Medical Definition of GRAIN. 1. a: a seed or fruit of a cereal grass b: the seeds or fruits of various food plants including the cereal grasses and in commercial and statutory usage other plants (as the soybean) c: plants producing grain.2. : a small hard particle or crystal (as of sand or salt).efinition of GRAIN for Kids. 1. : the edible seed or seeds of some grasses (as wheat, corn, or oats) or a few other plants (as buckwheat). 2. : plants that produce grain.
The Circulatory System. Veins, arteries and capillaries are the three types of blood vessels in the circulatory system. The blood vessels carry blood, which contains antibodies, nutrients, oxygen, electrolytes and hormones, throughout the body to be used by the organs.eins. 1  Veins carry blood from throughout the body back to the heart. 2  The blood in the veins is dark red in color because it contains very little oxygen. 3  The walls of veins are not as strong as the walls of the arteries because they farther away from the heart, meaning they are subject to less pressure from the heart.
There is a layer of muscle surrounding the arteries and the veins which help contract and expand the vessels. This creates enough pressure for blood to be pumped around the body. Blood vessels are part of the circulatory system, together with the heart and the blood.here is a layer of muscle surrounding the arteries and the veins which help contract and expand the vessels. This creates enough pressure for blood to be pumped around the body. Blood vessels are part of the circulatory system, together with the heart and the blood.