Circulatory Systems The exchange of nutrients and wastes MUST take place across _______ membranes MOIST (molecules must be dissolved to be transported across membranes) One celled creatures: Entire external surface exchanges with the environment Simple Multicellular Organisms (like Sponges & Cnidarians) All cells are exposed in some way to the environment. But more complex organisms have cells which are isolated from their external environment… they need a….. CIRCULATORY SYSTEM!!

The Presence of a Circulatory System: Reduces the distance a substance must travel to enter or leave a cell. Uses “Blood” to carry these molecules to the cells. O 2 diffuses from the air in the lungs (or similar structures) across epithelium to the “blood” which carries it to all parts of the body. Once in the capillaries, O 2 diffuses across epithelium again to the tissues. CO 2, of course, moves in the opposite dirction. There are 2 main types of Circulatory Systems: OPEN & CLOSED

Open vs Closed Circulatory Systems OPEN: Insects, most arthropods, mollusks “hemolymph” (a combination of blood and interstitial fluid) bathes all the internal organs while moving through “sinuses”. Circulation results from contraction of the “heart” and body movements. Relaxation of the heart draws blood back through pores called “ostia” CLOSED: Annalids, some Mollusks, Chordates Blood is confined to vessels, separated from interstitial fluid Larger vessels branch into smaller Heart  Arteries  Arterioles  Capillaries  Venules  Veins See transparencies…. (campbell activity)

Circulation & Blood: Control of the heart Cardiac cells are “MYOGENIC”:Can contract without external Input from the nervous system. Tempo is controlled by the Sinoatrial node (S.A. node): Located at right atrium by superior vena cava Contraction at the SA node causes both atria to contract Wave of contraction passes down to Atrioventricular (AV) node: Impulse is delayed by 0.1 second (atria need to be empty) Causes ventricles to contract “LUB” “DUP!” Campbell Activity again....

Vessel Structure Epithelial tissue Simple squamous May only be just one Single cell in capillaries! Smooth Muscle Thicker in the arteries than veins Connective Tissue Mostly elastic fibers Also thicker in arteries….

Pressure & Flow: Law of Continuity: Greatest in the ArteriesLowest in the Veins Fluid will flow faster in narrower pipes. HOWEVER…. Blood actually moves slower in capillaries because collectively they have greater total added diameter. Pressure in the veins is near zero The action of skeletal muscles (+ effect of the valves) is what actually keeps blood moving in veins. Only 5-10% of capillaries carry blood at any given time Exceptions include BRAIN, HEART, LIVER, & KIDNEYS. BARORECPTOR REFLEX: Vessels constrict in response to low blood pressure …and dilate in response to rising blood pressure. Distribution is controlled by: Hormonal signaling Precapillary sphincters