chapter 8 p2

Question 1

How does the body return deoxygenated blood in veins to the heart with low pressure and against gravity? What are the three main ways:

Answer 1

The majority of the veins have one-way valves at intervals.

Many of the bigger veins run between the big, active muscles in the body, for example in the arms and legs.

The breathing movements of the chest act as a pump.

Question 2

The majority of the veins have one-way valves at intervals.

Answer 2

These are flaps or infoldings of the inner lining of the vein. When blood flows in the direction of the heart, the valves open so the blood can pass through.
If the blood starts to flow backwards, the valves close to prevent this from happening.

Question 3

Many of the bigger veins run between the big, active muscles in the body, for example in the arms and legs.

Answer 3

When the muscles contract they squeeze the veins, forcing the blood towards the heart.
The valves prevent backflow when the muscles relax.

Question 4

The breathing movements of the chest act as a pump.

Answer 4

The pressure changes and the squeezing actions move blood in the veins of the chest and abdomen towards the heart.

Question 5

valves working

Answer 5

Question 6

Types of liquids in circulatory system:

Answer 6

Blood is the main transport medium of the human circulatory system, but it is only part of the story.
Tissue fluid is the other important player in the exchange of substances between the blood and the cells.
A third liquid, lymph, is also part of the complex system that makes up the circulation of the body.

Question 7

Blood and plasma:
p1

Answer 7

consists of a yellow liquid - plasma - which carries a wide variety of other components including dissolved glucose and amino acids, mineral ions, hormones, and the large plasma proteins including albumin (important for maintaining the osmotic potential of the blood), fibrinogen (important in blood clotting) and globulins (involved in transport and the immune system).
Plasma also transports red blood cells (which carry oxygen to the cells and also give the blood its red appearance) and the many different types of white blood cells.

Question 8

Blood and plasma:
p2

Answer 8

It also carries platelets (fragments of large cells called megakaryocytes found in the red bone marrow) which are involved in the clotting mechanism of the blood.
Plasma makes up 55% of the blood by volume - and much of that volume is water.
Only the plasma and the red blood cells are involved in the transport functions of the blood.
The other components have different functions.

Question 9

human blood diagrams

Answer 9

Question 10

The composition of the blood is closely related to its functions in the body, many of which involve transport. They include transport of:

Answer 10

• oxygen to, and carbon dioxide from, the 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.
• contributes to maintenance of a steady body temperature and acts as a buffer, minimising pH changes.

Question 11

Tissue fluid: p1
Oncotic Pressure

Answer 11

The substances dissolved in plasma can pass through the fenestrations in the capillary walls, with the exception of the large plasma proteins.
The plasma proteins, particularly albumin, have an osmotic effect.
They give the blood in the capillaries a relatively high solute potential (and so a relatively low water potential) compared with the surrounding fluid.
As a result, water has a tendency to move into the blood in the capillaries from the surrounding fluid by osmosis.
The tendency of water to move into the blood by osmosis is termed oncotic pressure and it is about -3.3 kPa.

Question 12

Tissue fluid: p2Hydrostatic Pressure

Answer 12

However, as blood flows through the arterioles into the capillaries, it is still under pressure from the surge of blood that occurs every time the heart contracts - This is hydrostatic pressure.
At the arterial end of the capillary, the hydrostatic pressure forcing fluid out of the capillaries is relatively high at about 4.6 kPa (Figure 3).
It is 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. Tissue fluid has the same composition as the plasma, without the red blood cells and the plasma proteins.
Diffusion takes place between the blood and the cells through the tissue fluid.

Question 13

Tissue fluid p1
return

Answer 13

As the blood moves through the capillaries towards the venous system, the balance of forces changes.
The hydrostatic pressure falls to around 2.3 kPa in the vessels as fluid has moved out and the pulse is completely lost.
The oncotic pressure is still -3.3 kPa, so it is 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 time the blood returns to the veins, 90% of the tissue fluid is back in the blood vessels.

Question 14

Answer 14

Question 15

Lymph Composition

Answer 15

Some of the tissue fluid does not return to the capillaries.
10% of the liquid that leaves the blood vessels drains into a system of blind-ended tubes called lymph capillaries, where it is 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.

Question 16

Lymph Transport

Answer 16

The lymph capillaries join up to form larger vessels.
The fluid is transported through them by the squeezing of the body muscles. One-way valves like those in veins prevent the backflow of lymph.
Eventually the lymph returns to the blood, flowing into the right and left subclavian veins (under the clavicle, or collar bone).

Question 17

Lymph Nodes

Answer 17

Along the lymph vessels are the lymph nodes.
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. This is why doctors often examine the neck, armpits, stomach or groin of their patients - these are the sités of some of the major lymph nodes (which people often refer to as lymph glands’).

Question 18

diagram of the lymphatic system

Answer 18

Question 19

Erythrocyte Structure p1

Answer 19

Erythrocytes have a biconcave shape.
This shape has a larger surface area than a simple disc structure or a sphere, increasing the surface area available for diffusion of gases.
It also helps them to pass through narrow capillaries.
In adults, erythrocytes are formed continuously in the red bone marrow.

Question 20

Erythrocyte Structure p2

Answer 20

By the time mature erythrocytes enter the circulation they have lost their nuclei, which maximises the amount of haemoglobin that fits into the cells.
It also limits their life, so they only last for about 120 days in the bloodstream.
Erythrocytes contain haemoglobin, the red pigment that carries oxygen and also gives them their colour.

Question 21

Haemoglobin

Answer 21

Haemoglobin is a very large globular conjugated protein made up of four peptide chains, each with an iron-containing haem prosthetic group.
There are about 300 million haemoglobin molecules in each red blood cell and each haemoglobin molecule can bind to four oxygen molecules.
The oxygen binds quite loosely to the haemoglobin forming oxyhaemoglobin. The reaction is reversible.

Question 22

Oxyhaemoglobin reaction:

Answer 22

Question 23

Oxygen Uptake:
p1

Answer 23

When the erythrocytes enter the capillaries in the lungs, the oxygen levels in the cells are relatively low.
This makes a steep concentration gradient between the inside of the erythrocytes and the air in the alveoli.
Oxygen moves into the erythrocytes and binds with the haemoglobin.

Question 24

Oxygen Uptake:
p2

Answer 24

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.
This is known as positive cooperativity.
Because 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.

Question 25

Oxygen Release:

Answer 25

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 erythrocytes 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 26

oxygen dissociation curve

Answer 26

An oxygen dissociation curve is an important tool for understanding how the blood carries and releases oxygen.
The percentage saturation haemoglobin in the blood is plotted against the partial pressure of oxygen (pO,).

Question 27

Oxygen dissociation curves show p1

Answer 27

• show the affinity of haemoglobin for oxygen.
• A very small change in the partial pressure of oxygen in the surroundings makes a significant difference to the saturation of the haemoglobin with oxygen, because once the first molecule becomes attached, the change in the 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 the haem groups are bound to oxygen and so the haemoglobin is saturated and cannot take up any more.
• This means that at the high partial pressure of oxygen in the lungs the haemoglobin in the red blood cells is rapidly loaded with oxygen.

Question 28

Oxygen dissociation curves show p2

Answer 28

Equally, a relatively small drop in oxygen levels in the respiring tissues means oxygen is released rapidly from the haemoglobin to diffuse into the cells.
This effect is enhanced by the relatively low pH in the tissues compared with the lungs.
When you are not very active, only about 25% of the oxygen carried in your erythrocytes is released into the body cells.
The rest acts as a reservoir for when the demands of the body increase suddenly.

Question 29

Partial Pressure

Answer 29

Partial pressure is a useful way of talking about the concentration of a chemical when it is one of a mixture of gases.
The whole mixture of gases has an overall pressure and each of the chemicals in the mixture can be thought of as contributing part of that pressure.
If you were shut in an airtight room, as time went by the overall pressure of the air around you would not change but the partial pressure of oxygen (pO,) would fall, and the partial pressure of carbon dioxide would rise (the partial pressure of nitrogen would not change).
If you walk up a mountain the overall pressure of the air does decrease as the altitude increases.
In this case the partial pressures of oxygen, carbon dioxide and nitrogen are all falling (but the proportions of each are not changing).

Question 30

Bohr effect.

Answer 30

As the partial pressure of carbon dioxide rises (in other words, at higher partial pressures of CO,), haemoglobin gives up oxygen more easily (Figure 2).
This change is known as the Bohr effect.

Question 31

The Bohr effect is important in the body because as a result:

Answer 31

• 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 32

Answer 32

Question 33

Fetal haemoglobin:

Answer 33

When a fetus is developing in the uterus it is completely dependent on its mother to supply it with oxygen.
* Oxygenated blood from the mother runs close to the deoxygenated fetal blood in the placenta.
* If the blood of the fetus had the same affinity for oxygen as the blood of the mother, then 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 (Figure 3).
* So it removes oxygen from the maternal blood as they move past each other.

Question 34

Transporting Carbon Dioxide in Blood p1

Answer 34

• Carbon dioxide is transported from the tissues to the lungs in three different ways:
• About 5% is carried dissolved in the plasma.
• 10-20% is 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.
• Most of the carbon dioxide that diffuses into the blood from the cells is transported to the lungs in the form of hydrogen carbonate ions.

Question 35

Carbon Dioxide Reaction

Answer 35

Carbon dioxide reacts slowly with water to form carbonic acid (H2CO3-).
The carbonic acid then dissociates to form hydrogen ions and hydrogen carbonate ions.

Question 36

Transporting Carbon Dioxide in Blood p2

Answer 36

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.
This enzyme 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 37

Chloride Shift p1

Answer 37

• 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.
• This is known as the chloride shift.
• 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.

Question 38

Chloride Shift p2

Answer 38

When the blood reaches the lung tissue where there is a relatively 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 this is broken down by 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.

Question 39

haemoglobin’s Role

Answer 39

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

Answer 40