The Circulatory System In Animals

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Mammalian Circulation

The mammalian circulatory system is a mass transport system that supplies cells with reactants for metabolism, such as oxygen and glucose.
It is a closed circulatory system, meaning blood flows inside enclosed vessels; in an open circulatory system (e.g. insects), blood is pumped directly into body cavities.
Mammals have a double circulatory system: for each complete circuit of the body, blood passes through the heart twice.
The left side of the heart pumps blood around the body — the systemic circulatory system.
The right side of the heart pumps deoxygenated blood to the lungs for gas exchange — the pulmonary circulatory system.
Key vessels include the coronary arteries (supply the heart with oxygenated blood), aorta (carries oxygenated blood out of the heart to the body), vena cava (carries deoxygenated blood into the heart from the body), pulmonary artery (carries deoxygenated blood to the lungs), pulmonary vein (carries oxygenated blood from the lungs to the heart), renal artery (supplies kidneys with oxygenated blood) and renal vein (carries deoxygenated blood away from the kidneys).
The pulmonary vein is the only vein in the circulatory system that contains oxygenated blood.

The Human Heart

The human heart has four chambers: the top two are the left and right atria (singular: atrium) and the bottom two are the left and right ventricles.
The left and right sides are separated by a wall of muscular tissue called the septum, which ensures blood from the two sides does not mix.
The left side contains oxygenated blood; the right side contains deoxygenated blood.
Valves ensure blood flows forward in the right direction: the right atrioventricular (tricuspid) valve separates the right atrium and ventricle; the left atrioventricular (bicuspid) valve separates the left atrium and ventricle; the right semilunar (pulmonary) valve separates the right ventricle and pulmonary artery; the left semilunar (aortic) valve separates the left ventricle and aorta.
Blood enters the heart via the vena cava (from the body) and the pulmonary vein (from the lungs); blood leaves via the pulmonary artery (to the lungs) and the aorta (to the body).
The muscular walls of the atria are thinner than those of the ventricles because the atria pump blood over a much shorter distance.
The muscle of the left ventricle is thicker than that of the right ventricle because blood leaving the left ventricle travels all around the body, requiring higher pressure.

The Cardiac Cycle

The cardiac cycle is the series of events in one heart beat, including muscle contraction (systole) and relaxation (diastole).
Contraction of heart muscle decreases volume in the chamber, which increases pressure; relaxation increases volume and decreases pressure.
Atrial systole: the atria contract, atrial volume decreases and pressure increases, forcing the AV valves open; blood is forced into the ventricles, which are relaxed (ventricular diastole).
Ventricular systole: the ventricles contract, ventricular volume decreases and pressure increases; the AV valves close (preventing backflow) and the semilunar valves open, forcing blood into the arteries; the atria are relaxed (atrial diastole) and continue to fill with blood.
Diastole: both atria and ventricles are relaxed; pressure in the ventricles drops below that in the aorta and pulmonary artery, so the SL valves close; blood returns via the vena cava and pulmonary vein; pressure in the atria rises above the ventricles, forcing the AV valves open and blood flows passively into the ventricles.
Valves open when pressure behind them is greater than pressure in front; they close when pressure in front is greater than behind.
During the cardiac cycle: atrial systole — AV valves open, SL valves closed; ventricular systole — AV valves closed, SL valves open; diastole — AV valves open, SL valves closed.
On a cardiac cycle graph: A = end of diastole (AV valve open); A–B = atrial systole; B = beginning of ventricular systole (AV valve shuts); C = ventricular systole (aortic valve opens); D = beginning of diastole (aortic valve closes); D–E = early diastole; E = diastole (AV valve opens).

Investigating Heart Rate

Heart rate can be investigated by studying the effect of exercise on human pulse rate, or the effect of caffeine on the heart rate of Daphnia (water fleas).
For exercise studies: record resting heart rate, exercise at a set intensity for a set time, record heart rate immediately after, allow recovery, repeat at different intensities and with several individuals to calculate a group average.
Limitations of exercise studies: exercise intensity must be quantified; heart rate must be taken at the same point each time as it slows immediately after exercise; participants may become tired, so heart rate may not reflect only the exercise intensity.
Daphnia are suitable for heart rate investigations because they have transparent bodies, so internal organs such as the heart can be observed using a light microscope.
The Daphnia method: prepare caffeine solutions and a distilled water control, place a Daphnia in a cavity slide, observe under low power, count heart beats over 20 seconds (multiply by 3 for beats per minute), return the Daphnia to the stock culture, and repeat with other individuals and concentrations.
Ethical considerations for Daphnia: handle gently, keep examination periods short, return promptly to the holding tank, and avoid extreme conditions such as very strong caffeine solutions.
Cardiac output (CO) is the total volume of blood pumped by the heart per unit of time: cardiac output = heart rate × stroke volume.
Heart rate is the number of cardiac cycles per minute; stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle.

Blood Vessels

Arteries transport blood away from the heart at high pressure; they have thick walls containing smooth muscle and elastic fibres to withstand high pressures.
Elastic fibres in arteries allow stretch to expand around blood when the heart beats, and recoil to maintain blood pressure when the heart is relaxed.
Arteries have a narrow lumen to help maintain high blood pressure, and a folded inner lining (endothelium) that can stretch to allow increased blood flow.
Arterioles branch from arteries and transport blood into capillaries; their walls contain a lower proportion of elastic fibres and a large number of muscle cells.
Muscle in arterioles can contract to adjust blood flow to specific organs, e.g. during exercise blood flow to the intestine is reduced while flow to the muscles increases.
Veins transport blood back to the heart at low pressure; their walls are thin with fewer smooth muscle and elastic fibres because thick walls with elastic recoil are not needed.
Veins have a much larger lumen than arteries, allowing a high volume of blood to flow through, and contain valves to prevent backflow of blood.

Capillaries & Tissue Fluid

Capillaries are tiny blood vessels that carry blood from larger vessels to the cells, forming networks called capillary beds that function as exchange surfaces.
Capillary walls (endothelium) are one cell thick, reducing the diffusion distance for gas exchange between blood and tissues.
The cells of the capillary wall have gaps (pores) between them, allowing small molecules from the blood to leak out into the tissues.
The capillary lumen has a very small diameter, approximately the same as a single red blood cell, forcing blood to travel slowly and providing more time for diffusion.
Tissue fluid is formed as blood passes through capillaries and some plasma leaks out through gaps in the walls to surround the cells; its composition is similar to plasma but contains fewer large proteins as these are too large to pass out.
Exchange of substances between cells and blood occurs via tissue fluid, e.g. waste carbon dioxide leaves the cells, dissolves in the tissue fluid, and diffuses into the capillary.
Tissue fluid formation and return depends on the balance between hydrostatic pressure (outward force) and osmotic pull from dissolved solutes such as plasma proteins (inward force).
At the arterial end, hydrostatic pressure is greater than the osmotic pull, so water and small molecules are forced out of the capillary down a hydrostatic pressure gradient, forming tissue fluid; large plasma proteins remain in the blood.
At the venous end, the osmotic pull is higher than the hydrostatic pressure (which has decreased due to loss of plasma volume and flow resistance), so fluid is drawn back into the capillary down its water potential gradient.
Factors affecting tissue fluid formation: high blood pressure forces extra fluid out of capillaries, and low blood protein content increases the water potential of blood and reduces osmosis, both leading to a build-up of fluid in the tissues.

Cardiovascular Disease Data

Cardiovascular disease (CVD) is any disease of the heart and blood vessels, e.g. coronary heart disease (CHD) (blockage of the coronary arteries that can lead to a heart attack), stroke (blood clots or bleeding affecting blood supply in the brain), and congenital heart disease (birth defects affecting heart structure).
A risk factor for CVD is any factor that can increase the risk of developing CVD; examples include high blood pressure, smoking, stress, diet high in cholesterol and saturated fat, genetic factors, and age and biological sex (risk increases with age and is much more likely to affect men).
High blood pressure can damage the walls of blood vessels, leading to the formation of fatty deposits and blood clots that can reduce blood flow.
Chemicals in cigarettes can narrow blood vessels and increase blood pressure; high stress can raise heart rate and increase blood pressure.
When interpreting data on CVD risk factors, you may be asked to describe what the data show, state a conclusion, and comment on validity (e.g. sample size, statistical analysis, control of additional variables, bias).
Conflicting evidence is that which shows a different pattern to evidence gained elsewhere; when it arises, more research is needed to show which pattern is correct.
When evaluating CVD studies, consider: is the sample representative (large enough, human subjects, demographics matching the target population)? Has statistical analysis been carried out (error bars/standard deviations)? Has an experimental control been included (placebo)? Is there more data to back up the findings? Does the study control additional variables? Is the study biased?
A common area of confusion is correlation vs causal relationship: data may show a correlation or association between a risk factor and CVD, but this does not prove that the risk factor causes CVD.

Required Practical: Dissecting Mass Transport Systems

Dissecting the mammalian heart allows observation of chambers, valves, and associated vessels to better understand form and function.
Safety: wear a lab coat, gloves, and eye protection; use dissection tools safely (cut away from the body, keep fingers away from blades).
Apparatus includes a dissection board, scalpel, dissecting scissors, forceps/tweezers, pins, mounted needle, biological specimen (e.g. pig or sheep heart), and disinfectant and disposal container.
Method: place the specimen front upwards (identified by the coronary arteries crossing the front between the ventricles); observe external features (coronary arteries, fatty deposits, atria, major vessels); make two vertical cuts through the centre of the left and right ventricles and atria; gently open out the muscle tissue; pin back the outer sides of the ventricles to expose internal features.
Internal structures to identify: left and right ventricle walls (compare muscle thickness), thin muscle walls of the atria, the septum, atrioventricular valves, and the entrances to the major blood vessels.
Limitations: dead tissue may not accurately reflect living organs (atria and vessels may be removed during butchering; older or frozen tissue may become dry or stiffened); smaller structures such as valve flaps may be difficult to locate; dissecting only one specimen may limit reliability.
Ethical considerations: specimens should come from reputable sources; biological waste must be disposed of responsibly; some students may have ethical or religious concerns about animal use.
Link visible features, such as wall thickness, to functional significance, e.g. generating high pressure in the left ventricle.

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Übungsfragen

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1.What is the name given to the wall of muscle that separates the left and right sides of the mammalian heart?
Easy
- ASeptum
- BEndothelium
- CPericardium
- DValve
2.Which of the following blood vessels carries oxygenated blood away from the lungs and towards the heart?
Easy
- APulmonary vein
- BPulmonary artery
- CVena cava
- DAorta
3.In a double circulatory system, blood passes through the heart twice for each complete circuit of the body.
Easy
True or false?
4.Which valve prevents the backflow of blood from the left ventricle into the left atrium?
Easy
- ALeft atrioventricular (bicuspid) valve
- BRight atrioventricular (tricuspid) valve
- CLeft semilunar (aortic) valve
- DRight semilunar (pulmonary) valve
5.During which stage of the cardiac cycle are the atrioventricular valves closed and the semilunar valves open?
Medium
- AVentricular systole
- BAtrial systole
- CDiastole
- DAtrial diastole
6.Which of the following are adaptations of capillaries for efficient exchange? (select all that apply)
Medium
- AWalls are one cell thick
- BPresence of valves to prevent backflow
- CLumen is approximately the diameter of a red blood cell
- DWalls contain gaps or pores between cells
- EThick layer of smooth muscle in the wall
7.Match each blood vessel with its correct function.
Medium
- Coronary arteries
- Aorta
- Pulmonary artery
- Renal vein
- Supplies the heart with oxygenated blood
- Carries oxygenated blood away from the heart to the body
- Carries deoxygenated blood away from the heart to the lungs
- Carries deoxygenated blood away from the kidneys to the heart
8.Which of the following is a feature of arterioles that allows them to control blood flow to different organs?
Medium
- AA high proportion of muscle fibres in their walls
- BA high proportion of elastic fibres in their walls
- CThe presence of valves
- DA very wide lumen

Mehr Fragen

9.Tissue fluid contains fewer large proteins than plasma.
Easy
True or false?
10.During ventricular systole, what causes the semilunar valves to open?
Medium
- APressure in the ventricles rises above the pressure in the aorta and pulmonary artery
- BPressure in the atria rises above the pressure in the ventricles
- CPressure in the ventricles drops below the pressure in the aorta and pulmonary artery
- DThe atrioventricular valves contract
11.Which of the following statements about the cardiac cycle are correct? (select all that apply)
Medium
- AAtrial systole and ventricular diastole occur at the same time
- BThe AV valves open when pressure in the atria exceeds pressure in the ventricles
- CThe semilunar valves open when pressure in the ventricles is lower than in the arteries
- DVentricular systole and atrial diastole occur at the same time
- EThe AV valves close when pressure in the ventricles exceeds pressure in the atria
12.Place the following events of the cardiac cycle in the correct order.
Medium
- Atrial systole forces blood into the ventricles
- Ventricular systole begins, AV valves close
- Semilunar valves open and blood is ejected
- Diastole begins, semilunar valves close
- AV valves open and blood flows passively into the ventricles
13.Which of the following is a feature of veins that helps blood return to the heart?
Easy
- AValves to prevent backflow
- BThick elastic walls to maintain pressure
- CNarrow lumen to increase pressure
- DA high proportion of muscle fibres for contraction
14.The elastic tissue in the wall of an artery allows it to stretch and recoil, helping to maintain blood pressure when the heart is relaxed.
Medium
True or false?
15.In the cardiac cycle graph, which point represents the opening of the semilunar (aortic) valve?
Medium
- AWhen ventricular pressure exceeds aortic pressure
- BWhen atrial pressure exceeds ventricular pressure
- CWhen ventricular pressure falls below aortic pressure
- DWhen aortic pressure exceeds ventricular pressure
16.Which of the following factors would increase the formation of tissue fluid? (select all that apply)
Hard
- AHigh blood pressure
- BLow blood protein content
- CIncreased osmotic pull by plasma proteins
- DBlockage of a vein
- EDecreased hydrostatic pressure in the capillary
17.During diastole, which of the following occurs?
Medium
- AThe atria and ventricles are both relaxed
- BThe atria contract while the ventricles relax
- CThe ventricles contract while the atria relax
- DThe semilunar valves are open
18.The pulmonary artery carries oxygenated blood away from the heart.
Easy
True or false?
19.Which of the following is the correct order of blood flow through the heart?
Medium
- ARight atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta
- BRight atrium → right ventricle → pulmonary vein → lungs → pulmonary artery → left atrium → left ventricle → aorta
- CLeft atrium → left ventricle → pulmonary artery → lungs → pulmonary vein → right atrium → right ventricle → aorta
- DRight atrium → left atrium → right ventricle → left ventricle → pulmonary artery → aorta
20.Which of the following statements about the mammalian circulatory system are correct? (select all that apply)
Medium
- AIt is a closed system, with blood contained within vessels
- BIt is an open system, with blood pumped into body cavities
- CIt is a double circulatory system, with blood passing through the heart twice per circuit
- DThe left side of the heart pumps blood to the lungs
- EThe right side of the heart pumps blood to the body
21.Match each blood vessel with the organ(s) it is associated with.
Medium
- Coronary arteries
- Aorta
- Vena cava
- Pulmonary artery
- Heart
- Heart, lungs
22.Which of the following is the correct definition of stroke volume?
Easy
- AThe volume of blood pumped out of the left ventricle during one cardiac cycle
- BThe volume of blood pumped out of the right ventricle during one cardiac cycle
- CThe total volume of blood pumped by the heart per minute
- DThe volume of blood in the atria at the end of diastole
23.The pulmonary vein is the only vein in the circulatory system that contains oxygenated blood.
Easy
True or false?
24.Which of the following are functions of the elastic tissue in the wall of an artery? (select all that apply)
Medium
- AAllows the artery to stretch when the heart beats
- BEnables recoil to maintain blood pressure when the heart is relaxed
- CPrevents backflow of blood
- DControls blood flow to different organs
- EReduces diffusion distance for gas exchange
25.Place the following events of the cardiac cycle in the correct order, starting with atrial systole.
Medium
- Atrial systole forces blood into the ventricles
- Ventricular systole forces blood into the arteries
- Diastole: both atria and ventricles relax
- Atria fill with blood again
26.Which of the following is a feature of capillaries that allows them to function efficiently as exchange surfaces?
Medium
- AWalls are one cell thick
- BWalls contain a high proportion of elastic fibres
- CThey have a large lumen
- DThey contain valves to prevent backflow
27.A person has a heart rate of 75 bpm and a stroke volume of 70 cm³. What is their cardiac output in dm³ min⁻¹?
Medium
- A5.25 dm³ min⁻¹
- B5250 dm³ min⁻¹
- C0.93 dm³ min⁻¹
- D1.07 dm³ min⁻¹
28.Match each structure with its correct function.
Medium
- Septum
- Right atrioventricular valve
- Left semilunar valve
- Coronary arteries
- Separates the left and right sides of the heart
- Prevents backflow from the right ventricle to the right atrium
- Prevents backflow from the aorta to the left ventricle
- Supplies the heart muscle with oxygenated blood
29.Which of the following are limitations of using a heart rate monitor during an investigation into the effect of exercise on heart rate? (select all that apply)
Medium
- AHeart rate begins to slow immediately after exercise
- BExercise intensity may not be quantified
- CParticipants may become tired over the course of the experiment
- DThe monitor may not be accurate
- EThe monitor provides a continuous reading
30.A cross-section of an artery shows a layer marked X that contains smooth muscle and elastic fibres. Which layer is X?
Medium
- AEndothelium
- BTunica media
- CConnective tissue
- DLumen
31.An artery wall has layers 25 μm, 495 μm, 375 μm and 125 μm thick. A vein wall has layers 25 μm, 245 μm, 245 μm and 125 μm thick. Calculate the ratio of the total thickness of the artery wall to the total thickness of the vein wall. Express your answer in the form __ : 1.
Medium
- A1.6 : 1
- B1.8 : 1
- C2.0 : 1
- D1.5 : 1
32.Figure 1 shows the blood circulation in a mammal. Which blood vessel carries oxygenated blood away from the lungs?
Medium
- APulmonary artery
- BPulmonary vein
- CVena cava
- DAorta
33.Babies born with a small hole in the septum between the right and left ventricles often have difficulty supplying enough oxygen to their tissues. Why is this?
Hard
- AOxygenated and deoxygenated blood mix, reducing the oxygen concentration of blood pumped to the body.
- BBlood flows from the left ventricle to the right ventricle, reducing the volume pumped to the lungs.
- CThe hole increases pressure in the left ventricle, causing the aortic valve to close.
- DThe hole allows blood to bypass the lungs entirely, so no gas exchange occurs.
34.Figure 1 shows changes in pressure in parts of a human heart during 0.8 seconds. Which letter represents the point at which the semilunar valves open?
Medium
- AA
- BB
- CC
- DD
35.During the cardiac cycle, what happens to the aortic pressure between 0.22 and 0.32 seconds?
Medium
- AIt decreases as the left ventricle relaxes.
- BIt increases as the left ventricle contracts.
- CIt remains constant as blood flows into the aorta.
- DIt decreases as the semilunar valves close.
36.Which of the following is a function of the coronary arteries?
Medium
- ATo carry deoxygenated blood to the lungs
- BTo supply the heart muscle with oxygenated blood
- CTo carry oxygenated blood to the body
- DTo bring deoxygenated blood from the body to the heart
37.Arterioles have a higher proportion of muscle fibres compared to elastic fibres. Why is this beneficial during exercise?
Hard
- AMuscle fibres allow arterioles to stretch and recoil, maintaining blood pressure.
- BMuscle fibres can contract to redirect blood flow to exercising muscles.
- CMuscle fibres increase the volume of blood that can flow through the arteriole.
- DMuscle fibres prevent backflow of blood towards the heart.
38.Which of the following statements about blood pressure in veins compared to arteries is correct?
Medium
- ABlood pressure in veins is higher and fluctuates more.
- BBlood pressure in veins is lower and does not fluctuate.
- CBlood pressure in veins is higher and does not fluctuate.
- DBlood pressure in veins is lower and fluctuates more.
39.Which of the following are adaptations of capillaries for efficient exchange? (Select all that apply.)
Medium
- AWalls are one cell thick
- BPresence of valves
- CSmall diameter lumen
- DGaps between endothelial cells
- EThick elastic layer
40.Which of the following factors would increase the formation of tissue fluid? (Select all that apply.)
Hard
- AIncreased hydrostatic pressure at the arterial end
- BDecreased concentration of plasma proteins
- CIncreased osmotic pull at the venous end
- DBlockage of lymphatic vessels
- EIncreased heart rate
41.Complete the sentence about the cardiac cycle.
Easy
The contraction phase of the heart is called ____, while the relaxation phase is called ____.
42.An individual has a cardiac output of 5.0 dm³ min⁻¹ and a heart rate of 70 bpm. Calculate their stroke volume in cm³.
Medium
43.Complete the paragraph on tissue fluid formation.
Easy
At the arterial end of a capillary, the hydrostatic pressure is ____ than the hydrostatic pressure in the tissue fluid. This forces fluid ____ the capillary. Large molecules such as plasma ____ remain in the blood.
44.A cardiac cycle lasts 0.8 seconds. Calculate the heart rate in beats per minute (bpm).
Medium
45.Name the valve that separates the right atrium from the right ventricle.
Medium
46.Complete the sentence about the mammalian circulatory system.
Easy
Mammals have a ____ circulatory system, meaning blood flows inside enclosed vessels, and it is a ____ circulatory system, meaning blood passes through the heart twice per circuit.
47.Name the blood vessel that carries deoxygenated blood from the body into the heart.
Easy
48.A student counts 24 heartbeats in 20 seconds for Daphnia. Calculate the heart rate in beats per minute.
Medium
49.Explain why the muscle wall of the left ventricle is thicker than that of the right ventricle.
Hard
50.Explain how the structure of capillaries enables them to function efficiently as exchange surfaces.
Hard
51.Explain the formation of tissue fluid and why proteins are not present in it.
Hard
52.Explain how high blood pressure may affect tissue fluid.
Hard
53.Name the equation used to calculate cardiac output.
Medium
54.An individual has a cardiac output of 5.0 dm³ min⁻¹ and a heart rate of 70 bpm. Calculate their stroke volume in cm³.
Medium
- A64.3 cm³
- B85.6 cm³
- C78.5 cm³
- D71.4 cm³
55.A cardiac cycle lasts 0.8 seconds. Calculate the heart rate in beats per minute (bpm).
Medium
- A75 bpm
- B83 bpm
- C91 bpm
- D67 bpm
56.A student counts 24 heartbeats in 20 seconds for Daphnia. Calculate the heart rate in beats per minute.
Medium
- A86 bpm
- B72 bpm
- C65 bpm
- D79 bpm

Lernnotizen

## Mammalian Circulation - **Mass transport system**: delivers oxygen and glucose to cells. - **Closed circulatory system**: blood flows inside enclosed vessels (unlike open systems, e.g. insects). - **Double circulatory system**: blood passes through the heart twice per complete circuit. - **Systemic**: left side pumps oxygenated blood around the body. - **Pulmonary**: right side pumps deoxygenated blood to the lungs. - **Key vessels**: - Coronary arteries – supply heart muscle with oxygenated blood. - Aorta – carries oxygenated blood away from heart to body. - Vena cava – carries deoxygenated blood into heart from body. - Pulmonary artery – carries deoxygenated blood to lungs. - Pulmonary vein – carries oxygenated blood from lungs to heart (only vein with oxygenated blood). - Renal artery – supplies kidneys with oxygenated blood. - Renal vein – carries deoxygenated blood away from kidneys. ## The Human Heart - **Four chambers**: left and right atria (top), left and right ventricles (bottom). - **Septum**: muscular wall separating left and right sides, prevents mixing of oxygenated and deoxygenated blood. - **Valves** ensure one-way flow: - Right AV valve (tricuspid) between right atrium and ventricle. - Left AV valve (bicuspid) between left atrium and ventricle. - Right semilunar (pulmonary) valve between right ventricle and pulmonary artery. - Left semilunar (aortic) valve between left ventricle and aorta. - **Muscle thickness**: - Atria walls thinner than ventricles (pump blood short distance). - Left ventricle thicker than right (pumps blood all around body). ## The Cardiac Cycle - **Systole** = contraction; **diastole** = relaxation. - **Volume changes**: contraction → decreased volume → increased pressure; relaxation → increased volume → decreased pressure. - **Events**: - **Atrial systole**: atria contract, AV valves open, blood forced into ventricles; ventricles relaxed. - **Ventricular systole**: ventricles contract, AV valves close, semilunar valves open, blood forced into arteries; atria relax. - **Diastole**: all relaxed, semilunar valves close, AV valves open, blood flows passively into ventricles. - **Valve movements**: open when pressure behind > pressure in front; close when pressure in front > pressure behind. - **Cardiac output (CO)** = heart rate × stroke volume. - Heart rate = beats per minute. - Stroke volume = volume pumped out of left ventricle per cardiac cycle. ## Investigating Heart Rate - **Exercise effect**: record resting heart rate, exercise at set intensity, record immediately after, allow recovery, repeat at different intensities, average across individuals. - **Daphnia**: transparent, heart visible under microscope; expose to caffeine solutions, count beats in 20 s, multiply by 3 for bpm. Ethical: handle gently, minimise harm. ## Blood Vessels - **Arteries**: thick walls with smooth muscle and elastic fibres, narrow lumen, folded endothelium. Withstand high pressure; elastic recoil maintains pressure. - **Arterioles**: more muscle, less elastic; contract to adjust blood flow (e.g. during exercise). - **Veins**: thin walls, large lumen, valves prevent backflow; low pressure. ## Capillaries & Tissue Fluid - **Capillaries**: one cell thick endothelium, gaps/pores, narrow lumen (red blood cell diameter) → slow flow, more time for diffusion. - **Tissue fluid formation**: - At arterial end, hydrostatic pressure > osmotic pull → water and small molecules forced out. - Large plasma proteins remain in blood. - At venous end, osmotic pull > hydrostatic pressure → fluid drawn back in. - **Affected by**: high blood pressure (more fluid forced out), low blood protein (reduces osmosis, fluid build-up). ## Cardiovascular Disease Data - **CVD** includes coronary heart disease, stroke, congenital heart disease. - **Risk factors**: high blood pressure, smoking, stress, diet (high cholesterol/saturated fat), genetic factors, age, biological sex. - **Data interpretation**: correlation ≠ causation; consider sample size, representativeness, statistical analysis, control of variables, bias. ## Required Practical: Dissecting Mass Transport Systems - **Heart dissection**: identify chambers, valves, vessels; compare wall thickness. - **Method**: place heart front up, make vertical cuts, open out, pin back, identify structures. - **Safety**: lab coat, gloves, eye protection, cut away from body. - **Limitations**: dead tissue may not reflect living organ; smaller structures hard to locate. - **Ethical**: reputable sources, responsible disposal.

Mass transport system: A system that ensures cells receive a constant supply of reactants for metabolism, e.g. oxygen and glucose.
Closed circulatory system: A circulatory system in which blood flows inside enclosed vessels.
Double circulatory system: A circulatory system in which blood passes through the heart twice for each complete circuit of the body.
Systemic circulatory system: The left side of the heart pumps blood around the body.
Pulmonary circulatory system: The right side of the heart pumps deoxygenated blood to the lungs for gas exchange.
Septum: A wall of muscular tissue that separates the left and right sides of the heart, ensuring blood doesn't mix.
Atrioventricular (AV) valve: Valve between atrium and ventricle; right AV valve is the tricuspid valve, left AV valve is the bicuspid valve.
Semilunar (SL) valve: Valve between ventricle and artery; right semilunar valve is the pulmonary valve, left semilunar valve is the aortic valve.
Systole: Contraction of the heart.
Diastole: Relaxation of the heart.
Cardiac output (CO): The total volume of blood pumped by the heart per unit of time; CO = heart rate × stroke volume.
Stroke volume: The volume of blood pumped out of the left ventricle during one cardiac cycle.
Heart rate: The number of times the heart beats per minute, or the number of cardiac cycles per minute.
Tissue fluid: The liquid which surrounds cells, formed from plasma leaking out through gaps in capillary walls.
Hydrostatic pressure: Pressure exerted by fluid, forcing fluid out of capillaries at the arterial end.
Osmotic pull: The inward force generated by dissolved solutes, e.g. plasma proteins, drawing fluid back into capillaries.
Cardiovascular disease (CVD): Any disease of the heart and blood vessels, e.g. coronary heart disease, stroke, congenital heart disease.
Risk factor: Any factor that can increase the risk of developing a disease, e.g. high blood pressure, smoking, stress, diet, genetic factors, age, biological sex.

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