Gas Exchange in humans

The ventilation system

Ventilation (V) refers to the flow of air into and out of the alveoli, while perfusion (Q) refers to the flow of blood to alveolar capillaries

Parts of the ventilation system

• trachea

• bronchi

• bronchioles

• alveoli

• lungs

• diaphram

Exchange surfaces must:

• permeability: be permeable to the substances that must pass across them.

• Composed of thin tissue layer: be thin, so that there is a short distance for exchange by diffusion or other means.

• Have a moist surface: be moist, so that materials can dissolve if necessary.

• Large surface area: have a large surface area so there is maximum area for exchange.

• Concentration gradient: have a means of maintaining a concentration gradient so that substances can flow down a gradient to where they are needed. 

  • for diffusion to happen there needs to be a difference in concentration of the gas between two areas

Exchange surfaces must (acronymn)

SMART

• Surface area

• Moist

• Absorptive

• Rich blood system

• Thin tissue layer
  

Concentration gradient

refers to the difference in concentration of a substance between two areas.

• The bigger the difference in concentration, the steeper the concentration gradient, and the faster the rate of diffusion

Adaptations in the exchange surfaces of animals to maintain steep concentration gradients include:

1. dense network of blood vessels

2. continuous blood flow

3. ventilation

Why? a dense network of blood vessels

• means that there is much opportunity for substances to be exchanged between the surface and the blood

Why? continuous blood flow

• ensures that, as soon as substances move into the blood, they are transported away by the continuous blood flow,

• ensuring a low concentration of that substance in the blood supply adjacent to the exchange surface

Why? ventilation

ensures the air or water rich in the desired gas is moved across the exchange surface.

• Mammals inhale air into the lungs, and exhale to remove air from the lungs.

Why? Large surface area

the surface where exchange/diffusion takes place has large surface area- example villi in intestines, gills in fishes, alveoli in lungs etc

Breathing vs respiration

breathing = ventilation

• movement of air in and out of the lungs (where gas exchange happens)

• inspiration = inhale, expiration = exhale

Cell respiration

• in the mitochondria of a cell

Alveoli and adaptations

• alveoli increase their surface area for gas exchange

• membranes are very tiny - both of alveoli and the capillaries - so the diffusion path is short

how many membranes must an oxygen molecule pass through to enter an erythrocyte?

5

type 2 vs type 1 pneumocyte (in the alveoli)

type 1

• single layer of cells (at walls)

• extremely thin

• permeable (where exchange happens)

type 2

• secrete fluid to moisten the inner surface of the alveolus

• can divide to form type I pneumocytes

How does gas exchange happen?

Gas exchange occurs via a process called diffusion

gas exchange and surface area to volume ratio

As organisms increase in size, surface area-to-volume ratio decreases and there is a greater need for specialised structures to facilitate efficient gas exchange.

• In such larger organisms, exchange across the body surface alone would not supply or remove gases efficiently.

alveoli characteristics

• permeability

• a moist surface for gases to diffuse

• a large surface area

• a short diffusion distance (thus thin wall)

As we exhale, what happens to the lung volume and lung pressure?

Lung volume decreases, pressure in the lungs increases.

• When you exhale, air leaves the lungs. Think of what happens when you squeeze a balloon.

Inhalation/exhalation and pressure

• Higher pressure in the lungs forces air out, lower pressure brings air in.

• Pressure increases when volume decreases, pressure decreases when volume increases.

What happens to lung volume and lung pressure for the process of inhaling to occur?

Lung volume increases, pressure in the lungs decreases.

Describe inspiration

• diaphram contracts

• abdominal muscles relax

• external intercostal muscles contract (ribcage up and out)

• internal intercostal muscles relax

• air pressure in thorax increases

• volume of thoratic cavity decreases so that air can enter the lungs

Describe expiration

• diaphram relaxes

• abdominal muscles contract - pressure from abdominal muscles pushes diaphram into dome shape

• external intercostal muscles relax

• internal intercostal muscles contract (ribcage down and in)

• air pressure in thorax decreases

• volume of thoratic cavity increases so air leaves the lungs

trachea and bronchi role

• The trachea and bronchi carry air to the alveoli in bronchioles in the lungs.

antagonistic muscles

• External and internal intercostal muscles

• diaphragm

• abdominal muscles

required for inspiration and expiration because muscles only work by contracting.