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Substance Exchange

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Substance Exchange

Substance exchange in biology is the exchange of substances (e.g. glucose) between the internal and external environments of cells. Put simply, this is how organisms get oxygen and nutrients from the external environment and how they get rid of metabolic waste. Multicellular organisms require specialised organ systems for substance exchange because their surface area to volume ratio is too small to rely on cell membranes as an exchange surface.

Substance Exchange in plants

Plants also exchange substances with the environment, mainly taking part in gas exchange and mass flow of solutes.

Gas exchange in plants

Carbon dioxide is exchanged for oxygen in leaves to allow photosynthesis. The underside of leaves has microscopic pores called stomata (singular stoma) that are key in facilitating gas exchange.

The stomata are not permanent pores as they are regulated by specialised cells termed guard cells as per the diagram below. Apart from the gas exchange, water also diffuses out of the leaf via stomata. This is transpiration, an undesirable event in plant biology.

Figure 1

Plants also need oxygen for respiration. Hence plant parts adapt to maximising absorption of oxygen by diffusion - mainly via increasing the surfaces area through branching and air spaces in the plant body.

Mass flow

Knowing how plants absorb water from their roots and produce glucose via photosynthesis, you may question how these get transported to distant parts of plants such as flowers.

Plants have their own transport systems to fulfil this purpose. The transport system of plants, termed mass transport, is composed of two systems - the xylem and phloem. Both systems facilitate mass transport through a process called translocation.

Figure 2

The main difference between xylem and phloem is that xylem transports water, whereas phloem transports substances made by photosynthesis (assimilates).

Substance Exchange in animals

Transports systems in animals also have a pump-like organ (heart) and more complex vessels. These adaptations satisfy the greater metabolic demand in animals.

Role of capillaries in Substance Exchange

Animals have many blood vessels making up their transport systems. Substance exchange directly occurs in the capillaries.

Capillaries are adapted for efficient diffusion by being thin (only one cell thick) and forming a large network called capillary beds.

Figure 3

Substance exchange between blood and muscle

There is a very high metabolic rate in muscle cells because they generate a lot of energy for contraction. They require a lot of glucose and oxygen from the blood and release a lot of carbon dioxide. A rich network of capillaries surrounds muscle cells for that purpose.

Figure 4

Another key role of capillaries is facilitating gas exchange. We will explore the role of capillaries in gas exchange in the following examples.

Gas exchange in animals

Oxygen and carbon dioxide are the two substances exchanged during gas exchange.

Gas exchange occurs over a gas exchange surface. The gas exchange surfaces differ across animals with low oxygen demand (e.g., insects and fish) and animals with high oxygen demand (e.g., humans).

The gas exchange surfaces in fish are composed of structures called gills. Gills are divided into filaments that contain many lamellae.

Figure 5

Gas exchange occurs in the lamellae, which follow the counter-current system. This means that blood and water in the lamellae flow in opposite directions.

Figure 6

In insects that lack capillaries, gas exchange occurs in the trachea.

Figure 7

As organisms with high oxygen demand, humans have more complex gas exchange systems and protein pigments that transport oxygen (e.g. haemoglobin).

The human gas exchange system consists of multiple organs. Gas exchange takes place in specialised, grape-like structures in the lungs known as the alveoli.

Figure 8

The alveoli are adapted for gas exchange via their thin (i.e. one cell thick) walls and close proximity with capillaries.

Figure 9

Substance Exchange - Key takeaways

  • Substance exchange occurs in all organisms. Substance exchange in plants is called mass flow systems, whereas animals have circulatory systems.
  • The mass flow systems of plants comprise xylem and phloem. Xylem transports water and mineral ions from the roots to aerial plant parts, whereas phloem transports products from photosynthesis from the leaves to the rest of the plant.
  • Capillaries are the main vessels involved in substance exchange in animals. Substance exchange occurs in the capillaries via diffusion. Capillaries also play a significant role in gas exchange, especially in fish and humans.

Frequently Asked Questions about Substance Exchange

As single-celled organisms have a high surface area to volume ratio, they simply rely on the cell membrane to exchange substances through processes such as diffusion, osmosis and active transport.

Solutes such as glucose dissolve in the plasma after being absorbed by the small intestines. As the plasma has a higher glucose concentration than body cells, glucose diffuses into body cells via capillaries to be respired to generate energy (ATP).

Capillaries are one cell thick to shorten the diffusion distance between the blood and neighbouring cells. They also form a network known as a capillary bed around neighbouring cells to increase the surface area needed for efficient diffusion.

As a result of their large sizes, the surface area to volume ratio of multicellular organisms is too small to enable diffusion alone for efficient substance exchange. Therefore, multicellular organisms have their own transport systems to achieve so. Plants have mass flow systems, whereas animals have circulatory systems.

The placenta is an organ that supplies nutrients to the fetus from the mother and removes metabolic wastes from the fetus into the mother’s blood. The placenta is surrounded by a rich network of maternal blood vessels. It comprises villi (finger-like projections similar to those of the small intestines) to increase the surface area and allow efficient substance exchange.

Final Substance Exchange Quiz

Question

Substance exchange in multicellular organisms occurs solely via diffusion. (True/ False)

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Answer

​​​​False - the surface area to volume ratio of multicellular organisms is too small for substance exchange to occur via diffusion. Instead, multicellular organisms have mass transport systems adapted for this function


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Question

Stomata, singular stoma, are located on the underside of leaves. (True/ False)


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Answer

True

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Question

Name the process where water from plants is inevitably lost when stomata are opened for gas exchange.


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Answer

Transpiration

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Question

Do plants also require oxygen? Describe the adaptations of plants in maximising the absorption of oxygen.


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Answer

Plants also require oxygen for respiration. Leaves have sufficient oxygen because it is a byproduct of photosynthesis. Other plant parts adapt to maximising the absorption of oxygen by branching and having air spaces between cells. These adaptations serve to increase the surface area of plants to facilitate the diffusion of oxygen.


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Question

Translocation is the term used to describe mass flow in the phloem only. (True/ False)


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Answer

False - translocation is the term used to describe mass flow in BOTH the xylem and phloem


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Question

The xylem transports water, whereas the phloem transports substances made by photosynthesis (True/ false). 


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Answer

True

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Question

Briefly explain why animals have a circulatory system that consists of a pump-like organ (heart) and more complex vessels for substance exchange.


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Answer

Animals have greater metabolic demands. Thus, circulatory systems serve to maximise the efficiency of substance exchange.

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Question

Fill in the blanks.

The gas exchange surfaces in fish are composed of structures called ____ that are divided into ______ that contain many ______.


Gas exchange occurs in the _____ which follow the ____________ ______. This means that ____ and _____ inside flow in ______ directions.

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Answer

The gas exchange surfaces in fish are composed of structures called gills. Gills are divided into filaments that contain many lamellae.


Gas exchange occurs in the lamellae, which follow the counter-current system. This means that the blood and water inside flow in opposite directions.

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Question

Instead of capillaries, gas exchange in insects occurs in the trachea as they form direct contact with respiring cells. (True/ False)


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Answer

True

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Question

Outline how the alveoli are adapted for gas exchange.

 

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Answer

The alveoli are adapted for gas exchange via their thin (i.e. one cell thick) walls and close proximity with capillaries.

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Question

The heart is made up of specialised muscle cells that contract voluntarily (True/ False)

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Answer

False - the heart is made up of specialised muscle cells that contract INVOLUNTARILY


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Question

Multicellular organisms have a large surface area to volume ratio (True/ False)


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Answer

False - multicellular organisms have a SMALL surface area to volume ratio

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Question

Fill in the blanks.

Multicellular organisms require circulatory systems due to their _____ surface area to volume ratio. Therefore they cannot rely on _____ alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large _____ require substances to travel large ______ to enter. Subsequently, the surface for substances to enter proportionately______, ultimately becoming too _________if diffusion is the only means of exchanging substances .


Animals need a _____ because of their high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to ______ like plants, they have the capacity for _______ to obtain food. A biological pump is then essential for maximal _____ of the exchange of metabolic substances across cells.

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Answer

Multicellular organisms require circulatory systems due to their small surface area to volume ratio. Therefore they cannot rely on diffusion alone like unicellular organisms (e.g. yeasts) for the exchange of substances. This is because their large sizes/volumes require substances to travel large distances to enter. Subsequently, the surface for substances to enter proportionately reduces, ultimately becoming too time-consuming if diffusion is the only means of exchanging substances.


Animals need a heart because of the high metabolic rate necessary for generating sufficient energy (ATP). As animals are unable to photosynthesise like plants, they have the capacity for locomotion to obtain food. A biological pump is then essential for maximal efficiency of the exchange of metabolic substances across cells.


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Question

Double circulatory systems are found in organisms where oxygen demand is low (True/ False)

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Answer

False - single circulatory systems are found in organisms where oxygen demand is low

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Question

Venules are a type of blood vessel (True/ False)


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Answer

True

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Question

The circulatory system produces hormones such as insulin (True/ False)


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Answer

False - insulin is produced in the pancreas, the circulatory system TRANSPORTS insulin to target cells

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Question

Outline the four different functions of circulatory systems.

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Answer

  • supply respiring cells with nutrients such as glucose (e.g. from small intestines to respiring tissues)
  • maintain a constant supply of oxygen to cells undergoing aerobic respiration (e.g., from lungs to respiring tissues)
  • rid metabolic waste products (e.g., carbon dioxide in respiring tissues back to the lungs)
  • transporting substances made from one part of the body to another (e.g., hormones such as insulin made in the pancreas to muscle cells)

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Question

The blood pressure in the systemic circulation is higher than in the pulmonary circulation (True/ False)


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Answer

True

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Question

Oxygenated and deoxygenated blood are mixed together to allow cells to receive as much oxygen as possible (True/ False)


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Answer

False - oxygenated and deoxygenated blood are separated to allow cells to receive as much oxygen as possible

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Question

Fill in the blanks.

A single circulatory system has only ____ circulatory route that involves _____ sets of capillaries. The first set of capillaries facilitate the exchange of gases  _____ and ______,  whereas the second set is where the exchange of substances between the ______ and ______ occurs. Blood is said to travel _______ through the heart on one complete circuit. The single circulatory system is common in organisms where _____ ______ is ____, such as fish, echinoderms and earthworms.


On the other hand, a double circulatory system has ____circulatory routes. The ____ route carries ____ blood to the body, then _______ back to the heart after cellular gas exchange. Whereas the ______ route carries _______ blood to the lungs and back to the _____ upon oxygenation. Blood is said to travel _____ through the heart on one complete circuit. The double circulatory system is found in all _____- blooded animals and also _____.

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Answer

A single circulatory system has only one circulatory route that involves two sets of capillaries. The first set of capillaries facilitate the exchange of the gases oxygen and carbon dioxide,  whereas the second set is where the exchange of substances between the blood and cells occurs. Blood is said to travel once through the heart on one complete circuit. The single circulatory system is common in organisms where oxygen demand is low, such as fish, echinoderms and earthworms.


On the other hand, a double circulatory system has two circulatory routes. The systemic route carries oxygenated blood to the body, then deoxygenated blood back to the heart after cellular gas exchange. Whereas the pulmonary route carries deoxygenated blood to the lungs and back to the heart upon oxygenation. Blood is said to travel twice through the heart on one complete circuit. The double circulatory system is found in all warm-blooded animals and also reptiles.

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Question

The circulatory system moves ammonia produced via deamination in the liver to the kidneys (True/ False)

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Answer

True


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Question

The heart as a biological pump is crucial for maximal efficiency of the exchange of metabolic substances across cells (True/ False)


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Answer

True

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Question

Describe the two main advantages of a double circulatory system.

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Answer

  • Ensures no mixing of blood allows cells to receive as much oxygen as possible and directs blood flow more precisely to the organs that need the most oxygen and nutrients.
  • Enables pressure differences - the systemic circulation has a higher pressure to receive oxygenated blood rapidly. The pulmonary circulation has a lower pressure to prevent damage to vessels and allow gas exchange.

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Question

In single circulatory systems, blood travels once through the heart on one complete circuit (True/ False)

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Answer

True


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Question

Lymphocytes are found in the blood (True/ False)


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Answer

False - lymphocytes are found in the lymph

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Question

The diagram of the heart on paper usually has the left side of the heart on the right-hand side and the right side of the heart on the left-hand side. (True/ False)


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Answer

True

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Question

How is the heart adapted to prevent over-distending with blood?


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Answer

It is surrounded by an inelastic pericardial membrane.

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Question

Explain the need for two sides of the heart.


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Answer

Each side of the heart is composed of separate pumps lying side by side. The left side receives oxygenated blood from the lungs and pumps it to the body and the right side receives deoxygenated blood from the body and pumps it to the lungs. Blood has to pass through the lungs first to be oxygenated before passing through the rest of the body. As there is a large drop in blood pressure after passing through the lungs, the heart needs to pump this blood to increase its pressure for it to pass through the rest of the body. Hence, the heart is split into two separate pumps lying side by side instead of one pump.


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Question

Name the two veins that connect to the heart and outline their roles.


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Answer

  • Vena cava – brings deoxygenated blood from both the lower and upper body 
  • Pulmonary vein – brings oxygenated blood from the left and right lungs 

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Question

Name the two arteries that connect to the heart and outline their roles.


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Answer

  • Aorta – large and arching, also branches into several smaller arteries to transport oxygenated blood up to the head and the rest of the lower body
  • Pulmonary artery – branches into two to transport deoxygenated blood into the left and right lungs

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Question

Describe how the heart obtains its own blood supply.


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Answer

The heart obtains its own blood supply by having specialised arteries that branch from the aorta called coronary arteries. These arteries supply oxygenated blood to cardiac muscles to facilitate their necessary contraction and relaxation.

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Question

Name the structure of the heart that stops blood of the left and right sides of the heart from mixing together.


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Answer

Septum

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Question

Fill in the blanks

The upper chambers that are smaller in size are the ____ (singular _____). They are connected to the veins, with the _____ ______ connected to the vena cava and whereas the _____ _____  connected to the pulmonary vein. 

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Answer

The upper chambers that are smaller in size are the atria (singular atrium). The atria are connected to the veins, with the right atrium connected to the vena cava and whereas the left atrium connected to the pulmonary vein. 


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Question

 Explain why the atria have thin walls.


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Answer

The atria have thin walls as they only pump blood into the ventricles that are small structures, whose lower pressure of pumping prevents the ventricles from bursting.

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Question

Fill in the blanks

The lower chambers that are larger in size are the ______. The ventricles are connected to the _______, with the ____ ________ connected to the pulmonary artery whereas the ____ ______ is connected to the aorta.

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Answer

The lower chambers that are larger in size are the ventricles. The ventricles are connected to the arteries, with the right ventricle connected to the pulmonary artery whereas the left ventricle is connected to the aorta.

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Question

Explain why the walls of the left ventricles are thicker than the right ventricles.


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Answer

The walls of the left ventricles are thicker than the right ventricles because the blood is pumped from the left ventricles to the extremities. This blood also needs to overcome the elastic recoil of arteries. As for the thinner walls of the right ventricles, the lungs that receive blood from the right ventricles not only are closer to the heart and are smaller than the rest of the body, but the capillaries of the lungs are extremely delicate.

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Question

Valves serve to prevent backflow of blood. Valves in the arteries that connect to the heart are called semilunar valves. As for atrioventricular valves between the atria and ventricles, the right side of the heart has bicuspid valves whereas the left side of the heart has tricuspid valves. (True/ False)


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Answer

False - the right side of the heart has tricuspid valves whereas the left side of the heart has bicuspid valves

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Question

Outline the role of tendinous cords found connected to atrioventricular valves.

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Answer

To prevent pressure from the heart from turning the valves inside out when the ventricles contract.

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Question

Name the process given to the coordination of the different components of the heart in the organ’s series of contractions and relaxations.


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Answer

The cardiac cycle


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Question

Outline the four roles of the heart.


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Answer

  • Pumping oxygenated blood to the other body parts.
  • Pumping hormones and other vital substances to different parts of the body.
  • Receiving deoxygenated blood and carrying metabolic waste products from the body and pumping it to the lungs for oxygenation.
  • Maintaining blood pressure

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Question

The pressure in the atria is greater than in the ventricles. What is the direction of blood flow in this case?

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Answer

Blood flows from the atria to the ventricles as the direction of blood flow in the heart is from a region of high pressure to a region of low pressure.

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Question

Choose the correct options

When the heart muscle (contracts/ relaxes), this results in a region of (high/ low) pressure. Whereas when the heart muscle (contracts/ relaxes), this creates a region of (high/ low) pressure. (Systole/ diastole) is the term to describe the contraction of the heart muscle, whereas (systole/ diastole) is when the heart muscle relaxes.

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Answer

When the heart muscle contracts, this results in a region of high pressure. Whereas when the heart muscle relaxes, this creates a region of low pressure. Systole is the term to describe the heart muscle contraction, whereas diastole is when the heart muscle relaxes.


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Question

Valves ensure blood flows in one direction (True/ False)



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Answer

True

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Question

Valves contract and relax, whereas the heart muscle opens and closes. (True/ False)


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Answer

False - valves open and close

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Question

Choose the correct options.

Pressure changes affect the function of valves. Valves (open/close) when the pressure difference is against the direction of blood flow and (open/ close) when the pressure difference follows the direction of blood flow.


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Answer

Pressure changes affect the function of valves. Valves close when the pressure difference is against the direction of blood flow and open when the pressure difference follows the direction of blood flow.


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Question

Name the three stages of the cardiac cycle.


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Answer

Atrial systole, ventricular systole, and ventricular diastole.

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Question

Describe the events that occur during atrial systole.


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Answer

Muscles in both atria contract

  • Atrial pressure > ventricular pressure

  • Blood flows from atria to ventricles

  • Atrioventricular valves open, semilunar valves shut

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Question

Describe the events that occur during ventricular systole.


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Answer

Muscles in both ventricles contract

  • Pressure in ventricles > atria

  • Atrioventricular valves shut

  • Pressure in ventricles > arteries

  • Blood flows from atria to arteries, semilunar valves open

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Question

Describe the events that occur during ventricular diastole.


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Answer

The whole of the heart muscle relaxes

  • Pressure in veins > atria and ventricle

  • Blood flows in

  • Pressure in arteries > ventricles

  • Semilunar valves close

Show question

Question

Blood flow in ventricular systole is higher in pressure than in atrial systole. (True/ False)


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Answer

True

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