Transport systems in animals: open, closed, single and double – Circulatory System in Animals (Senior 6 Biology) | Ganzaa

Circulatory System in Animals

Lessons

Senior 6 · Biology

Circulatory System in Animals

5 lessons

Lesson 1 of 5

Transport systems in animals: open, closed, single and double

25 min

Why does a cow need a heart, but a flatworm does not?

A flatworm living under a stone in a stream is so thin that every cell lies within a fraction of a millimetre of the water. Oxygen diffuses in and carbon dioxide diffuses out fast enough. A cow grazing in Nyagatare is a different matter: its muscle cells may be many centimetres from the air in its lungs and from the food in its gut. Diffusion alone would take hours or years to supply them. Large, active animals need a mass transport system that moves a fluid in bulk, carrying substances close to every cell.

In Senior 1 and 2 you studied the lungs and the blood. This unit follows the whole circulatory system: its types, the mammalian heart and how it beats, the blood vessels and body fluids, the transport of oxygen and carbon dioxide, blood clotting and cardiovascular disease.

Definition

Mass flow (mass transport): the bulk movement of a fluid, and everything dissolved or suspended in it, in one direction, driven by a pressure difference. Example: blood pumped through an artery.

Circulatory system: a system of a pump (heart), a transport fluid (blood or haemolymph) and, usually, vessels, that moves substances around the body by mass flow.

Open circulatory system: a system in which the transport fluid, haemolymph, leaves the vessels and bathes the organs directly in a body cavity, the haemocoel. Example: insects.

Closed circulatory system: a system in which the blood stays inside vessels all the time, and substances reach the cells by diffusing across capillary walls. Example: earthworms, octopuses, all vertebrates.

Why diffusion is not enough

Two physical facts set the limit.

Surface area to volume ratio. For a cube of side $L$ , the surface area is $6 L^{2}$ and the volume $L^{3}$ , so $\frac{\text{SA}}{V} = \frac{6}{L}$ . As an organism gets bigger, it has less surface for each unit of volume that needs supplying.
Diffusion time rises with the square of distance. The mean time for a molecule to diffuse a distance $x$ is roughly $t \approx \frac{x^2}{2D}$ where $D$ is the diffusion coefficient; for oxygen in water, $D \approx 2 \times 10^{-9}\,\mathrm{m^2\,s^{-1}}$ . Double the distance and the time goes up four times.

Activity makes the problem worse: an active, warm-blooded animal uses oxygen fast. So large, active animals have a pump, vessels and a fluid that carries oxygen, glucose, amino acids, carbon dioxide, urea, hormones, antibodies and heat.

Worked example 1: how long would diffusion take?

Using $t \approx \frac{x^2}{2D}$ and $D = 2 \times 10^{-9}\,\mathrm{m^2\,s^{-1}}$ for oxygen in water, calculate the time for oxygen to diffuse (a) $0.1\,\mathrm{mm}$ , (b) $1\,\mathrm{cm}$ , (c) $0.5\,\mathrm{m}$ .

(a) $x = 1 \times 10^{-4}\,\mathrm{m}$ : $t = \frac{(1 \times 10^{-4})^2}{2 \times 2 \times 10^{-9}} = \frac{1 \times 10^{-8}}{4 \times 10^{-9}} = 2.5\,\mathrm{s}$ .
(b) $x = 1 \times 10^{-2}\,\mathrm{m}$ : $t = \frac{1 \times 10^{-4}}{4 \times 10^{-9}} = 2.5 \times 10^{4}\,\mathrm{s}$ , about 7 hours.
(c) $x = 0.5\,\mathrm{m}$ : $t = \frac{0.25}{4 \times 10^{-9}} = 6.25 \times 10^{7}\,\mathrm{s}$ , about 2 years.

Conclusion: diffusion is fast over the width of a few cells, but hopeless over the size of a cow. Mass flow brings blood within about $0.1\,\mathrm{mm}$ of every cell, and diffusion does the last step.

Open and closed systems

Figure 1: An open circulatory system (an insect, from the side) and a closed one. Orange arrows: haemolymph. Red: blood in an artery. Blue: blood in a vein.

In an open system, the heart pumps haemolymph at low pressure into the haemocoel. It flows slowly among the organs and returns to the heart through the ostia, openings with valves that stop backflow. Insects can manage with this because their haemolymph carries little oxygen: a separate system of air tubes, the tracheae, takes oxygen directly to the cells. In a closed system, the blood is kept in vessels at high pressure, flows fast, and can be directed to the organs that need it most by widening or narrowing arteries.

	Open system	Closed system
Examples	Insects, spiders, most molluscs (snails)	Earthworms, octopuses and squid, all vertebrates
Fluid	Haemolymph, in direct contact with the cells	Blood, separated from cells by capillary walls; tissue fluid bathes the cells
Pressure and speed	Low pressure, slow flow	High pressure, fast flow
Control of flow	Little: fluid goes wherever the cavity allows	Precise: arteries and capillary beds can be opened or closed
Suits	Small animals, or those with a separate oxygen supply (tracheae)	Large, active animals with high oxygen demand

Single and double circulation

Figure 2: Single circulation in a fish and double circulation in a mammal. In the mammal, the heart is drawn as seen from the front, so its right side is on your left.

In a fish, blood passes through the heart once in each circuit of the body. The heart pumps it through the narrow capillaries of the gills, and this lowers its pressure a great deal; the blood then flows on to the body at low pressure. This is enough for a fish, whose body is supported by water and whose temperature is that of the water.

In mammals and birds, the blood passes through the heart twice: the pulmonary circulation (right ventricle to lungs to left atrium) and the systemic circulation (left ventricle to body to right atrium). After the lungs, the blood returns to the heart and is pumped again, so it reaches the body at high pressure. A complete septum keeps oxygenated and deoxygenated blood apart. This suits endotherms, which need a fast, high-pressure supply of oxygen to their respiring cells. The pulmonary circulation runs at a lower pressure, which protects the delicate lung capillaries and keeps fluid out of the alveoli.

Amphibians and most reptiles have a double circulation with a heart of three chambers (two atria and one ventricle), in which some mixing of blood can occur.

Worked example 2: comparing surface area to volume ratios

Model a small flatworm as a cube of side $1\,\mathrm{mm}$ and a goat's body as a cube of side $0.5\,\mathrm{m}$ . Calculate each $\frac{\text{SA}}{V}$ in $\mathrm{m^{-1}}$ , and explain what the difference means.

Flatworm: $L = 1 \times 10^{-3}\,\mathrm{m}$ , so $\frac{\text{SA}}{V} = \frac{6}{L} = \frac{6}{1 \times 10^{-3}} = 6\,000\,\mathrm{m^{-1}}$ .
Goat: $\frac{6}{0.5} = 12\,\mathrm{m^{-1}}$ .
The flatworm has $\frac{6\,000}{12} = 500$ times as much surface per unit volume. It can exchange gases across its whole body surface; the goat cannot, so it needs lungs with a huge internal surface and a circulatory system to link them to every cell.

Limitation: real animals are not cubes (a flatworm is flat, which raises its ratio further), but the trend holds for any shape.

Warning

Common mistakes

"Insects have no blood." They have haemolymph, which carries food, wastes and hormones; it does not carry much oxygen, because the tracheae do that.
"Double circulation means two hearts." It means one heart working as two pumps side by side, with blood passing through it twice per circuit.
Mixing up sides. In a diagram of the heart seen from the front, the heart's right side is on your left.

CrashCourse: "Circulatory & Respiratory Systems" (CrashCourse Biology #27). Watch for why large animals need a circulatory system and for the route of blood through the double circulation.

Tip

Exam technique. When comparing single and double circulation, always mention pressure: "In a single circulation, blood loses pressure in the gill capillaries, so it reaches the body at low pressure; in a double circulation, it returns to the heart after the lungs and is pumped again at high pressure." Pressure is the point the examiner is looking for.

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Practice: transport systems in animals

Explain why a unicellular organism such as Amoeba does not need a circulatory system.
Calculate $\frac{\text{SA}}{V}$ for cubes of side 2 cm and 8 cm, and state by what factor the ratio changes.
Using $t \approx \frac{x^2}{2D}$ with $D = 2 \times 10^{-9}\,\mathrm{m^2\,s^{-1}}$ , calculate the time for oxygen to diffuse $2\,\mathrm{mm}$ .
Give three differences between an open and a closed circulatory system.
Trace the path of a red blood cell from the right atrium of a mammal back to the right atrium, naming every chamber and main region it passes through.
Explain why a double circulation is an advantage to a mammal but a single circulation is enough for a fish.
An insect's haemolymph contains no respiratory pigment. Suggest why this does not limit the insect's activity.

Show answersHide answers

It is so small that it has a large surface area to volume ratio and short diffusion distances, so diffusion across its cell surface meets its needs.
2 cm: $\frac{6}{2} = 3\,\mathrm{cm^{-1}}$ ; 8 cm: $\frac{6}{8} = 0.75\,\mathrm{cm^{-1}}$ . The ratio falls by a factor of 4 as the side increases by a factor of 4.
$t = \frac{(2 \times 10^{-3})^2}{2 \times 2 \times 10^{-9}} = \frac{4 \times 10^{-6}}{4 \times 10^{-9}} = 1\,000\,\mathrm{s}$ , about 17 minutes.
Any three: open has haemolymph in a haemocoel, closed keeps blood in vessels; open is low pressure and slow, closed high pressure and fast; open has little control over distribution, closed can direct blood to particular organs; open fluid bathes the cells directly, closed exchanges through capillary walls via tissue fluid.
Right atrium → right ventricle → pulmonary artery → lung capillaries → pulmonary vein → left atrium → left ventricle → aorta → arteries → body capillaries → veins → vena cava → right atrium.
The mammal is an endotherm with a high metabolic rate, so it needs oxygen delivered fast at high pressure; after the lungs the blood is re-pumped by the left ventricle. A fish is supported by water and does not use respiration to keep warm, so its oxygen demand is lower, and low-pressure blood from the gills is enough.
Oxygen reaches the cells directly as a gas through the tracheal system, not in the haemolymph, so the haemolymph does not need to carry oxygen.

Key points

Large, active animals have a small $\frac{\text{SA}}{V}$ and long diffusion distances ( $t \propto x^2$ ), so they need mass transport.
Open systems (insects) bathe organs in haemolymph at low pressure; closed systems keep blood in vessels at high pressure.
Fish have a single circulation; mammals a double circulation (pulmonary and systemic), which returns blood to the heart after the lungs so it reaches the body at high pressure.

Sources and credits

7.8.2: Overview of the Circulatory System, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for open and closed systems and single and double circulation. Adapted by Ganzaa.
Mass diffusivity, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for the diffusion coefficient of oxygen in water. Adapted by Ganzaa.
Photo: Fish gills, Krishna satya 333, CC BY-SA 4.0, via Wikimedia Commons.
Video: Circulatory & Respiratory Systems - CrashCourse Biology #27, CrashCourse, embedded from YouTube.

Figures 1 and 2, the table, the worked examples and the practice questions are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why does a cow need a heart, but a flatworm does not?

2Why diffusion is not enough

3Open and closed systems

4Single and double circulation

Why does a cow need a heart, but a flatworm does not?

Why diffusion is not enough

Open and closed systems

Single and double circulation