3.1.1 Exchange surfaces

All of exchange surfaces - revision

Need for exchange

Allows for diffusion across plasma membranes for materials such as oxygen, glucose and carbon dioxide and to remove excretory materials such as urea and heat with their environment

High SA:V ratio

Organisms with a high surface area to volume ratio have a larger surface area relative to their volume which means substances diffuse across a shorter distance through plasma membranes faster - smaller organisms

Low SA:V ratio

Organisms with a lower surface area to volume ratio have a smaller surface area relative to their volume so substances have to diffuse across a larger distance through plasma membranes so diffusion is slower - larger organisms

Calculating SA:V ratio

calculate surface area l x w x 6

calculate volume l x w x d

( for a cube )

Why do multicellular organisms require specialised exchange surfaces ?

• cells are not in direct contact with their external environment

• diffusion distances between cells and their environment are larger

• larger organisms have higher metabolic rates so they need more oxygen and glucose

How do single celled organisms exchange ?

diffusion occurs across the cell membrane directly from their environment

In single-celled organisms, substances diffuse directly across the cell membrane.

What are exchange surfaces ?

Specialised structures which allow for materials to be transported between cells and the surrounding environment

4 Key features of exchange surfaces

• large surface area

• thin cell walls

• extensive / rich blood supply and/or ventilation

• surrounded by selectively permeable membranes

purpose of large surface area

larger area in which substances can be exchanged

purpose of thin cell walls

minimise diffusion distance

purpose of extensive blood supply and/or ventilation

maintains a steep concentration gradient

purpose of selectively permeable membranes

controls what substances are exchanged

Why is the gas exchange system located inside the body ?

• air is not dense enough to support these delicate structures

• body would lose water and dry out

Pathway of air

• Air (containing oxygen) enters the nose / mouth

• Air travels down the trachea

• Air travels into the bronchi (both) and branches into each bronchus (one)

• Air travels into the bronchioles which lead to tiny air sacs - alveoli

Gas exchange

• Surrounding the alveoli is an extensive network of capillaries

• Oxygen travels from inside the alveoli into erythrocytes (RBC’s) containg haemoglobin

• Oxygen is carried by erythrocytes to body cells where it is used for respiration which produces carbon dioxide as a waste product into the blood

• From the blood in the capillaries carbon dioxide is exchanged into the alveoli and into the bronchioles, bronchus, trachea and then outside from the mouth/nose into the air

What is ciliated epithelium ?

tissue located throughout most of the airways

Structure of ciliated epithelium

• Ciliated epithelial cells

• Goblet cells

function of goblet cells

Secrete mucus which traps dust and microbes

function of cilia on ciliated epithelial cells

waft mucus up toward the mouth so it can be swallowed

Trachea - Structure and adaptations (4)

• Rings of cartilage to prevent airway from collapsing (keeps it open)

• Smooth muscle can constrict or relax to constrict or dilate the airway and control air flow

• Elastic tissue containing elastic fibres which allows for stretching and recoiling

• Lined with ciliated epithelial cells and goblet cells

Bronchi - Structure and adaptations(4)

Same as trachea

• Reinforced with to keep airways open

• Smooth muscle. can constrict or relax to constrict and dilate the airway to control air flow

• Elastic tissue with elastic fibres which allows for stretching and recoiling

• Lined with ciliated epithelial cells and goblet cells

Bronchioles - Structure and adaptations (4)

• NO cartilage, can change shape

• Smooth muscle constricts and relaxes to constrict and dilate airways to control air flow

• Elastic tissue with elastic fibres which allows for stretching and recoiling

• Simple squamous epithelium (only larger bronchioles have a ciliated epithelium)

How do the alveoli carry out gas exchange?

(brief)

• Oxygen diffuses from the alveoli into the pulmonary capillary and and binds to haemoglobin in erythrocytes

• Carbon dioxide disassociates from haemoglobin and diffuses from the blood into the alveoli

Adaptations of alveoli for gas exchange (8)

• Wall consists of squamous epithelial cells

• Large surface area

• Dense network of capillaries

• Ventilation of air

• Collagen fibres

• Elastic fibres

• Partially permeable membrane

• Moist inner surface

Purpose : wall consists of squamous epithelial cells

Allows for rapid diffusion

Purpose : Large surface area

Increases rate of gas exchange

Purpose : Dense network of capillaries

Brings blood closer to air for more efficient gas exchange

Purpose : Ventilation of air

Maintains a steep concentration gradient

Purpose : Collagen fibres

Strong collagen prevents alveoli from bursting and limits overstretching

Purpose : Elastic fibres

Allow stretching and recoiling

Purpose : Moist inner surface

Allows for gases to dissolve and lung surfactant helps alveoli remain inflated

Pulmonary blood vessels (3)

• Pulmonary arteries

• Pulmonary veins

• Pulmonary capillaries

function of pulmonary artery

delivers deoxygenated blood away from the heart into the pulmonary capillaries

function of pulmonary vein

delivers deoxygenated blood from capillaries to the heart

function of pulmonary capillary

site of gas exchange between alveoli and blood

Adaptations of the pulmonary capillaries for gas exchange (5)

• thin walls

• erythrocytes pressed against capillary walls

• large surface area

• movement of blood

• slow blood movement

Purpose : thin walls

shorter diffusion distance

Purpose : erythrocytes against capillary walls

reduces diffusion distance

Purpose : large surface area

increases diffusion speed

Purpose : movement of blood

maintains steep diffusion gradient

Purpose : slow blood movement

allows more time for diffusion

What happens during inspiration ?

1. The external intercostal muscles contract while the internal intercostal muscles relax, moving the ribcage up and out.

2. The volume of the thoracic cavity increases.

3. The diaphragm contracts and flattens, further increasing the volume of the thoracic cavity.

4. The lung pressure decreases below atmospheric pressure.

5. Air flows into the lungs down the pressure gradient.