CIRCULATORY SYSTEM OF BIRDS: FUNCTIONS AND STRUCTURE - BIOLOGY - 2025

The circulatory system of birds is made up of the heart (with four cavities, similar to that of mammals), arteries and veins that carry nutrients, oxygen, carbon dioxide, metabolic waste, hormones and temperature.

This model of the circulatory system is quite efficient, as it allows birds to satisfy their metabolic needs to be able to fly, run, swim or dive. This system not only distributes the oxygen contained in the blood to the cells of the body, it also removes the waste product of metabolic processes and maintains the bird's body temperature (Lovette & Fitzpatrick, 2016).

Birds, like mammals, have a four-chamber heart (two ventricles and two atria), where a complete process of separation of oxygenated blood from non-oxygen-bearing blood takes place. The right ventricle pumps blood to the lungs, while the left ventricle must generate pressure to pump blood through the body (D'Elgin, 1998).

Birds tend to have larger hearts than mammals in proportion to their body size. The heart of birds is relatively large, as it must meet the metabolic needs required to fly.

Hummingbirds, despite their small size, are birds that have a larger heart compared to the proportions of the rest of their body. This is because the constant flapping of their wings demands high energy consumption.

Structure of the circulatory system in birds

Heart

The heart is the most important organ in the circulatory system of any vertebrate animal. In the case of birds, it is divided into four cavities responsible for separating oxygenated blood from that which is not. The heart has the important job of distributing oxygen and nutrients to the body through the blood (Reilly & Carruth, 1987).

The heart of birds is similar to that of mammals, however their structure is slightly different due to their lifestyle and needs. Birds have proportionally larger hearts than those of mammals, this means that the average volume that the heart of a mammal occupies is 0.4% of its body mass, while in birds it is 4%.

Smaller birds have especially large hearts compared to their size, as they require more energy to fly. On the other hand, the heart of birds pumps more blood per minute than the heart of mammals.

The speed of the heartbeat is slower, but the volume of blood pumped is greater in birds than in mammals. However, the heart of birds has a single aortic arch located on the right side of the body, while the heart of mammals has the same arch on the left side.

Veins and arteries

The blood located inside the bird's body flows through different types of blood vessels known as arteries, arterioles, capillaries and veins. Each of these channels fulfills different functions, as can be seen below.

• Arteries: carry oxygenated blood from the heart to the cells of the body.
• Arterioles: they distribute blood directly to the tissues and organs that need it most, through processes of vasoconstriction and vasodilation.
• Capillaries: they carry out an exchange between nutrients, gases and waste products between the blood and the cells of the body.
• Veins: they can be large or smaller (venules) and are responsible for conducting blood back to the heart to be oxygenated again and pumped back to the rest of the body.

Some of the most important arteries in the circulatory system of birds are the following:

• Carotid: carries blood to the head and brain.
• Brachialis: carry blood to the wings.
• Pectorals: carry blood that goes directly to the pectoral muscles, necessary for flight.
• Systemic arch: also called aorta, it is responsible for carrying blood to all parts of the body, except the lungs.
• Pulmonary arteries: carry blood that goes to the lungs.
• Celiac: they are the most important branch that emerges from the descending aorta. They are responsible for carrying blood to the organs and tissues in the upper abdomen.
• Renal arteries: carry blood that goes to the kidneys.
• Femoral: carry the blood that goes to the legs and the caudal artery is responsible for irrigating the tail.
• Posterior mesenteric: they are responsible for carrying blood to the organs and tissues in the lower abdomen.

The blood distributed by the arteries around the body, flows back to the heart, directly into the first chamber or right atrium through the veins.

From the right atrium, blood without oxygen is displaced to the right ventricle, which pumps the blood directly to the lungs to be oxygenated again (PoultryHub, 2017).

Oxygenation of the blood

In the lungs, the blood is again oxygenated and travels to the left atrium of the heart, from which it is pumped to the left ventricle.

This last cavity through which the blood passes, is the strongest and most muscular of all, since it has the task of pumping the blood through the arteries that supply the entire body. Therefore, the left ventricle has a thick wall of muscle that allows it to fulfill this important task (Farner & King, 1972).

With each beat of the heart, the process of oxygenation of the blood is repeated. Only mammals and birds have four cavities in their hearts that allow them to separate the oxygenated blood from that which is no longer so. In other animals, the heart has a maximum of two chambers and the blood is mixed.

For the oxygenated blood distribution process to be more efficient, it is important that the oxygenated blood is in constant circulation through the bird's body, and the oxygen-depleted blood quickly returns to the heart to be oxygenated again.

An efficient blood distribution process implies a faster metabolic process and more energy for the bird (Scanes, 2015).

References

1. D'Elgin, T. (1998). The Circulatory System. In T. D'Elgin, The Everything Bird Book: From Identification to Bird Care, (p. 18). Holbrook: Adams Media Corporatio.
2. Farner, DS, & King, JR (1972). Avian Biology, Volume 2. New York-London: Academic Press.
3. Lovette, IJ, & Fitzpatrick, JW (2016). Circulatory System. In IJ Lovette, & JW Fitzpatrick, Handbook of Bird Biology (pp. 199-200). Oxford: Wiley.
4. (2017, February 1). Poultry Hub. Retrieved from Circulatory System: poultryhub.org
5. Reilly, EM, & Carruth, G. (1987). Circulatory System. In EM Reilly, & G. Carruth, The bird watcher's diary (p. 30). Harper & Row.
6. Scanes, CG (2015). The Cardiovascular System. In CG Scanes, Sturkie's Avian Physiology (pp. 193-198). London: Elsevier.