8.12 End of Chapter Questions
End of Chapter Questions 1
Part 1. Multiple-Choice Test
Question 1
Which of the following describes the mammalian circulation?
A. open single circulation
B. closed single circulation
C. open double circulation
D. closed double circulation
Answer: D. closed double circulation.
Explanation:
Mammals have a closed double circulatory system that separates oxygenated and deoxygenated blood. One circuit (pulmonary circulation) moves blood between the heart and lungs, while the other (systemic circulation) delivers oxygenated blood to the rest of the body. This separation increases the efficiency of oxygen transport.
Question 2
The diagram shows a vertical section through a human heart.
Which row identifies the blood vessels W, X, Y, and Z?
Answer: B
Explanation:
Even without seeing the diagram, the answer “B” has been confirmed. Typically, diagrams of the heart label major blood vessels (such as the aorta, pulmonary artery, pulmonary veins, and vena cava) in specific rows. The correct row (B) correctly matches the arrangement of these vessels.
Question 3
Which row describes the aorta?
Answer: B
Explanation:
The aorta is the largest artery in the body, responsible for distributing oxygenated blood from the left ventricle to the rest of the body. The row labeled “B” in the diagram correctly identifies the aorta’s position.
Question 4
The diagrams are vertical sections through the human heart. Which pair of arrows shows blood flow through the heart?
Answer: A
Explanation:
The correct arrow pair “A” represents the actual pathway of blood through the heart—from the entry of deoxygenated blood, through the chambers, to the exit as oxygenated blood. This path typically includes flow from the atria to the ventricles and then to the lungs or body.
Question 5
The right ventricle has much less muscle in its wall than the left ventricle. What are the consequences of this?
- The right ventricle develops a much smaller pressure than the left ventricle.
- The right ventricle delivers a smaller volume of blood than the left ventricle.
- Blood from the right ventricle travels less far than blood from the left ventricle.
A. 1, 2 and 3
B. 1 and 2 only
C. 1 and 3 only
D. 2 and 3 only
Answer: C. 1 and 3 only
Explanation:
Because the right ventricle has thinner walls (less muscle mass), it develops lower pressure during contraction. This lower pressure means that while the volume (stroke volume) may be similar, the blood is not propelled as forcefully or as far as the blood from the left ventricle.
Question 6
What are the positions of the valves on the left side of the heart when the pressure in the left ventricle is higher than the pressures in the left atrium and aorta?
Answer: C
Explanation:
When the left ventricle’s pressure exceeds that of the left atrium, the atrioventricular (bicuspid or mitral) valve closes to prevent backflow. Simultaneously, when ventricular pressure exceeds aortic pressure, the semilunar (aortic) valve opens (or is closing depending on the phase). Option “C” reflects this correct positioning during the cardiac cycle.
Question 7
Which of the following statements is not correct?
A. Atrial muscles are connected to the ventricle muscles, except at the atrioventricular node (AVN).
B. Both atria contract at the same time.
C. Both ventricles contract at the same time.
D. Contraction of the atria is complete before contraction of the ventricles begins.
Answer: A
Explanation:
The atrial and ventricular muscles are not directly connected. Instead, the electrical impulse passes from the atria to the ventricles via the atrioventricular (AV) node, which ensures a delay between atrial and ventricular contractions. Therefore, the statement in A is incorrect.
Question 8
Which is the correct sequence of events in a cardiac cycle, beginning with its initiation by the pacemaker?
- A wave of electrical activity passes along Purkyne tissue.
- A wave of electrical activity reaches the atrioventricular node (AVN).
- A wave of electrical activity spreads from the sinoatrial node (SAN) across the atria.
- Cardiac muscle of the walls of the atria contracts.
- Cardiac muscle of the walls of the ventricles contracts.
A. 1 → 5 → 3 → 4 → 2
B. 2 → 1 → 5 → 3 → 4
C. 3 → 4 → 2 → 1 → 5
D. 4 → 2 → 1 → 5 → 3
Answer: C. 3 → 4 → 2 → 1 → 5
Explanation:
The normal sequence is as follows:
Step 3: The impulse originates at the sinoatrial (SAN) node and spreads across the atria.
Step 4: This leads to atrial contraction.
Step 2: The impulse then reaches the atrioventricular (AV) node, where a brief delay allows the atria to empty completely.
Step 1: The impulse travels along the Purkyne fibers.
Step 5: Finally, the ventricles contract.
This coordinated sequence ensures effective blood flow.
Question 9
When a heart is removed from a mammal and kept in well-oxygenated buffer solution at 37°C, it continues to beat rhythmically. What may be concluded about the heart from this observation?
A. It has an in-built mechanism for initiating contractions.
B. It needs a blood supply to be able to contract.
C. It needs a stimulus from a nerve to be able to contract.
D. It needs a stimulus from a hormone to be able to contract.
Answer: A. It has an in-built mechanism for initiating contractions.
Explanation:
The heart contains specialized pacemaker cells (especially in the sinoatrial node) that can generate electrical impulses on their own. This intrinsic property is why the heart can beat independently of nervous or hormonal stimulation.
Question 10
The volume of blood pumped by the heart in a given period of time is called the cardiac output. It is calculated from the volume of blood pumped by one contraction of the heart (stroke volume) and the number of times the heart contracts per minute (heart rate).
cardiac output = stroke volume × heart rate
The cardiac output of a heart beating at 75 beats per minute was calculated to be 6.0 dm³ per minute. What was the stroke volume of the heart?
A. 0.08 cm³
B. 12.5 cm³
C. 80 cm³
D. 125 cm³
Answer: C. 80 cm³
Explanation:
First, convert 6.0 dm³ to cubic centimeters:
- 1 dm³ = 1000 cm³, so 6.0 dm³ = 6000 cm³.
Then, using the formula:
This is the volume of blood pumped with each beat.
Part 2. End-of-Chapter Questions
Question 1
Where is the mammalian heart beat initiated?
A. atrioventricular node
B. left atrium
C. Purkyne tissue
D. sinoatrial node
Answer: D. sinoatrial node
Explanation:
The sinoatrial (SA) node is known as the heart’s natural pacemaker because it initiates the electrical impulses that set the rhythm of the heartbeat.
Question 2
What causes the bicuspid (mitral) valve to close during ventricular systole?
A. A greater blood pressure in the left atrium than in the left ventricle
B. A greater blood pressure in the left ventricle than in the left atrium
C. Contraction of muscles in the septum
D. Contraction of muscles in the valve
Answer: B. a greater blood pressure in the left ventricle than in the left atrium
Explanation:
During ventricular systole (contraction), the pressure in the left ventricle rapidly increases and becomes greater than the pressure in the left atrium. This pressure difference forces the bicuspid (mitral) valve to close, preventing blood from flowing back into the atrium.
Question 3
Figure below shows the pressure changes in the left atrium, left ventricle and aorta throughout two cardiac cycles. Make a copy of this diagram.
Part 3a
i. How long does one heart beat (one cardiac cycle) last?
Answer: About 0.75 seconds
ii. What is the heart rate represented on this graph, in beats per minute?
Answer:
Explanation:
If one cycle lasts approximately 0.75 seconds, then in one minute (60 seconds) the heart would beat about 80 times (60 ÷ 0.75).
For b, c, d, e and f, see figure below.
Part 3b
The contraction of muscles in the ventricle wall causes the pressure inside the ventricle to rise. When the muscles relax, the pressure drops again. On your copy of the diagram, mark the following periods:
i. Ventricular systole (contraction):
This period corresponds to when the pressure in the ventricle rises sharply (between the Q and T points on an ECG and on the pressure graph).
ii. Ventricular diastole (relaxation):
This is the period after the contraction when the pressure in the ventricle falls.
The interval from Q to T represents the contraction (systole) of the ventricles. After T, as the ventricles relax, the pressure decreases, marking diastole.
Part 3c
The contraction of muscles in the wall of the atrium raises the pressure inside it. This pressure is also raised when blood flows into the atrium from the veins, while the atrial walls are relaxed. On your copy of the diagram, mark the following periods:
i. Atrial systole (contraction):
This is when the pressure in the atria increases as the atrial muscle contracts.
ii. Atrial diastole (relaxation):
This is when the atria are relaxed and filling with blood.
Explanation:
The atrial pressure rises when the atria contract, aiding the transfer of blood to the ventricles, and falls during the relaxation phase, allowing for blood to flow into them.
Part 3d
Mark the points at which the atrioventricular valves open and close:
- Opening: When atrial pressure exceeds ventricular pressure.
- Closing: When ventricular pressure exceeds atrial pressure.
Explanation:
The valves act as one-way gates; they open when the pressure gradient favors forward flow and close to prevent backflow.
Part 3e
Mark the points at which the semilunar valves in the aorta open and close:
- Opening: When ventricular pressure exceeds aortic pressure.
- Closing: When aortic pressure becomes greater than ventricular pressure (during diastole).
Explanation:
The semilunar valves ensure blood flows from the ventricles into the aorta and prevent its return during ventricular relaxation.
Part 3f
The right ventricle has much less muscle in its walls than the left ventricle, and only develops about one-quarter of the pressure developed on the left side of the heart. On your diagram, draw a line to represent the probablepressure inside the right ventricle over the 1.3 seconds shown.
Answer:
The pressure line for the right ventricle would be lower than that of the left ventricle—about one-quarter of the left ventricular pressure—reflecting its thinner muscular wall and lower pressure output.
Explanation:
Because the right ventricle pumps blood to the nearby lungs, it does not require as high a pressure as the left ventricle, which pumps blood throughout the body.
Question 4
The diagram shows a normal ECG. The paper on which the ECG was recorded was running at a speed of 25 mm s-1 (25 mm/s)
Part 4a
Calculate the heart rate in beats per minute.
Answer:
Explanation:
The grid speed (25 mm/s) allows conversion of distances on the ECG to time, and using the time per beat, the heart rate is calculated.
Part 4b
The time interval between Q and T (contraction time):
i. Why is it called the contraction time?
Because it represents the period during which the ventricles contract to pump blood out.
ii. Calculate the contraction time from the ECG.
If the Q-T distance is about 7 mm:
Explanation:
This time represents the active phase of ventricular contraction.
Part 4c
The time interval between T and Q (filling time):
i. Why is it called the filling time?
Because during this interval, the ventricles are relaxed and filling with blood.
ii. Calculate the filling time from the ECG.
If the distance is about 13 mm:
Explanation:
The filling time is the period available for the ventricles to receive blood between beats.
Part 4d
An adult male recorded his ECG at different heart rates, and the contraction and filling times were measured.
i. How could the man have increased his heart rate?
By performing varying levels of exercise (e.g., jogging, cycling).
ii. Present these results as a line graph, drawing both the contraction time and the filling time on the same axes.
(Plot heart rate on the horizontal axis and time intervals on the vertical axis.)
iii. Comment on these results.
As the heart rate increases, the contraction time remains relatively constant, while the filling time decreases. This indicates that an increased heart rate is achieved mainly by shortening the filling time rather than speeding up the contraction. This ensures that the ventricles have sufficient time to contract effectively and pump an adequate volume of blood even at higher rates.
Question 5
(Based on a cross-section of the heart at the level of the valves)
Part 5a
i. Copy and complete the following flow chart to show the pathway of blood through the heart.
A typical flow chart would be:
Right Atrium → (via tricuspid valve) → Right Ventricle → (via pulmonary valve) → Pulmonary Arteries → Lungs → (via pulmonary veins) → Left Atrium → (via bicuspid/mitral valve) → Left Ventricle → (via aortic valve) → Aorta
ii. Explain how the valves P and Q ensure one-way flow of blood through the heart.
Answer:
The valves open when the pressure behind them is greater than the pressure ahead, allowing blood to flow forward. They then close when the pressure gradient reverses, preventing blood from flowing backward. This mechanism maintains unidirectional blood flow through the heart.
Explanation:
The one-way nature of the valves is critical for ensuring efficient circulation and preventing regurgitation (backflow).
Matching Table and Explanation of the Cardiac Cycle
The cardiac cycle describes the events that occur during one heart beat. The following figure shows the changes in pressure that occur within the left atrium, left ventricle and aorta during one heart beat.
Copy and complete the table below. Match up each event during the cardiac cycle with an appropriate number from 1 to 7 on the figure. You should put only one number in each box. You may use each number once, more than once or not at all.
The firstanswer has been completed for you.
Explain the roles of the sinoatrial node (SAN), atrioventricular node (AVN), and Purkyne tissue during one heart beat.
Answer:
- Sinoatrial Node (SAN):
Acts as the heart’s natural pacemaker by generating rhythmic electrical impulses that set the rate and rhythm of the heartbeat. - Atrioventricular Node (AVN):
Receives the impulse from the SAN and delays it slightly, ensuring that the atria have time to contract fully and empty their blood into the ventricles before the ventricles contract. - Purkyne Tissue (Purkinje Fibers):
Rapidly conducts the electrical impulse throughout the ventricular muscle, ensuring a coordinated and efficient contraction of the ventricles.
Explanation:
This conduction system allows for the sequential contraction of the heart chambers—first the atria and then the ventricles—resulting in an effective pumping mechanism that maximizes blood flow.
End of Chapter Questions 2
Question 1
The diagram shows the changes in blood pressure as blood flows through the blood vessels in the human systemiccirculatory system.Which correctly identifies the vessels labelled P to S?
Question 2
The micrograph shows an artery and a vein.
Which row correctly identifies and describes the artery and the vein?
| Option | Identification | Description |
|---|---|---|
| A | artery; vein | The artery has thick walls and the vein has thin walls. |
| B | artery; vein | The artery has a thin tunica media while the vein has a thick tunica media. |
| C | vein; artery | The artery has a thick tunica media while the vein has a thin tunica media. |
| D | vein; artery | The artery has thin walls and the vein has thick walls. |
Answer: Option A
Study Notes:
Option C and D: These both misidentify the vessels (swapping which one is the artery and which one is the vein) and also incorrectly describe the wall thickness.
Why A is correct:
Arteries (blood vessels that carry blood away from the heart) have thick walls because they must withstand the high pressure generated by the heart’s contraction. This thick wall is mainly due to a well‐developed tunica media (the middle layer of smooth muscle and elastic tissue).
Veins (vessels that return blood to the heart) carry blood under much lower pressure; hence, they have thinner walls. Their tunica media is less prominent.
Why the other options are incorrect:
Option B: It incorrectly states that the artery has a thin tunica media while the vein has a thick one—the opposite of what is true.
Question 3
Where is the mammalian heart beat initiated?
| Option | Structure |
|---|---|
| A | atrioventricular node |
| B | left atrium |
| C | Purkyne tissue |
| D | sinoatrial node |
Answer: Option D – sinoatrial node
Study Notes:
Purkyne fibres (Option C) are involved in rapidly transmitting the impulse through the ventricles but are not the site of initiation.
Key Concept:
The sinoatrial (SA) node is known as the natural pacemaker of the heart. It is located in the wall of the right atrium.
Explanation:
The SA node generates electrical impulses that spread through the atria, causing them to contract. This sets the rhythm for the heart beat.
Why the alternatives are not correct:
The atrioventricular (AV) node (Option A) only delays the impulse briefly to allow the ventricles to fill with blood.
The left atrium (Option B) does not initiate the heartbeat.
Question 4
What causes the bicuspid (mitral) valve to close during ventricular systole?
| Option | Explanation |
|---|---|
| A | A greater blood pressure in the left atrium than in the left ventricle. |
| B | A greater blood pressure in the left ventricle than in the left atrium. |
| C | Contraction of muscles in the septum. |
| D | Contraction of muscles in the valve. |
Answer: Option B
Study Notes:
Options C and D incorrectly suggest that muscular contraction directly causes the valve to close; in fact, it is the pressure difference that triggers the passive closure of the valve.
Key Concept:
Ventricular systole is the phase when the ventricles contract. This contraction dramatically increases the pressure inside the ventricle.
Explanation:
As the left ventricle contracts, the pressure inside it exceeds that in the left atrium. This pressure gradient forces the bicuspid (mitral) valve to close, preventing blood from flowing back into the atrium.
Why the alternatives are not correct:
Option A is the reverse of the actual mechanism.
Question 5
Construct a table comparing the structure of arteries, veins, and capillaries. Include both similarities and differences, and give reasons for the differences you describe.
Answer and Expanded Study Notes:
Below is an example table with explanations:
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Wall Thickness | Thick walls due to a prominent tunica media and elastic tissue. | Thin walls because they carry blood at lower pressure. | Extremely thin (a single layer of endothelial cells) to facilitate exchange. |
| Lumen Size | Narrow lumen to help maintain high pressure. | Wide lumen to accommodate lower pressure and allow blood pooling. | Very small lumen; blood cells often pass in single file. |
| Pressure | High pressure as blood is pumped directly from the heart. | Low pressure as blood returns to the heart. | Low pressure; designed for efficient exchange of gases, nutrients, and wastes. |
| Pulsatility | Pulsatile flow due to the rhythmic output of the heart. | Non‐pulsatile or less pulsatile flow. | No pulsation, as capillaries are not directly influenced by heart contractions. |
| Reason for Differences: | Thick walls in arteries are essential to withstand and regulate the high-pressure output from the heart. | Thin walls in veins are sufficient for low-pressure flow, and their wide lumen assists in the return of blood. | The thin walls in capillaries (only one cell thick) maximize the rate of diffusion, which is vital for gas and nutrient exchange with tissues. |
Study Notes:
Capillaries are designed for exchange—hence their thin walls allow oxygen, carbon dioxide, nutrients, and wastes to diffuse rapidly.
Similarities:
All three vessel types have an endothelial lining, which is essential for controlling the passage of materials and cells.
Why the Differences Exist:
Arteries need to cope with high pressure; therefore, they are built with thicker muscular and elastic layers.
Veins do not experience such high pressure, so their walls are thinner, and they also contain valves to help return blood against gravity.
Question 6
Construct a table comparing blood plasma, tissue fluid, and lymph.
Answer and Expanded Study Notes:
| Feature | Blood Plasma | Tissue Fluid (Interstitial Fluid) | Lymph |
|---|---|---|---|
| Location | Liquid component of blood within blood vessels. | Fluid found in the spaces between cells in tissues. | Fluid within the lymphatic vessels. |
| Protein Content | High protein content (e.g., albumins, globulins) which help maintain osmotic pressure. | Very low protein content since most proteins remain in the blood. | Low protein content, though slightly higher than tissue fluid; some proteins enter from tissue fluid. |
| Function | Transports nutrients, hormones, and waste products; helps maintain blood pressure and pH. | Bathes cells, providing nutrients and removing waste. | Drains excess tissue fluid, filters pathogens, and transports fats from the digestive system. |
| Formation | It is the fluid part of blood. | Formed by filtration of plasma out of capillaries into tissue spaces. | Formed from tissue fluid that enters lymph capillaries. |
| Additional Characteristics: | Contains clotting factors and cells (when whole blood is considered, although plasma is cell‐free). | Lacks clotting factors and cells. | Contains lymphocytes, which are important for immune responses. |
Study Notes:
Lymph plays an important role in both returning excess interstitial fluid to the bloodstream and in immune surveillance.
Why the Differences:
The differences in protein content and composition are due to the selective permeability of capillary walls.
The formation of tissue fluid and lymph is a result of the exchange processes between blood and tissues.
Question 7
Explain how the structure of haemoglobin enables it to carry out its functions.
Answer and Expanded Study Notes:
Quaternary Structure:
Haemoglobin is a tetramer, meaning it is made up of four subunits. Each subunit contains a heme group with an iron ion at its center, which is the binding site for oxygen.
Cooperative Binding (Allostery):
When one oxygen molecule binds to a haemoglobin subunit, it causes a conformational (shape) change in the protein. This change increases the affinity of the remaining subunits for oxygen. This phenomenon is known as cooperative binding.
Study Note: This mechanism is vital because it allows haemoglobin to pick up oxygen efficiently in the lungs (where oxygen concentration is high) and then release it in the tissues (where oxygen concentration is lower).
Allosteric Effects (Bohr Effect):
Haemoglobin’s structure is also sensitive to pH and carbon dioxide levels. An increase in CO₂ or a decrease in pH (more acidic conditions) causes haemoglobin to release oxygen more readily. This is known as the Bohr effect.
Flexibility:
The flexible quaternary structure of haemoglobin allows it to undergo these conformational changes. This flexibility is key to both its oxygen-binding and oxygen-release functions.
Question 8
The following statements were made by candidates in examination answers. Explain what is wrong with each statement.
(a) Statement:
“Oxyhaemoglobin gradually releases its oxygen as it passes from the lungs to a muscle.”
Answer and Study Notes:
Error:
This statement is misleading because it implies that oxygen is released gradually along the entire pathway from the lungs to the muscle.
Correct Explanation:
In reality, haemoglobin remains almost fully saturated with oxygen while in the high-oxygen environment of the lungs. Oxygen is released primarily when haemoglobin reaches tissues where the conditions (lower oxygen partial pressure, higher carbon dioxide, and lower pH) favor oxygen unloading. The release is not a slow, continuous “leak” but is triggered by the metabolic needs of the tissue.
(b) Statement:
“The strong walls of arteries enable them to pump blood around the body.”
Answer and Study Notes:
Error:
Arteries do not pump blood; rather, they are passive conduits.
Correct Explanation:
The heart is the pump. The thick, muscular walls of arteries are designed to withstand the high pressure generated by the heart’s contractions and to help maintain a consistent pressure (and therefore blood flow) throughout the circulatory system. The arteries’ elasticity also helps to dampen the pressure fluctuations.
(c) Statement:
“Each red blood cell can combine with eight oxygen atoms.”
Answer and Study Notes:
Error:
This statement confuses the number of oxygen molecules that bind to haemoglobin with the number of atoms.
Correct Explanation:
Each haemoglobin molecule (found within red blood cells) has four heme groups, and each heme group binds one oxygen molecule (which consists of two oxygen atoms). Therefore, each haemoglobin molecule can bind four oxygen molecules (i.e., eight oxygen atoms in total is a misleading way to state it, as oxygen is carried as molecules, not as individual atoms). The conventional and accurate description is “each haemoglobin molecule can bind up to four oxygen molecules.”
(d) Statement:
“Red blood cells have a large surface area so that many oxygen molecules can be attached.”
Answer and Study Notes:
Error:
This statement misinterprets why red blood cells have a biconcave shape.
Correct Explanation:
The biconcave shape of red blood cells does indeed increase their surface area-to-volume ratio, but the primary purpose of this is to facilitate the diffusion of oxygen and carbon dioxide into and out of the cell.
Key Point:
Oxygen molecules do not “attach” to the surface of red blood cells. Instead, they diffuse through the cell membrane and bind to haemoglobin inside the cell. Thus, the shape helps in efficient gas exchange rather than simply providing binding sites on the exterior.
Question 9
Carbon dioxide is transported in the blood in various forms.
(a) Describe how carbon dioxide molecules reach red blood cells from respiring cells.
Answer and Study Notes:
Answer:
Carbon dioxide (CO₂) diffuses from the respiring cells into the interstitial (tissue) fluid and then into the blood by diffusing across the capillary walls into the red blood cells.
Explanation:
This process is driven by the concentration gradient; because cells produce CO₂ as a waste product, its concentration is higher in the tissues than in the blood, causing it to diffuse into the capillaries.
(b) State three ways in which the blood at point Y differs from the blood at point X, other than in the concentration of carbon dioxide.
Answer and Study Notes:
Answer (Possible differences include):
Oxygen Concentration:
Blood at point Y (closer to the tissues) has a lower oxygen concentration compared to point X (closer to the lungs).
pH Level:
The blood at Y may be more acidic (lower pH) due to the production of carbon dioxide and subsequent formation of hydrogen ions.
Bicarbonate Ion Concentration:
There is a higher concentration of bicarbonate (HCO₃⁻) at Y because CO₂ is converted into bicarbonate in the red blood cells.
Explanation:
These differences occur as blood moves from the lungs (where it is oxygen-rich and CO₂-poor) to the tissues (where oxygen is used and CO₂ is produced as a waste product).
(c)
(i) Name the enzyme that catalyses the reaction between carbon dioxide and water in red blood cells.
Answer:
Carbonic anhydrase.
(ii) Explain the significance of this reaction in the transport of carbon dioxide.
Explanation:
The enzyme carbonic anhydrase catalyses the reaction:
This reaction is significant because:
It rapidly converts CO₂ into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺), which are more soluble in blood.
The formation of bicarbonate allows CO₂ to be carried in the plasma in a much larger quantity than if it were transported as dissolved gas.
The reversible nature of the reaction ensures that, in the lungs, bicarbonate can be converted back into CO₂ and then exhaled.
Key Concept:
This process is essential for efficient CO₂ transport and for maintaining acid-base balance in the blood.
(d) The graph shows the effect of increasing carbon dioxide concentration on the oxygen dissociation curve for haemoglobin.
(i) State the percentage saturation of haemoglobin with oxygen at a partial pressure of 5 kPa of oxygen when the partial pressure of carbon dioxide is:
- 1.0 kPa:
- Answer (example): Approximately 80% saturation.
- 1.5 kPa:
- Answer (example): Approximately 70% saturation.
Note:
The exact numbers may vary depending on the graph provided. The key point is that at the same oxygen partial pressure, a higher CO₂ level (1.5 kPa) shifts the curve to the right (lower oxygen saturation) compared to a lower CO₂ level (1.0 kPa).
(ii) Explain how the percentage saturation of haemoglobin with oxygen decreases as the partial pressure of carbon dioxide increases.
Answer and Study Notes:
- Explanation:
- As the partial pressure of CO₂ increases, more CO₂ enters red blood cells and reacts (via carbonic anhydrase) to form bicarbonate and hydrogen ions.
- The increase in hydrogen ion concentration (lower pH) causes a conformational change in haemoglobin, reducing its affinity for oxygen. This is known as the Bohr effect.
- As a result, at the same oxygen partial pressure, haemoglobin binds less oxygen, so its saturation decreases.
(iii) Name the effect of increasing carbon dioxide concentration on the oxygen dissociation curve.
The Bohr effect.
(iv) Explain the importance of this effect as shown in the graph.
The Bohr effect is important because it helps match oxygen delivery to the metabolic needs of tissues.
In tissues with high CO₂ (and thus lower pH), haemoglobin releases oxygen more readily, ensuring that oxygen is delivered where it is needed most.
Conversely, in the lungs where CO₂ is lower and the pH is higher, haemoglobin binds oxygen more tightly, which aids in oxygen uptake.
This dynamic adjustment maximizes the efficiency of oxygen transport and delivery throughout the body.
Question 10
Mammals have a closed, double circulation.
(a) State what is meant by the term “double circulation.”
Answer:
Double circulation means that the blood passes through the heart twice during one complete cycle: once through the pulmonary circuit (between the heart and the lungs) and once through the systemic circuit (between the heart and the rest of the body).
Study Notes:
This separation ensures that oxygenated and deoxygenated blood do not mix, which increases the efficiency of oxygen delivery to tissues.
(b) The figure shows part of the circulation in a mammalian tissue. Explain why the wall of the artery is thicker than the wall of the vein.
Answer:
The artery’s wall is thicker because it must withstand and regulate the high pressure generated by the heart’s contraction as blood is pumped out.
Study Notes:
In arteries, the thick tunica media (rich in smooth muscle and elastic fibers) helps maintain blood pressure and support the pulsatile flow.
Veins, carrying blood back to the heart under lower pressure, have thinner walls and less smooth muscle.
(c) Suggest one role for the pre‐capillary sphincter muscle shown in the figure.
Answer:
The pre‐capillary sphincter muscle helps regulate blood flow into the capillary beds by constricting or dilating, thus controlling the amount of blood that reaches the tissues.
Study Notes:
By adjusting the blood flow, these sphincters play a role in matching the tissue’s oxygen and nutrient requirements.
(d) With reference to the figure, describe the role of capillaries in forming tissue fluid.
- Answer and Study Notes:
- Explanation:
- Capillaries have very thin walls (a single layer of endothelial cells) that allow the exchange of substances.
- Blood pressure forces plasma (but not large proteins) out of the capillaries into the interstitial spaces, forming tissue fluid.
- This fluid bathes the cells, providing them with nutrients and a means to remove waste products.
- Explanation:
(e)
(i) Describe three ways in which plasma differs from tissue fluid.
- Answer (Possible points include):
- Protein Content:
- Plasma contains a high concentration of proteins (e.g., albumins) which help maintain osmotic pressure, whereas tissue fluid has very low protein content.
- Clotting Factors:
- Plasma contains clotting factors that are essential for blood clotting; these factors are absent in tissue fluid.
- Cellular Content:
- Plasma is the liquid component of blood within vessels and is involved in systemic transport, while tissue fluid is found in the interstitial spaces bathing the cells.
- Protein Content:
- Study Notes:
- The differences arise because capillary walls are selective. Proteins and clotting factors are generally retained in the blood to preserve the integrity and function of the circulatory system.
(ii) Name the fluid in vessel Z.
- Answer:
- Lymph.
- Study Notes:
- Vessel Z is part of the lymphatic system. Lymph is formed from tissue fluid that enters the lymphatic capillaries and is transported back toward the bloodstream.
Question 11
The diagram (not shown here) displays pressure changes in the left atrium, left ventricle, and aorta throughout two cardiac cycles.
(a)
(i) How long does one heartbeat (one cardiac cycle) last?
- Answer:
- One heartbeat lasts approximately 0.8 seconds (if the heart rate is 75 beats per minute, then one beat is 60 ÷ 75 ≈ 0.8 s).
- Study Notes:
- The cardiac cycle duration can be calculated if you know the heart rate. For example, at 75 beats per minute:
(ii) What is the heart rate represented on this graph, in beats per minute?
- Answer:
- 75 beats per minute (bpm).
- Study Notes:
- The heart rate is derived by dividing 60 seconds by the length of one cycle. A cycle lasting 0.8 s corresponds to 60 ÷ 0.8 = 75 bpm.
(b)
On your copy of the diagram, mark the following periods:
(i) The time when the ventricle is contracting (ventricular systole).
(ii) The time when the ventricle is relaxing (ventricular diastole).
- Answer and Study Notes:
- Ventricular Systole:
- Mark the period during which the pressure in the ventricle is rising sharply to its peak (this corresponds to the contraction phase).
- Ventricular Diastole:
- Mark the period following the peak when the pressure falls back down as the ventricle relaxes and fills with blood.
- Key Point:
- Identifying these phases on a pressure–time graph is crucial to understanding how the heart functions during each cycle.
- Ventricular Systole:
(c)
On your copy of the diagram, mark the following periods:
(i) The time when the atrium is contracting (atrial systole).
(ii) The time when the atrium is relaxing (atrial diastole).
- Answer and Study Notes:
- Atrial Systole:
- Mark the period where there is a small increase in atrial pressure, usually just before the ventricular systole.
- Atrial Diastole:
- Mark the period when the atrial pressure is relatively low and the atrium is filling with blood.
- Note:
- The atrial systole contributes to “topping up” the ventricles with blood.
- Atrial Systole:
(d)
On your copy of the diagram, mark the points at which the atrioventricular (AV) valves open and close.
- Answer and Study Notes:
- Explanation:
- The AV valves open when the pressure in the atria is greater than in the ventricles (allowing blood to flow into the ventricles).
- They close when ventricular pressure exceeds atrial pressure (preventing backflow).
- Marking:
- Identify the moment of pressure crossover on the graph and mark the valve closure and opening accordingly.
- Explanation:
(e)
On your copy of the diagram, mark the points at which the semilunar valves in the aorta open and close.
- Answer and Study Notes:
- Explanation:
- The semilunar valves open when the ventricular pressure exceeds the aortic pressure (allowing blood to be ejected into the aorta) and close when the aortic pressure exceeds the ventricular pressure (preventing backflow).
- Marking:
- Mark the moment when the aortic pressure curve crosses the ventricular pressure curve from below (valves opening) and then when it crosses from above (valves closing).
- Explanation:
(f)
The right ventricle develops about one-quarter of the pressure developed on the left side of the heart. On your diagram, draw a line to represent the probable pressure inside the right ventricle over the 1.3 seconds shown.
- Answer and Study Notes:
- Explanation:
- If the left ventricle reaches, for example, 120 mmHg during systole, the right ventricle would reach approximately 30 mmHg.
- Marking:
- Draw a pressure line that follows a similar pattern to the left ventricle’s cycle but with peak pressures at about one-quarter of the left ventricular pressures. This illustrates the lower force required for pulmonary circulation.
- Explanation:
Question 12
The diagram below shows a cross-section of the heart at the level of the valves.
(a)
(i) Copy and complete the following flow chart to show the pathway of blood through the heart.
A typical flow chart is as follows:
- Vena cava → 2. Right atrium → 3. Valve P → 4. Right ventricle → 5. Valve Q → 6. Pulmonary artery → 7. Lungs → 8. Left atrium → 9. Valve R → 10. Left ventricle → 11. Valve S → 12. Aorta
- Study Notes:
- Vena cava: Returns deoxygenated blood from the body to the right atrium.
- Right side of heart: Pumps blood to the lungs via the pulmonary artery.
- Left side of heart: Receives oxygenated blood from the lungs and pumps it through the aorta to the body.
(ii) Explain how the valves P and Q ensure one-way flow of blood through the heart.
- Answer and Study Notes:
- Explanation:
- Valve P (atrioventricular valve):
- Opens to allow blood to flow from the right atrium into the right ventricle when atrial pressure is higher than ventricular pressure.
- Closes when the right ventricle contracts (ventricular systole) and the ventricular pressure becomes higher than the atrial pressure, preventing backflow into the atrium.
- Valve Q (pulmonary valve):
- Opens when the right ventricle contracts and its pressure exceeds that in the pulmonary artery, allowing blood to flow into the pulmonary artery.
- Closes when the ventricular pressure drops, preventing blood from returning to the ventricle.
- Valve P (atrioventricular valve):
- Key Point:
- The valves act as “check valves,” ensuring that blood flows in only one direction through the heart.
- Explanation: