Birds have a very efficient circulatory and respiratory system that enables them to fly. Their blood cells and blood composition are adapted to meet the high metabolic demands of flight. Birds have nucleated red blood cells, unlike mammalian red blood cells that lack a nucleus. Their blood also contains five main types of white blood cells that play key roles in the bird’s immune system. In this article, we will explore the different types of avian blood cells, their structure and function.
Avian Red Blood Cells
Avian red blood cells, also known as erythrocytes, are nucleated. This means the cells contain a nucleus, unlike mammalian erythrocytes that lack nuclei. The presence of a nucleus allows the cells to synthesize protein and RNA. However, it also means the cells are larger, so birds have a lower red blood cell count than mammals. A microliter of bird blood contains around 2.5-3.5 million red blood cells, while a microliter of human blood contains 5 million red blood cells.
The main function of avian red blood cells is to transport oxygen from the lungs to the tissues and organs of the body. Birds have adapted to meet the high oxygen demands of flight through:
- A high hemoglobin content – Each cell contains around 30-36 pg of hemoglobin which binds to oxygen.
- A large heart and vascular system – This circulates blood with oxygen quickly around the body.
- Systemic lungs – Bird’s lungs deliver oxygenated blood directly via arteries to organs and muscles.
- High capillary density – Organs like the flight muscles contain many capillaries to extract oxygen.
The presence of a nucleus means avian red blood cells can live longer than mammalian cells, surviving for 300-600 days. Old and defective cells are removed by macrophages in the spleen and bone marrow.
Avian White Blood Cells
Birds have five main types of white blood cells (leukocytes) that play crucial roles in the immune system:
Heterophils
Heterophils are the most abundant white blood cells in birds, making up 50-75% of the total leukocyte count. They are equivalent to neutrophils in mammals. Heterophils phagocytose and kill bacteria and fungi, releasing antimicrobial enzymes from cytoplasmic granules. They also release web-like chromatin traps to contain infections.
Lymphocytes
Lymphocytes make up 10-30% of avian white blood cells. They are key mediators of the adaptive immune system. There are two main types:
- B lymphocytes – Produce antibodies to neutralize pathogens.
- T lymphocytes – Kill infected cells and regulate the immune response.
Lymphocytes develop immune memory after an infection, enabling a stronger response to subsequent encounters with the same pathogen.
Monocytes
Monocytes account for 2-10% of the leukocyte population in birds. Like heterophils, they are phagocytes that engulf and digest pathogens, dead cells and cell debris. However, monocytes remain in the bloodstream for longer before maturing into macrophages in tissues.
Eosinophils
Eosinophils normally represent around 5% of circulating white blood cells in birds. They defend against parasites such as worms. Eosinophils attack parasites with cytotoxic proteins and free radicals that damage the parasite.
Basophils
Basophils are the rarest avian leukocytes, accounting for under 1% of the total. They contain granules with chemicals like histamine and heparin. When activated, basophils release these chemicals to induce inflammation as part of the immune response.
Avian Blood Platelets
Birds have disc-shaped anuclear platelets, like mammals. Avian thrombocytes are about a third the size of human platelets. Their main role is to stop bleeding by aggregating and releasing clotting factors at the site of blood vessel injury. Each microliter of bird blood contains around 10,000-30,000 thrombocytes.
Avian Blood Plasma
Plasma makes up around half of the total blood volume in birds. It is the liquid part of blood containing water, proteins (e.g. albumin, antibodies), electrolytes, nutrients, hormones and clotting factors. Avian plasma helps regulate pH, osmotic pressure, and circulation of nutrients, gases and waste.
Blood Component | Cell Type | Amount in blood | Main functions |
---|---|---|---|
Red blood cells | Erythrocytes | 2.5-3.5 million per microliter | Transport oxygen |
White blood cells |
|
Varies by cell type | Immune system functions e.g. phagocytosis, antibody production |
Platelets | Thrombocytes | 10,000-30,000 per microliter | Blood clotting |
Plasma | Liquid portion | Around 50% of blood volume | Transport of proteins, electrolytes, nutrients etc. |
Key Differences From Mammalian Blood
While bird blood is similar to mammalian blood, some key differences include:
- Nucleated red blood cells – Allows protein synthesis but fewer cells per volume.
- Presence of heterophils – Equivalent to neutrophils in mammals.
- No natural eosinophil degranulation – Requires antigen-antibody complex.
- Thrombocytes are smaller – Birds have lower platelet counts.
- Plasma uric acid – Uric acid is the main nitrogenous waste product.
- Lower fibrinogen – Slower blood clotting times.
- Higher body temperature – Average 41°C compared to 37°C in mammals.
Role of the Avian Circulatory System
Birds have a four-chambered heart like mammals. Oxygenated blood from the lungs flows back to the left atrium, then into the left ventricle which pumps it around the body. Deoxygenated blood returns from the body into the right atrium, then into the right ventricle which pumps it back to the lungs.
Some key adaptations make the avian circulatory system highly efficient:
- Powerful cardiac output – The heart beats faster and has thicker ventricular walls than in mammals.
- Extensive capillary beds – Organs like the flight muscles contain dense networks of capillaries.
- Countercurrent exchange – Arteries transfer heat to veins in the legs to minimize heat loss.
- Separation of oxygenated and deoxygenated blood – Prevents oxygen loss.
These adaptations enable sufficient oxygen transport for the metabolic demands of flight. Birds flapping in fast, sustained flight can increase their oxygen consumption up to 15-20 times their resting levels.
How Do Birds Form Blood Clots?
Birds form blood clots, known as hemostatic plugs, through the coagulation cascade. This involves:
- Platelet aggregation – Platelets stick together and release clotting factors at the injury site.
- Coagulation – Clotting factors like thrombin convert fibrinogen to fibrin threads forming the clot meshwork.
- Clot retraction – The clot shrinks as platelets contract.
Birds have some small differences in their clotting system compared to mammals. They have lower levels of fibrinogen and factor XIII. Birds also lack factor VII. As a result, the initial clotting time is slightly longer than in mammals, but clots are eventually formed to stop bleeding.
Do Birds Have Blood Types?
Birds do not have the same blood group systems as mammals. However, they do have naturally occurring antibodies that can cause transfusion reactions. Chickens have five main blood group systems:
- E – The most important system with over 20 antigen specificities.
- A – Similar to the mammalian ABO system but with different antigens.
- B – Two identified antigens BG-1 and BG-2.
- L – Three antigen groups in chickens.
- M – Only found in pigeons.
There are also other minor blood groups. It’s important to cross-match donor and recipient blood before transfusions. Even with compatible groups, reactions can occasionally occur due to other blood proteins.
How Do Birds Adapt To High Altitudes?
Birds that fly or live at high altitudes adapt in several ways to low oxygen levels:
- Increased red blood cell production – More erythrocytes to transport oxygen.
- Increased hemoglobin levels in cells – Binds and carries more oxygen.
- Enlarged hearts and lungs – Improves oxygen circulation.
- Increased capillary networks – Extracts oxygen efficiently.
- Controlled hypothermia – Lowers metabolic rate and oxygen needs.
Species like bar-headed geese can fly over the Himalayas above 9,000 meters where oxygen levels are extremely low. Their adaptations enable remarkable feats of high altitude flight.
Conclusion
In summary, avian blood is well adapted to meet the high aerobic demands of flight. Birds have nucleated red blood cells, five main leukocytes, smaller platelets and uric acid in their plasma. A powerful circulatory system transports oxygen thanks to adaptations like countercurrent exchange. Birds form clots to stop bleeding via platelet aggregation and the coagulation cascade. And species can evolve adaptations to fly in low oxygen conditions at high altitudes. Understanding the unique properties of avian blood provides fascinating insights into the evolution of birds for sustained flight.