Hemolymph

Hemolymph is the fluid in the circulatory system of most invertebrates, equivalent to the blood in vertebrates. It fills the hemocoel, the interior of the animal's body, and surrounds the organs directly. Hemolymph is composed of water, inorganic salts (mostly sodium, chlorine, potassium, magnesium, and calcium), and organic compounds (mostly carbohydrates, proteins, and lipids). The primary oxygen transporter molecule is hemocyanin.

Composition and Functions

Hemolymph performs many functions in the invertebrate body. It plays a crucial role in nutrient transport, waste removal, immune defense, and hydraulic control of movement. Unlike vertebrates, invertebrates lack red and white blood cells. Instead, hemolymph contains various types of hemocytes, which play a role in immune responses.

Circulation

In many invertebrates, the heart is a tube running the length of the body, contracting in sequence to propel the hemolymph into the aorta, from where it diffuses to the tissues. In others, like insects, the heart is localized to the abdomen, with the aorta extending into the head. The circulatory system can be open, as in many invertebrates, where the hemolymph bathes the organs directly, or closed, as in some annelids and most cephalopods, where the hemolymph is confined to vessels.

Hemolymph in Different Species

The composition of hemolymph varies across different species. For instance, in arthropods, the primary respiratory pigment is hemocyanin, while in some spiders and scorpions, it is hemerythrin. In some smaller insects, no respiratory pigment is necessary, as oxygen can diffuse into the tissues directly.

See Also

• Blood
• Invertebrate
• Circulatory system
• Hemocoel
• Hemocyanin