Topic 8 Transport in Mammals.

Question 1

What is the circulatory system (otherwise known as the blood system)?

Answer 1

The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood.

Question 2

What is a major role of the circulatory system?

Answer 2

The circulatory system’s main function is to transport oxygen from the lungs’ alveoli to body tissues, as cells require a steady supply for aerobic respiration. This oxygen is carried by red blood cells, bound to the protein hemoglobin.

Question 3

Mammals have a closed double circulatory system consisting of a heart, blood and blood vessels including arteries, arterioles, capillaries, venules and veins. What does this mean?

Answer 3

Mammals have a closed double circulatory system consisting of a heart, blood and blood vessels including arteries, arterioles, capillaries, venules and veins meaning that for every one circuit of the body, the blood passes through the heart twice.
This means that the mammalian heart must have four chambers to keep oxygenated and deoxygenated blood separate.

Question 4

What is a closed blood system?

Answer 4

A circulatory system made up of vessels containing blood.

Question 5

What is the advantage of a double circulatory system?

Answer 5

Double circulatory systems can maintain a higher blood pressure which increases the speed at which the blood flows so nutrients can be delivered and waste can be removed more quickly.

Question 6

What is the systemic circulation and pulmonary circulation?

A diagram representing the two pathways involved in the double circulatory system—pulmonary circulation and systemic circulation.

Answer 6

Systemic circulation is the part of the circulatory system that carries blood from the heart to all of the body except the gas exhange surface and back to the heart.

Pulmonary circulation is the part of the circulatory system that carries blood from the heart to the gas exchange surface and then back to the heart.

https://images.nagwa.com/figures/explainers/912123271719/5.svg

Question 7

What is the purpose of the right side of a mammalian heart?

Answer 7

The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs (the pulmonary circulation).

Question 8

What is the purpose of the left side of a mammalian heart?

Answer 8

The left side of the heart receives oxygenated blood from the lungs and pumps it to the body (the systemic circulation).

Question 9

Fish have a single circulatory system. What does this mean?

Answer 9

This means that their heart only has two chambers (consisting of an upper atrium and a lower ventricle), and blood passes through the heart only once during its circuit around the body.

Question 10

Describe the pathway of blood with the single circulatory system in fish.

Answer 10

Heart → gills → body → heart.
Oxygen is absorbed as blood
passes the gills, thus fish do not have lungs.

Question 11

What is the function of the heart (step-by-step)?

Answer 11

• Deoxygenated blood coming from the body flows into the right atrium via the vena cava.
• The right atrium contracts and blood moves through a one-way valve (tricuspid valve) to the right ventricle.
• The right ventricle contracts and blood exits the heart through a one-way valve (semilunar valve) to the lungs via the pulmonary artery.
• Blood becomes oxygenated in the lungs and then returns to the heart via the pulmonary vein, entering the left atrium.
• The left atrium contracts and blood moves through a one-way valve (bicuspid valve) into the left ventricle.
• The left ventricle contracts and oxygenated blood exits the heart past the semilunar valve through the aorta (artery) and travels around the body, becoming deoxygenated.

Question 12

In what direction from the heart is blood pumped in arteries and veins?

Answer 12

Blood is pumped from the heart in arteries and returns to the heart in veins.

Question 13

What are the four main blood vessels of the pulmonary and systemic circulations?

Answer 13

• Pulmonary artery.
• Pulmonary vein.
• Aorta (artery).
• Vena cava (vein).

Question 14

Describe the functions of the main blood vessels of the pulmonary circulations:
- Pulmonary artery.

Answer 14

The pulmonary artery is responsible for carrying blood low in oxygen or deoxygenated blood from the heart to the lungs, where it is oxygenated and releases carbon dioxide through the process of respiration. It starts from the right ventricle of the heart and branches into the left and right pulmonary arteries, which further divide into smaller arterioles within the lungs.

Question 15

Describe the functions of the main blood vessels of the pulmonary circulations:
- Pulmonary vein.

Answer 15

The pulmonary vein’s main function is to carry oxygenated blood or oxygen-rich blood from the lungs back to the heart. After blood is oxygenated in the lungs, it flows through the pulmonary veins into the left atrium of the heart. From there, it is pumped into the left ventricle and then circulated throughout the body via systemic circulation.

Question 16

Describe the functions of the main blood vessels of the systemic circulations:
- Aorta (artery).

Answer 16

The aorta is one of the largest systemic criculation arteries in the human body and its main function is to distribute oxygenated blood throughout all parts of the body, ensuring cellular function and metabolism. It starts from the left ventricle of the heart and carries oxygenated blood away from the heart to supply all parts of the body with nutrients and oxygen. The aorta branches out into smaller arteries that further divide into arterioles, which eventually reach every tissue and organ in systemic circulation.

Question 17

Describe the functions of the main blood vessels of the systemic circulations:
- Vena cava (vein).

Answer 17

The vena cava consists of two main veins: superior vena cava and inferior vena cava. The superior vena cava collects deoxygenated blood from above the diaphragm (upper body) and returns it to the right atrium of the heart. The inferior vena cava gathers deoxygenated blood from below diaphragm (lower body) regions and delivers it back to the right atrium. The primary function of both vena cava’s is to return deoxygenated blood from various parts of systemic circulation back to the heart for reoxygenation.

Question 18

Describe the structure of:
- Red blood cells (erythrocytes).

A diagram representing the structure of red blood cells.

Answer 18

• Red blood cells are shaped like a biconcave disc.
• Red blood cells are very small. The diameter of a human red blood cell is about 7 μm.
• Red blood cells are very flexible.
• Red blood cells have no nucleus, no mitochondria and no endoplasmic reticulum.

https://biology-igcse.weebly.com/uploads/1/5/0/7/15070316/1209710_orig.png

Question 19

Describe the function of:
- Red blood cells (erythrocytes).

Answer 19

Red blood cells, also known as erythrocytes, are primarily responsible for transporting oxygen from the lungs to the rest of the body tissues and carrying carbon dioxide back to the lungs for exhalation.
They contain haemoglobin, a protein that binds to oxygen in the lungs and releases it in tissues where it is needed.
Red blood cells lack a nucleus and most organelles such as mitochondria and endoplasmic reticulum, allowing them to have more space for haemoglobin and thus enhancing their oxygen-carrying capacity.

Question 20

Explain why are red blood cells are shaped like a biconcave disc?

Answer 20

Red blood cells are shaped like a biconcave disc. The dent in each side of the cell increases the surface area to volume ratio (surface area : volume) of the cell. This large surface area means that oxygen can difffuse quickly into or out of the cell.

Question 21

Explain why are red blood cells very small?

Answer 21

Red blood cells are very small. The diameter of a human red blood cell is about 7 μm.
This small size means that no haemoglobin molecule within the cell is very far from the cell surface membrane, and the haemoglobin molecules can therefore quickly exchange oxygen with the fluid outside the cell.
It also means that narrow capillaries can still allow red blood cells to move through them, so bringing oxygen as close as possible to cells which require it.

Question 22

Explain why are red blood cells flexible?

Answer 22

Red blood cells are very flexible. This is possible because the cells have a specialised cytoskeleton, made up of a mesh-like network of protein fibres. This allows them to be squashed into different shapes and still return back to their normal biconcave shape.

Question 23

Explain why red blood cells have no nucleus, no mitochondria, and no endoplasmic reticulum?

Answer 23

Red blood cells have no nucleus, no mitochondria and no endoplasmic reticulum meaning that there is more room for haemoglobin, so maximising the amount of oxygen which can be carried by each red blood cell.

Question 24

Descibe the lifespan of a red blood cell.

Answer 24

Red blood cells do not live very long (only four months). Old ones are broken down in the liver, and new ones are constantly made in the bone marrow.

Question 25

Describe the structure of:
- White blood cells.

Answer 25

• White blood cells all have a nucleus.
• Most white blood cells are larger than red blood cells, although one type, lymphocytes, may be slightly smaller.
• White blood cells are either spherical or irregular in shape, not a biconcave disc.

Question 26

Answer the following questions:
- Where are white blood cells made?
- What is the main purpose of white blood cells?
- What are the two types of white blood cells?

Answer 26

• White blood cells are made from stem cells in the bone marrow.
• The main purpose of white blood cells is fighting disease.
• They can be divided into two main types:
  - Phagocytes.
  - Lymphocytes.

Question 27

What are phagocytes and what are the two types of phagocytes?

Answer 27

Phagocytes are cells that destroy invading microorganisms or pathogens by phagocytosis.

The two types of phagocytes are:

• Neutrophils: which is the most common type of phagocyte and can be recognised by its lobed nucleus and granular cytoplasm.
• Monocytes: are cells that can develop into a different type of phagocyte called a macrophage.

Question 28

Describe the function of the following white blood cell:
- Monocytes.

Answer 28

Monocytes are a type of phagocytic white blood cell with a large oval-shaped nucleus. They circulate in the bloodstream and can differentiate into macrophages when they migrate from the blood into tissues. Their primary functions include detecting and destroying pathogens through phagocytosis as well as antigen presentation.

Question 29

What is a macrophage?

Answer 29

A type of phagocytic cell found in tissues throughout the body and they act as antigen-presenting cells.

Question 30

What is antigen presentation?

Answer 30

Antigen presenting involves the display of antigens on the surface of specialized cells known as antigen-presenting cells (such as macrophages). This process is essential for the activation of T cells (a type of lymphocyte).

Question 31

How do macrophages work as antigen-presenting cells?

Answer 31

Macrophages are long-lived cells that help start immune responses. Instead of completely destroying pathogens, they break them down and display pieces of them (antigens) on their surface. This allows lymphocytes to recognize and respond to these antigens.

Question 32

Describe the function of the following white blood cell:
- Neutrophils.

Answer 32

Neutrophils are a type of white blood cell containing a lobed nucleus and a granular cytoplasm.
Neutrophils have receptor proteins on their membranes to identify pathogens as non-self.
Neutrophils engulf and destroy foreign pathogens through a process called phagocytosis.

When there is an infection, large numbers of neutrophils are released from the bone marrow and accumulate at the site of infection. However, they are short-lived and die after digesting pathogens causing the formation of pus.

Question 33

Describe the function of the following white blood cell:
- Lymphocytes.

Answer 33

Lymphocytes are a type of white blood cell with a large round nucleus that almost fills the cell and only a small amount of cytoplasm. They destroy microorganisms through the secretion of chemicals called antibodies which attach to antigens and destroy the invading pathogens.

Question 34

What is haemoglobin?

A diagram representing the structure of a haemoglobin protein.

Answer 34

Haemoglobin is a water-soluble globular protein which consists of two alpha-globin and two beta-globin polypeptide chains, each containing one haem group.
It carries oxygen in the blood as oxygen can bind to the haem (Fe²+) group and oxygen is then released when required.
Each haemoglobin molecule can combine with four oxygen molecules (O₂) or eight oxygen atoms.

https://www.researchgate.net/profile/Mehmet-Yegin-2/publication/351969451/figure/fig22/AS:1028814037590016@1622299789021/Structure-of-Hemoglobin-Protein-Hemoglobingentr-https-hemoglobingentr-Access.png

Question 35

What is the balanched chemical equation for the binding of oxygen to haemoglobin?

Answer 35

Hb + 4O₂ → HbO₈.
Haemoglobin + Oxygen → Oxyhaemoglobin.

Question 36

The Haemoglobin Dissociation Curve shows that:
The affinity of oxygen for haemoglobin varies depending on the partial pressures of oxygen (pO₂) which is a measure of oxygen concentration.
Explain what is meant by this.

Answer 36

The greater the concentration of dissolved oxygen in cells the greater the partial pressure. Therefore, as partial pressure of oxygen (pO₂) increases, the percentage saturation of haemoglobin with oxygen increases or the affinity of haemoglobin for oxygen increases, that is oxygen binds to haemoglobin tightly.

During respiration, oxygen is used up therefore the partial pressure decreases, decreasing the affinity of oxygen for haemoglobin.

Partial pressure is a measure of the concentration of a gas.

Question 37

Define affinity.

Answer 37

Affinity refers to how strongly a haemoglobin molecule binds to an oxygen molecule. High affinity means the haemoglobin molecule binds tightly and easily to the oxygen molecule, while low affinity means the binding is weaker and less likely to occur.

Question 38

Where in the body is the pO₂ highest?

Answer 38

In the body, the pO₂ is highest in the alveolar capillaries of the lungs. This is because the alveoli are the sites where oxygen from inhaled air diffuses into the blood, and thus they have a higher concentration of oxygen compared to other parts of the body. Therefore, haemoglobin can carry oxygen and become highly saturated with oxygen.

Lung tissues have a higher pO₂ and a lower pCO₂.

Question 39

Where in the body is the pO₂ lowest?

Answer 39

• In metabolically active tissues, the pO₂ is low because oxygen demand is high as it is used for cellular respiration. The higher the metabolic activity of the tissue, the lower the pO₂.
• After oxygen has been delivered to the tissues, the pO₂ in venous blood is lower compared to arterial blood. This is because venous blood carries deoxygenated blood from the tissues back to the heart.

Respiring tissues have lower pO₂ and higher pCO₂. This occurs because oxyhemoglobin (HbO₈) dissociates or separates more readily in response to the increased oxygen demand for aerobic respiration.

Question 40

Why is The Haemoglobin Dissociation Curve S-Shaped (Sigmoidal)?

Answer 40

The hemoglobin dissociation curve is S-shaped, or sigmoidal, due to the cooperative binding of oxygen by hemoglobin molecules.

Question 41

What is meant by the term cooperative binding?

Answer 41

When one haem group binds to oxygen, it changes the shape of the hemoglobin molecule.The change in shape makes it easier for the other three haem groups to bind to oxygen. This means that once one oxygen molecule binds, the affinity of the remaining sites for oxygen increases. This is called cooperative binding.

One haemoglobin molecule contains four heme groups. Each heme group can bind one molecule of oxygen, allowing hemoglobin to carry up to four oxygen molecules or eight oxygen atoms.

Question 42

Why does The Haemoglobin Dissociation Curve level off or plateau?

Answer 42

The hemoglobin dissociation curve levels off at high pO₂ because hemoglobin approaches its maximum oxygen-binding capacity as all haem groups are fully occupied.

Question 43

The Haemoglobin Dissociation Curve shows the effects of an increase or decrease of pO₂. Describe these differences.

Answer 43

• A small increase in pO₂, results in a large increase in percentage concentration of haemoglobin with oxygen. This shows how haemoglobin takes up oxygen in the alveolar capillaries of the lungs.
• A small decrease in pO₂, results in a large decrease in percentage concentration of haemoglobin with oxygen. This shows how haemoglobin releases oxygen to respiring tissues.

Question 44

What happens at high partial pressures of carbon dioxide (pCO₂)?

This question can also be rephrased to:
What is the Bohr shift?

What is the effect of increased pCO₂ on The Haemoglobin Dissociation Curve.

Answer 44

At high pCO₂, haemoglobin’s affinity for oxygen decreases due to the Bohr shift, leading to the release of oxygen.

This results in The Haemoglobin Dissociation Curve shifting to the right because a higher pO₂ is needed.

Question 45

1. Describe the pathway of carbon dioxide.
2. How does high pCO₂ decrease the affinity of Hb (haemoglobin) to O₂ (oxygen)?

Answer 45

Carbon dioxide produced by respiring cells diffuses into the blood plasma and then into red blood cells. Inside the red blood cells, the enzyme carbonic anhydrase catalyzes the formation of carbonic acid from carbon dioxide and water.

High pCO₂ decreases the affinity of hemoglobin (Hb) for oxygen through the following process:

• Formation of Carbonic Acid.
• Dissociation of Carbonic Acid into Hydrogen Ions and Bicarbonate.

Question 46

Describe the Formation of Carbonic Acid.

Answer 46

• Formation of Carbonic Acid:
  Increased pCO₂ levels lead to increased formation of carbonic acid (H₂CO₃) in the blood. Carbon dioxide reacts with water (H₂O) to form carbonic acid.
  CO₂ + H₂O ⇌ H₂CO₃
  Carbon dioxide + Water ⇌ Carbonic Acid.

This reaction is catalysed by an enzyme called carbonic anhydrase which is found in the cytoplasm of red blood cells.

Question 47

Describe the Dissociation of Carbonic Acid into Hydrogen Ions and Bicarbonate.

Answer 47

• Dissociation into Hydrogen Ions and Bicarbonate:
  Carbonic acid (H₂CO₃) then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). This increases the concentration of hydrogen ions, which lowers the blood pH (making it more acidic).

H₂CO₃ ⇌ H⁺ + HCO₃⁻
Carbonic Acid ⇌ Hydrogen Ions + Bicarbonate Ions.

Question 48

What is the effect of increased hydrogen ions on haemoglobin?

Answer 48

Increased hydrogen ions (H⁺) in the blood results in a lower pH (more acidic conditions), lead to the following effects on haemoglobin:

• Reduced Oxygen Affinity: The increased concentration of hydrogen ions interacts with haemoglobin, causing a change in its structure that reduces haemoglobin’s affinity for oxygen. Haemoglobin combines with the hydrogen ions to form haemoglobonic acid.
  Hb + H⁺ ⇌ HHb
  Haemoglobin + Hydrogen Ions ⇌ Haemoglobonic Acid.
• Facilitated Oxygen Release: As a result of this decreased affinity, haemoglobin releases oxygen more readily. This helps ensure that oxygen is delivered to tissues where it is needed most, particularly in areas with high metabolic activity that produce more hydrogen ions.

This phenomenon is known as the Bohr shift.

Question 49

What is the chloride shift?

Answer 49

The chloride shift is the movement of chloride ions (Cl⁻) into red blood cells from the blood plasma, to balance out the movement of bicarbonate ions (HCO₃⁻) into the plasma from the red blood cells.

Question 50

Describe the the exchange of ions is known as the chloride shift.

Answer 50

• In tissues, carbon dioxide (CO₂) diffuses into red blood cells and is converted to carbonic acid (H₂CO₃) by the enzyme carbonic anhydrase. Carbonic acid then dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺).
• To prevent a buildup of bicarbonate ions inside the red blood cells, which would lead to an imbalance, bicarbonate ions move out of the cells into the plasma.
• To maintain neutrality, chloride ions (Cl⁻) from the plasma move into the red blood cells to balance the outgoing bicarbonate ions.

This exchange of ions is known as the chloride shift.

Question 51

What are the three ways in which blood transports carbon dioxide?

Answer 51

• As bicarbonate ions in the blood plasma. About 85% of carbon dioxide transported by the blood is carried in this way.
  Explained in the Bohr shift.
• As dissolved carbon dioxide molecules in blood plasma. About 5% of carbon dioxide transported by the blood is carried in this way.
  Carbon dioxide remains as carbon dioxide and simply dissolves in the blood plasma.
• As carbaminohaemoglobin. About 10% carbon dioxide transported by the blood is carried in this way.

Question 52

Explain how carbon dioxide is transported as carbaminohaemoglobin (CO₂Hb).

Answer 52

Carbon dioxide is transported as carbaminohemoglobin when it binds directly to the terminal amine groups (-NH₂) of haemoglobin.

This binding occurs independently of the oxygen-binding sites on haemoglobin, allowing haemoglobin to carry both CO₂ and O₂, although CO₂ binding slightly reduces hemoglobin’s affinity for oxygen.

Question 53

When carbon dioxide is bound to hemoglobin as carbaminohemoglobin, it is released from the body in the following steps:

Answer 53

As blood reaches the lungs, the partial pressure of CO₂ in the alveoli is lower than in the blood. This gradient causes CO₂ to diffuse from the blood into the alveoli.

The decrease in CO₂ concentration in the blood leads to a reduction in carbaminohaemoglobin. As CO₂ detaches from haemoglobin, the haemoglobin molecule’s structure changes, facilitating the release of CO₂.

Once released from haemoglobin, CO₂ is carried in the dissolved form or as bicarbonate ions to the alveoli, where it is expelled from the body during exhalation.

Question 54

What is cardiac muscle?

Answer 54

Cardiac muscle is the type of muscle that makes up the walls of the heart.

Question 55

What are coronary arteries?

Answer 55

Coronary arteries are arteries that branch from the aorta and spread over the walls of the heart supplying the cardiac muscle with nutrients and oxygen.

Question 56

Explain the importance of the septum.

Answer 56

The left and right sides of the heart are separated by the septum, which makes sure that oxygenated and deoxygenated blood remains separate.

Question 57

What are atrium (singular: atria)?

Answer 57

The atria are the two upper chambers of the heart. They receive blood from the veins and pump it into the lower chambers (ventricles). The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs.

Question 58

What are ventricles?

Answer 58

Ventricles are the two lower chambers of the heart that pump blood out to the lungs and the rest of the body.
The right ventricle pumps deoxygenated blood to the lungs, while the left ventricle pumps oxygenated blood to the rest of the body.

Question 59

What are the names of the valves located between the atria and ventricles?

What are the types of semilunar valves?

Answer 59

The atria and ventricles have valves between them, which are known as the atrioventricular valves (AV). The one on the left is the mitral or bicuspid valve, and the one on the right is the tricuspid valve.

The two types of semilunar valves are the pulmonary valve and aortic valve.

Question 60

Give the summarized functioning of the heart.

Answer 60

Vena cava → Right atrium → Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left atrium → Left ventricle → Aorta → The body.

Question 61

Explain the differences in the thickness of the walls of the artia and ventricles.

Answer 61

The walls of the atria are thinner because they only need to push blood a short distance into the ventricles. The pressure they generate is relatively low.
The walls of the ventricles are thicker because they need to generate much higher pressure to pump blood out of the heart and into the lungs (right ventricle) or the rest of the body (left ventricle).

Question 62

Explain the differences in the thickness of the walls of the left ventricle and right ventricle.

Answer 62

The left ventricle has thicker walls than the right ventricle. This is because it needs to generate higher pressure to pump oxygenated blood through the aorta and throughout the entire body.
The right ventricle has thinner walls as it only needs to pump deoxygenated blood to the lungs, which requires less pressure.

Question 63

What is the normal heart rate in beats per minute?

Answer 63

The normal heart rate is 75 beats per minute (bpm).

Question 64

What is the cardiac cycle and what is the average length of one cardiac cycle?

Answer 64

The cardiac cycle is the sequence of events that takes place during one heartbeat with the average length of one cardiac cycle being 0.8 seconds.

Question 65

What are the three stages of the cardiac cycle?

Answer 65

• Atrial systole (about 0.1 seconds).
• Ventricular systole (about 0.3 seconds).
• Diastole (about 0.4 seconds).

Diastole is the phase of the cardiac cycle when the heart muscle relaxes and the chambers fill with blood.
Systole is the phase when the heart muscle contracts and pumps blood out of the chambers.

Question 66

What happens during atrial systole?

Answer 66

• The atria contract, pushing blood into the ventricles.
• The AV valves (tricuspid and bicuspid/mitral) open, allowing blood flow from the atria to the ventricles.
• The semilunar valves (pulmonary and aortic) are closed to prevent backflow into the ventricles.

This phase ensures that the ventricles are fully filled with blood before they contract.

Question 67

What happens during ventricular systole?

Answer 67

• The ventricles contract, increasing pressure within them.
• The AV valves close to prevent backflow into the atria.
• The pressure of the blood forces the semilunar valves open, allowing blood to be ejected from the ventricles into the pulmonary artery (right ventricle) and the aorta (left ventricle).

This phase ensures that blood is effectively pumped out of the heart to the lungs and the rest of the body.

Question 68

What happens during diastole?

Answer 68

• The heart muscle relaxes, allowing the chambers to expand.
• The atria and ventricles fill with blood from the veins (the atria fill from the vena cava and pulmonary veins, and the ventricles fill from the atria).
• The AV valves are open, while the semilunar valves are closed to prevent backflow.

This phase ensures that the heart chambers are filled with blood, preparing for the next contraction.

Question 69

Cardiac muscles are ____. What does that mean?

Answer 69

Cardiac muscles are myogenic meaning that it naturally contract and relaxes without having to receive impulses from a nerve to make it do so.

Question 70

How is the cardiac cycle initiated and coordinated?

Answer 70

The cardiac cycle is initiated and coordinated by the heart’s electrical conduction system:
1. Initiation:
Sinoatrial Node (SAN) also called the pacemaker.
2. Coordination:
Atrioventricular Node (AVN).
Bundle of His and Purkyne Tissue.

Question 71

What happens at the Sinoartial Node (SAN)?

Why does the electrical excitation wave generated by the SAN not spread to the ventricles?

Answer 71

The cycle begins with the SAN, located in the right atrium, which generates an electrical excitation wave. This wave initiates the heartbeat as it spreads across the atria and causes both atria to contract simultaneously, pushing blood into the ventricles (atrial systole).

The electrical excitation wave generated by the SAN does not spread to the ventricles because of a non-conducting tissue preventing impulses from reaching there so atria and ventricles do not conduct at the same time.

Question 72

What happens at the Atrioventricular Node (AVN)?

Answer 72

The electrical excitation wave travels from the SAN to the AVN, located at the junction between the atria and ventricles.
The AVN delays the wave slightly before it travels down the ventricles. This delay means that the ventricles recieve the signal to contract after the atria recieve the signal to empty blood into the ventricles; allowing the ventricles to fill completely with blood from the atria.
The AVN then transmits the impulse to the ventricles through the Bundle of His.

Question 73

What happens at the Purkyne Tissue?

Answer 73

After the AVN, the electrical excitation wave travels through the Bundle of His, which divides into the Purkyne tissues. These tissues spread the electrical excitation wave from the base of the ventricles and upwards through the ventricle walls causing them to contract.

The ventricles then relax. Then the muscles in the SAN contracts again, and the whole sequence runs through once more.

Question 74

How can heart activity be monitored?

Answer 74

Heart activity can be monitored by using an ECG, measuring pulse rate, or listening to the sounds of valves closing using a stethoscope.