"Blood" - what is it, definition of the term
«Blood» is the red, protein‑rich circulatory fluid of vertebrates, composed primarily of plasma, erythrocytes, leukocytes and platelets; it transports oxygen, nutrients, hormones and waste products, maintains osmotic balance and thermoregulation, and serves as the primary food source for ectoparasites such as ticks, bugs, lice and fleas.
Detailed information
The circulatory fluid consists of plasma, a water‑based solution containing proteins, electrolytes, nutrients, hormones and waste products. Red cells, rich in hemoglobin, transport oxygen; white cells provide immune defense; platelets mediate clot formation. Viscosity and osmotic pressure are regulated by albumin and fibrinogen, maintaining vascular integrity.
When hematophagous arthropods attach, they insert mouthparts that pierce the host’s skin and encounter the fluid. Salivary secretions of ticks, bugs, lice and fleas contain anticoagulant compounds such as apyrase, thrombin inhibitors and vasodilators. These agents prevent clotting, enlarge blood vessels and facilitate continuous ingestion. The volume of fluid taken varies by species:
- Ticks ingest up to several milliliters during a single feeding episode lasting days.
- True bugs (e.g., bed bugs) consume a few microliters per bite, repeated multiple times.
- Lice acquire only enough to sustain metabolism, typically a fraction of a microliter per feeding.
- Fleas draw a minute quantity, sufficient for rapid digestion and egg production.
Pathogen transmission occurs when the fluid carries microorganisms. Ticks transmit spirochetes and viruses; bugs can spread Trypanosoma spp.; lice are vectors for Rickettsia and Borrelia; fleas convey Yersinia pestis. The fluid’s composition influences pathogen survival: plasma proteins protect microbes, while immune cells may limit infection. Anticoagulants also aid pathogen entry by maintaining an open feeding site.
Host responses include localized inflammation, platelet aggregation and coagulation cascade activation. Persistent feeding can deplete iron stores, reduce hematocrit and trigger anemia. Repeated infestations increase exposure risk to vector‑borne diseases, emphasizing the need for control measures targeting both the arthropods and their blood‑feeding behavior.