood cells) play critical roles in the immune response
and in the repair of connective tissue.
• Platelets enable coagulation, a process that involves thrombin enzyme,
fibrinogen, and clotting factors.
• Hereditary factors and vitamin K deficiency can impair coagulation or
other functions of the blood.
