Heart rate is controlled by hormones, body temperature, and by the opposing actions of two sets of nerves. Blood Flow Blood vessels contain 3 layers of tissue. Outer layer connective tissue with elastic fibers, middle layer of smooth muscle and elastic fibers, and an inner layer of endothelium of simple squamous epithelium. Arteries carry blood away from the heart, while veins carry blood toward the heart. Veins contain valves to prevent the back flow of blood on its way to the heart.
Capillaries connect veins to arteries.
It is here that any and all exchanges take place between the blood and the body. Materials may leave due to passive diffusion or hydrostatic pressure. Capillaries are one cell thick. The fluid is called lymph and it is mainly composed of fatty materials, large proteins and ion. The vessels have valves in them and the fluid is pushed along by the skeletal muscles.