Explain to students about the use of animals, such as giraffes, in NASA studies. Giraffes are valuable models when studying the cardiovascular system because they are very tall animals that must overcome large gravitational effects when pumping blood from the heart up to the head. Giraffes, humans and other tall animals have high blood pressure at the heart level in order to maintain blood flow to the brain. When humans are in the microgravity environment of space, blood and other fluids move from the heart and lower body into the head because their vascular systems no longer have to overcome the force of gravity to pump blood to the brain. A similar thing may happen when giraffes lie down with the their head at the same level as their heart.
Blood moves from the tissues in the giraffe's lower body up into the head.
So much fluid can pool in the brain that the blood can no longer flow properly. This tendency to shift fluids may be one of the reasons that giraffes never let their heads rest at the same levels as their hearts, even when they're lying down. This fluid shift also occurs when humans lie in bed for long periods of time, or during space flight. Introduce the concepts of water displacement and buoyancy, drawing on the students' experiences with swimming and taking a bath. Ask students why some objects float in water while others sink.