Biology - An Illustrated Guide to Science (794127), страница 30
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Bony extensions of the spineprotect these vital organs.● Ureters connect the kidneys to thebladder. There are no valves in theureters, so fluid drains downward bygravity. Backflow of urine to thekidneys is prevented by the constantproduction of fluid by the kidneys.● Blood supply to the kidneys is throughthe renal artery, a branch of the aorta.This blood is at high pressure, which isessential for efficient kidney function.The renal vein, which opens into thevena cava, drains blood from thekidneys at low pressure.●inferiorvena cavaaortaleft kidneyright kidneyureterrenal veinrenalarteryBladder and uretersThe bladder acts as a storage organ,holding urine until it is convenient torelease it to the outside world.● The urine then flows along theurethra. A valve in the urethraprevents backflow of urine into thebladder.bladderurethra© Diagram Visual Information Ltd.●172HUMAN BIOLOGYKey wordsaortaBowman’scapsuleglomeruluskidneynephronureterExcretion: kidneystructureKidneyLongitudinal sectioncortexGross anatomyThe human kidn ey is supplied withblood by a branch of the aorta calledthe renal artery and is drained ofblood by a branch from the vena cavacalled the renal vein.● Urine produced by the kidney isconducted by the u reter to thebladder for storage.●branch ofrenal arteryrenal veinrenal arteryrenal pelvismedullaCortex and medullaThe outer part of the kidney is calledthe cortex.
It surrounds the medulla,which includes in the middle a spaceconnected to the ureter.● The renal artery branches repeatedlyin the kidney, delivering blood to thecortex.●NephronsNephron s are long tubules that start inthe cortex with a small knot ofcapillaries called a glom eru lu s. Theglomerulus is surrounded by a cupshaped structure called the Bowm an 'scapsu le, which serves as a filter toremove organic wastes, excessinorganic salts, and water.
Fluids fromblood in the glomerulus are collectedin the Bowman’s capsule and furtherprocessed along the nephron to formurine.● A proximal convoluted tubule receivesthe fluids from the renal corpuscle.The proximal tubule leads to a longtubular loop called the loop of Henle,which is concerned with absorbingwater from the urine before it isreleased. The ascending limb of theloop of Henle returns the urine to thecortical region of the kidney where itenters the distal convoluted tubule.This tubule carries the urine to thecollecting duct, which empties into therenal pelvis and then into the ureterand bladder.ureterglomeruliblood flowNephronproximalconvolutedtubuleglomerulusdistalconvolutedtubulecortexBowman’s capsulebranch ofrenal arterybranch of renal veincollecting ductcapillariesKidneymedullaLongitudinal sectioncortexnephronrenalpelvisrenal pelvis© Diagram Visual Information Ltd.●medullaloop of Henle173Excretion: kidneyfunctionHUMAN BIOLOGYKey wordsBowman’scapsuleconcentrationgradientglomeruluskidneyNephron functiondistalconvolutedtubuleglomerulusBowman’s capsulenephronselectivereabsorptionultrafiltrationureaureterUltrafiltrationThe human kidn ey is supplied withblood by a branch of the aorta calledthe renal artery.
This enters the kidneyat high pressure, and plasma is forcedout of the capillaries in theglom eru lu s. This process is calledu ltrafiltration .● The glomerular filtrate that collects inthe space of the Bowm an ’s capsu lecontains a wide range of usefulmaterials (salts, sugar, etc.) as well aswaste products like u rea.● If this liquid were passed out as urine,the body would be losing valuablematerials and a huge amount of waterevery day.●proximalconvoluted tubulecapillaryloop of HenleSelective reabsorptionAs the fluid passes along the nephrontoward the collecting duct that willtake it to the u reters and then to thebladder, useful materials arereabsorbed.● Glucose and many salts are reabsorbedin the first part of the n ephron calledthe proximal convoluted tubule.Glucose is actively reabsorbed.
Manyother substances diffuse back into theblood along a con cen tration gradien t.● The loop of Henle is important forreabsorbing water. Sodium ions arepumped into and out of the loop in aparticular pattern to cause water tofollow them by osmosis. This processcan create a highly concentrated urinethat can conserve water in times ofstress. When water is plentiful, thisprocess is modified, and mammalsproduce large volumes of dilute urine.First part of nephron: schemecollectingductBowman’scapsuleURINEglomeruluscapillaryproximalconvolutedtubuleblood flowfiltrate flowfiltrationreabsorption of glucose,amino acids, water, salts© Diagram Visual Information Ltd.●174HUMAN BIOLOGYKey wordsepidermiskeratinMalpighian layerExcretion: skin structureSkinVertical sectionepidermisSkin layers●The skin is made up of three distinctlayers: the epiderm is, the dermis, anda layer of fat called subcutaneous fat.Malpighian layercornified layergranular layerhair follicleEpidermishaircapillary loopThe epidermis is the outer layer of theskin and includes everything from theMalpighian layer outward.● Mature epidermal cells are dead andare constantly lost from the surface.The Malpighian layer replaces thesecells at the base of the epidermis sothat there is a constant migration ofcells from inside to outside.
Duringthis migration, the cells are filled withkeratin , which helps to waterproof theskin.● Melanocytes in the skin produce thepigment melanin in response to UVlight. This darkens the skin andprotects the delicate dermis fromradiation.sebaceous gland●nerve ending (pressure)hair erector musclenerve ending(touch)sweat porenerveending(touch)nerveending(pain)DermisThe dermis is much thicker than theepidermis and contains a much widerrange of structures.● Sweat glands in the dermis producesweat, which is released most activelywhen the temperature rises.
It coolsthe skin by absorbing energy toevaporate.● Hair follicles are embedded in thedermis. Sebaceous glands, foundwhere the hair exits the skin, producea fatty secretion called sebum. Bacterialiving on the skin surface digest this tocreate acids that inhibit the growth ofcertain pathogenic organisms.● The dermis also contains a wide rangeof sensory nerve endings.● Blood vessels in the dermis canexpand and contract to regulate heatloss from the skin.© Diagram Visual Information Ltd.●Subcutaneous fat●Subcutaneous fat gives the skin aplump, padded look.subcutaneous fatdermissweat ductsweat gland with capillaries175Coordination: nervoussystemHUMAN BIOLOGYKey wordsassociationneuroncentral nervoussystemmotor neuronneuronNervous systembraincentral nervous systemcranial nervesneurotransmitterperipheralnervous systemsensory neuronspinal cordsynapseCNS and PNSThe human nervous system dividesinto two main parts: the cen traln ervou s system (CNS) and theperipheral n ervou s system (PNS).● The CNS includes the brain and spinalcord.
The PNS includes everything elseand consists of paired nerves that arisefrom the spin al cord. A single nerve isa bundle of elongated cells calledn eu ron s.peripheral nervous system●spinal cordNeuronsNeurons are the cells of the nervoussystem that carry nerve impulses.There are three basic types:● Sen sory n eu ron s convey messagesfrom the sense organs into the CNS.● Motor n eu ron s carry messages fromthe CNS out to muscles or glands.● Association n eu ron s carry messagesaround inside the CNS.
They aredifferent from sensory and motorneurons in that they do not possessmyelin sheaths, the insulatingenvelopes that surround the core of anerve fiber and facilitate thetransmission of nerve impulses.● Neurons pass messages betweenthemselves across synapses. A syn apseforms where two nerve cells are inclose contact. Messages pass acrossthe small gap as secretions ofchemicals called n eu rotran sm itters.● Impulses passing along neurons arewaves of electrical activity created bythe movement of sodium ions intoand out of the nerve fiber.spinal nerves© Diagram Visual Information Ltd.●176HUMAN BIOLOGYKey wordsimpermeableneuronstimulusCoordination: nerveimpulseSchematic longitudinal section of nerve fiber to showpassage of an impulse along the membraneResting state (inside negative, outside positive)Electrical balanceA nerve fiber is long and tubular inshape.
It is normally im perm eable tosodium ions, and these are pumped tothe outside of the cell so that anelectrical potential exists across thecell membrane.● This means that the outside is morepositive, with a higher concentrationof sodium ions than the inside.●DepolarizationWhen a n eu ron is stimulated, themembrane is changed to allow ions topass, and sodium ions rush in toequalize the potential difference. Thisis called depolarization.
A number ofother ions inside also move,particularly potassium ions, which flowout of the neuron.● The inside of the neuron is nowneutral or slightly negative. This lastsfor a very short time, and soon theactive pumping of sodium ions andreversion of the membrane back toimpermeability reasserts the positivecharge outside the cell.nervefiber(neuron)membrane●Initiationof nerveimpulsenervefiber(neuron)area of depolarizationA wave of depolarizationA nerve impulse is a wave ofdepolarization that moves along theneuron.● Depolarization in one part stimulatesthe next part of the neuron todepolarize.
This moves the impulseforward. Behind the depolarization,the cell is repolarizing the membraneagain to leave it ready to receive thenext stim u lu s.© Diagram Visual Information Ltd.●Propagationof impulsenervefiber(neuron)area where charge acrossmembrane has been restoredsodium ioninflow of sodium ionspotassium ionoutflow of potassium ionsarea ofdepolarizationdirection of impulse177Coordination: synapseHUMAN BIOLOGYKey wordsConnections between association neuron and motor neuronaxondendriteneuronneurotransmittersynapseassociationneuronvesicleLinks between cellsNerve cells do not exist in isolation.They must pass messages betweenthemselves in order to function.● The point at which n eu ron s meet iscalled a syn apse.
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