CIRCULATION AND GAS EXCHANGE CHAPTER 42 CIRCULATION AND GAS EXCHANGE

I. CIRCULATION IN ANIMALS A.    TRANSPORT SYSTEMS FUNCTIONALLY CONNECT THE ORGANS OF EXCHANGE WITH THE BODY CELLS: AN OVERVIEW MOST OF ANIMAL CELLS ARE TOO FAR FROM THE OUTSIDE ENVIRONMENT TO BE SERVICED BY DIFFUSION OR ACTIVE TRANSPORT MANY ANIMALS HAVE AN INTERNAL TRANSPORT SYSTEM THAT PROVIDES A LIFELINE BETWEEN THE AQUEOUS ENVIROMENT OF LIVING CELLS AND THE ORGANS EX. THE LUNGS: EXCHANGE CHEMICALS WITH THE OUTSIDE ENVIRONMENT.

B.    MOST INVERTEBRATES HAVE A GASTROVASCULAR CAVITY OR A CIRCULATORY SYSTEM FOR INTERNAL TRANSPORT CNIDARAIANS AND FLATWORMS HAVE GASTROVASCULAR CAVITIES THAT FUNCTION IN CIRCULATION AS WELL AS DIGESTION. ARTHROPODS AND MOST MOLLUSKS HAVE OPEN CIRCULATORY SYSTEMS, IN WHICH TISSUES ARE BATHED DIRECTLY IN HEMOLYMPH PUMPED BY A HEART INTO SINUSES. ANNELIDS, SOME MOLLUSKS AND VERTEBRATES HAVE CLOSED CIRCULATORY SYSTEMS, WITH BLOOD CONFINED TO VESSELS, SOME OF WHICH PULSATE AND FUNCTION AS HEARTS.

C. VERTEBRATES PHYLOGENY IS REFLECTED IN ADAPTATIONS OF THE CARDIOVASCULAR SYSTEM IN VERTEBRATES, BLOOD FLOWS IN A CLOSED CARDIOVASCULAR SYSTEM CONSISTING OF BLOOD VESSELS AND A TWO- TO FOUR-CHAMBERED HEART. THE HEART HAS ONE ATRIUM OR TWO ATRIA, WHICH RECEIVE BLOOD FROM VEINS, AND ONE OR TWO VENTRICLES, WHICH PUMP BLOOD INTO ARTERIES. ARTERIES BRANCH ARTERIOLES, WHICH CONVEY BLOOD TO CAPILLARIES, THE SITE OF CHEMICAL EXCHANGE BETWEEN BLOOD AND INTERSTITIAL FLUID. CAPILLARIES REJOIN INTO VENULES THAT CONVERGE INTO VEINS. IN FISHES, THE HEART HAS A SINGLE ATRIUM AND A SINGLE VENTRICLE THAT PUMPS BLOOD TO GILLS FOR OXYGENATION;

THE BLOOD THEN TRAVELS TO OTHER CAPILLARY BEDS OF THE BODY BEFORE RETURNING TO THE HEART. AMPHIBIANS AND MOST REPTILES HAVE A THREE-CHAMBERED HEART IN WHICH THE SINGLE VENTRICLE PUMPS BLOOD VIA TWO CIRCUITS TO GAS EXCHANGE SURFACES AND TO OTHER PARTS OF THE BODY. THE TWO CIRCUITS RETURN BLOOD TO SEPARATE ATRIA. THIS DOUBLE CIRCULATION REPUMPS BLOOD RETURNING FROM THE CAPILLARY BEDS OF THE RESPIRATORY ORGAN, THUS ENSURING A STRONG FLOW OF BLOOD TO THE REST OF THE BODY. BIRDS AND MAMMALS, BOTH ENDOTHERMS, HAVE FOUR-CHAMBERED HEARTS THAT KEEP OXYGEN-RICH AND OXYGEN-POOR BLOOD COMPLETELY SEPARATED.

D. DOUBLE CIRCULATION IN MAMMALS DEPENDS ON THE ANATOMY AND PUMPING CYCLE OF THE HEART. HEART VALVES DICTATE A ONE-WAY FLOW OF BLOOD THROUGH THE HEART. THE HEART RATE (PULSE) IS THE NUMBER OF TIMES THE HEART BEATS EACH MINUTE. THE CARDIAC CYCLE, ONE COMPLETE SEQUENCE OF THE HEART'S PUMPING AND FILLING, CONSISTS OF PERIODS OF CONTRACTION, CALLED SYSTOLE, AND PERIODS OF RELAXATION, CALLED DIASTOLE. THE CARDIAC OUTPUT IS THE VOLUME OF BLOOD PUMPED INTO THE SYSTEMIC CIRCULATION PER MINUTE.

THE INTRINSIC CONTRACTION OF THE CARDIAC MUSCLE IS COORDINATED BY A CONDUCTION SYSTEM ORIGINATING IN THE SINOATRIAL (SA) NODE (PACEMAKER) OF THE RIGHT ATRIUM. THE PACEMAKER INITIATES A WAVE OF CONTRACTION THAT SPREADS TO BOTH ATRIA, HESITATES MOMENTARILY AT THE ATRIOVENTRICULAR (AV) NODE, AND THEN PROGRESSES TO BOTH VENTRICLES. THE PACEMAKER IS INFLUENCED BY NERVES, HORMONES, BODY TEMPERATURE, AND EXERCISE.

I. BLOOD IS A CONNECTIVE TISSUE WITH CELLS SUSPENDED IN PLASMA WHOLE BLOOD CONSISTS OF CELLULAR ELEMENTS (CELLS AND PIECES OF CELLS CALLED PLATELETS) SUSPENDED IN A LIQUID MATRIX CALLED PLASMA. PLASMA IS A COMPLEX AQUEOUS SOLUTION OF INORGANIC ELECTROLYTES, PROTEINS, NUTRIENTS, METABOLIC WASTE PRODUCTS, RESPIRATORY GASES, AND HORMONES. PLASMA PROTEINS INFLUENCE BLOOD PH, OSMOTIC PRESSURE, AND VISCOSITY, AND FUNCTION IN LIPID TRANSPORT, IMMUNITY (ANTIBODIES), AND BLOOD CLOTTING (FIBRINOGENS).

RED BLOOD CELLS, OR ERYTHROCYTES, TRANSPORT OXYGEN. FIVE TYPES OF WHITE BLOOD CELLS, OR LEUKOCYTES, FUNCTION IN DEFENSE BY PHAGOCYTOSING BACTERIA AND DEBRIS OR BY PRODUCING ANTIBODIES. PLURIPOTENT STEM CELLS IN RED BONE MARROW GIVE RISE TO ALL TYPES OF BLOOD CELLS.

II. GAS EXCHANGE IN ANIMALS A.    GAS EXCHANGE SUPPLIES OXYGEN FOR CELLULAR RESPIRATION AND DISPOSES OF CARBON DIOXIDE CELLULAR RESPIRATION DEMANDS A CONSTANT SUPPLY OF 02 AND REMOVAL OF C02. ANIMALS REQUIRE LARGE, MOIST RESPIRATORY SURFACES FOR THE ADEQUATE DIFFUSION OF RESPIRATORY GASES BETWEEN THEIR CELLS AND THE RESPIRATORY MEDIUM, EITHER AIR OR WATER.

B. GILLS ARE RESPIRATORY ADAPTATIONS OF MOST AQUATIC ANIMALS GILLS ARE OUTFOLDINGS OF THE BODY SURFACE SPECIALIZED FOR GAS EXCHANGE. THE EFFICIENCY OF GAS EXCHANGE IN SOME GILLS, INCLUDING THOSE OF FISHES, IS INCREASED BY VENTILATION AND COUNTERCURRENT FLOW OF BLOOD AND WATER.

C. TRACHEAL SYSTEMS AND LUNGS ARE RESPIRATORY ADAPTATIONS OF TERRESTRIAL ANIMALS THE TRACHEA OF INSECTS ARE TINY BRANCHING TUBES THAT PENETRATE THE BODY, BRINGING 02 DIRECTLY TO CELLS. MOST TERRESTRIAL VERTEBRATES, LAND SNAILS, AND SPIDERS HAVE INTERNAL LUNGS. IN MAMMALS, AIR INHALED THROUGH THE NOSTRILS PASSES THROUGH THE PHARYNX INTO THE TRACHEA, BRONCHI, BRONCHIOLES, AND DEAD-END ALVEOLI, WHERE GAS EXCHANGE OCCURS.

LUNGS MUST BE VENTILATED BY BREATHING. FROGS VENTILATE BY POSITIVE PRESSURE, PUMPING AIR INTO THEIR LUNGS. MAMMALS VENTILATE WITH NEGATIVE PRESSURE BY CONTRACTING AND RELAXING RIB MUSCLES AND THE DIAPHRAGM, WHICH CHANGES THE VOLUME AND HENCE THE PRESSURE OF THE CHEST CAVITY AND LUNGS RELATIVE TO THE ATMOSPHERE.