Biology F211

-single cell organisms gaseous exchange occurs via diffusion or osmosis.

Cells constantly have to;
Replace substances which are used up e.g food and oxygen
Remove other substances e.g co2 and waste products

-These substances can pass in and out by DIFFUSION & OSMOSIS

Diffusion- net movement of a particle across a partially permeable membrane. From an area of high concentration to a low concentration.

 Small Organisms (Single Celled) - Large surface area to volume

LUNGS DIFFUSION GRADIENT
Maintaining high concentration of molecules entering (on the supply side) and the other side has a low concentration of molecules leaving (on the demand side).

-Blood brings carbon dioxide from the tissues to the lungs so conc high on supply side of exchange surfaces low on demand side to maintain a steep diffusion gradient.

-This is achieved by the action of the blood transport and the ventilation movements.
-Blood brings co2 from the tissue to the lungs - conc of co2 higher in the blood than the air in alveoli.
-Blood carries o2 away from aveoli so conc on o2 is lower in the blood
-The heart pumps blood the blood along the pulmonary artery to the lungs in the lungs, artery divides up to form finer and finer vessels
-These evetually carry blood into tiny cappillaries which lets 1 red blood cell squeeze through
these cappillaries lie over the surface of the alveoli

• The breathing movements of the lungs ventilate the lungs, they replace used air with fresh air.
• This brings more o2 into the lungs and ensures the conc of o2 in the air of the alveolus remains higher than the conc in the blood.
• Ventilation also removes air containing co2 from the alveoli thus ensures the conc of co2 in the alveoli remains lower than in the blood

• This constant supply of gas to one side of the gaseous exchange surface and its removal from one side ensures that diffusion, and therefore ensures exchange can continue.

• Large surface area
• Large surface area alveoli 
• A barrier permeable to oxygen and carbon dioxide the plasma membrane that surround the thin layer of cytoplasm of the cells form the barrier to exchange. These readily allow the diffusion of oxygen and carbon dioxide
• For a efficent gaseous exchange surface organisms must have the following;

    1. LARGE SURFACE AREA

    2. CONCENTRATION GRADIENT 

              3.THIN SURFACES

              4. MOIST SURFACE

• Thin barrier to reduce diffusion distance

• The alevolus 1 cell thick
• Capillary wall 1 cell thick
• Both walls consist of squamous cells- this means flattened or very thin cell
• Capillaries are in close contact with the alveolus walls
• Capillaries are so narrow that red blood cells are squeezed against the capillary wall, making them closer to the air in the alveoli and reducing the rate at which they flow past in the blood
• The total barrier to diffusion is only two flattened cells thick and is less than 1 nanometre thick

For the trachea, bronchi and bronchioles effective "air passages" they should be;

• Large airways - must be large enough to allow sufficient air to pass through without obstruction
• Must divide into smaller air ways to get the oxygen to the alveoli
• The airways must be strong enough to withstand high pressure
• Must be flexible to allow movement
• Must be able to stretch and recoil

• The trachea and bronchi are the same but differ in size as the trachea has thicker walls with several layers of tissue
• Much of the wall has cartilage
• The trachea has many incomplete c rings whereas the bronchi is less regular
• On the inside surface of the cartilage is a layer of glandular tissue, connective tissue, elastic fibres, smooth muscle and blood vessels.

• Inner layer the epithelium layer has two types of cell which is cilliated epithelium and goblet cells.
• The bronchioles are narrower than the bronchi, larger bronchioles have some cartilade but smaller ones dont. The walls are mostly of smooth muscle and elastic fibres. smallest bronchioles have clusters of alveoli (air sacs) at their ends

• Smooth muscle can contract and when it contracts it constricts the airway
  this makes the lumen of the airway narrower

• Effect of smooth muscle is most obvious in the bronchioles the constricting of the lumen can restrict the flow of air to and from the alveoli

• Controlling the flow of air is important as there might be harmful substances in the air.
• Contraction of smooth muscle and control of air flow is not a voluntary act. 

Elastic Fibres:

• When the airway contricts due to the smooth muscle contracting the elastic fibres deforms
• Elastic fibres then recoil when the smooth muscle relaxes, back to its orginal shape
• This helps to dilate (widen) the airway

Goblet cells and glandular tissue:

• Goblet cells and glandular tissues uner the epithelium secrete mucus
• Mucus traps tiny particles from the air these particles maybe bacteria or pollen.This will reduce the risk of infection

Cilliated Epithelium:

• Epithelium consists of ciliated cells. these cells have many hair like structures projecting from their membrane these are Cilia
• Cilia move in a synchronised pattern to waft mucus up the the air way to the back of the throat. once there the mucus is swallowed, and the acidity kills the bacteria