peripheral blood
Meaning of peripheral blood: The blood a needle can reach, defined by contrast with the marrow where its cells were made rather than by where it flows.
Definition of peripheral blood
Peripheral blood is the blood moving through the arteries, veins and capillaries of the body, as distinct from blood and blood forming cells still inside the bone marrow. It is what a routine sample from a vein contains: red cells, white cells and platelets suspended in plasma. Nearly every ordinary blood test refers to this compartment.
Why does the word peripheral appear at all?
Because the contrast it draws is useful. Blood cells are manufactured in the marrow, which holds far more of them than the circulation does and holds them at every stage of maturity. Only finished cells normally leave. Saying peripheral blood makes clear that a sample came from a vein rather than from a marrow needle, and that what is counted is the finished product rather than the factory. The word adds little to an everyday sentence, but on a laboratory report it separates two very different specimens.
How do the two compartments differ?
| Feature | Peripheral blood | Bone marrow |
|---|---|---|
| Cells present | Mature forms almost exclusively | Every stage, from earliest to mature |
| How it is sampled | Needle into a vein | Needle into the hip bone or breastbone |
| Discomfort | Minor | Considerable, usually with local anesthetic |
| What it reveals | The end result of production | Whether production itself is working |
The relationship between the two makes a blood count informative. Few red cells in the circulation alongside a busy marrow points to loss or destruction; the same shortage with a quiet marrow points to a failure of production. Immature cells in the circulation are abnormal in an adult, and blasts found there signal leukemia rather than a variation of normal.
What is a blood film?
A drop spread thinly on a glass slide, stained and examined by eye. Automated counters report numbers, while the peripheral blood smear reports shape, size and inclusions that no counter reads reliably. Sickled red cells, fragments sheared off in damaged small vessels, malaria parasites inside red cells and the abnormal white cells of leukemia are all found this way, which keeps the film among the cheapest and most informative tests in medicine.
Why is it used for transplants?
Stem cells normally stay in the marrow, but a course of a growth factor drug pushes large numbers of them out into the circulation. They can then be collected from a vein by a machine that separates the wanted cells and returns the rest, replacing the operating room harvest that was once necessary. Grafts gathered this way take hold faster, though they carry more mature donor immune cells and with them a higher rate of graft-versus-host disease.
Also known as
Used in a sentence
A peripheral blood sample was sent for a full blood count and film.
Good to know
The term marks a specimen type as much as a location, separating a sample drawn from a vein from a marrow aspirate, and immature cells found in the circulation of an adult are abnormal.
Word origin
Where “peripheral blood” comes from
Peripheral comes from the Greek periphereia, a carrying around, built from peri, meaning around, and pherein, meaning to carry.