M3 Transport in Animals

Question 1

Why do organisms need specialised transport systems?

Answer 1

• The metabolic demands of most multicellular animals are high so diffusion over long distances is not enough to supply to quantities needed.
• The SA:V ratio gets smaller as multicellular organisms get bigger so the amount of surface area available to absorb or remove substances becomes smaller.
• Molecules such as hormones or enzymes may be made in one place but needed in another.
• Food will be digested in one organ system, but needs to be transported to every cell for use in respiration and other aspects of cell metabolism.
• Waste products for metabolism needs to be removed from cells and transported to excretory organs.

Question 2

Features of a circulatory system

Answer 2

• Liquid transport medium that circulates around the system (blood)
• Vessels that carry the transport medium.
• A pumping mechanism to move the fluid around the system.

When substances are transported in a mass of fluid with a mechanism for moving the fluid around the body it is known as a mass transport system.

Question 3

What is an open circulatory system?

Answer 3

• Blood is pumped straight from the heart into the body cavity if the animal.
• This open cavity is called the haemocoel.
• In the haemocoel the transport medium is under low pressure. It comes directly into contact with the tissues and the cells.
• This is where exchange takes place between the transport medium and the cells. The transport medium returns to the heart through an open-ended vessel.

Mainly found in invertebrate

Question 4

What is insect blood called?

Answer 4

Haemolymph
- It doesn’t carry oxygen or carbon dioxide
- It transports food and nitrogenous waste products and the cells involved in defence against disease
- The body cavity is split by a membrane and the heart extends along the length of the thorax and the abdomen of the insect.
- The haemolymph circulates but steep diffusion gradients cannot be maintained for efficient diffusion.
- The amount of haemolymph flowing to a particular tissue cannot be varied to meet changing demands.

Question 5

What is a closed circulatory system?

Answer 5

• Blood is enclosed in the blood vessels and does not come directly into contact with the cells of the body.
• The heart pumps blood around the body under pressure and relatively quickly, and the blood returns directly to the heart.
• Substances leave and enter the blood by diffusion through the walls of the blood vessels.
• The amount of blood flowing to a particular tissue can be adjusted by widening or narrowing blood vessels.
• Most closed circulatory systems contain a blood pigment that carries the respiratory gases.

Question 6

What is a single circulatory system?

Answer 6

• Blood flows through the heart and is pumped out to travel all the body before returning to the heart.
• In a single closed circulatory system the blood passes through two sets of capillaries before it returns to the heart:
  • in the first it exchanges oxygen and carbon dioxide
  • in the second set substances are exchanged between the blood and the cells
• As a result the blood pressure drops so the blood returns to the heart slowly, limiting the efficiency of the system.

Question 7

Describe the circulatory system of fish

Answer 7

• Fish have a relatively efficient single circulatory system, meaning they can be very active.
• They have a countercurrent gaseous exchange mechanism in their gills that allows them to take a lot of oxygen from the water.
• Their body weight is supported by the water and they do not maintain their own body temperature.
• This greatly reduced the metabolic demand on their bodies, and combined with their efficient gaseous exchange, explains how fish can be so active with a single closed circulatory system.

Question 8

Describe a double closed circulatory system

Answer 8

A double circulatory system has two separate circulations:
- blood is pumped from the heart to the lungs to pick up oxygen and unload carbon dioxide and then returns to the heart
- blood flows through the heart and is pumped out to travel all around the body before returning to the heart again
- Blood travels through the heart for each circuit of the body. Each circuit only passes through one capillary network, meaning a relatively high pressure and fast flow of blood can be maintained.

Question 9

Function of elastic fibres in blood vessels

Answer 9

Composed of elastin and can stretch and recoil, providing vessel walls with flexibility.

Question 10

Function of smooth muscle in blood vessels

Answer 10

Contracts/relaxes which changes the size of the lumen

Question 11

Function of collagen in the blood vessels

Answer 11

Provides structural support to maintain the shape and volume of the vessel

Question 12

Describe the function of arteries

Answer 12

• The arteries carry blood away from the heart to the tissues of the body.
• They carry oxygenated blood (except pulmonary artery).
• The blood in the arteries is under higher pressure than blood in the veins .

Question 13

Describe the structure of arteries

Answer 13

• Artery walls contain elastic fibres, smooth muscle and collagen.
• The elastic fibres enable them to withstand the force of the blood pumped out of the heart and stretch (within limits maintained by collagen) to take the larger blood volume.
• In between the contractions of the heart, the elastic fibres recoil and return to their original length.
• This helps to give out the surfaces of blood pumped from the heart to give a continuous flow.
• However, you can still feel a pulse when the heart contracts, which the elastic fibres cannot completely eliminate.
• The lining of the artery (endothelium) is smooth so the blood flows easily over it.

Question 14

Describe the function of arterioles

Answer 14

• Arterioles link the arteries and the capillaries.
• They have more smooth muscle and less elastin in their walls than arteries, as they have little pulse surge, but can constrict or dilate to control the flow of blood into individual organs.
• When the smooth muscle in the arteriole contracts it constricts the vessel and prevents blood flowing in a capillary bed (vasoconstriction).
• When the smooth muscle in the wall of an arteriole relaxes, blood flows through into the capillary bed (vasodilation).

Question 15

Describe the function of capillaries

Answer 15

• The capillaries are microscopic blood vessels that link the arterioles with the venules.
• They form an extensive network through all the tissues of the body, needed for the exchange of substances.
• The lumen of a capillary is so small that red blood cells have to travel single file.
• Substances are exchanged through the capillary walls between tissue cells and the blood.
• The gaps between the endothelial cells that make up the capillary walls in most areas of the body are relatively large. This is where many substances pass out of the capillaries into the fluid surrounding the cells.
• In most organs of the body blood entering the capillaries is oxygenated, by the time it leaves the capillaries for the venules it has less oxygen and more carbon dioxide.

Question 16

How are the capillaries adapted for their role?

Answer 16

• They provide a very large surface area for the diffusion of substances into and out of the blood.
• The total cross-sectional area of the capillaries is always greater than the arteriole supplying them so the rate of blood flow falls. The relatively slow movement of blood through the capillaries gives more time for the exchange of materials by diffusion between the blood and the cells.
• The wins are a single endothelial cell thick, giving a very thin layer for diffusion.

Question 17

Describe the role of the veins

Answer 17

• The veins carry blood away from the cells towards the heart (except the pulmonary vein and the umbilical vein).
• Deoxygenated blood flows from the capillaries into very small veins called venules, and then into larger veins. It enters the heart through the inferior and superior vena cava.
• Veins do not have a pulse as the surges of blood are lost as the blood passes through the capillaries.
• The blood pressure in the veins is very low compared to the arteries. Medium sized veins have arteries to prevent the back flow of blood.

Question 18

Describe the structure of veins

Answer 18

• The walls contain lots of collagen, and relatively little elastic fibre.
• Veins have a wide linen and smooth lining (endothelium) so blood flows easily.
• Venules link capillaries with the veins. They have very thin walls with just a little smooth muscle. Several venules join to form a vein.

Question 19

What are the adaptations of veins?

Answer 19

Deoxygenated blood in the veins must be returned to the heart to be pumped to the lungs, however blood is under low pressure and needs to move against gravity:
- They have one-way valves at intervals (flaps or unfolding a of the inner lining of the vein). When blood flows in the direction of the heart, valves open so the blood can pass through to prevent the back flow of blood.
- Many of the bigger veins run between the big, active muscles in the body. When the muscles contract they squeeze the veins, forcing blood towards the heart.
- Breathing movements of the chest act as a pump. The pressure changes and the squeezing actions move blood in the veins of the chest and abdomen towards the heart.

Question 20

What are platelets?

Answer 20

• Fragments of large cells called megakarocytes found in the red bone marrow.
• They are involved in the blood clotting mechanism.

Question 21

What are the functions of the blood?

Answer 21

The transport of (in plasma):
- oxygen and carbon dioxide, to and from respiring cells
- digested food from the small intestine
- nitrogenous waste products from the cells to the excretory organs
- chemical messages (hormones)
- food molecules from storage compounds to the cells that need them
- platelets to damaged areas
- cells and antibodies involved in the immune response

• Blood also contributes to maintainable of a steady body temperature, and acts as a buffer to minimise pH changes.

Question 22

Define oncotic pressure

Answer 22

The tendency of water to move into the blood by osmosis (about -3.3kPa)

Question 23

Describe the pressures in the capillaries

Answer 23

• The substances dissolved in plasma can pass through gaps in the capillary walls, except large plasma proteins.
• The plasma proteins have an osmotic effect, they give blood in the capillaries a relatively high solute potential compared to the surrounding fluid.
• Therefore water has a tendency to move into the blood in the capillaries from the surrounding fluid by osmosis.

Question 24

Define hydrostatic pressure

Answer 24

The pressure from the surge of blood that occurs every time the heart contracts

Question 25

Describe the pressures in the arteries

Answer 25

• As blood flows through the arterioles into the capillaries, it is still under hydrostatic pressure.
• At the arterial end of the capillary the hydrostatic pressure forcing fluids out of the capillaries is relatively high (higher than the oncotic pressure attracting water in by osmosis) so fluid is squeezed out of the capillaries.
• This fluid fills the spaces between the cells and is called tissue fluid.
• Diffusion takes place between the blood and the cells through the tissue fluid.

Question 26

Define tissue fluid

Answer 26

Tissue fluid has the same composition as plasma, without the red blood cells and plasma proteins

Question 27

Describe the pressures in the veins

Answer 27

• As the blood moves through the capillaries towards the venous system, the balance of forces changes.
• The hydrostatic pressure falls to around 2.3kPa in the vessels as fluid has moved out and the pulse is completely lost.
• The oncotic pressure is still the same, but now stronger than the hydrostatic pressure, so water moves back into the capillaries by osmosis as it approaches the venous end of the capillaries.
• By the same blood returns to the veins, 90% of tissue fluid is back in the veins.

Question 28

What is the lymph?

Answer 28

• 10% of tissue fluid does not return to the capillaries, it drains into a system of blind-ended tubes called lymph capillaries, where is it known as lymph.
• Lymph is similar in composition to plasma and tissue fluid but has less oxygen and fewer nutrients.
• It also contains fatty acids, which have been absorbed into the lymph from the villi of the small intestine.
• The lymph capillaries join up to from larger vessels. The fluid is transported through them by the squeezing of the body muscles.
• One-way valves like those in the veins prevent the backflow of lymph.
• Eventually the lymph returns to the blood, flowing into the right and left subclavian veins.

Question 29

Describe the lymph nodes

Answer 29

• Lymphocytes build up in the lymph node when necessary and produce antibodies, which are then passed into the blood.
• Lymph nodes also intercept bacteria and other debris from the lymph, which are ingested by phagocytes found in the nodes.
• The lymphatic system plays a major role in the defence mechanisms of the body.
• Enlarged lymph nodes are a sign that the body is fighting off an invading pathogen.

Question 30

What are the adaptations of erythrocytes for transporting oxygen?

Answer 30

• Biconcave shape, making a larger surface area than a simple disc structure or sphere, increasing the surface available for diffusion of gases. It also helps them pass through narrow capillaries.
• Erythrocytes are formed continuously in the red bone marrow, by the time mature erythrocytes enter the circulation they have lost their nuclei, which maximises the amount of haemoglobin that fits into cells, but limits their life.
• Erythrocytes contain haemoglobin, the red pigment that carries oxygen and gives them their colour. Haemoglobin is a large globular conjugated protein made up of four peptide chains, each with an iron-containing haem prosthetic group. Each haemoglobin molecule can bind to four oxygen molecules.
  The oxygen binds loosely to the haemoglobin forming oxyhaemoglobin. The reaction is reversible.

Question 31

What is the reaction between haemoglobin and oxygen?

Answer 31

Hb + 4O2 ⇌ Hb(O2)4

Question 32

Describe how oxygen is carried

Answer 32

• When erythrocytes enter capillaries in the lungs, the oxygen levels in the cells are low creating a steep concentration gradient between the erythrocytes and air in the alveoli.
• Oxygen moves into the erythrocytes and binds with the haemoglobin. The arrangement of the haemoglobin molecule means that as soon as one oxygen molecule binds to a haem group, the molecule changes shape, making it easier for the next oxygen molecules to bind - cooperative binding.
• As the oxygen is bound to the haemoglobin, the free oxygen concentration in the erythrocyte stays low, so a steep diffusion gradient is maintained until all of the haemoglobin is saturated with oxygen.
• When the blood reaches the body tissues, the situation is reversed. The concentration of oxygen in the cytoplasm of the body cells is lower than in the erythrocytes.
• As a result, oxygen moves out of the erythrocyte down a concentration gradient.
• Once the first oxygen molecule is released by the haemoglobin, the molecule again changes shape and it becomes easier to remove the remaining oxygen molecules.

Question 33

What do oxygen dissociation curves show?

Answer 33

• The percentage saturation haemoglobin in the blood is plotted against the partial pressure of oxygen.
• They show the affinity of haemoglobin for oxygen.
• A small change in partial pressure of oxygen in the surroundings makes a significant difference to the saturation of haemoglobin with oxygen, because once the first molecule becomes attached, the change in shape of the haemoglobin molecule means other oxygen molecules are added rapidly.
• The curve levels out at the highest partial pressures of oxygen because all haem groups are bound to oxygen.
• Therefore at the high partial pressure of oxygen in the lungs, the haemoglobin in the red blood cells is rapidly loaded with oxygen.

Question 34

How does carbon dioxide affect the affinity of oxygen?

Answer 34

As the partial pressure of carbon dioxide rises, haemoglobin gives up oxygen more easily (the Bohr effect). This occurs because:
- in active tissues with a high partial pressure of carbon dioxide, haemoglobin gives up its oxygen more readily
- in the lungs where the proportion of carbon dioxide in the air is relatively low, oxygen binds to the haemoglobin molecules easily

Question 35

Describe fetal haemoglobin

Answer 35

• When a feria is developing it is dependant on its mother to supply oxygen.
• Oxygenated blood from the mother runs close to the deoxygenated feral blood in the placenta.
• If the blood of the fetid had the same affinity for oxygen as the blood of the mother, little or no oxygen would be transferred to the blood of the fetus.
• However, fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin at each point along the dissociation curve. So it removes oxygen from the maternal blood as they move past each other.

Question 36

What are the three ways carbon dioxide is transported from the tissues to the lungs?

Answer 36

• 5% carried dissolved in plasma
• 10-20% combined with the amino groups in the polypeptide chains of haemoglobin to form a compound called carbaminohaemoglobin
• 75-85% is converted into hydrogen carbonate ions (HCO3-) in the cytoplasm of the red blood cells

Question 37

How is carbon dioxide changed into hydrogen carbonate ions?

Answer 37

• Carbon dioxide reacts slowly with water to from carbonic acid. The carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions.
• In the blood plasma this reaction happens slowly. However in the cytoplasm of the red blood cells there are high levels of the enzyme carbonic anhydrase, which catalyses the reversible reaction between carbon dioxide and water to form carbonic acid.
• The carbonic acid then dissociates to form hydrogen carbonate ions and hydrogen ions.

Question 38

What is the chloride shift?

Answer 38

• The negatively charged hydrogen carbonate ions move out of the erythrocytes into the plasma by diffusion down a concentration gradient and negatively charged chloride ions move into the erythrocytes, which maintains the electrical balance of the cell.

Question 39

How is carbon dioxide transported as hydrogen carbonate ions?

Answer 39

• By removing the carbon dioxide and converting it to hydrogen carbonate ions, the erythrocytes maintain a steep concentration gradient for carbon dioxide to diffuse from the respiring tissues into the erythrocytes.
• When the blood reaches the lungs tissue where there is a low concentration of carbon dioxide, carbonic anhydrase catalyses the reverse reaction, breaking down carbonic acid into carbon dioxide and water.
• Hydrogen carbonate ions diffuse back into the erythrocytes and react with hydrogen ions to form more carbonic acid.
• When it is broken down by the carbonic anhydrase it releases free carbon dioxide, which diffuses out of the blood into the lungs.
• Chloride ions diffuse out of the red blood cells back into the plasma down an electrochemical gradient.
• Haemoglobin in the erythrocytes also plays a role in this process. It acts as a buffer and prevents changes in the pH by accepting free hydrogen ions in a reversible reaction to form haemoglobinic acid.

Question 40

Describe the structure of the heart

Answer 40

• The heart consists of two pumps, joined and working together.
• Deoxygenated blood from the body flows into the right side of the heart, which pumps it to the lungs.
• Oxygenated blood from the lungs returns to the left side of the heart, which pumps it to the body. The blood from the two sides of the heart does not mix.
• The heart is made of cardiac muscle which contracts and relaxes in a regular rhythm. It does not get fatigued.
• The coronary arteries supply the cardiac muscle with the oxygenated blood it needs to keep contracting and refacing all the time.
• The heart is surrounded by in elastic pericardial membranes, which help prevent the heart from over-distending with blood.

Question 41

What is the name of the right atrioventricular valve?

Answer 41

Tricuspid valve

Question 42

What is the name of the left atrioventricular valve?

Answer 42

Bicuspid valve

Question 43

Describe the pathway of deoxygenated blood

Answer 43

1. Deoxygenated blood enters the right atrium of the heart from the superior and inferior vena cava at relatively low pressure.
2. The atria have thin muscular walls. As blood flows in, slight pressure builds up until the tricuspid valve opens to let blood pass into the right ventricle.
3. When both the atrium and ventricle are filled with blood the atrium contracts, forcing all the blood into the right ventricle and stretching the ventricle walls.
   4, As the right ventricle starts to contract, the tricuspid valve closes, preventing any backflow of blood to the atrium. The tendinous cords make sure the valves are not turned inside out by the pressures exerted when the ventricle contracts.
4. The right ventricle contracts fully and pumps deoxygenated blood through the semilunar valves into the pulmonary artery, which transports it to the capillary beds of the lungs.
5. The semilunar valves prevent the backflow of blood into the heart.

Question 44

Describe the pathway of oxygenated blood

Answer 44

1. Oxygenated blood from the lungs enters the left atrium from the pulmonary vein.
2. As pressure in the atrium builds the bicuspid valve opens between the left atrium and the left ventricle so the ventricle also fills with oxygenated blood.
3. When both the atrium and the ventricle are full the atrium contracts, forcing all oxygenated blood into the left ventricle.
4. The left ventricle then contracts and pumps oxygenated blood through semilunar valves into the aorta and around the body.
5. As the ventricle contacts the tricuspid valve closes, preventing any backflow of blood.

Question 45

How do the left and right sides of the heart compare?

Answer 45

• The muscular wall of the left wide of the heart is much thicker than the right.
• The lungs are close to the heart and much smaller than the rest of the body so the right side has to pump blood a relatively short distance and only has to overcome the resistance of pulmonary circulation.
• The left side has to produce sufficient force to overcome the resistance of the aorta and the arterial systems of the whole body and move the blood under pressure to the whole body.
• The septum is the inner dividing wall of the heart which prevents the mixing of deoxygenated and oxygenated blood.
• The right and left side of the heart fill and empty together.

Question 46

What is the cardiac cycle?

Answer 46

The events of a single heartbeat, which lasts about 0.8 seconds

Question 47

What occurs in diastole?

Answer 47

• The heart relaxes.
• The atria and then the ventricles fill with blood.
• The volume and pressure of the blood in the heart build as the heart fills, but the pressure in the arteries is at a minimum.

Question 48

What occurs in systole?

Answer 48

• The atria contact (atrial systole), closely followed by the ventricles (ventricular systole).
• The pressure inside the heart increases dramatically and blood is forced out of the right side of the heart to the lungs, and from the left side to the main body circulation.
• The volume and pressure of the blood in the heart is low, until the end of systole, and the blood pressure in the arteries is at a maximum.

Question 49

What makes the sounds of the heartbeat?

Answer 49

• Blood pressure closing the valves
• The first sound in the lub-dub is as blood is forced against the atrioventricular valves as the ventricles contract.
• The second comes as backflow of blood closes the semilunar valves in the aorta and pulmonary artery as the ventricles relax.

Question 50

How is the cardiac cycle myogenic?

Answer 50

• The cardiac muscle has its own rhythm at around 60 bpm.
• This prevents the body wasting resources maintaining the basic rate.
• The average resting heart rate is higher as other factors including exercise, excitement and stress also affect our heart rate.

Question 51

How is the basic rhythm of the heart maintained by a wave of electrical excitation?

Answer 51

1. A wave of electrical excitation begins in the pacemaker area called the sino-atrial node (SAN), causing the atria to contract and initiating the heartbeat. A layer of non-conducting tissue prevents the excitation passing directly to the ventricles.
2. The electrical activity from the SAN is picked up by the atrio-ventricular node (AVN). The AVN imposes a slight delay before stimulating the bundle of His, a bundle of conducting tissue made up of Purkyne fibres, which penetrate through the septum between the ventricles.
3. The bundle of His splits into two branches and conducts the wave of excitation to the apex (bottom) of the heart.
4. At the apex the Purkyne fibres spread out through the walls of the ventricles on both sides. The spread of excitation triggers the contraction of the ventricles, starting at the apex. Contraction starting at the apex allows more efficient emptying of the ventricles.

The way in which the wave of excitation spreads through the heart from the SAN, with the AVN delay, makes sure that the atria have stopped contracting before the ventricles start.

Question 52

How can you measure the spread of electrical excitation?

Answer 52

• A recording of the electrical activity of the heart called an electrocardiogram (ECG).
• An ECG measures tiny electrical differences in your skin, which result from the electrical activity of the heart.

Question 53

What is tachycardia?

Answer 53

When the heartbeat is very rapid, over 100bpm (evenly spaced).
This is often normal eg. during exercise, but abnormal if it is caused by problems in the electrical control of the heart.

Question 54

What is bradycardia?

Answer 54

When the heart rate slows down to below 60bpm (evenly spaced).
Many people have bradycardia because they are fit, however severe bradycardia may need an artificial pacemaker.

Question 55

What is an ectopic heartbeat?

Answer 55

Extra heartbeats out of a normal rhythm.
They are usually normal but can be linked to serious conditions when they are very frequent.

Question 56

What is atrial fibrillation?

Answer 56

• An abnormal rhythm of the heart - an example of arrhythmia.
• Rapid electrical impulses are generated in the atria. They contact very fast (fibrillation), however if they don’t contract properly and only some of the impulses are passed on to the ventricles, which contract much less often.
• As a result the heart does not pump blood very effectively.