04 Circulation Web

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Chapter 42; pp. 867-879

Animal Circulatory Systems

� Types of circulatory systems:

gastrovascular, open, closed.

� Vascular system: arteries, veins, capillaries.

� Capillary - tissue fluid exchange.

Absence of a Circulatory System

� Very small animals may not need a circulatory

system.

� Small size may permit nutrients and other

substances to reach all the body parts by simple

diffusion

figure 49-01.jpg

Cnidarian Gastrovascular Systems

Fig. 33.5

Some larger animals such as

sea anemones, jellyfish, and

flatworms lack a true

circulatory system.

The gastrovascular cavity

extends to most areas of the

body in these animals andserves as a circulatory

system as well as a

digestive cavity.

Larger Animals Without a Separate Circulatory System

Flatworm Gastrovascular System

Fig. 33.10

Medusa Internal Transport

Fig. 42.2

� Radial canals branch from the centrally located stomach and

extend out to the circular canal at the margin of the bell.

� Nutrients and other material are carried around the body in

these canals and then diffuse out to nearby tissues.

Circulatory Systems

� Two types of circulatory system are found:

Open Circulatory Systems

Closed Circulatory Systems

For larger or more active animals, some form of

more efficient circulatory system is necessary for

internal transport.

Open Circulatory System

� Hemolymph leaves theheart in short, branchedarteries that open up intolarge spaces called sinuses.

� Hemolymph percolatesaround organs, directlybathing the cells.

� Hemolymph then returns tothe heart directly or throughshort veins.

Fig. 42.3a

� Advantage - Since cells are bathed by hemolymph,

the exchange of materials is direct between the

hemolymph and tissues. There is no diffusion

barrier.

� Disadvantage - There is little opportunity for fine

control over distribution of the hemolymph to body

regions. No mechanism for reducing flow to aspecific part of an organ.

� Open circulatory systems tend to be found in more

inactive animals.

� Most molluscs have an open system, but thehighly active cephalopods (squid and octopus)

have evolved a closed system.

� Insects have circumvented limitation of their opensystem by their tracheal system for oxygen supply.

Origin of the Hemocoel

� The open sinuses where the hemolymph circulates

is called the hemocoel.

� This space is not a coelomic cavity.

� It is a persistent blastocoel that has never been

filled in by expanding mesodermal tissue during

development.

Cross-sections

through

developingembryos.

See Fig. 47.9

for steps in

development.

Closed Circulatory System

� The blood is containedwithin a completely closed

system of vessels.

� Vessels form a closed

loop, usually with some

sort of pumping organ like

a heart or contractile

vessels.

� Vessels branch into

smaller and smaller tubes

that penetrate among the

cells of tissues.

Fig. 42.3b

Closed Circulatory System

� Fine-scale control over the distribution of blood

to different body regions is possible.

� Muscular walls of vessels can constrict and dilate

to vary the amount of flow through specific

vessels.

� Blood pressures are fairly high and the circulation

can be vigorous.

Advantages:

Fig. 33.23

figure 49-03.jpg

Earthworm Anatomy

Earthworm CirculationExtensive

capillary beds:

Body wall

Gut wall

Excretory tubules

Coelomic Cavities - Circulatory Function

� Coelomic cavities are filled with fluid that can

transport materials around the body.

� Nematode worms have an extensive body cavity,

the pseudocoel, but lack a separate circulatory

system.

Ascaris

Cross-Section

Pseudocoel

(fluid-filled space)

The Vertebrate Vascular System: Arteries,

Veins, and CapillariesArteries and arterioleshave a layer of smooth

muscle tissue which

allows them to contract

(vasoconstrict) and

expand (vasodilate),

altering their diameter

and thus blood flow.

Walls of arteries andarterioles have many

elastic fibers enabling

them to withstand high

pressures.Fig. 42.9

Artery and Vein

Artery

Note the much

thinner walls inveins.

Blood Pressure and

Flow Velocity

� As arteries branch, the

cross-sectional area increases

causing blood pressure and

flow velocity to fall.

� In mammals there is a an

800-fold increase in cross-

sectional area from the aorta

to the capillaries.

� Velocity in the aorta is

around 40-50 cm/s but drops

to < 0.1 cm/s in capillaries.

Fig. 42.11

Capillaries

� Capillaries are very small,

about the diameter of a red

blood cell (8µm or less).

� Capillary walls are a single

layer of very thin endothelial

cells, attached at their edges

and surrounded by a basementmembrane (extracellular

matrix).

Endothelial cells

Blood cells,

proteins.

Vesicles; large,lipid-insoluble

(proteins)Filtration; fluid and

small, lipid-insoluble

molecules (water,

amino acids,NaCl, glucose,

Diffusion;

lipid-soluble

molecules

(O2, CO2,

lipids)

Capillary Density in Tissues

� Penetration of tissues by capillaries is soextensive that in active tissues each capillary

serves a volume of tissue only about 10 times

its own volume.

� No cell is very far from the blood supply.

Capillary - Tissue Fluid Exchange

� Capillary beds are the site of exchange of materials between bloodand the interstitial fluid that bathes the tissues.

� Fluid exchange between blood and interstitial fluid is determinedby the balance between the positive blood pressure (hydrostatic

pressure) and the net negative osmotic potential in the bloodplasma.

� The osmotic potential of the blood plasma in the capillary is more

negative than the osmotic potential of the surrounding interstitialfluid.

� Proteins in the blood plasma that cannot easily leave the capillaryare the source of this difference in osmotic potential. Otherwise,the interstitial fluid and blood plasma have similar concentrationsof ions and other small molecules.

Osmotic Gradient Between Interstitial Fluid and Blood

Similar to

Fig. 42.14

Blood hydrostatic pressure exceeds the opposing negative

colloidal osmotic potential of the blood plasma.

Water, containing small dissolved molecules, is forced

out of the capillary through small pores in the capillary

wall by the excess hydrostatic pressure.

Capillary Fluid Exchanges

Blood pressure(hydrostatic)

32 mm Hg

Plasma colloidalosmotic potential

-22 mm Hg

Net pressure

10 mm Hg

Venous End

� At the venous end of the capillary, the balance of

forces reverses and the blood plasma¶s negative

colloidal osmotic potential exceeds the hydrostaticpressure of the blood.

� Water, containing small dissolved molecules,

moves back into the capillary by osmosis.

Capillary Fluid Exchanges

Frictional

resistance Blood pressure(hydrostatic)

15 mm Hg

-22 mm Hg

Net pressure

-7 mm Hg

Blood pressure(hydrostatic)

32 mm Hg

-22 mm Hg

Net pressure

10 mm Hg

� Note that the net force is less at the venous end(7 mm Hg inwards vs 10 mm Hg outwards at arterial end ).

� Less water re-enters the capillary than originally

left at the arterial end.

� The surplus fluid is taken up by the lymphatic

system

The Lymphatic System

Fig. 43.5

� A separate system of

vessels, the lymphatic

system, returns excesstissue fluid to the blood.

� Lymphatic ducts drain

into the venous system

near the heart.

� The bulk flow of fluid out of the capillary

exchanges material much faster than would be

possible by simple diffusion alone.

� Nutrients and O2 are released to the tissues

rapidly.

� Wastes from cell metabolism are more rapidlycleared away by the circulatory system.

Control of Capillary Circulation

� Arteries, arterioles, and metarterioles that feedblood to the capillaries contain a circular layer of smooth muscle in their walls.

� Contraction of these smooth muscles(vasoconstriction) is important in controlling theblood flow through capillary beds.

� Relaxation of smooth muscles results invasodilation, an expansion of the vessel diameter that increases blood flow.

figure 49-18.jpg

Precapillary sphincters are

rings of smooth muscle that

surround the junction of a

capillary with an arteriole or

metarteriole.

Contraction of precapillary

sphincters can completely

shut off blood flow to a

capillary bed.

Fig. 42.13

Circulatory Patterns in

Vertebrates

The circulatory pattern has been modified

during evolution of the major groups of vertebrates.

Based on Fig. 42.3

(and capillaries)

Fish Heart

Frog Circulation

� The frog has completely separated atria, but the

ventricle is a single chamber.

� There is surprisingly little mixing of oxygenated and

deoxygenated blood as it passes through the single

ventricle.

� Oxygenated blood from the pulmonary vein

probably soaks into the spongy walls of the ventricle

so it doesn¶t mix much with deoxygenated blood.

Based on Fig. 42.3

Frog Heart

� There is incomplete mixing

of the oxygenated anddeoxygenated blood.

� The most deoxygenatedblood passes into the

pulmocutaneous arteries.� The most oxygenated blood,

from the lung (pulmonaryvein) preferentially entersthe carotid arteries.

Mammals and Birds

� Mammals and birds have completely divided atria

and ventricles so no mixing of oxygenated anddeoxygenated blood is possible.

� There is a complete double circulation pattern first

through the pulmonary circuit and then throughthe systemic circuit.

Based on Fig. 42.3

� Deoxygenated blood enters the right atrium and

goes on into the pulmonary circuit to the lungs.

� Oxygenated blood comes back to the left atrium in

the pulmonary vein and then goes on to the

systemic circuit

Mammals and Birds

Cardiac cycle

Fig. 42.7Animation

Structure of blood vessels

Fig. 42.9

Fig. 42.10

Blood flow in veins

One-way flow of blood (toward heart) isdetermined by valves.

components

Fig 42 15

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