Blood is a vital fluid in the human body, composed of red blood cells, white blood cells, platelets, and plasma. It plays essential roles in transporting oxygen and nutrients to cells, removing waste products, and defending against infections. The red blood cells contain hemoglobin, which binds to oxygen, while white blood cells are crucial for the immune response. Plasma, the liquid component of blood, makes up about 55% of its volume and contains water, electrolytes, proteins, and hormones. Platelets are small cell fragments that help in blood clotting, preventing excessive bleeding. Together, these components ensure the body functions properly and maintains homeostasis.