The circulatory system includes the lymphatic system, which circulates lymph. This metabolite then circulates in the bloodstream. Water-cooled engines contain passages in the engine block where cooling fluid circulates (the water jacket). Compressed air circulates through it to maintain moderate temperatures. LDL circulates and is absorbed by the liver and peripheral cells. During oven roasting, hot air circulates around the meat, cooking all sides evenly. A pump circulates liquid through the circuit. In addition, the air circulates in one direction through the parabronchioles in the lungs. The doughnut is held still while water circulates inside it. A small fan circulates the air in the cooking chamber. Plasma also circulates hormones transmitting their messages to various tissues. Glucose circulates in the blood of animals as blood sugar. Blood introduced directly into the veins circulates and functions as blood, not as nutrition. Leptin circulates in blood in free form and bound to proteins. The flow "circulates" through the network, hence the name of the problem.