Topic 3: Exchange: Human gas exchange

System of lungs and air passages. Oxygen enters and carbon dioxide leaves (the blood). Via gas exchange services called alveoli.

Lungs are located inside the chest in the thoracic cavity (thorax) protected by the rib cage

Why are the lungs inside the body?

• decreases water loss (the body would otherwise lose water and dry out)

• It is not dense enough, it will not support and protect the delicate structure of the lungs if they were situated outside the body

Ciliated epithelium tissue is located throughout most airways

1. Goblet cells- produce and secrete mucus that traps dust and microbes

2. Cilia on epithelial cells- what the mucus upwards to the mouth so it can be swallowed

Trachea

Rings of cartilage, goblet cells, cilia. Release mucus, help to trap pathogens and dust particles.

Bronchus

Are two main branches extending from the trachea that carry air into each lung

Both the trachea and bronchus

1. Rings of large cartilage to keep airway open

2. Smooth muscle contract or relaxed to constrict or dilate the airway and change airflow

3. Elastic tissue contains elastic fibres with elastic that allows stretching and recoiling

4. Line with ciliated epithelial cells and goblet cells

Bronchioles

Smaller area is branching from the bronchi that carry air to the alveoli.

1. No cartilage, so can change shape

2. Smooth muscle can contract or relaxed to constrict/dilate airway and change airflow

3. Elastic tissue contains elastic fibres with elastin that allows stretching and recoiling

4. Simple squamous epithelium (only larger bronchioles have ciliated epithelium)

Alveoli

Minute air sacks, between the alveoli are collagen fibres that are elastic and help to stretch

Mechanisms of breathing (allows it enter and leave the lungs, providing the body with oxygen and removing carbon dioxide)

• ventilation (consist of inspiration and expiration) when air is constantly moved in and out of the lungs – maintains diffusion of gases across alveolar epithelium

• Inspiration (inhalation) – when air pressure of the atmosphere is greater than air pressure inside the lungs

• Expiration (exhalation) – air pressure in the lungs is greater than that of the atmosphere

Muscles involved in ventilation

Ribs enclose the thorax. Muscles attached to the rib cage contract and relax, they move the ribs to change the volume of the thoracic cavity. This affects the pressure in the lungs and controls ventilation.

Diaphragm

Doom sheet muscle below belongs. Contracts and goes flat. This is a sheet of muscle that moves the rib cage up and out when it contracts.

Internal intercostal muscles

Found between the ribs but pull the rib cage down and in when they contract → expiration

External intercostal muscles

Found between the ribs and pulled the rib cage up and out when they contract → inspiration

Inhaling

1. Internal intercostal muscles relaxed and external intercostal muscles contracted

2. Diaphragm muscle is contracted (flat)

3. The lungs are inflated (pressure decreases below atmospheric). Volume of thoracic cavity increases.

4. Air flows into the lungs down pressure gradient

Exhaling

1. Internal intercostal muscles contracted an external intercostal muscles relaxed

2. Diaphragm muscle is relaxed and domed

3. The lungs deflate (pressure increases above atmospheric). Volume of thoracic cavity decreases.

4. Air is forced out of the lungs down the pressure gradient

Gas exchange in the alveoli

• Have good supply of blood

• Around 300 million in the lungs

• Air breathed in is rich in oxygen

• Blood moving past has low oxygen content due to body cells using for respiration therefore concentration gradient → simple diffusion between two layers of cells, walls of alveoli and walls of capillary

• Oxygen molecules that eventually picked up by red blood cells

• Red blood cells are slowed as they passed through pulmonary capillaries

• The distance between the Alveo light and red blood cells is reduced as the red blood cells are flattened against the capillary wall

Oxygen diffusers from the alveoli into the pulmonary capillaries where it binds to haemoglobin in red blood cells

Carbon dioxide disassociates from Heber globin and diffusers from the blood into the alveoli

Surfactant

• Alveoli are minute, Bubble-like air sacs lined with moisture

• They are liable to collapse due to surface tension – if their sides touch, they could stick together

• Alveolar epithelium secretes a surfactant (mixture of phospholipids) which reduces the surface tension and keeps the alveoli open

• Unborn babies do not start to secrete these until 22 weeks of pregnancy

Structure of alveoli

• Each alveolus is lined with a single layer of flattened epithelial cells

• Around each alveolus is a network of pulmonary capillaries lined with a single layer of endothelial cells

• The capillaries are narrow so that red blood cells are flattened and squeezed through one at a time

Adaptations of alveoli for gas exchange

1. Wall consists of one layer of squamous epithelial cells – allows rapid diffusion

2. Large surface area – this increases rate of gas exchange

3. Partially permeable – this means that only certain gases can move across the wall

4. Surrounded by dense network of capillaries – brings blood Close for gas exchange

5. Ventilation of air – this maintains steep diffusion gradient

6. Elastic fibres – allow stretching and recoiling

7. Collagen fibres – these contain strong collagen that prevents alveoli from bursting and limits overstretching

8. Moist inner surface – this allows gases to dissolve, and lung surfactant helps alveoli remain inflated

The pulmonary wrestles are those involved in circulation of the lungs

1. The pulmonary artery – this delivers deoxygenated blood from the heart to pulmonary capillaries

2. The pulmonary vein – this delivers oxygenated blood from capillaries to heart

3. The pulmonary capillaries – these are the site of gas exchange between blood and alveoli

Adaptations of the pulmonary capillaries for gas exchange

1. Thin walls (one endothelial cell thick) – this maintains a short diffusion distance

2. Red blood cells pressed against capillary walls – this reduces diffusion distance

3. Large surface area – this increases diffusion speed

4. Movement of blood – this maintains steep diffusion gradient

5. Slow blood movement – this allows more time for diffusion