special surfaces for exchange

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.

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).

ACHIVED BY VENTILATION which ;

• ventilation ensures replacement of used air with fresh air MORE oxygen in the lungs
• high concentration of carbon dioxide in blood however low in the alveoli
• low concentration of oxygen in the blood - high concetration of oxygen in alveoli
• removes carbon dioxide

• 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.

• air can pass into the lungs through the nose along the trachea, bronchi and bronchioles.
• each part of the airway is adapted to its function of allowing passage of air
• finally air reaches tiny, air filled sacs called alveoli
• the wall of the alveoli are the surface where the exchange of gases takes place
• lungs protected by the ribs movements of the ribs together with the action of the diaphragm help produce breathing movements ventilation

gases pass both ways through thin walls of the alveoli oxygen passes from the air in the alveoli to the blood in the capillaries. Carbon dioxide passes from the blood to the air in the alveoli

Large surface area

• large surface area alveoli are very small but there are so many the total surface area is much larger than that of our skin the total surface area is 70m2 about half the size of a tennis court

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

The trachea, bronchi and bronchioles all are airways for the passage of air. for them to be effective passages they all must;

• 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

• 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. some people may have an allergic rection with substances in the air, and their bronchioles constrict makking it diffucult to breathe this is one of the causes of asthma

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

Ciliated 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

Inspiration

• Diaphragm contracts to become flatter and pushes the digestive organs down
• External intercostal muscles contract to raise ribs the volume of the chest cavity increases
• pressure in the chest cavity drops below atmospheric pressure
• air moves into the lungs

Expiration

• Diaphragm relaxes, its pushes up by displaced by organs underneath
• external intercostal muscles relax and ribs fall
• volume of chest cavity decreases
• pressure in the lungs increases and rises above atmospheric pressure
• air moves out of the lungs

Spirometer is used to measure the volume of air that moves in and out of the lungs

How air passes into the lungs and the contracting muscles

• pressure changes in the lungs cause air flow. Air flows from an area of high pressure to an area of low pressure.
• the volume of air change within the lungs causes a pressure change so flow will occur.
• For air to flow into the lungs the pressure inside the lungs must fall below atmospheric pressure
• the main muscles invovled in breathing are the diaphragm, intercostal muscles (internal and external).
• when the diaphragm is at rest pressure inside pushes the abdominal contents upwards causing the diaphragm to dome up into the thorax. Then the diaphragm contracts it pulls downwards increasing the volume inside the lungs hence decreasing the pressure. Causes the air to flow into the lungs down the pressure gradient.
• movement of the diaphragm pushes the abdominal contents down.This is why during quiet breathing you see the abdominal moving more than the chest.

Tidal volume is the volume of air breathed in and out during a single breath.

Vital capacity is the maximum volume of the air that can be breathed out of the lungs

Residual volume is the air that always remains in the lungs, even after the biggest possible exhalation (about 1.5dm3).

Dead space is the air in the bronchioles, bronchi and trachea. There is no gas exchange between this air and the blood.

Inspiratory reserve volume is how much more air can be breathed in (inspired) over and above the normal tidal volume when you take in a big breath. you call this reserve when excercising.

Expiratory reserve volume is how much more air can be breathed out (expired) over and above the amount that is breathed in a tidal volume breath.