Blood is specialized connective tissue comprising about 8% of the body mass. It has a thick, liquid extracellular matrix that contains cells, dissolved proteins, and electrolytes, making it five times more viscous than water. Blood is warm, around 38°C, and has an alkaline pH ranging from 7.35 to 7.45.

The primary function of blood is to transport oxygen and carbon dioxide between tissues and the lungs. Oxygenated blood is bright red, while oxygen-depleted blood is darker. It also carries nutrients from the digestive tract and secretions like hormones from endocrine glands to various tissues. The proteins and solutes dissolved in the blood act as buffering agents to regulate the pH of interstitial fluids. Blood also absorbs and redistributes the heat generated by active skeletal muscles.

In the case of injury, blood proteins induce clotting to prevent excessive blood loss. Specialized blood cells can clear foreign particles and secrete proteins, like antibodies, to protect against disease.