The Avian Heart and Circulatory System

The heart is the driving force behind the avian circulatory system, and the vessels of the vascular system are the highways for oxygen, energy, and waste transport. Behind the feathers, a complex and fascinating collection of processes unfolds within these systems to support the diverse array of bird species at home on planet Earth.

Of course, hearts, veins, and other cardiovascular components are not unique to birds. Mammals, reptiles, fish, and all other vertebrates have some form of circulatory system. However, the metabolic demands of sustained flight have created an especially powerful and efficient heart and vascular system in our feathered friends.

In this guide, we’ll take a deep dive into the form and function of the avian heart and blood vessels, so read along with us to learn all about these crucial physiological systems and processes.

The Avian Heart – An Engine for Flight

Structure and Size

The avian heart is very similar in form and function to the mammal heart, although it is proportionally bigger and stronger to power a bird’s high metabolic rate.

Structurally, a bird’s heart is a muscular organ comprising four chambers separated by one-way valves. The chambers on the left side receive and distribute oxygenated blood, while the right side receives deoxygenated blood and pumps it through the lungs for gas exchange.

The size of a bird’s heart varies with its body size but is proportionally larger in small birds with high metabolic rates. Larger species and birds that do not fly long distances tend to have smaller hearts relative to their body mass.

Beating Fast and Strong

Heart rates vary dramatically across bird species and according to activity levels. In Ostriches, resting heart rates may be as low as 23 beats per minute but rise dramatically when the birds are alarmed or stressed. Meanwhile, Manakins and Hummingbird’s hearts can reach over 1200 beats per minute!

A study on Eurasian Blackbirds highlighted the variation even within the same species. The researchers discovered that heart rates were significantly lower at night and that average heart rates varied considerably across seasons.

Eurasian Blackbird. Heart rates vary dramatically across bird species and according to activity levels

Blood Vessels – Highways for Oxygen

Arteries and Veins

The avian circulatory system consists of the heart and a network of blood vessels for efficiently transporting blood throughout the body. The arteries transport blood away from the bird’s heart while the veins return deoxygenated blood to the right atrium.

Oxygenated blood enters the aorta (the largest artery in the body) after leaving the left ventricle. From there, blood is diverted into smaller arteries, such as the coronary artery, which supplies the heart itself, and the brachiocephalic arteries, which supply the brain and wings.

Although arteries generally transport oxygenated blood, the pulmonary artery differs by transporting deoxygenated blood from the right ventricle to the lungs.

Various veins return blood to the heart. The precavae collect blood returning from the head and wings, while the postcava collects blood returning from the posterior (lower half) of the body. Both these large veins empty into the right atrium.

Capillaries

The arteries and veins are the highways of the avian vascular system, evolved to efficiently transport blood between the tissues and the heart. However, they are too large to allow the exchange of oxygen and metabolic products required to support the intense metabolic demands of an active bird. This is the role of much smaller vessels called capillaries.

Capillaries are extremely narrow, allowing them to branch out so extensively that tissues throughout the body have a nearby source of blood. These tiny vessels also have very thin walls, which enable an easy flow of gases through the epithelial layer by diffusion. Larger products move between the cells that make up the walls.

Blue Manakin. The avian circulatory system consists of the heart and a network of blood vessels for efficiently transporting blood throughout the body

Circulatory Adaptations for Avian Lifestyles

High-Altitude Adaptations

Life at the top of the world presents a special challenge to animals with such high metabolic rates because air pressure is much lower, making it harder for birds to extract the oxygen they need. However, some birds have evolved to live above 16,000 feet (approximately 5000m), and some even migrate at heights over 20,000 feet (about 6000m).

Surviving in such low-oxygen environments requires an advanced respiratory system but also relies on a highly efficient circulatory system. Andean Geese, which spend their entire lives in thin air, have a large stroke volume (amount of blood pumped from the left ventricle), while Bar-headed Geese, which fly over the world’s highest mountain range, respond by increasing their heart rate.

Endurance Flyers

Long-distance migration places enormous demands on a bird’s cardiovascular system, but migratory species can prepare for the trip in some remarkable ways.

Some migratory birds increase their heart mass and elevate their blood’s oxygen-carrying capacity (hemoglobin levels) to improve their endurance.

Regulating the Rhythm: Heart Rate Control in Birds

Autonomic Control

A bird’s heart rate is under the involuntary control of the autonomic nervous system, which is regulated by a part of the brain known as the hypothalamus. Sympathetic stimulation caused by increased physical exertion increases the heart rate rapidly, but exercise is not the only factor that affects heart rate.

In some birds, heart rate can more than treble due to stress or excitement alone. Once the threat has passed or the bird is at rest, the bird’s heart rate naturally decreases due to the balancing effect of the parasympathetic nervous system.

Hormonal Influences

The nervous system controls discussed above rely on hormones that elevate heart rate, constrict blood vessels, and have other effects on cardiovascular function.

Incidentally, these hormones are transported from glands via the bloodstream. The effects of hormones can be acute in the case of sudden physical activity or a ‘fight or flight’ response or occur on a more seasonal basis.

Bar-headed Goose. Surviving in such low-oxygen environments requires an advanced respiratory system but also relies on a highly efficient circulatory system

From Egg to Adult – The Developing Avian Heart

Growth Changes

The avian heart and vascular system develop rapidly within the egg. In chickens, the vascular system is recognizable on just the second day of incubation, and before the second day is over, the heart will be linked to this system and begin to beat. Until the fourth day, the heart occurs outside the body, but it will be incorporated into the chest by the end of the first week.

The initial blood flow system functions to transport nutrients from the yolk and for respiration within the egg. However, these are only useful as long as the developing chick remains within its shell.

Before hatching, the young bird will puncture the air cell at the end of the egg and use its lungs for the first time before emerging into the outside world.

Impact of Growth on Cardiac Function

Growth places high demands on the circulatory system, even though nestlings and young birds aren’t as physically active as their parents.

An early study on domestic chicken chicks found that heart rate increased rapidly over the first three days after hatching and continued to increase to a maximum at weeks three and four. After that, their heart rate decreased to normal adult levels.

Challenges and Disorders

Common Cardiac Disorders

Birds are vulnerable to various cardiovascular disorders caused by factors such as poor diet, exposure to harmful chemicals, bacterial and parasitic infections, congenital defects, and environmental factors such as altitude.

These ailments are often difficult to diagnose and may become progressively worse or present suddenly. Cardiac disorders can reduce a bird’s ability to perform high-energy activities and may result in lethargy, shortness of breath, coughing, or fainting.

Environmental and Dietary Influences

External factors can have important consequences for avian cardiovascular health. Toxic substances such as oil and inhalable particulate matter can seriously affect birds that live in polluted areas or occur after pollution events.

Diet is another external factor that can affect the avian circulatory system. Diets that are high in calories can cause obesity, while an excess of fatty foods can lead to build-ups in the arteries called atherosclerosis. Excess salt in the diet is also a risk factor for birds and may even cause heart failure.

A Baby Chick hatching. An early study on domestic chicken chicks found that heart rate increased rapidly over the first three days after hatching and continued to increase to a maximum at weeks three and four

Summary

From early development in the egg to the peak of physical performance and endurance in diving, sprinting, and high-altitude migrations, birds rely on their circulatory systems every ‘flap’ of the way.

Their cardiovascular system is a wonderful collection of mechanical and chemical processes that follows the same general blueprint but has also evolved to suit specialized needs.

With so many bird species under pressure from environmental degradation and habitat loss, understanding avian cardiovascular physiology and protecting birds from environmental risk factors is more important than ever before.