Blood is a vital fluid in the human body that carries oxygen and nutrients to cells while removing waste products. It consists of red blood cells, white blood cells, platelets, and plasma. Red blood cells transport oxygen using a protein called hemoglobin, while white blood cells play a crucial role in the immune system, defending against infections. The average adult has about 5 to 6 liters of blood, which makes up roughly 7-8% of body weight. Blood is produced in the bone marrow and circulates through the body via a network of blood vessels, including arteries, veins, and capillaries.