* Excretion: elimination waste substances from blood. It is performed by the urinary system and other organs.Urinary system: formed by:

-Kidneys: they are two bean-shaped organs behind the abdomen, on each side of the spinal cord. Each one has an external section (renal cortex), an internal one divided into pyramidal sections (renal medulla) and a cavity, the renal pelvis. A renal arter transports blood with waste substances to each kidney, and a renal vein transports filtered blood (without waste substances) from each kidney. The kidneys are formed by nephrons, that filter the blood forming urine Each nephron is formed by a ball of blood capillaries called the glomerulus of Malpighi, surrounded by a hollow capsule (Bowman's capsule), and a thin and twisted renal tube with the shape of a loop. The renal tubes from several nephrons flow into thicker tubes, the collecting ducts, that end in the renal pelvis, transporting urine to the ureters.

The formation of the urine has two stages:

-Glomerular filtration: passage of simple waste substances (urea, uric acid...), water and simple nutrients (mineral salts, glucose, amino acids, etc.) from blood contained in the Malpighian glomerulus to the Bowman's capsule. The filtered liquid is transported by the renal tube.

-Tubular reabsorption: useful substances (most of the water, mineral salts, glucose, amino acids, etc.) are reabsorbed, passing from the renal tube to the blood capillaries again. The filtered liquid is transported by renal tube and it forms the urine, whose chemical composition is similar to the blood plasma, but without useful nutrients and with more waste products (urea, uric acid, etc.).

     -Ureter: thin tubes that start from each kidney and transport urine to the urinary bladder.

     -Urinary bladder: elastic and muscular bag that stores urine. When it contains a large quantity of               urine,it contracts due to a nervous reflex (micturition reflex) and releases urine into the urethra. The micturition reflex can be controlled voluntarily.

     -Urethra: duct that transports urine to the outside. It has a sphincter at its end that remains closed           except when the micturition reflex occurs.

Other organs that participate in excretion are:

-lungs: they expel CO2 produced in cell respiration.

-liver: it eliminates some waste substances from digestion (urea, uric acid), medicines and drugs.

-sweat glands: they produce sweat, a liquid formed by water and waste substances, similar to urine but more diluted. Sweat also contributes to regulating body temperature, because when it evaporates, it cools down the skin.

THECIRCULATORYSYSTEM

*Function:ToTransportO2AndNutrientsToTheBodyCellsAndCollectCO2AndWasteSubstancesFromTheBodyCellsToTransportThemToTheOrgansResponsibleForTheirElimination. Components:

1.Blood:RedLiquidTransportedByTheBloodVessels.It'sFormedBy:

Ø BloodPlasma (55 %OfBlood),MadeUpOfWater (90%)WithSubstancesDissolved (mineralSalts,Nutrients,WasteSubstances,CO2..).ItTransportsNutrients,WasteSubstancesAndCO2AndItRegulatesTheBodyTemperature

Ø BloodCells (45 %OfBlood):FormedInTheBoneMarrow.TheyAre:

§RedBloodCells (orErythrocytes):Disk-shapeCellsWithoutANucleus.TheyAreTheMostAbundantBloodCells.TheyContainARedPigment,Hemoglobin,ThatTransportsOxygen

§ WhiteBloodCells (orLeukocytes):CellsWithANucleus.TheyCanBePhagocytes (responsibleForThePhagocytosisOfPathogenicAgents)OrLymphocytes (theyProduceAntibodies),ThatProtectFromInfections.

§ Platelets (thrombocytes):FragmentsOfCellsWithoutANucleus,ThatAllowBloodToCoagulate

 2.BloodVessels:TheyAreTheDuctsThatCarryBloodAroundTheBody (forThisReason,HumanBloodCirculationIsCalledClosed).ThereAreThree                   types:Arteries,VeinsOrBloodCapillaries.

3.Heart:HollowMuscularOrganThatPumpsTheBloodThroughTheBloodVessels.ItIsFoundInTheChest,BetweenTheTwoLungs,SlightlyToTheLeft.It'sFormedByAMuscle,TheMyocardium,CoveredExternallyByAMembrane(pericardium),AndInternallyByTheEndocardium.ItIsDividedIntoTwoParts,TheRightAndTheLeft,CompletelySeparatedByInterventricularSeptum.TheLeftHalfCarriesOxygenatedBloodAndTheRightHalf,DeoxygenatedBlood.EachHalfOfTheHeartIsSubdividedIntoTwoChambers:TheUpperSmallerChambersAreTheAtriaAndTheLargerLowerOnesAreTheVentricles.EachAtriumIsCommunicatedWithTheCorrespondingVentricleThroughTheAtrioventricularValve,ThatAllowsBloodToPassFromTheAtriumIntoTheVentricleButNotTheOtherWayAround.TheValveLocatedBetweenTheRightAtriumAndTheRightVentricleIsNamedTricuspidValveAndTheOneOnTheLeftSideIsCalledTheBicuspidOrMitralValve.SeveralVeinsArriveToBothAtriums:TwoVenaeCavae(orCavaVeins)ToTheRightAtriumAndFourPulmonaryVeinsToTheLeftOne.TwoArteriesOriginateFromTheVentricles:ThePulmonaryArteryFromTheRightVentricleAndTheAortaArteryFromTheLeftOne.AtTheStartOfThoseArteriesThereAreSigmoidValves(thePulmonaryValveAndTheAorticValve),FormedByThreeMembranesThatPreventBloodFromComingBackIntoTheHeartOnceItHasBeenPumpedOut.

HeartbeatAndBloodCircuits 

The heart performs a constant movement called the heartbeat to pump blood thorough the body, divided into different stages that make up the cardiac cycle:

-AtrialSystole:AtriaContract,AtrioventricularValvesAreOpenedAndBloodIsPropelledIntoTheVentricles.

-VentricularSystole:VentriclesContract,SigmoidValvesOpen AndBloodExitsTheHeartThroughThePulmonaryAndAortaArteries.AtrioventricularValvesAreClose (firstCardiacNoise)ToPreventTheBloodFromReturningToTheAtria.

-Diastole→Discontraction:AtriaAndVentriclesRelaxedAndBloodEntersTheAtriaThroughTheCavaAnd PulmonaryVeins.SigmoidValvesClose(secondCardiacNoise)ToPreventBloodFromComingBackIntoTheVentricles.

TheCirculationOfBloodInHumansIsDouble (ItTravelsTwiceThroughTheHeartDescribingTwoCircuits)AndComplete (oxygenatedBloodNeverMixesWithDeoxygenatedBloodThanksToTheInterventricularSeptum).TheTwoBloodCircuitsAre:

§ PulmonaryCircuit:BetweenTheHeartAndTheLungsWhereBloodLeavesCO2AndBecomesOxygenated.

rightVentricle→PulmonaryArtery →Lungs→PulmonaryVeins →LeftAtrium

§ SystemicCircuit:BetweenTheHeartAndTheOrgans,CarryingOxygenAndNutrientsAndCollectingCarbonDioxideAndWasteProducts.

leftVentricle→aorta →Organs →CavaVeins →RightAtrium

(deoxygenatedBlood/OxygenatedBlood)