Section 8: Transport In Animals

Question 1

Why do multicellular organisms need a transport system?

Answer 1

Multicellular organisms are relatively big and have a low surface area to volume ratio and a higher metabolic rate (the speed at which chemical reactions take place in the body). A lots of multicellular organisms are also very active. This means that a large number of cells are all respiring very quickly, so they need a constant, rapid supply of glucose and oxygen. Carbon dioxide also needs to be removed from cells quickly.
To ensure that every cell has a good enough supply of useful substances and has its waste products removed, multicellular organisms need a transport system. The circulatory system in mammals uses blood to carry glucose and oxygen around the body. It also carries hormones, antibodies and waste products.

Question 2

What type of circulatory system can organisms have?

Answer 2

Organisms can have a single circulatory system and others have a double circulatory system.

Question 3

What happens in a single circulatory system?

Answer 3

In a single circulatory system, blood only passes through the heart once fo each complete circuit of the body.

Question 4

What happens in a double circulatory system?

Answer 4

In a double circulatory system, the blood passes through the heart twice for each complete circuit of the body.

Question 5

What is a closed circulatory system?

Answer 5

In a closed circulatory system, the blood is enclosed inside blood vessels. All vertebrates have a closed circulatory system.

Question 6

What is an open circulatory system?

Answer 6

In an open circulatory system, blood isn’t enclosed in blood vessels all the time. Instead, it flows freely through the body cavity. Some invertebrates have an open circulatory system.

Question 7

Describe the circulatory system in fish (single/double)

Answer 7

Fish have a single circulatory system. The heart pumps blood to the gills (to pick up oxygen) and then on through the rest of the body (to deliver the oxygen) in a single circuit.

Question 8

Describe the circulatory system in mammals.

Answer 8

Mammals have a double circulatory system. The heart is divided down the middle, so it’s really like two hears joined together. The right side of the heart pumps blood to the lungs (to pick up oxygen). From the lungs, it travels to the left side of the heart, which pumps it to the rest of the body. When blood returns to the heart it enters the right side again.
So, our circulatory system is just two linked loops. One sends blood to the lungs - called the pulmonary system, and the other sends blood to the rest of the body - called the systematic system.
The advantage of the mammalian double circulatory system is that the heart can give the blood an extra push between the lungs and the rest of the body. This makes the blood travel faster, so oxygen is delivered to the tissues moe quickly.

Question 9

Describe the circulatory system in fish (closed/open).

Answer 9

In fish, the heart pumps blood into arteries. This branch out into millions of capillaries. Substances like oxygen and glucose diffuse from the blood in the capillaries into the body cells, but the blood stays inside the blood vessels as it circulates. Veins take the blood back to the heart.

Question 10

Describe the circulatory system of an insect.

Answer 10

An insect’s heart is segmented. It contracts in a wave, starting from the back, pumping the blood into a single main artery. The artery opens up into the body cavity. The blood flows around the insect’s organs, gradually making its way back into the heart segments through a series of valves.
The circulatory system supplies the insect’s cells with nutrients, and transports things like hormones around the body. It doesn’t supply the insect’s cells with oxygen though - this is done by a system of tubes called the tracheal system.

Question 11

What are the five main types of blood vessel?

Answer 11

The five main types of blood vessels are arteries, arterioles, capillaries, venules and veins.

Question 12

What are arteries and what do they do?

Answer 12

Arteries carry blood from the heart to the rest of the body. Their walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain the high pressure. The inner lining (endothelium) is folded, allowing the artery to expand - this also helps it to maintain the high pressure. All arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs.
Arteries have elastic tissue in their walls, a thick muscle layer, a folded endothelium and a lumen in the centre.

Question 13

What are arterioles and what do they do?

Answer 13

Arteries branch into arterioles, which are much smaller than arteries. Like arteries, arterioles have a layer of smooth muscle, but they have less elastic tissue. The smooth muscle allows them to expand or contract, thus controlling the amount of blood flowing to tissues.

Question 14

What are capillaries and what do they do?

Answer 14

Arterioles branch into capillaries, which are the smallest of the blood vessels. Substances like glucose and oxygen are exchanged between cells and capillaries, so they’re adapted for efficient diffusion, e.g. their walls are only one cell thick.

Question 15

What are venules and what do they do?

Answer 15

Capillaries connect to venules, which have very thin walls that can contain some muscle cells. Venules join together to form veins.

Question 16

What are veins and what do they do?

Answer 16

Veins take blood back to the heart under low pressure. They have a wider lumen than equivalent arteries, with very little elastic or muscle tissue. Veins contain valves to stop the blood flowing backwards. Blood flow through the veins is helped by contraction of the body muscle surrounding them. All veins carry deoxygenate blood (because oxygen has been used up by body cells) except for the pulmonary veins, which carry oxygenated blood to the heart from the lungs.

Question 17

What is tissue fluid and what does it do?

Answer 17

Tissue fluid is the fluid that surrounds cells in tissues. It’s made from substances that leave the blood plasma, e.g. oxygen, water and nutrients. (Unlike blood, tissue fluid doesn’t contain red blood cells or big proteins, because they’re too large to be pushed out through the capillary walls). Cells take in oxygen and nutrients from the tissue fluid, and release metabolic waste into it. In a capillary bed (the network of capillaries in an area of tissue), substances move out of the capillaries, into the tissue fluid, by reassure filtration.

Question 18

What is pressure filtration?

Answer 18

At the start of the capillary bed, nearest the arteries, the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid. This difference in hydrostatic pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid. As fluid leaves, the hydrostatic pressure reduces in the capillaries - so the hydrostatic pressure is much lower at the end of the capillary bed that’s nearest to the venules.
As water leaves the capillaries, the concentration of plasma proteins in the capillaries increases and the the water potential decreases. Plasma proteins in the capillaries generate a form of pressure called oncotic pressure - so at the venule end of the capillary bed there’s a high oncotic pressure and a low water potential. Because the water potential in the capillaries is lower than the water potential in the tissue fluid, some water re-enters the capillaries from the tissue fluid at the venule end by osmosis.

Question 19

What are lymph vessels?

Answer 19

Not all of the tissue fluid re-enters the capillaries at the vein end of the capillary bed - some excess tissue fluid is left over. This extra fluid eventually gets returned to the blood though the lymphatic system - a kind of drainage system, made up of lymph vessels.
The smallest lymph vessels are the lymph capillaries. Excess tissue fluid passes into lymph vessels. Once inside, it’s called lymph. Valves in the lymph vessels stop the lymph going backwards. Lymph gradually moves towards the main lymph vessels in the thorax. Here, it’s returned to the blood, near the heart.

Question 20

What are the two heart valves?

Answer 20

The atrioventricular (AV) valves and the semi-lunar (SL) valves.

Question 21

What is the function of the atrioventricular valves?

Answer 21

The atrioventricular (AV) valves link the atria to the ventricles.

Question 22

What is the function of the semi-lunar (SL) valves?

Answer 22

The semi-lunar (SL) valves link the the ventricles to the pulmonary artery and aorta.

Question 23

How do the AV and SL valves work?

Answer 23

The valves only open one way - whether they’re open or closed depends on the relative pressure of the heart chambers. If there’s a higher pressure behind a valve, it’s forced open, but if pressure is higher in front of the valve it’s forced shut. This means that the flow of blood is unidirectional - it only flows in one direction.

Question 24

What are the steps involved in heart dissection?

Answer 24

1. Make sure you are wearing an apron and lab gloves because heart dissections can be messy.
2. Place the heart you are given on your dissecting tray.
3. Look at the outside of the heart and try to identify the four main vessels attached to it. Feel inside the vessels to help you - arteries are thick and rubbery, whereas veins are much thinner.
4. Identify the right and left atria, the right and left ventricles and the coronary arteries. Draw a sketch of the outside of the heart and label it.
5. Using a clean scalpel, carefully cut along the lines shown to look inside each ventricle. You could measure and record the thickness of the ventricle walls and note any differences between them.
6. Net cut open the atria and look inside them too. Note whether the atria walls are thicker or thinner than the ventricle walls.
7. Then find the atrioventricular valves, followed by the semi-lunar valves. Look at the structure of the valves and see if you can see how they only open one way. Draw a sketch to show the valves and the inside of the ventricles and atria.
8. Make sure you wash your hands and disinfect all work surfaces once you’ve completed your dissection.

Question 25

What is the cardiac cycle?

Answer 25

The cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating round the body. The volumes of the atria and ventricles change as they contract and relax, altering the pressure in each chamber. This causes valves to open and close, which directs the blood flow through the heart. If you listen to a human heartbeat you can hear a ‘lub-dub’ sound. The first ‘lub’ sound is caused by the atrioventricular valves closing. The second ‘dub’ sound is caused by the semi-lunar valves closing.

Question 26

Explain the cardiac cycle.

Answer 26

The ventricles are relaxed. The atria contract, decreasing the volume of the chambers and increasing the pressure inside the chambers. This pushes the blood into the ventricles though the atrioventricular valves. There’s a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria.
The atria relax. The ventricles contract (decreasing their volume), increasing their pressure. The pressure becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow. The pressure in the ventricles is also higher than in the aorta and pulmonary artery, which forces open the SL valves and blood is forced out into these arteries.
The ventricles and the atria both relax. The higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back-flow into the ventricles. Blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein. In turn this starts to increase the pressure of he atria. As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open. This allows blood to flow passively (without being pushed by atrial contraction) into the ventricles from the atria. The atria contract, an the whole process begins again.

Question 27

How do you calculate cardiac output?

Answer 27

In the exam you could be asked to calculate cardiac output. Cardiac output is the volume of blood pumped by the heart per minute (measured in centimetres cubed millimetres -1). It’s calculated using the formula:
cardiac output = heart rate time x stroke volume
Heart rate – the number of beats per minute.
Stroke volume – the volume of blood pumped during each heartbeat, measured in centimetres cubed