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Blood is a liquid tissue. Suspended in the watery plasma are seven types of cells and cell fragments.
If one takes a sample of blood, treats it with an agent to prevent clotting, and spins it in a centrifuge,
  • the red cells settle to the bottom
  • the white cells settle on top of them forming the "buffy coat".
The fraction occupied by the red cells is called the hematocrit. Normally it is approximately 45%. Values much lower than this are a sign of anemia.

Functions of the blood

Blood performs two major functions:

The formation of blood cells (cell types and acronyms are defined below)

All the various types of blood cells

Link to a diagram showing the actual appearance of these cells (96K)
These stem cells Which path is taken is regulated by

For example, Interleukin-7 (IL-7) is the major cytokine in stimulating bone marrow stem cells to start down the "lymphoid" path leading to the various lymphocytes (mostly B cells and T cells).

Some of the cytokines that drive the differentiation of the "myeloid" leukocytes are
Link to a discussion of how transplants of hematopoietic stem cells are used in therapy.

Red Blood Cells (erythrocytes)

The most numerous type in the blood. RBC precursors mature in the bone marrow closely attached to a macrophage.

This scanning electron micrograph (courtesy of Dr. Marion J. Barnhart) shows the characteristic biconcave shape of red blood cells.

Thus RBCs are terminally differentiated; that is, they can never divide. They live about 120 days and then are ingested by phagocytic cells in the liver and spleen. Most of the iron in their hemoglobin is reclaimed for reuse. The remainder of the heme portion of the molecule is degraded into bile pigments and excreted by the liver. Some 3 million RBCs die and are scavenged by the liver each second.

Red blood cells are responsible for the transport of oxygen and carbon dioxide.

Oxygen Transport

In adult humans the hemoglobin (Hb) molecule The reaction is reversible.

Carbon Dioxide Transport

Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ions:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

95% of the CO2 generated in the tissues is carried in the red blood cells:

Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)

When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.


Anemia is a shortage of Anemia has many causes. One of the most common is an inadequate intake of iron in the diet.

Blood Groups

Red blood cells have surface antigens that differ between people and that create the so-called blood groups such as the ABO system and the Rh system.
Link to a discussion of blood groups.

White Blood Cells (leukocytes)

White blood cells


There are several kinds of lymphocytes (although they all look alike under the microscope), each with different functions to perform . The most common types of lymphocytes are

Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus [View] where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they


Monocytes leave the blood and become macrophages and dendritic cells.

This scanning electron micrograph (courtesy of Drs. Jan M. Orenstein and Emma Shelton) shows a single macrophage surrounded by several lymphocytes.

Macrophages are large, phagocytic cells that engulf


The most abundant of the WBCs. This photomicrograph shows a single neutrophil surrounded by red blood cells.

Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis.

This is a never-ending task, even in healthy people: Our throat, nasal passages, and colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm. But that is because neutrophils keep them in check.


can reduce the numbers of neutrophils so that formerly harmless bacteria begin to proliferate. The resulting opportunistic infection can be life-threatening.


The number of eosinophils in the blood is normally quite low (0–450/΅l). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.


The number of basophils also increases during infection. Basophils leave the blood and accumulate at the site of infection or other inflammation. There they discharge the contents of their granules, releasing a variety of mediators such as:

which increase the blood flow to the area and in other ways add to the inflammatory process. The mediators released by basophils also play an important part in some allergic responses such as


Platelets are cell fragments produced from megakaryocytes.

Blood normally contains 150,000–450,000 per microliter (΅l) or cubic millimeter (mm3). This number is normally maintained by a homeostatic (negative-feedback) mechanism [Link].

If this value should drop much below 50,000/΅l, there is a danger of uncontrolled bleeding because of the essential role that platelets have in blood clotting.

Some causes:

When blood vessels are cut or damaged, the loss of blood from the system must be stopped before shock and possible death occur. This is accomplished by solidification of the blood, a process called coagulation or clotting.

A blood clot consists of
Details of the clotting process are in a separate page. Link to it.


Plasma is the straw-colored liquid in which the blood cells are suspended.

Composition of blood plasma
Component Percent
Water ~92
Proteins 6–8
Salts 0.8
Lipids 0.6
Glucose (blood sugar) 0.1

Plasma transports materials needed by cells and materials that must be removed from cells:

Most of these materials are in transit from a place where they are added to the blood (a "source") to places ("sinks") where they will be removed from the blood.

Serum Proteins

Proteins make up 6–8% of the blood. They are about equally divided between serum albumin and a great variety of serum globulins.

After blood is withdrawn from a vein and allowed to clot, the clot slowly shrinks. As it does so, a clear fluid called serum is squeezed out. Thus:

Serum is blood plasma without fibrinogen and other clotting factors.

The serum proteins can be separated by electrophoresis.

Serum Lipids

Because of their relationship to cardiovascular disease, the analysis of serum lipids has become an important health measure.

The table shows the range of typical values as well as the values above (or below) which the subject may be at increased risk of developing atherosclerosis.
LIPID Typical values (mg/dl) Desirable (mg/dl)
Cholesterol (total) 170–210 <200
LDL cholesterol 60–140 <100
HDL cholesterol 35–85 >40
Triglycerides 40–160 <160
More on cholesterol How cholesterol is taken into cells.

Blood Transfusions

In the United States, in 2001, some 15 million "units" (~475 ml) of blood were collected from blood donors.

Ensuring the safety of donated blood

A variety of infectious agents can be present in blood. and could be transmitted to recipients. To minimize these risks, Most of these tests are performed with enzyme immunoassays (EIA) — Link — and detect antibodies against the agents. However, it takes a period of time for the immune system to produce antibodies following infection, and during this period ("window"), infectious virus is present in the blood. For this reason, blood is now also checked for the presence of the RNA of these RNA viruses: by the so-called nucleic acid-amplification test (NAT).

Thanks to all these precautions, the risk of acquiring an infection from any of these agents is vanishingly small. Despite this, some people — in anticipation of need — donate their own blood ("autologous blood donation") prior to surgery.

Blood Typing

Donated blood must also be tested for certain cell-surface antigens that might cause a dangerous transfusion reaction in an improperly-matched recipient. This is discussed in a separate page — link to it.

Blood Substitutes

Years of research have gone into trying to avoid the problems of blood perishability and safety by developing blood substitutes. Most of these have focused on materials that will transport adequate amounts of oxygen to the tissues.

Although some have reached clinical testing, none has as yet proved acceptable for routine use.

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16 July 2008