Blood is a kind of body fluid in human and other animals. It transports essential substances (such as nutrients and oxygen) to cells and transfers metabolic waste from these same cells.

In vertebrates, it consists of blood cells suspended in the plasma. Plasma accounts for 55% of blood, mainly water (92% of volume), and contains dissipative proteins, glucose, mineral ions, hormones, carbon dioxide (plasma is the main medium for the transport of excreted products) and blood cells themselves. Albumin is the main protein in plasma, whose function is to regulate the colloidal osmotic pressure of blood. Blood cells are mainly red blood cells (also known as RBC or red blood cells), white blood cells (also known as WBC or white blood cells) and platelets (also known as platelets). The most abundant cell in vertebrate blood is red cell. They contain hemoglobin, an iron-containing protein, which promotes oxygen transport by reversibly combining with this breathing gas and promoting its solubility in the blood. In contrast, most of carbon dioxide is transported out of the cell as bicarbonate ions in the plasma.

Hemoglobin in vertebrate blood is bright red when oxidized and dark red when deoxidized. Some animals, such as crustaceans and mollusks, use haemocyanin instead of hemoglobin to carry oxygen. Insects and some mollusks use a liquid called lymph instead of blood, the difference is that the closed-loop system does not contain lymph. In most insects, this "blood" does not contain oxygen carrying molecules such as hemoglobin, because their bodies are small enough to allow their tracheal system to provide oxygen.

The mandible vertebrates have an adaptive immune system, which is mainly based on leukocytes. White blood cells help fight infection and parasites. Platelets are important in clotting. Arthropods use haemolymph cells as part of their immune system.

The pumping function of the heart makes blood circulate around the human body through blood vessels. In animals with lungs, arterial blood carries oxygen from the inhaled air into the body's tissues, while venous blood carries carbon dioxide (the metabolic waste produced by cells) from the tissues into the lungs to exhale.

Medical terms related to blood usually begin with "hemato" or "hemato" (also known as "heemo -" and "Haemato -) in Greek (Haima). In terms of anatomy and histology, blood is considered to be a special form of connective tissue, considering that blood originates from bone and has potential molecular fibers in the form of fibrinogen.