Hummingbirds have a two-chambered heart, consisting of one ventricle and one atrium. This is different from the four-chambered hearts found in humans and other mammals.
Anatomy of the Hummingbird Heart
The hummingbird heart is small and compact, weighing less than half a gram on average. It beats extremely fast, up to 1,260 beats per minute while at rest. This rapid heart rate enables the high metabolism that powers their fast wing beats and allows them to hover in place while drinking nectar.
The main structures of the hummingbird heart are:
- Atrium – Upper receiving chamber
- Ventricle – Lower pumping chamber
Oxygenated blood from the lungs enters the left atrium, then moves to the ventricle. The ventricle contracts to pump blood out to the body through the aorta. Deoxygenated blood returns from the body to the right atrium, then fills the ventricle again before being pumped back to the lungs.
Unique adaptations in the hummingbird circulatory system include:
- Thicker left ventricle wall to handle the high pumping pressure
- Higher red blood cell count to deliver more oxygen
- Ability to starve organs of oxygen when needed to accommodate flying
Differences From Mammal Hearts
In contrast to the four-chambered hearts of mammals like humans, the hummingbird heart has:
- One ventricle instead of two
- One atrium instead of two
- No septum to separate oxygenated and deoxygenated blood
This means some mixing of oxygenated and deoxygenated blood occurs. However, the arrangement allows their small heart to pump blood efficiently while meeting the extreme oxygen demands of hummingbird flight.
Embryonic Development of the Hummingbird Heart
When hummingbirds develop as embryos, their hearts begin as two separate parallel tubes called primordial tubes. These fuse together to form a single tube, which gets divided into atrium, ventricle and outflow tract. This process happens between embryonic days 3 and 6.
Some key developmental steps include:
- Embryonic day 3 – Paired primordial tubes visible
- Embryonic day 4 – Tubes migrate together
- Embryonic day 5 – Tubes fuse into a single heart tube
- Embryonic day 6 – Heart tube folding forms chambers
Regulatory genes control the chamber specification. For example, the TBX5 gene drives development of the left ventricle. After the chambers form, the valves and septa develop to separate flows between the atrium and ventricle.
Comparison to Bird Embryonic Heart Development
The embryonic heart development of hummingbirds follows a similar process to other birds. Differences arise late in development that distinguish the hummingbird heart:
| Feature | Hummingbird | Other Bird |
|---|---|---|
| Early heart tube formation | Same | Same |
| Early chamber specification | Same | Same |
| Septum formation | Partial or absent | Full septum present |
| Outcome | Two chambers | Four chambers |
This table summarizes key differences in heart development between hummingbirds and other birds leading to the two-chambered hummingbird heart.
Unique Properties of Hummingbird Hearts
The hummingbird’s two-chambered heart has many unique properties that enable their sustained hovering flight:
Extreme Heart Rate
Hummingbirds have the highest heart rate relative to body size of all animals. While at rest, the hummingbird heart beats at about 500 beats per minute. During flight, it can reach up to 1,260 beats per minute.
For comparison, human hearts average 60-100 beats per minute at rest. Hummingbird hearts beat at about 10-15 times faster than humans when adjusted for their smaller body size.
High Cardiac Output
Cardiac output is the volume of blood pumped by the heart per minute. Hummingbird cardiac output is the highest of all vertebrates relative to their body size.
| Animal | Cardiac Output | Heart Rate |
|---|---|---|
| Hummingbird | 700 ml/min/kg | 1260 bpm |
| Shrew | 450 ml/min/kg | 800 bpm |
| Mouse | 300 ml/min/kg | 600 bpm |
| Human | 70 ml/min/kg | 60-100 bpm |
This table shows hummingbirds have 10x higher cardiac output per weight than humans, enabling their unique metabolic demands.
High Blood Oxygen Content
Hummingbird blood has about double the oxygen carrying capacity by volume compared to human blood. Their blood has more red blood cells (40-50% by volume vs. 40-45% in humans). This helps deliver oxygen at a higher rate.
How the Heart Supports Hummingbird Flight
The unique two-chambered hummingbird heart provides the extreme cardiovascular performance required to support their metabolically demanding hovering flight:
- Rapid heartbeat provides quick oxygen circulation
- Powerful pumping generates high cardiac output
- Rich blood oxygen content sustains muscles
- Adaptable oxygen flow allows brief organ starvation
Without their specialized cardiovascular system, hummingbirds would not be able to hover and fly in the same way. The two-chambered heart configuration allows for compact size while still providing enough power output and blood flow.
Oxygen Regulation During Hovering
While hovering, hummingbirds can divert blood away from the digestive system and kidneys. This allows them to essentially starve organs of oxygen temporarily so more oxygen reaches the flight muscles. The high oxygen capacity of their blood helps prevent damage.
Thermoregulation Benefits
The exceptional hummingbird cardiovascular system also helps maintain their high body temperature and prevent overheating during flight. Rapid circulation dissipates heat, while increased blood flow to surface capillaries releases heat.
Conclusion
In summary, hummingbirds have a highly specialized two-chambered heart that enables their unique hovering flight ability. The rapid heartbeat, high cardiac output, and rich oxygen blood allow them to meet extreme metabolic demands. Their cardiovascular system powers sustained muscle exertion and adapts blood flow on demand, providing key advantages that pair with their aerodynamic body design to make hovering possible.
