Gas exchange in humans

Question 1

What are the main features of gas exchange in humans?

Answer 1

1. Large surface areato allow faster diffusion of gases across the surface
2. Thin wallsto ensure diffusion distances remain short
3. Good ventilation with airso that diffusion gradients can be maintained
4. Good blood supplyto maintain a high concentration gradient so diffusion occurs faster

Question 2

How does large surface area increase the rate of gas exchange?

Answer 2

• This large surface area is provided by the numerous alveoli in the lungs and is critical for enhancing the capacity of gas exchange. More alveoli means more space for gases to be exchanged simultaneously, significantly increasing efficiency.
• In terms of individual alveolus, there is a large surface area in contact between capillaries and alveoli

Question 3

How does good ventilation increase the rate of gas exchange?

Answer 3

Ventilation, the process of air entering and exiting the lungs, is crucial in maintaining a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli. This keeps up concentration gradients for oxygen and carbon dioxide

Question 3

How does thin walls increase the rate of gas exchange?

Answer 3

• The alveoli are only one cell thick, therefore oxygen and carbon dioxide only have to travel a short distance between the alveolar air space and the blood in the capillaries. This minimises the time taken for diffusion, making the process quick and efficient.
• The capillaries are also one cell thick which means to get from the alveoli into the blood the oxygen only has to go through a distance of two cells
• Alveolar walls are also moist, to prevent the cells from drying out and to allow the gases to dissolve in the water on the alveolar walls. This reduces diffusion distance.

Question 4

How does good blood supply increase the rate of gas exchange?

Answer 4

• A dense network of capillaries surrounds each alveolus, providing a continuous flow of blood. This ensures that there is always blood ready to receive oxygen and offload carbon dioxide
• This reduces diffusion distance and maintains the high concentration gradient

Question 5

Function of the ribs

Answer 5

Bone structures that protect vital organs and blood vessels and expand and contract

Question 6

Function of the intercoastal muscles

Answer 6

Muscles between the ribs that control their movement causing inhalation and exhalation

Question 7

Function of the diaphragm

Answer 7

Sheet of connective tissue and muscle at the bottom of the thorax that helps change the volume of the thorax to allow inhalation and exhalation

Question 8

Function of the trachea

Answer 8

Windpipe that connects the mouth and nose to the lungs. It is surrounded by rings of cartilage. The function of the cartilage is to support the airwaysandkeep them openduring breathing. If they were not present then the sides could collapse inwards when the air pressure inside the tubes drops

Question 9

Function of the larynx

Answer 9

Also known as the voice box, when air passes through here we are able to make sounds. It functions as a lid to stop food going down the trachea when swallowing

Question 10

What are bronchi?

Answer 10

Large tubes branching off the trachea with one bronchus going to each lung

Question 11

What are bronchioles?

Answer 11

Bronchi split to form smaller tubes called bronchioles

Question 12

What are alveoli?

Answer 12

Tiny air sacs where gas exchange takes place

Question 13

Why are there two sets of intercoastal muscles?

Answer 13

Because muscles are only able to pull on bones, not push on them. Therefore, there must be two sets of intercostal muscles; one to pull the rib cage up and another set to pull it down

Question 14

What is the concentration of oxygen in the inspired air vs the expired air?

Answer 14

Inspired air: 21%
Expired air: 16%

Question 15

What is the reason that there is more oxygen in inspired air than expired air?

Answer 15

Oxygen is removed from the blood by respiring cells so blood returning to the lungs has a lower concentration than the air in the alveoli meaning oxygen diffuses into the blood in the lungs down the concentration gradient

Question 16

What is the concentration of carbon dioxide in the inspired air vs the expired air?

Answer 16

Inspired air: 0.04%
Expired air: 4%

Question 17

What is the reason that there is more carbon dioxide in expired air than inspired air?

Answer 17

Carbon dioxide is produced by respiration and diffuses into the blood from respiring cells; the blood transports the carbon dioxide to the lungs where it diffuses into the alveoli as it is in a higher concentration in the blood than in the alveoli

Question 18

What is the concentration of water vapour in the inspired air vs the expired air?

Answer 18

Inspired air: Lower
Expired air: Higher

Question 19

What is the reason that there is more water vapour in expired air than inspired air?

Answer 19

Water evaporates from the moist lining of the alveoli into the expired air as a result of the warmth of the body

Question 20

What is the concentration of nitrogen in the inspired air vs the expired air?

Answer 20

78% both

Question 21

What is the reason that there is the same amount of nitrogen in inspired air and expired air?

Answer 21

Nitrogen isn’t a product or reactant for respiration so it isn’t used by the body, this is why the concentration doesn’t change

Question 22

Experiment to investigate difference in inspired and expired air

Answer 22

The experiment uses limewater to test for carbon dioxide as limewater turns cloudy when carbon dioxide is present
- When we breathe in, the air is drawn through boiling tube A
- When we breathe out, the air is blown into boiling tube B
- Lime waterisclearbut becomescloudy(or milky) when carbon dioxide is bubbled through it
- The lime water inboiling tube A will remain clear, but the limewater inboiling tube B will become cloudy
- This shows us that thepercentage of carbon dioxide in exhaled air is higher than in inhaled air

Question 23

What happens during inhalation?

Answer 23

During inhalation the external set of intercostal muscles contract to pull the ribs up and out (this means the internal intercoastal muscles relax as they are antagonistic pairs). This increases the volume of the chest cavity (thorax), decreasing air pressure which creates a negative pressure which draws air in. Additionally, the diaphragm contractsmeaning it flattens and thisincreases the volumeof the chest cavity (thorax), which also leads to adecrease in air pressureinside the lungs relative to outside the body,meaning air enters the lungs to equilibrate the pressure

Question 24

What happens during exhalation?

Answer 24

During exhalation, the external set of intercostal muscles relax so the ribs drop down and in (this means the internal intercoastal muscles contract because they are antagonistic pairs). This decreases the volume of the chest cavity (thorax) increasing air pressure which creates a positive pressure, forcing air out to equilibrate pressure. Additionally the diaphragm relaxes andmoves upwards back into its domed shape and thisdecreases the volumeof the chest cavity (thorax), which consequently leads to anincrease in air pressureinside the lungs relative to outside the body,forcing air out

Question 25

What happens when we need to increase the rate of gas exchange?

Answer 25

When we need to increase the rate of gas exchange (for example during strenuous activity) the internal intercostal muscles will also work topull the ribs down and into decrease the volume of the thorax more, forcing air out more forcefully and quickly – this is calledforced exhalation. There is actually a greater need to rid the body of increased levels of carbon dioxide produced during strenuous activity and this allows agreater volume of gases to be exchanged

Question 26

What is respiration and breathing?

Answer 26

Respiration is a series of chemical reactions that releases energy from food
Breathing is the mechanism for moving air in and out of the lungs; this supports the chemical reactions of respiration

Question 27

Why does frequency and depth of breathing increase when exercising?

Answer 27

The brain detects increasing CO2 levels and sends signals to the lungs to increase breathing and the heart to pump faster. This is to maintain high concentrations of oxygen and low concentrations of carbon dioxide in alveoli, therefore maintaining the steep concentration gradient between blood and alveoli.

Question 28

How does the increased rate of breathing help the body during exercise?

Answer 28

• Increased rate of breathing means you can expel carbon dioxide from your body faster, making the carbon dioxide concentration gradient steeper. This helps carbon dioxide diffuse out of your blood more quickly. Increased depth of breathing means that you can breathe in a higher volume of oxygen per breath, increasing your oxygen concentration gradient. This allows more oxygen to diffuse into your blood.
• This is because muscles are working harder and aerobically respiring more and theyneed more oxygen to be delivered to them(and carbon dioxide removed) to keep up with the energy demand. If they cannot meet the energy demand they will alsorespire anaerobically, producinglactic acid

Question 29

What happens when exercise is finished?

Answer 29

After exercise has finished, the lactic acid that has built up in muscles needs to beremovedas itlowers the pH of cellsand candenature enzymescatalysing cell reactions. It can only be removed by combining it with oxygen - this is known as‘repaying the oxygen debt’. This can be tested by seeing how long it takes after exercise for the breathing rate and depth to return to normal -the longer it takes, the more lactic acid produced during exercise and the greater the oxygen debt that needs to be repaid

Question 30

How does the brain detect that it needs to increase breathing?

Answer 30

• Therate of respirationincreases in muscle cells when exercising heavily
• CO2is aproductof aerobic respiration, soCO2levels increasein the muscle cells. This CO2diffuses out of the cells into theblood plasma, CO2in solution causes a slight drop in pH so the blood becomes slightly more acidic
• The blood flows around the circulatory system and passes to the brain where the increased carbon dioxide levels are detected bychemoreceptors (cells that detect chemical changes in the body)in the brain

Question 31

What does the brain do during exercise to reduce CO2 levels?

Answer 31

• The brain sends nerve impulses to the diaphragm and the intercostal muscles toincrease therate and depth of muscle contraction. Therate of inspiration increases, along with the thevolume of airmoved in and out with each breath. A signal is also sent to the heart to pump faster so that oxygen and nutrients can be transported faster to body cells.
• The result isgreater absorption of oxygenandremoval rate of carbon dioxide

Question 32

How to investigate effects of physical activity on breathing?

Answer 32

• This can be investigated bycounting the breaths taken during one minute at restandmeasuring average chest expansion over 5 breathsusing a tape measure held around the chest
• Exercise for a set time (at least 3 minutes)
• Immediately after exercising,count the breaths taken in one minute and measure the average chest expansion over 5 breaths
• Following exercise, thenumber of breaths per minute will have increasedand thechest expansion will also have increased

Question 33

What do goblet cells do?

Answer 33

Goblet cells in the lung epithelium produce and secrete mucus

Question 34

What does mucus do to protect the respiratory system?

Answer 34

The mucus traps bacteria, viruses and any particles that enter our lungs. The mucus is pushed out of the lungs by cilia and coughing

Question 35

What does cilia do to protect the respiratory system?

Answer 35

Cilia are the small hairs on the surface of epithelial cells that beat to push mucus and other small trapped particles towards the nose and throat where it can be removed. Cilia doesn’t trap debris it only pushes it out