Circulatory System

Vertebrates have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly hemoglobin molecules in blood cells transport oxygen. The human closed circulatory system is sometimes called the …

…small intestine into the capillaries. The blood then carries most of the nutrients to the liver. The liver removes certain nutrients from the blood and stores them. It later releases the nutrients into the blood when the body needs them. The liver also changes some nutrients into substances needed by the body. Blood leaving the liver contains nutrients that the cells use in the production of energy, enzymes, and new building materials for the body.