blood
Meaning of blood: Counted as a tissue rather than a fluid, because the cells suspended in it are as much a part of the body as any organ.
Definition of blood
Blood is the fluid tissue that circulates through the heart and blood vessels, made up of red cells, white cells and platelets suspended in a straw-colored liquid called plasma. Each part has its own job: red cells carry oxygen, white cells defend against infection, platelets help stop bleeding, and plasma carries everything dissolved in it.
What is blood made of?
| Component | What it does |
|---|---|
| Plasma | Rather more than half the volume; water carrying proteins, salts, hormones, nutrients and waste |
| Red cells | Carry oxygen using hemoglobin, and give blood its color |
| White cells | Several distinct types that fight infection and manage inflammation |
| Platelets | Cell fragments that clump at a wound and start a blood clot |
The cells are made in the bone marrow and are replaced continuously, since a red cell survives only a matter of months in circulation and most white cells far less.
What does the circulation actually carry?
Blood collects oxygen in the lungs and nutrients from the gut and delivers both to every tissue, then picks up carbon dioxide and other waste and takes them to the lungs, kidneys and liver for disposal. It carries hormones from the glands that make them to the organs they act on, distributes heat from the working core toward the skin, and moves immune cells and antibodies to wherever they are needed. All of this depends on an intact network of vessels, so a damaged blood vessel interrupts the supply to whatever lies downstream of it.
Why is a blood sample so widely useful?
Because blood passes through every organ, it picks up traces of what those organs are doing. Liver enzymes, kidney waste products, thyroid hormones, blood sugar, markers of inflammation and the cell counts themselves can all be measured from a single tube, which is why one sample often answers questions about several unrelated systems at once. That breadth has limits. Many results shift with hydration, recent food, time of day, pregnancy and medicines, and a value outside the reference range is a prompt to look further rather than a diagnosis by itself.
Why do blood groups matter?
Red cells carry inherited markers on their surface, and the immune system attacks cells carrying markers it does not recognize as its own. Blood group systems describe these markers, which is why a blood transfusion is matched before it is given. The same incompatibility can arise between a pregnant person and a fetus.
Used in a sentence
A blood sample was drawn to evaluate the patient's red and white cell counts.
Good to know
A single blood sample can reflect the function of organs throughout the body rather than one system, which is why the same specimen is often used to assess several unrelated conditions at once.