Fish do have blood, a specialized fluid tissue responsible for maintaining the internal environment of the body. Its primary purpose is to act as a transport system, delivering necessary gases, nutrients, and hormones to tissues. Simultaneously, it carries metabolic waste products away for excretion. This circulating fluid connects the gills, where oxygen is absorbed, to every other cell in the fish’s anatomy.
Composition and Key Characteristics
Fish blood is a complex mixture consisting of a fluid component called plasma and various cellular components suspended within it. Plasma, which is mostly water, contains dissolved proteins, electrolytes, and nutrients that are distributed throughout the body. The cellular elements include red blood cells (RBCs), which are responsible for oxygen transport, white blood cells (WBCs) for immune defense, and thrombocytes, which function in clotting to prevent blood loss.
The red blood cells (RBCs) of fish are nucleated, a defining characteristic that distinguishes them from those of mammals. This means mature fish RBCs retain their nucleus. Despite this difference, fish RBCs contain hemoglobin, the iron-containing protein that binds to oxygen and makes fish blood red when oxygenated. These nucleated RBCs also play a role in immune responses, such as acting as phagocytes and modulating leukocyte activity.
The Single-Loop Circulatory System
Fish manage blood movement using a closed, single-loop circulatory system. The fish heart has only two chambers—one atrium and one ventricle—and receives only deoxygenated blood. The ventricle pumps this blood forward through a single circuit that passes through the gills first.
The blood flows from the heart to the gills, where it passes through fine capillaries to pick up oxygen and release carbon dioxide. Once oxygenated, the blood continues directly from the gills to the rest of the body’s tissues through the systemic circulation, without returning to the heart. This continuous, unidirectional flow results in a significant pressure drop after the blood passes through the restrictive gill capillaries. Consequently, fish operate with lower overall blood pressure and flow rates.
Temperature and Oxygen’s Impact on Blood Appearance
Fish blood is fundamentally red due to hemoglobin, but external factors like water temperature and oxygen levels influence its physiological performance. In cold-water environments, oxygen solubility is higher and the fish’s metabolic rate is often lower. This reduced need for oxygen transport can lead to a lower synthesis of hemoglobin and a reduced volume of red blood cells, resulting in a lower hematocrit.
The most extreme example of this adaptation is the Antarctic icefish (family Channichthyidae), the only known vertebrate to completely lack hemoglobin and red blood cells as an adult. Their blood is transparent or colorless, a trait made possible by the extremely cold, oxygen-rich waters of the Southern Ocean. The icefish compensates for this massive reduction in oxygen-carrying capacity by having a high-volume circulatory system and a very large heart, relying solely on the oxygen physically dissolved in their blood plasma to meet their low metabolic demands.