Types of Circulation

8/17/2019 Types of Circulation

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TYPES OF CIRCULATION

2 Types of closed blood circulation systems:

The 2 types of circulation

systems in animals are:

• Open Circulatory

Systems (OpenCirculation Systems)e.g. in insects,where there may besome vessels butthe circulatory flui !"hemolymph", alsowritten"haemolymph" !flows out of the

vessels that form thecirculatory systeman bathes tissuesirectly. That is,instea of the bloocapillaries in closedcirculatorysystems (seebelow), the mainvessels in opencirculatorysystems openinto open sinuses.

• Closed Circulatory

Systems (CloseCirculation Systems)e.g.in vertebrates suchas fish an mammals, bloo iscontaine withinthe structure of theheart an bloovessels that have arange of si#es an

structures,incluing arteries, ar terioles, capillaries,venules an veins.$arger bloo vesselsmove bloo arounthe boy, ta%ing itfrom organ to organ,through the bloodsystem, which isalso callethe vascularsystem. Substances

pass betweenthe bloo anthe tissues itsupplies by movingthrough the thinwalls of the smallestbloo vessels, whicharecalle capillaries.

Are all thecirculatorysystems in animals'blood circulationsystems' ?

• Closed circulatory

systems in animalsare blood circulation systems.&owever, thecirculatory flui inanimals thathave opencirculatorysystems is calle"hemolymph" or"haemolymph"rather than "bloo"because in thoseanimals there is noistinction betweenthe "bloo" an the"interstitial flui" inanimals that haveclose circulatorysystems !hemolymph beingthe combination ofthe e'uivalent ofthose fluis.&emolymph hasistinctivecharacteristics thatiffer from bloo e.g.it containshemocyanin, acopper!baseprotein that turnsblue whenoygenate, asoppose to the iron!base hemoglobin invertebrate re bloocells.

• lso, in larger

animals incluinghumans, bloocirculation is not theonly circulation

system ! at least,bloo is not the onlyflui circulating. Thehuman lymphaticsystem inclues anetwor% of lymphnoes an lymphaticvessels that containa flui calle lymph.The lymphaticsystem ! also callesimply the lymphsystem, is not a

close system. (Thelymphatic system issometimesescribe incon*unction with thebloo system, or itmay be stuie aspart of the immunesystem.)

More about Closed

Blood Circulation

Systems:

ll close bloo circulatory

systems inclue + essential

parts

(three components),

specifically:

• loo

• loo -essels

• &eart

s state above, the bloo

(a flui) is containe within a

networ% of blood vessels,

of which there are several

ifferent types accoring to

their si#e an position within

the bloo circulation system.

n orer to perform the

essential functions of bloo,

the bloo must move aroun

the bloo system.

/ovement of bloo aroun

the networ% of bloo vessels

happens because the heartpumps the bloo through the

heart itself an therefore

onwars through the close

networ% of bloo vessels

which, together with the

bloo an the heart, form the

close circulatory bloo

system. Common to allclose circulatory systems

&owever, there are ifferent

types of close bloocirculation systems an also

ifferent structures of hearts

(bloo pumps) in animals

with ifferent types of bloo

circulation systems.

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• Single Circulation Systems (Single Blood

Circulation)

• Double Circulation Systems (Double Blood

Circulation)

2.0 Double Circulatory System

1.0 Single Circulatory System

is! !a"e single circulatory systems in #!ic! blood

passes t!roug! t!e !eart only once eac! time it

completes a full circuit around t!e fis!$s body% including

t!roug! its gills and all ot!er organs and tissues.

Blood flo# rate and blood pressure fall #!en blood

lea"es a fis!$s gills (blood is o&ygenated as it passes

t!roug! fis!es$ gills ' ust as adult !uman blood is

o&ygenated as it passes t!roug! t!e lungs).

T!e lo# blood pressure in t!e single circulatory systems

present in fis! is insufficient for efficient idney function

in mammals.

*ammals !a"e double

circulatory systems '

meaning t!at blood passes

t!roug! t!e !eart t#ice in

order to complete a singlecomplete circuit around

t!e #!ole body% including

t!roug! t!e lungs and all

ot!er parts (incl. organs

and tissues) of t!e body.

Double circulatory

systems include t#o

circuits of blood flo#ing

to and from t!e !eart.

T!ey are:

+ulmonary circulation is

t!e circuit by #!ic! blood

flo#s from t!e !eart to t!e

lungs% t!en bac to t!e

!eart.

Systemic circulation is t!e

circuit by #!ic! blood

lea"es t!e left "entricle

(,-) of t!e !eart "ia t!e

aorta% passes t!roug! t!e

organs and tissues of t!e

body (e&cept for t!e

lungs)% t!en returns to t!e

!eart.

Separation of t!e blood

circulation system into

t!ese t#o circuits enables

blood at sufficient

"elocity (flo# rate) and

sufficient (!ig! enoug!)

blood pressure to

efficiently transport

essential substances to t!e

cells of acti"e mammals.

T!is is necessary for t!e

!ealt!y functioning of

t!eir tissues organs.

/n order to pass all t!e

#ay around a double

blood circulatory system

e.g. as in !umans% blood

must complete bot! t!e

pulmonary circulation and

t!e systemic circulation

circuits% t!erefore it passes

t!roug! t!e !eart t#ice.


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T!e t!ree components

of closed circulatory

systems:

Blood

Blood is t!e fluid t!at

mo"es t!roug! t!e blood

system. /t includes many

different particles%especially

Blood +lasma ' consisting

mainly of #ater

containing solutes suc! as

electrolytes% plasma

proteins and !ormones

ed Blood Cells '

responsible for

transporting o&ygen totissues around t!e body

!ite Blood Cells ' !a"e

important immune

function roles e.g. in

p!agocytosis and

production of antibodies

Blood +latelets ' blood

platelet plug formed

during blood clotting(coagulation) in cases of

damaged blood "essels

Blood Vessels

Blood "essels are t!e

biological 3tubes3 t!roug!

#!ic! blood flo#s. T!rey

contain t!e blood #it!in

t!e blood system and

ensure t!at it only flo#s in

one direction t!roug! t!e blood system (e.g. due to

sufficient blood pressure

in arteries and due to t!e

presence of "al"es in t!e

main "eins of t!e body).

/n order of flo# from

lea"ing t!e !eart to

returning to t!e !eart% t!e

main types of blood

"essels are: arteries%

arterioles% capillaries%

"enules and "eins. T!e

main larger blood "essels

(not all t!e indi"idual

capillaries% of #!ic! t!ere

are too many) !a"e

indi"idual names


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pumped on#ards it #ould continue to t!e tissues of t!e body at muc! lo#er

pressure and flo# rate t!an it does in double circulation systems.

• T!e separation of o&ygenated (more accurately 3o&ygen'ric!3) anddeo&ygenated (more accurately 3o&ygen'poor3) blood is possible in t!e cases

of double circulation systems t!at include a $double pump$% i.e. a four c!amber%

!eart.

• See for e&ample t!e diagram of a !uman !eart at t!e top of t!is page% #!ic!

s!o#s t!e separation of o&ygenated and deo&ygenated blood #it!in a !uman

!eart. Specifically% it illustrates:

• 5&ygen'poor blood% #!ic! is often called deo&ygenated blood ' entering t!e

rig!t atrium of t!e !eart "ia t!e inferior "ena ca"a and t!e superior "ena ca"a

t!en lea"ing t!e rig!t "entricle of t!e !eart "ia t!e pulmonary artery s!o#n in blue #it! blue arro#s.

• 5&ygen'ric! blood% #!ic! is often called o&ygenated blood or $reo&ygenated

blood$ ' entering t!e left atrium of t!e !eart "ia t!e pulmonary "eins t!en

lea"ing t!e left "entricle of t!e !eart "ia t!e aorta s!o#n in pale red #it! red

arro#s.

Ad)anta"es of do#le circ#lation o)er sin"le circ#lation s$ste%s

• Single circulation systems are ade8uate for many species e.g. different types of

fis!.

• Double circulation is ad"antageous for mammals because it increases t!e

pressure and !ence t!e flo# rate of blood supplied to t!e tissues of t!e body

"ia t!e systemic circulation.

*o!&

Double circulation systems deli"er greater blood flo# rate to tissues around t!e body

because t!e !eart pumps t!e (o&ygenated or $o&ygen'ric!$) blood returned to it from

t!e lungs. T!is is useful because #!en blood passes t!roug! t!e lungs its pressure is

reduced. T!erefore if it #ere not returned to t!e !eart t!en pumped out again (at

!ig!er pressure t!an t!e pressure at #!ic! it reac!ed t!e !eart from t!e lungs)% it

#ould pass on#ards around t!e body muc! more slo#ly.

Wh$ does this %atter&

• Some mammals are "ery large animals. Sufficient blood pressure to send

blood from t!e !eart t!en e"entually bac to t!e !eart is needed. T!e

necessary blood pressure is !ig!er in larger animals in #!ic! t!e blood must

be pumped from t!e !eart #it! enoug! force to send t!e blood considerable

distances around t!e body before it arri"es bac at t!e !eart.


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• 4nimals t!at !a"e double circulation systems need blood to be supplied to

t!eir tissues 8uicly due to t!eir relati"ely !ig! metabolic rates i.e. rates of

energy used in t!e c!emical reactions #it!in t!eir bodies (compared #it! e.g.

fis!). T!is necessitates sufficient blood flo# because t!e transport functions of

blood include supplying t!e tissues #it! t!e c!emical substances needed tocontinue acti"ities including t!e essential c!emical reactions.

• 4nimals t!at !a"e double circulation systems also need to maintain relati"ely

!ig! body temperatures ' #!ic! re8uires sufficient blood flo# due to t!e

blood$s role in maintaining body temperature as necessary for !omeostasis.


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T!e Structure and unctions of Blood

Note+ T!e structure and function of t!e !eart and ot!er aspects of t!e "ascular system

is part of training in t!erapies suc! as massage incl. /ndian 7ead *assage% S#edis!

*assage% acupressure massage% aromat!erapy% acupuncture% s!iatsu% and ot!ers. T!is page is intended to include information suitable for most basic (first le"el) courses in

t!ese t!erapies% and some /T9C Diplomas.

T!is page is di"ided into t!e follo#ing sections:

1. The F#nctions of Blood

(generally ' as opposed to t!e functions of particular components of blood).

2. The Co%'osition of Blood

(incl. t!e different types of blood cells and t!eir properties and functions).

. Process of O,$"enation of Tiss#es d#e to Circ#lation of Blood

. T$'es of Le#coc$tes (!ite Blood Cells)

-( F#nctions of Blood

1. Trans'orts

o

Dissol"ed gases (e.g. o&ygen% carbon dio&ide)o aste products of metabolism (e.g. #ater % urea)

o 7ormones

o 9n6ymes

o ;utrients (suc! as glucose% amino acids% micro'nutrients

("itamins minerals)% fatty acids% glycerol)

o +lasma proteins (associated #it! defence% suc! as blood'clotting and

anti'bodies)

o Blood cells (incl. #!ite blood cells $leucocytes$% and red blood cells

$eryt!rocytes$).

2. .aintains Bod$ Te%'erat#re

. Controls '*

T!e p7 of blood must remain in t!e range .% ot!er#ise it begins to

damage cells.

. Re%o)es to,ins fro% the od$

T!e idneys filter all of t!e blood in t!e body (appro&. = pints)% < times

e"ery 2 !ours. To&ins remo"ed from t!e blood by t!e idneys lea"e t!e

body in t!e urine.

(To&ins also lea"e t!e body in t!e form of s#eat.)

?. Re"#lation of Bod$ Fl#id Electrol$tes

9&cess salt is remo"ed from t!e body in urine% #!ic! may contain around

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10g salt per day

(suc! as in t!e cases of people on #estern diets containing more salt t!an t!e

body re8uires).

/( Co%'osition of Blood

Blood consists of many components (constituents).

T!ese include:

??@ +lasma

?@ Components (sometimes called 3formed elements3)% i.e. $Blood Cells$. 5f

t!ese%

AA@ are eryt!rocytes (red blood cells)

and 1@ are leucocytes (#!ite blood cells) and t!rombocytes (blood platelets)

T!is is summarised in t!e follo#ing diagram% and described in furt!er detail belo#.


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T!e summary c!art abo"e includes: eryt!rocytes (red blood

cells)% t!rombocytes ( blood platelets) and leucocytes (#!ite blood cells). /t also

includes categories of leucocytes: agranulocytes and granulocytes (also no#n

as polymorp!onucleocytes)% #!ic! may also be sub'di"idedinto lymp!ocytes% monocytes% basop!ils%neutrop!ils and eosinop!ils.

T!e follo#ing table includes furt!er general information about t!e constituents of

blood.

Structure unctions

+lasma ;ormal blood plasma is A0'A2 @ #ater.

T!is is t!e stra#'coloured fluid in #!ic!

t!e blood cells are suspended% and

consists of:

• Dissol"ed substances including

electrolytes suc! as sodium%

c!lorine% potassiun% manganese%

and calcium ions

• Blood plasma proteins (albumin%

globulin% fibrinogen)

• 7ormones

•T!e medium in #!ic!t!e blood cells are

transported around

t!e body (by

t!e blood "essels) and

are able to operate

effecti"ely.

• 7elps to maintain

optimum body

temperature

t!roug!out t!eorganism.

• 7elps to control t!e

p7 of t!e blood and

t!e body tissues%

maintaining t!is

#it!in a range at

#!ic! t!e cells can

t!ri"e.

• 7elps to maintain an

ideal balance ofelectrolytes in t!e

blood and tissues of

t!e body.

9ryt!rocyte

s

0Red

lood

cells1

• /mmature eryt!rocytes !a"e a

nucleus but mature eryt!rocytes

!a"e no nucleus.

• 9ryt!rocytes !a"e a 3prost!etic

group3 (meaning 3in addition to3 '

in t!is case% in addition to t!ecell). T!e acti"e component of t!is

• Carry o&ygen

(process described in

more detail ' belo#).

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prost!etic group is 7aem.

7aem relies on t!e presence of

iron (e).

7aem combines #it! o&ygen to

form

o&y!aemoglobin:

... continued in section

belo#

• 9ryt!rocytes are e"entually

broen do#n by t!e spleen into

t!e blood pigments bilinubin and

bil"iridin% and iron. T!ese

components are t!en transported by t!e blood to t!e li"er #!ere t!e

iron is re'cycled for use by ne#

eryt!rocytes% and t!e blood

pigments form bile salts. (Bile

breas do#n fats.)

• 7a"e a longe"ity of appro&. 120

days.

• T!ere are appro&. .? ' ?.= million

eryt!rocytes per micro'litre of

!ealt!y blood (t!oug! t!ere are"ariations bet#een racial groups

and men#omen).

,eucocytes

0White

lood

cells1

• T!ere are different types of

leucocytes (described in more

detail ' belo#)% classified as:

o ranular: e.g. ;eutrop!ils%

9osinop!ils% Basop!ils.

o 4granular (do not contain

granules): e.g. *onocytes%,ymp!ocytes.

• 7a"e a longe"ity of a fe# !ours to

a fe# days (but some can remain

for many years).

• T!ere are appro&. ?%000 ' 10%000

leucocytes per micro'litre of

blood.

• *aor part of t!e

immune system.

Trombocyte• Blood platelets are cell fragments• 4ppro& dis's!aped fragments

• To facilitate bloodclotting ' t!e purpose

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s

(+latelets)

• Diameter 2' um

(1 micro'metre 1 um

0.000001m)

• 7a"e many granules but no

nucleus

• 7a"e a longe"ity of appro&. ?'A

days

• T!ere are appro&. 1?0%000 '

00%000 platelets per micro'litre of

blood

of #!ic! is to pre"ent

loss of body fluids.

2( The O,$"enation of Blood

T!e o&ygenation of blood is t!e function of t!e eryt!rocytes (red blood cells) and

taes place in t!e lungs.

T!e se8uence of e"ents of t!e blood becoming o&ygenated (in t!e lungs) t!en

o&ygenating t!e tissues (in t!e body) is as follo#s:

• T!e ig!t -entricle (of t!e !eart) sends de'o&ygenated blood to t!e lungs.

• !ile in t!e lungs:

1. Carbon Dio&ide diffuses out of t!e blood into t!e lungs% and

2. 5&ygen (breat!ed into t!e lungs) combines #it! !aemoglobin in t!e blood

as it passes t!roug! t!e lung capillaries.

• 5&y!aemoglobin returns to t!e !eart "ia t!e pulmonary "ein and t!en enters

t!e systemic circulation "ia t!e aorta.• T!ere is a lo# concentration of o&ygen in t!e body tissues. T!ey also contain

#aste products of t!e metabolism (suc! as carbon dio&ide).

• Due to t!e !ig! concentration of o&ygen in t!e blood and t!e lo#

concentration of o&ygen in t!e tissues%

• ... t!e !ig! concentration of carbon dio&ide in t!e tissues diffuses into t!e

blood. (A?@ of t!is carbon dio&ide dissol"es in t!e blood plasma.)

• Blood returns from t!e tissues bac to t!e !eart "ia t!e superior "ena ca"a

(from t!e upper'body) and t!e inferior "ena ca"a (from t!e lo#er'body)

3( T$'es of Le#coc$tes 0White Blood Cells1


12/19

L$%'hoc$tes+ .onoc$tes+ 4Baso'hils+ 4Ne#tro'hils+ 4Eosino'hils+

4ppro&. 2@

of leucocytes

are

lymp!ocytes.

T!ese produce

anti'bodies

and include:

E T'Cells

E B'Cells

E ;atural

Filler Cells

4ppro&. @

of leucocytes

are

monoocytes.

T!ese are also

no#n as

p!agocytes.

T!ey combat

microbes by

t!e process of

p!agocytosis.

0.?'1@ of

leucocytes are

basop!ils.

Diameter ='10

micro'metres.

,iberate

!eparin%

!istamine% and

seratonin in

allergic

reactions%intensifying

inflammatory

response.


13/19

Str#ct#re and F#nctions of L$%'hatic Tiss#e 0L$%'h1

;ote: T!is page is part of t!e section about t!e structure and function of

different Tiss#e T$'es% #!ic! is related to t!e section about *istolo"$ and

Cells(incl. structure of animal cells% cell di"ision% mitosis% meiosis). T!is 3Tissue

Types3 section is included to complete description of t!e no#ledge of 37istology 'T!e Cell3 re8uired by some courses in irst',e"el 4natomy and +!ysiology. To read

about ot!er tissue types see t!e list of on t!e left.

1.0 Introduction to Lymphatic Tissues:

What is the Lymphatic System ?

T!e L$%'hatic S$ste% is one of t!e t#o systems of circulation of fluid around t!e

body (t!e ot!er being blood circulation).

T!e lymp!atic system consists of lymp!atic fluid (called 3 l$%'h3) flo#ing t!roug! a

system of l$%'hatic )essels% including l$%'hatic ca'illaries% ot!er lymp!atic

"essels of "arious si6es (#!ic! can be compared #it! blood "essels)% and l$%'h

nodes (#!ic! are encapsulated masses of B Cells and T Cells).

T!e Blood S$ste% and t!e L$%'hatic S$ste% are interconnected. T!e clear fluid

t!at is no#n as 3lymp!3 #!en flo#ing t!roug! t!e lympatic "essels% initially passes

into t!ose "essels as 3interstitial fluid3 contained in spaces bet#een tissue cells '

spaces into #!ic! it !ad been filtered from blood. Hltimately% after tra"elling t!roug!lymp!atic "essels% t!e same lymp! passes bac into t!e blood system ' at t!e

unctions of t!e ugular and subcla"ian "eins (on bot! t!e rig!t' and left' sides of t!e

body).

2.0 The tissues that form the Lymphatic System

Se"eral different types of tissues form t!e structures of t!e lymp!atic system.

T!ey are listed according to t!e parts of of t!e lymp!atic system t!at t!ey form:

(a) Primary Lymphatic r!ans

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T!e locations at #!ic! stem cells di"ide and mature into B Cells and T Cells:

• Red Bone .arro! '

• The Th$%#s ' T!is is a t#o'lobed organ located in t!e c!est. T!e tissue of t!e

t!ymus itself consists of T cells% macrop!ages and epit!elial cells. 9ac! lobe isencapsulated by a layer of connecti)e tiss#e.

(") Secondary Lymphatic r!ans and Tissues

T!e locations at #!ic! most immune responses occur:

• L$%'h Nodes ' T!ese are small bean's!aped organs located t!roug!out t!e

body and consisting of B cells t!at de"elop into plasma cells ' #!ic! secrete

antibodies% T cells% and macrop!ages. 9ac! node is co"ered by a capsule

of dense connecti)e tiss#e.

• The S'leen ' T!is is t!e largest single mass of lymp!atic tissue in t!e !uman body.

T!e outer co"ering of t!e spleen is formed by dense connecti)e tiss#e.

T!e spleen itself consists of t#o types of tissue% called !hite '#l' and red

'#l'.

o !ite +ulp: lymp!atic tissue t!at consists mostly of lymp!octes and

macrop!ages.

o ed +ulp: Blood'filled sinuses and cords of splenic tissues t!at consists

of lymp!ocytes% macrop!ages% eryt!rocytes% granulocytes% and plasma

cells.

• L$%'hatic Nod#les ' ,ymp!atic ;odules differ from ,ymp! ;odes in

t!at l$%'hatic nod#les are notsurrounded by capsules (of dense connecti"e

tissue). T!ey are also no#n as %#cosa6associated l$%'hatic tiss#e

0.ALT1.

(c) Lymph (#$uid)

T!e fluid 3lymp!3 can be described as a tissue in its o#n rig!t ' in t!e same #ay as

t!e fluid 3blood3 can be described as 3 blood tissue3. ,ymp! is a clear fluid t!at is

similar to plasma but contains less protein. /t flo#s t!roug! lymp!atic "essels

t!roug!out t!e body and includes c!emicals and cells #!ose composition "aries

according to location #it!in t!e body.

Despite being a fluid% lymp! is classified as a connecti"e tissue.

%.0 Where in the "ody are the $ymphatic tissues ?

4s e&plained abo"e% t!ere are lymp!atic tissues t!roug!out t!e body.

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However, the structure of lymphatic tissues vary according to the particular type of

lymphatic tissue or organ they form a part of - (a), (b) or (c) above, and possibly also

it's location in the body - especially in the case of (c) Lymph (Fluid).

3(7 The Str#ct#re 0Ph$sical 8escri'tion1 of the l$%'hatic tiss#es

,ymp! is a clear fluid t!at is similar to (blood) plasma% but contains less protein.

&.0 The #unctions of $ymphatic tissues

T!e lymp!atic tissues t!roug!out t!e body operate in conunction #it! eac! ot!er to

perform t!e functions of t!e lymp!atic system. T!e lymp!atic system is part of t!e

body$s immune system. T!e functions of t!e lymp!atic system include:

• 8rainin" interstitial fl#id.

• Trans'ortin" dietr$ li'ids.

• Protectin" the od$ 0or"anis%1 a"ainst in)asion9infection

What is *o%eostasis &

Definition of !omeostasis

*o%eostasis is the %aintenance 0)ia the od$:s 'h$siolo"ical %echanis%s1 of

relati)el$ stale conditions !ithin the od$:s internal en)iron%ent e.g.

conditions such as body temperature, blood pressure , pH, concentrations of

chemicals such as specific hormones in the blood , etc. des'ite chan"es occ#rrin"

oth inside and o#tside the od$ e.g. due to eating, eercise, pregnancy,

variations in eternal conditions, etc..

S!ort -ersion: 7omeostasis is t!e body$s maintenance "ia acti"e processes of an

appro&. constant internal en"ironment despite e&ternal c!anges.

4spects of no#ledge about !omeostasis include:

1. Control of *o%eostasis

i.e. *o! does ho%eostasis occ#r & ...

7o# does t!e body maintain stable conditions in its internal en"ironment I ...

or% (less accurately)%

7o# is !omeostasis maintained I

2. 7o# Feedac; .echanis%s act to maintain !omeostasis

. E,a%'les of *o%eostasis

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. *o%eostasis< 8isorders and 8iseases

;ote: Some definitions state or imply t!at !omeostasis is t!e stable condition in

#!ic! t!e body is maintained by t!e action of feedbac systems maing appropriate

adustments as necessary. 5t!er definitionsE of !omeostasis state t!at !omeostasis is

t!e (acti"e) maintenance of stable conditions #it!in t!e body.

E 9.g. according to t!e 5&ford concise Colour *edical Dictionary% 3homeostasis the

physiological process by which the internal systems of the body ... are maintained at

e!uilibrium, despite variations in the eternal conditions.3

1. 'o does the "ody contro$ 'omeostasis ?

Short Answer: Feedback Mechanisms (see 2. below).

The following are notes about the body systems inoled.

4ll t!e body$s structures are ept in balance by one or more !omeostatic mec!anisms.

T!e body$s !omeostatic mec!anisms are controlled mainly by t!e Ner)o#s S$ste%%

and t!e Endocrine S$ste%. T!e role of parts of t!ese and ot!er tissues "aries

according to t!e specific !omeostatic mec!anism. 7o#e"er% in general:

• Structures #it!in t!e ner)o#s s$ste% detect "ariation from t!e balanced state%

i.e. parameters suc! as !eat or p7 being #it!in t!e range of acceptable "alues%

and communicate t!at information by sending signals in t!e form of ner"e

impulsesto t!e glands% organs or tissues in t!e body responsible for taing

action to restore t!e balanced state.

• /n many cases t!e glands of t!e endocrine s$ste% (endocrine glands) tae

action to restore t!e body (or a part or system thereof ) to a balanced state by

producing andor secreting !ormone molecules into t!e blood. T!is controls

ho%eostasis because !ormones are c!emicals t!at can mo"e around t!e body

and are targeted to interact #it! specific cells t!at !a"e receptors matc!ing t!e

specific !ormone. 7ormones are described as 3c!emical messengers3 because

by interacting #it! target cells t!ey stimulate t!ose cells to tae specific

action% e.g. antidiuretic hormone ("#H) directs the $idneys (to decrease the

volume of urine they produce)% #!ose o"erall effect is to maintain t!e stability

of t!e body$s internal en"ironment% i.e. ho%eostasis.

4s indicated abo"e% t!e o"erall (general) process of !omeostasis in"ol"es t!e body

detecting some ' usually small ' "ariation from t!e ideal stable% balanced% state. /t

re8uires (unconscious) decisions to be taen about t!e adustment(s) necessary and

instructions to be con"eyed to t!e structures of t!e body responsible for taing

restorati"e action. 4ction must be taen to c!ange t!e out'of'balance condition bac

#it!in t!e acceptable range e.g. range of "alues of temperature. T!roug!out and

beyond t!e process of adustment% conditions #it!in t!e body partareasystem must

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be continually monitored and any necessary adustments made to maintain t!e

stability of t!at aspect of t!e body$s internal en"ironment.

T!is is ac!ie"ed "ia feedac; %echanis%s (see belo#).

4 ey difference bet#een t!e contributions of t!e Ner)o#s S$ste% and

t!e Endocrine S$ste% in t!e feedbac systems t!at control and maintain !omeostasis

is t!at ner"e impulses are generally muc! faster t!an t!e effects of t!e !ormones '

#!ic! generallyE tra"el "ia blood circulation and can remain acti"e in t!e blood for

muc! longer periods of time ' from minutes up to as muc! as a fe# !ours in some

casesEE.

E *ost !ormones are circulating !ormones% as opposed to local !ormones t!at only

act locally% i.e. on t!e cell t!at secreted t!em or on adacent cells% #it!out entering t!e

bloodstream.

EE 7ormone !alf'life and duration of acti"ity differ from !ormone to !ormone. /n

general% peptide !ormones "ery s!ort of seconds'minutes% #!ile steroids and t!yroid

!ormone !a"e longer !alf'li"es of up to an !our or sometimes longer.

2. *p$ain +#eed"ac, -echanisms+

T!e basic principle of t!e feedbac mec!anisms (sometimes called 3feedbacsystems3) t!at maintain !omeostasis is t!at a parameter called a controlled

condition e.g. t!e concentration in t!e blood of a specific !ormone is monitored

by rece'tors t!at send (3input3) information to a control centre e.g. t!e

!ypot!alamus controls t!e concentration in t!e blood of certain !ormonest!at

responds by (3output3) sending signals e.g. in t!e form of ner"e impulses or c!emical

signals to effectors #!ic! are t!e cells or ot!er structures #it!in t!e body t!at cause

t!e controlled condition to c!ange as necessary to bring t!at particular aspect of t!e

body bac into a steady% stable% balanced state. !ile t!is is !appening t!e receptors

continue to send information to t!e control centre% #!ic! t!an t!erefore continue to

adust its output as appropriate to t!e current state of t!e controlled condition.

T!ere are t#o types of feedbac mec!anisms:

• Ne"ati)e Feedac; S$ste%s (;egati"e feedbac mec!anisms)

+roduce negati"e feedbac #!ic! means t!at negati"e feedbac

systems re"erse c!anges in t!e controlled condition% !ence negati"e feedbac

tends to bring conditions #it!in t!e body bac into balance.

• Positi)e Feedac; S$ste%s (+ositi"e feedbac mec!anisms)

+roduce positi"e feedbac so positi"e feedbac systems reinforce(increase) c!anges in t!e controlled condition.

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T!erefore positi"e feedbac systems operate #!en an e"ent #ill occur to

discontinue t!e feedbac system #!en appropriate ' e.g. positi"e feedbac

systems control infre8uent conditions suc! as o"ulation% c!ildbirt! and blood

clotting.

See !ormone regulation feedbac mec!anisms for more about positi"e "s. negati"e

feedbac systems.

%. *amp$es of 'omeostasis

T!ere are many possible e&amples of !omeostasis mec!anisms in t!e !uman body.

T!e follo#ing are included in some courses in biology% !uman biology and anatomy

p!ysiology.

• Te%'erat#re *o%eostasis (Ther%ore"#lation)• Blood =l#cose *o%eostasis ' the cycle of regulation of blood sugar levels is

described on the page about diabetes.

• Blood Water *o%eostasis (Os%ore"#lation)

. 'omeostasis/ isorders and isease

Juestion: !y is !omeostasis important I

4ns#er: *o%eostasis is necessar$ for "ood health. it!out it% disorders% disease

and deat! may result.

+ro"ided t!at t!e body$s controlled conditions are ept #it!in certain constraints e.g.

of temperature% concentration of bioc!emicals% etc.% t!e body stays !ealt!y and can

generally be e&pected to t!ri"e ' including gro#ing and de"eloping during c!ild!ood

and supporting de"eloping fetus(es) during pregnancy. 7o#e"er% if !omeostasis is not

maintained% t!e balance of t!e body$s processes may be disturbed% leading

to disorder0s1% disease or e"en death.

• 4 disorder is an abnormality or malfunction of some part of t!e body or a

system of t!e body.

9.g. !eat'stress can tae many forms depending on t!e se"erity and if t!e

person is too !ot or too cold. /n general most se"ere abnormalities !a"e

become recogni6ed diseases ' see belo#.

• 4 disease is a disorder characteri>ed $ s'ecific reco"ni>ale si"ns and

s$%'to%s ' ecept forms of ill-health that are (only) a direct result of

physical in%ury e.g. due to an accident .

9.g. 8iaetes %ellit#s is a disease due to failure of glucose !omeostasis.

• 8eath may result from some (t!oug! not all) untreated diseases ' including

t!ose due to t!e failure of !omeostasis.

9.g. if untreated% a person #it! diabetic etoacidosis ' #!ic! in"ol"es t!e

accumulation of etone bodies in t!e bloodtoget!er #it! increased blood

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acidity ' can 8uicly e&perience s!oc t!en fall into a coma% from #!ic! deat!

mig!t occur.

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Types of Circulation

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