Context: Blood is a liquid connective tissue. It circulates throughout the body via blood vessels due to the pumping action of the heart. You couldnt survive without the approximately 4.5 to 5 liters of blood that are constantly being pumped through your blood vessels. Blood consists of both liquid and cells. The liquid part of blood is called plasma. Plasma is a watery, golden-yellow fluid that contains many dissolved substances. Substances dissolved in plasma include glucose, proteins, and gases. Plasma also contains blood cells. There are three types of blood cells: red blood cells, white blood cells, and platelets. You can see all three types in Figure 18.8. 1. Red blood cells are shaped like flattened disks. There are trillions of red blood cells in your blood. Each red blood cell has millions of molecules of hemoglobin. Hemoglobin is a protein that contains iron. The iron in hemoglobin gives red blood cells their red color. It also explains how hemoglobin carries oxygen. The iron in hemoglobin binds with oxygen molecules so they can be carried by red blood cells. 2. White blood cells are larger than red blood cells, but there are far fewer of them. Their role is to defend the body in various ways. For example, white blood cells called phagocytes engulf and destroy microorganisms and debris in the blood. 3. Platelets are small, sticky cell fragments that help blood clot. A blood clot is a solid mass of cell fragments and other substances that plugs a leak in a damaged blood vessel. Platelets stick to tears in blood vessels and to each other, helping to form a clot at the site of injury. Platelets also release chemicals that are needed for clotting to occur. The main function of blood is transport. Blood in arteries carries oxygen and nutrients to all the bodys cells. Blood in veins carries carbon dioxide and other wastes away from cells to be excreted. Blood also transports the chemical messengers called hormones to cells throughout the body where they are needed to regulate body functions. Blood has several other functions as well. For example, blood: defends the body against infections. repairs body tissues. controls the bodys pH. helps regulate body temperature. Red blood cells carry proteins called antigens on their surface. People may vary in the exact antigens their red blood cells carry. The specific proteins are controlled by the genes they inherit from their parents. The particular antigens you inherit determine your blood type. Why does your blood type matter? Blood type is important for medical reasons. A patient cant safely receive a transfusion of blood containing antigens not found in the patients own blood. With foreign antigens, the transfused blood will be rejected by the persons immune system. This causes a reaction in the patients bloodstream, called agglutination. The transfused red blood cells clump together, as shown in Figure 18.9. The clumped cells block blood vessels and cause other life-threatening problems. There are many sets of antigens that determine different blood types. Two of the best known are the ABO and Rhesus antigens. Both are described below. You can also learn more about them by watching this video: ABO blood type is determined by two common antigens, often called antigen A and antigen B. If your red blood cells carry only antigen A, you have blood type A. If your red blood cells carry only antigen B, you have blood type B. If your red blood cells carry both antigen A and antigen B, you have blood type AB. If your red blood cells carry neither antigen A nor antigen B, you have blood type O. Another red blood cell antigen determines a persons Rhesus blood type. This blood type depends on a single common antigen, typically referred to as the Rhesus (Rh) antigen. If your red blood cells carry the Rhesus antigen, you have Rhesus-positive blood, or blood type Rh+. If your red blood cells lack the Rhesus antigen, you have Rhesus-negative blood, or blood type Rh-. Some diseases affect mainly the blood or its components. They include anemia, leukemia, hemophilia, and sickle- cell disease. Anemia is a disease that occurs when there is not enough hemoglobin 
 Question: __genetic disorder in which abnormal hemoglobin causes red blood cells to change shape
Output: sickle-cell disease