Chapter 42 Circulation and Gas Exchange In unicellular organisms exchange occurs directly with the external environment However for most multicellular organisms direct transfer of materials between every cell and the environment is not possible and instead rely on specialized systems 42 1 CIRCULATORY SYSTEMS LINK EXCHANGE SURFACES WITH CELLS THROUGHOUT THE BODY The molecular trade than an animal carries out with its environment must ultimately involve every cell in the body Small molecules can move between cells if there is a difference in concentration In addition diffusion can tend to be very slow for distances of more than a few millimeters due to being proportional to the square of the distance THIS RELATIONSHIP PLACES A SUBSTANTIAL CONSTRAINT ON THE BODY PLAN OF ANY ANIMAL To overcome the large distances natural selection has selected for placing many cells in direct contact with the environment flatworms cnidarians or a circulatory system GASTROVASCULAR CAVITIES Hydaras jellies and other cnidarians have a central gastrovascular cavity that function in the distribution of substances throughout the body as well as in digestion A gastrovascular cavity has an opening that allows it to come into contact with the environment In animals with a gastrovascular cavity fluid bathes both the inner and outer tissue layers in order to facilitate the exchange of gases and cellular waste In addition only the cells lining the cavity have direct access to nutrients released by digestion However nutrients can diffuse because the body wall is 2 cells thick Planarians and most other flatworms use a flat body and gastrovascular cavity to survive without a circulatory system OPEN AND CLOSED CIRCULATORY SYSTEMS A circulatory system has 3 basic components a circulatory fluid a set of interconnecting vessels and a muscular pump the HEART The Heart uses metabolic energy to elevate the circulatory s fluid hydrostatic pressure to flow Through the transport of fluid throughout the body the circulatory system connects the aqueous environment of the body cells to the organs that exchange gases absorb nutrients and dispose waste There are 2 types of circulatory systems OPEN CIRCULATORY SYSTEM uses hemolymph as a circulatory fluid and bathes the body cell Arthropods and some mollusks have this Heart contraction pumps the hemolymph through the circulatory vessels into interconnected sinuses where a chemical exchange occurs with the body cells Less costly due to a lack of hydrostatic pressure CLOSED CIRCULATORY SYSTEM uses blood confined in vessels The heart pumps blood into large vessels that branch into smaller ones to infiltrate the organs Chemical exchange occurs between the blood and interstitial fluid in addition the body cells Annelid cephlapods and all vertebrates have this Benefits include higher blood pressure that enables delivery of Oxygen ORGANIZATION OF VERTEBRATE CIRCULATORY SYSTEMS Closed circulatory system of humans and other vertebrates is often called the CARDIOVASCULAR SYSTEM Blood circulates through an extensive of vessels Arteries veins and capillaries are the 3 main types of blood vessels ARTERIES carry blood from the heart to the organs throughout the body Arteries then branch into arterioles and convey blood to capillaries which are found in capillary beds Chemicals dissolved gasses diffuse between the blood and the interstitial fluid around the tissue cells Capillaries then converge into venues which then converge into veins that carry blood back to the heart The chambers that receive the blood entering the heart are the atria and the ventricles pump blood out the heart Single Circulation In bony fish rays and sharks the heart consists of 2 chambers and atrium and ventricle The blood passes through the heart once in each complete circuit throughout the body SINGLE CIRUCLATION Blood is collected in an atrium before transfer to the ventricle and pumped into the capillary buds found in gills where it is oxygenated Blood leaves the gills and the blood is carried to capillary beds across the body OVERAL BLOOD THAT LEAVES THE HEART PASSES THROUGH 2 CAPILLARY BEDS BEFORE RETURNING TO THE HEART Double Circulation Found in amphibians reptiles and mammals The animals with double circulation have pumps for 2 circuits that are combined into the heart Having a heart simplifies coordination of the pumping cycles One pump delivers oxygen poor blood to the capillaries to be oxygenated This is called the PULMONARY CIRCUIT if the capillary beds are in the lungs After oxygen enriched blood leaves the gas exchange tissues and enters the other pump the left side of the heart which propels the blood towards the capillary beds in organs and tissues throughout the body After the exchange of CO2 and O2 oxygen poor blood returns to the heart to complete the systemic circuit DOUBLE CIRCULATION PROVIDES A VIGROUS FLOW OF BLOOD TO THE BRAIN MUSCLES and OTHER ORGANS BECAUSE THE HEART REPRESSURIZES THE BLOOD DESTINED TO THEM Evolutionary Variation in Double Circulation Amphibians have a heart with 2 atria and 1 ventricle The incomplete division of the ventricle allows the frog to adjust its circulation shutting of most blood flow to its temporarily ineffective lungs Turtles Snakes Lizards incomplete septum partially divides the single ventricle into 2 chambers This allows greater control of the relative amount of blood flowing to the lungs and the rest of the body Alligators Caimans other Crocodilians ventricles are divided by a complete septum by a complete septum but the pulmonary and systemic circuits connect where the arteries exit the heart It allows the arterial valves to shunt blood flow from the lungs while underwater Double circulation in birds and mammals involves Having 2 atria and 2 ventricles Blood flow cannot vary in the lungs without it varying throughout the body Natural selection allows birds and mammals to deliver up to 10 times as much fuel and Oxygen while removing up to ten times as much CO2 and wastes because they are endoderms and use more energy 42 2 COORDINATED CYCELS OF HEART CONTRACTION DRIVE DOUBLE CIRCULATION IN MAMMALS MAMMALIAN CIRCULATION Detailed diagram on pg 920 THE MAMMALIAN HEART A CLOSER LOOK Located behind the sternum breastbone the human heart is the sized of a clenched fist and consist of cardiac muscle Two atria have thin walls and serve as collection chambers for blood returning from lungs or body tissues Blood enters the atria when all heart chambers relax The