3 Hearts, 9 Brains, And Blue Blood: 5 Shocking Facts About The Octopus Circulatory System

The octopus is arguably the most alien creature on Earth, possessing a suite of biological features that defy conventional animal biology—and its cardiovascular system sits at the top of that list. Unlike humans and most mammals, this marine genius doesn't rely on a single, central pump; it uses a complex, three-hearted system to survive in the deep, oxygen-scarce ocean. This deep dive into the octopus's anatomy, updated for December 2025, will not only answer the fundamental question of how many hearts it has but also reveal the ingenious mechanism behind its blue blood and why its systemic heart literally stops beating when the animal swims.

The latest scientific understanding confirms that the octopus, a member of the cephalopod class, possesses exactly three hearts, each with a distinct and specialized purpose. This redundancy is not a biological accident; it is an evolutionary necessity that allows the octopus to maintain its high-energy, predatory lifestyle, making it one of the most fascinating subjects in marine biology today. Understanding this unique organ arrangement is key to unlocking the secrets of its incredible intelligence and survival capabilities.

The Complete Anatomical Profile: The Octopus Cardiovascular System

To truly appreciate the function of the octopus's three hearts, one must first understand the overall anatomical framework of this remarkable mollusk. The octopus is a highly evolved invertebrate, and its internal structure reflects a life requiring high-speed movement, complex problem-solving, and efficient oxygen delivery in a challenging environment. Unlike most mollusks, it boasts a completely closed circulatory system, a feature more commonly associated with vertebrates like fish and mammals.

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• Classification: Kingdom Animalia, Phylum Mollusca, Class Cephalopoda (meaning "head-foot").
• Circulatory System: Closed, meaning blood remains within vessels (arteries, veins, capillaries) throughout the body.
• Number of Hearts: Three (3).
• Types of Hearts: Two (2) Branchial Hearts and One (1) Systemic Heart.
• Blood Pigment: Hemocyanin (copper-based).
• Blood Color: Blue when oxygenated.
• Nervous System: One central brain, plus a large ganglion (often referred to as a "mini-brain") in each of its eight arms, totaling nine functional "brains."
• Lifespan: Most species live only 1 to 2 years in the wild, despite their complex biology.

The Trio of Hearts: Understanding the Function of Three

The three hearts of the octopus are not interchangeable; they perform a highly coordinated sequence of pumping to ensure maximal oxygen delivery. This is crucial because of the nature of the octopus's blood and the high metabolic demands of its active lifestyle. The three hearts are categorized into two functional groups: the branchial hearts and the systemic heart.

1. The Two Branchial Hearts (Gill Hearts)

The octopus has two branchial hearts, also known as gill hearts, which are positioned near the gills. Their role is purely dedicated to respiration.

• Location: One branchial heart is situated at the base of each gill.
• Primary Function: To receive deoxygenated blood from the body and pump it through the capillaries of the corresponding gill.
• The Process: As the blood is pushed through the gills, it releases carbon dioxide and picks up a fresh supply of oxygen.
• Why Two? This dedicated, high-pressure pump ensures that the blood can efficiently traverse the dense gill structure to maximize oxygen uptake, a vital step before the blood is sent to the rest of the body.

2. The Single Systemic Heart (Main Heart)

The third heart is the systemic heart, which is the main circulatory pump for the entire body.

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• Location: Generally located in the middle of the octopus's body.
• Primary Function: To receive the freshly oxygenated blood from the two branchial hearts and then pump it with force to all the organs, muscles, and tissues throughout the body, including the arms and the brain.
• High-Pressure Delivery: The systemic heart is responsible for generating the high blood pressure needed to circulate oxygenated blood quickly to support the octopus's high metabolic rate.

The systemic heart is the reason for a fascinating biological limitation: it temporarily stops beating when the octopus swims vigorously. Swimming causes a sharp drop in blood pressure, and to maintain the integrity of the closed circulatory system, the systemic heart pauses. This is why octopuses prefer to crawl along the seafloor or jet in short bursts rather than swim long distances—swimming is metabolically costly and inefficient due to this circulatory pause.

Why Octopus Blood is Blue: The Hemocyanin Advantage

Another striking feature of the octopus circulatory system is its unusual blood color. Unlike humans, whose blood is red due to the iron-based protein hemoglobin, the octopus has blue blood.

The Role of Hemocyanin

The blue color is attributed to the oxygen-carrying protein called hemocyanin. Instead of iron, hemocyanin uses copper atoms to bind and transport oxygen molecules throughout the body.

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• Copper vs. Iron: When hemocyanin is oxygenated (carrying oxygen), the copper gives the blood a distinct blue hue. When deoxygenated, the blood is colorless or pale gray.
• Survival in Low Oxygen: This copper-based blood is not just a curiosity; it's a vital adaptation. Hemocyanin is significantly more efficient at transporting oxygen than hemoglobin in cold, low-oxygen conditions, especially at the high pressures of the deep ocean.
• The Evolutionary Edge: This "blue blood" adaptation is believed to have evolved because it allows the octopus to thrive in environments where oxygen is scarce and temperatures are low, giving it a crucial survival advantage over other marine life that rely on iron-based blood.

The Cephalopod Advantage: An Evolutionary Masterpiece

The complex, three-hearted, blue-blooded circulatory system is a testament to the evolutionary brilliance of the cephalopods, which include octopuses, squid, and cuttlefish. This unique arrangement solves a fundamental problem posed by their high-speed, predatory lifestyle and their specialized blood chemistry.

The systemic heart, which pumps blood to the body, receives blood only after it has passed through the gills, where the branchial hearts have done their work. The pressure drop that occurs when blood passes through the fine capillaries of the gills is a common problem in many circulatory systems. By having the two dedicated branchial hearts to boost the pressure *before* the gills, and then the powerful systemic heart to take over *after* oxygenation, the octopus ensures a robust and continuous flow of oxygen to its highly active muscles and its nine-part nervous system.

Without this three-heart system, the octopus could not sustain its high-energy activities, such as rapid jet propulsion, complex hunting maneuvers, and intricate camouflage changes. The combination of the multiple hearts and the super-efficient hemocyanin is what makes the octopus the eight-armed genius of the sea, capable of feats of intelligence and physical prowess unmatched by most other invertebrates.

In summary, the next time you encounter a fact about the octopus, remember its three hearts—two for the gills, one for the body—are the engine behind its extraordinary life, and its blue blood is the chemical key to its survival in the most challenging parts of the ocean. This creature is a living example of how evolution can craft a truly unique and efficient biological machine.