1 of 10© Boardworks Ltd 2009 2 of 10© Boardworks Ltd 2009 Why need a transport system? Single-celled organisms, such as bacteria and amoeba (below),

1 of 10 © Boardworks Ltd 2009


Why need a transport system?

Single-celled organisms, such as bacteria and amoeba (below), can obtain nutrients and excrete waste simply by diffusion.

nutrients waste products

Multi-cellular organisms, such as insects, fish and mammals, require a more specialized transport system. Why is this?


Surface area to volume ratio

In larger organisms, diffusion of substances would occur far too slowly to enable them to survive: the rate of diffusion increases with the square of the distance it has to travel.

Single-celled organisms have a very large surface area to volume ratio, because the diffusion path is so short.

This is not just because of its size, however; more important is an organism’s surface area to volume ratio.


Surface area and volume


Components of circulatory systems

Multi-cellular animals overcome the limitations of diffusion by having a specialized circulatory system. This comprises:

a heart

vessels through which the fluid can flow.

a fluid in which substances are transported

The two types of circulatory system are open (e.g. molluscs, arthropods) and closed (e.g. vertebrates, a few invertebrates).


Open circulatory systems

An open circulatory system consists of a heart that pumps a fluid called hemolymph through short vessels and into a large cavity called the hemocoel.

When the heart relaxes, the hemolymph blood is sucked back in via pores called ostia.

Hemolymph moves around the hemocoel due to the movement of the organism.

heart

hemocoel

In the hemocoel, the hemolymph directly bathes organs and tissues, enabling the diffusion of substances.


Closed circulatory systems

In a closed circulatory system, blood is fully enclosed within blood vessels at all times.

From the heart, blood is pumped through a series of progressively smaller vessels. In the smallest vessels, capillaries, substances diffuse in and out of the blood and into cells.

Blood then returns to the heart via a series of progressively larger vessels.

heart

capillaries


Closed circulatory systems


The mammalian circulatory system


Vertebrate circulatory system

• Adaptations in closed systemnumber of heart chambers differs

4 chamber heart is double pump = separates oxygen-rich & oxygen-poor blood; maintains high pressure

What’s the adaptive value of a 4 chamber heart?

2 3 4

low pressureto body

low O2

to body

high pressure & high O2

to body


Evolution of 4-chambered heart

• Selective forcesincrease body size

• protection from predation• bigger body = bigger stomach for herbivores

endothermy• can colonize more habitats

flight• decrease predation & increase prey capture

• Effect of higher metabolic rategreater need for energy, fuels, O2, waste

removal• endothermic animals need 10x energy• need to deliver 10x fuel & O2 to cells


Vertebrate cardiovascular system

• Chambered heartatrium = receive blood

ventricle = pump blood out

• Blood vesselsarteries = carry blood away from heart

• arterioles

veins = return blood to heart• venules

capillaries = thin wall, exchange / diffusion • capillary beds = networks of capillaries


Blood vesselsarteries

arterioles

capillaries

venules

veins

artery

arteriolesvenules

veins


Arteries: Built for high pressure pump

• Arteries thicker walls

• provide strength for high pressure pumping of blood

narrower diameter

elasticity • elastic recoil helps

maintain blood pressure even when heart relaxes


Veins: Built for low pressure flow• Veins

thinner-walled wider diameter

• blood travels back to heart at low velocity & pressure

• lower pressure– distant from heart– blood must flow by skeletal muscle contractions

when we move » squeeze blood through veins

valves• in larger veins one-way valves

allow blood to flow only toward heart

Open valve

Blood flows

toward heart

Closed valve


Capillaries: Built for exchange

• Capillariesvery thin walls

• lack 2 outer wall layers • only endothelium

– enhances exchange across capillary

diffusion• exchange between

blood & cells


Controlling blood flow to tissues

• Blood flow in capillaries controlled by pre-capillary sphincters

• supply varies as blood is needed• after a meal, blood supply to digestive tract increases• during strenuous exercise, blood is diverted from digestive tract to skeletal

muscles

capillaries in brain, heart, kidneys & liver usually filled to capacity

sphincters open sphincters closed


Exchange across capillary walls

Arteriole

Blood

flow

Venule

Lymphatic

capillary

Interstitial

fluid

Fluid & solutes flows out of capillaries to tissues due to blood pressure• “bulk flow”

Interstitial fluid flows back into capillaries due to osmosis plasma proteins osmotic pressure in capillary

BP > OP BP < OP

15% fluid returns via lymph

85% fluid returns to capillaries

Capillary


Blood• Plasma: liquid matrix of blood in which cells are suspended (90%

water)• Erythrocytes (RBCs): transport O2 via hemoglobin• Leukocytes (WBCs): defense and immunity• Platelets: clotting• Stem cells: pluripotent cells in the red marrow of bones


Circulation: true or false?

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