Purpose of circulation is to service the cells of the body,
Functions of the Cardiovascular System
1. Contractions of the heart generate blood pressure, which moves blood through blood vessels.
2. Blood vessels transport the blood from the heart into arteries, capillaries, and veins; and blood then returns to the heart so the circuit can be completed
3. Gas exchange occurs at the smallest-diameter vessels, the capillaries
4. The heart and blood vessels regulate blood flow, according to the needs of the body.
Lymphatic system: Assists the cardiovascular system because lymphatic vessels collect excess tissue fluid and return it to the cardiovascular system.
5.2
Artery: blood vessel that transports blood away from the heart.
Artery Endothelium: thin layer of cells. Surrounded by a relatively thick middle layer of smooth muscle and elastic tissue
Arterioles: small arteries barely visible to the naked eye
The Capillaries: Exchange
The structure of a capillary is adapted for exchange of materials with the cells of the body. Each capillary is an extremely narrow, microscopic tube with a wall composed only of endothelium; single layer of epithelial cells. Capillary beds are present in all regions of the body so no cells are far from gas exchange with body.
Precapillary sphincters: Rings of muscle that control the blood flow through a capillary bed.
Constriction of the sphincters closes the capillary bed. When a capillary bed is closed, the blood moves in an area whre gas exchange is needed, going directly from arteriole to venule through a pathway called an arteriovenous shunt.

Veins: to Heart
Veins: Blood vessels that return blood to the heart.
Venules: small veins that drain blood from the capillaries and then join to form a vein.

Vein and Venules have same three layer wall as artery but has less smooth muscle in the middle layer of a vein and less connective tissue in the outer layer. Therefore, wall of veins is thinner than artery.

Because the blood leaving the capillaries is usually under low pressure vein often have valves, which allow blood to flow only toward heart when open and prevent backward flow of blood when closed. Valves are extensions of the inner-wall layer and are found in the veins that carry blood against the force of gravity, especially the veins of lower extremities.

Heart: Cone-shaped, muscular organ located