The circulatory system of most vertebrates relies on a single, centralized pump, making the concept of multiple hearts a biological anomaly. The existence of creatures that defy this fundamental pattern challenges our understanding of circulatory efficiency and evolutionary adaptation. Investigating these unique systems offers a window into the physiological compromises required for survival in unusual environments.
The Fish with Multiple Hearts
The organism that possesses a complex array of pumping organs is the hagfish, a jawless fish belonging to the class Myxini. While it has one primary systemic heart, it utilizes several accessory pumps throughout its body, leading to its description as having multiple hearts. This ancient marine scavenger is one of the most primitive vertebrates. The accessory pumps are functionally distinct and help the hagfish manage its unusually low-pressure blood flow.
How Standard Fish Circulation Works
The majority of bony fish and cartilaginous fish possess a standard single-circuit circulatory system. In this arrangement, blood travels through the heart only once during a complete circuit of the body. The heart is two-chambered, consisting of a single atrium and a single ventricle. The atrium collects deoxygenated blood returning from the body tissues, and the ventricle pumps this blood forward. This deoxygenated blood is sent directly to the gills, where gas exchange occurs before the oxygenated blood is distributed to the rest of the body.
The single-circuit design is inherently a low-pressure system because the blood must pass through two sets of fine capillaries before returning to the heart. First, the blood flows through the high-resistance gill capillaries, and then it continues through the systemic capillaries of the body organs. The pressure drop after the gills means that blood flows relatively slowly through the rest of the body.
Components of the Hagfish Circulatory System
The hagfish manages its circulation using one main pump and several decentralized boosters. The most important is the branchial heart, the functional equivalent of the main vertebrate heart. This three-chambered organ, which includes a sinus venosus, atrium, and ventricle, is responsible for pumping blood to the gills. The branchial heart is highly unusual because it lacks nervous innervation, meaning its rhythm is controlled intrinsically by factors like venous return, rather than by the central nervous system.
Accessory Pumps
The hagfish utilizes several accessory pumps to maintain flow:
- The portal heart is a muscular structure composed of cardiac muscle located outside the protective pericardium. Its sole function is to pump venous blood from the gut directly through the liver, assisting in nutrient processing.
- Paired cardinal hearts are contractile chambers associated with the large cardinal veins.
- Paired caudal hearts are pumps located in the tail region that aid in returning blood from the posterior parts of the body.
Why the Hagfish Needs Multiple Pumps
The necessity for these multiple pumps stems from the hagfish’s exceptionally low blood pressure, the lowest recorded among all vertebrates. This low pressure is partly a consequence of its partially open circulatory system, which features massive, low-pressure subcutaneous blood sinuses situated beneath the skin. These sinuses can hold a significant portion of the hagfish’s high total blood volume.
A single heart could not generate enough force to efficiently move blood through this large, flaccid sinus network and back to the gills. The accessory hearts act as localized boosters, ensuring blood is propelled from distant, low-pressure areas back toward the main branchial heart. For example, the caudal hearts specifically move blood from the posterior subcutaneous sinuses into the caudal vein. Without these decentralized pumps, the low pressure would result in blood pooling and inadequate circulation.