AS Revision: F211, Exchange and Transport

F211 Biology M:2

Exchange and Transport

Good Exchange Surface Areas:

• Large surface area
• Partially permeable membrane
• Thin barrier for short diffusion pathway 
• Steep concentration gradient

LUNGS

Air passes through the nose into the trachea, bronchi and the bronchioles. Then into tiny air filled sacs called alveoli. 

Adaptions for exchange:

• Large surface area
• permeable plasma membranes
• thin barriers to reduce diffusion distance

Maintaining a diffusion gradient:

1. blood brings CO2 from the tissues to the lungs 
2. concentration of CO2 in the blood is higher than in the air of the alveoli.
3. oxygen is carried away from the lungs 
4. concentration of oxygen in the blood is lower than the contraction in the air inside the alveoli. 

Inhaling (Inspiration)	Exhaling (Expiration)
diaphragm contracts, flattens, digestive organs are then pushed down	diaphragm relaxes, and is pushed up by displaced organs below
external intercostal muscles rises up and contracts to raise ribs	external intercostal muscles relax and the ribs fall
Volume of chest cavity increases	Volume of chest cavity decreases
pressure drops below atmospheric pressure	pressure increases above atmospheric pressure
air moves into the lungs	air moves out of the lungs

TISSUES IN THE LUNGS

TISSUES IN THE LUNGS	FUNCTION
TRACHEA AND BRONCHI	bronchi is more narrow than the trachea much of the wall consists of cartilage  (C-rings in the trachea) inside cartilage: elastic fibres, smooth muscles, blood vessels etc.  inner epithelium layer: ciliated epithelium, has goblet cells
BRONCHIOLES	more narrow than the bronchi has some cartilage made up of mostly smooth muscle, and elastic fibres small bronchioles has clusters of alveoli on their ends
CARTILAGE	structural role supports the trachea and bronchi, keeping them open prevents collapse when air pressure is low during inhalation allows the oesophagus to expand during swallowing
SMOOTH MUSCLE	can contract smooth muscle contraction makes the lumen more narrow constricting lumen also constricts the flow of air
ELASTIC FIBRES	as the smooth muscle contracts the lumen diameter must decrease, and smooth muscle cannot reverse this effect,  when airway contracts, the elastic fibres are deformed smooth muscle relaxes the elastics fibres recoil.  helps to dilate the airways
GOBLET CELLS & GLANDULAR TISSUE	goblet cells and glandular tissue secrete mucus,  mucus traps small particles  trapping the bacteria reduces risks of infection
CILIATED EPITHELIUM	cells have tiny hair like structures cilia they way the mucus up to the back of the throat where it is then swallowed and killed by stomach acids

MEASURING LUNG CAPACITY

Tidal Volume: volume of air moved in and out during breathing whilst at rest.

Vital Capacity: largest volume of air that can be moved into and out of the lungs in any one breath. 

Residual Volume: volume of air that always remains in the lung, despite maximum possible exhalation

Dead Space: air between the bronchioles, bronchi and trachea, no gaseous exchange occurs here. 

Inspiratory reserve volume: how much more air can be breathed in over and above normal tidal volume

Expiratory reserve volume: how much more air can be breathed out, over and above the amount that is breathed in a tidal volume volume. 

Spirometer

• chamber filled with oxygen that floats on a tank of water
• person breaths through a disposable mouthpiece
• connected to the chamber of medical grade oxygen 
• breathing in, takes oxygen from the chamber, which then sinks down
• breathing out makes the chamber float up. 
• soda lime takes in all extra CO2

THE HEART

Right side of the heart: pumps deoxygenated blood to the lungs

Left side of the heart: pumps oxygenated blood to the rest of the body.

deoxygenated blood -> vena cava to the right atrium 

oxygenated blood -> pulmonary vein into the left atrium 

Cardiac Cycle

Ventricular systole -> diastole -> atrial systole

page. 57 -> graph, learn it and understand it

atrioventricular valves are found between the atria and the ventricles

semi lunar valves are found between the ventricles and the aorta and pulmonary artery

the sound of the heart is actually made by the sounds of the valves shutting. 

first ones are the atrioventricular valves, and the second ones are the semi lunar valves. 

Higher volumes -> lower pressure

Lower volume -> higher pressure. 

Control of the cardiac cycle:

SAN creates a wave of excitation at regular intervals, near the top of the right atrium =

the wave of excitation spreads across the walls of both atria along the membranes of the muscle tissue 

-> it causes cardiac muscles to contract and this is known as systole, base of the atria is a disc of tissue that cannot conduct the wave of excitation instead, at the inter ventricular septum is the AVN. 

The AVN is the only route through the disk of non conducting tissue -> wave of excitation is therefore delayed and this allows time for the atria to contract and for blood to flow down into the ventricles before they contract. 

Contraction of the ventricles: 

wave of excitation is carried away from the AVN and down the purkyne tissue, before running down the inter ventricular septum.

Wave of excitation then also moves out from the walls of the ventricles this means that the ventricles contract the base upwards. 

BLOOD VESSELS

ARTERIES

• carry blood away from the heart
• small lumen for high pressure
• thick wall, with collagen fibres to withstand pressure
• elastic tissue to allow of stretch and recoil 
• smooth muscle to allow for contractions and constriction
• constriction narrows the lumen
• endothelium can fold and unfold when the artery stretches

VEIN

• carry blood to the heart
• large lumen to ease blood flow
• thin layers of collagen, smooth muscle and elastic tissue. 
• no stretch and recoil, do not constrict to reduce blood flow.
• contain valves, so blood goes in one direction, and blood to go back to the heart

CAPILLARY

• thin walls
• allow exchange of materials between the blood and cells of tissues via the tissue fluid
• walls (single later of flattened endothelial cells - for short diffusion distance)
• narrow lumen, same diameter as a red blood cell, 7 µm
• narrow lumen -> so red blood cells are squeezed as they past the capillary walls and are more likely to give up their oxygen. 

Blood, Tissue Fluid and Lymph 

How does the fluid return back to the blood? 

it’s not only hydrostatic pressure acting upon the tissue fluid. 

there is also a water potential gradient. 

highest water potential value: 0kPa

Arteriole end:  has a high hydrostatic pressure and since the tissue fluid is less negative in terms of water potential, the water moves by osmosis into the blood. 

Venule end: blood has lost its hydrostatic pressure but combined with the osmotic force, it is sufficient enough to move the fluid back into the capillary. It carries with it any dissolved waste substances. 

Formation of Lymph

not all tissue fluid returns to the capillaries, some is drained into the lymphatic system -> 

• has loads of vessels similar to capillaries
• they start in tissues and drain the excess fluid into larger vessels
• these vessels join back into the bloody system
• similar to tissue fluid
• less oxygen and fewer nutrients
• more waste products and fatty materials
• also contains more lymphocytes (WBC)
• produced in the lymph nodes
• they filter bacteria and foreign material from the lymph fluid
• lymphocytes also engulf and destroy these bacterias

Carriage of Oxygen

Oxygen is transported in erythrocytes (RBC) which then contain the protein: haemoglobin. 

haemoglobin + oxygen ——————> oxyhaemoglobin

haemoglobin and oxygen transport:

release and take up of oxygen depends on the Pa, or partial pressure -> also called oxygen tension 

amount of oxygen in the surrounding tissues is not proportional to oxygen uptake by the haemoglobin. Instead there is a S-Shape, oxyhemoglobin disassociation curve. 

Low oxygen tension -> haemoglobin finds it hard to associate 

because the haem-group is in the centre of the haemoglobin molecule so at low partial pressure it is hard for the oxygen to reach the harm group and associate. 

Rise in oxygen tension -> diffusion gradient increases, and association takes place. 

when this association occurs, it alters the quaternary structure of the haemoglobin, so more oxygen diffuses in and associates with the other haem groups. 

once haemoglobin has bound to three of the oxygen molecules, it is hard for a fourth to associate so it is difficult to achieve 100% saturation. 

Fetal Haemoglobin

-> higher affinity for oxygen 

-> placenta has a lower Pa, so must hold on more,

Carriage of Carbon Dioxide

 CO² is transported in three ways:

1. 5% is transported in the blood plasma
2. 10% is transported when combined with haemoglobin to form carbaminohaemoglobin
3. 85% is transported in the form of hydrogen carbonate ions (HCO₃⁻)

How are Hydrogencarbonate ions formed?

Carbon dioxide diffuses into the blood, some enters red blood cells.

CO² + H²O -> H₂CO₃ 

(Catalysed by carbonic anhydrase)

H₂CO₃  -> HCO₃⁻ + H⁺

• Carbon acid disassociates to release hydrogen ions and hydrogen carbonate ions. 
• hydrogen carbonate diffuses out
• Cl⁻ diffuses in to maintain charge ->  chloride shift
• to avoid the red blood cells becoming acidic because of the Hydrogen ions, 
• haemoglobin takes up the H⁺ so that haemoglobinic acid is produced, 
• haemoglobin is a buffer

Bohr Shift 

Hydrogen ions are released from the disassociation of carbonic acid -> compete for space taken up by the oxygen. 

hydrogen ions displace the oxygen on the haemoglobin molecule. -> more O² is released. 

in tissues where there is more respiration -> more CO² available!

more hydrogen ions being produced, more oxygen released 

This is the Bohr Shift. 

TRANSPORT IN PLANTS 

vascular tissue: the transport system in plants which moves water. 

xylem: water and minerals travel upwards in xylem tissue 

phloem: sugars travel up or down in phloem tissue 

both are found in vascular bundles. 

	STRUCTURE	FUNCTION
XYLEM TISSUE	long cells,  thick walls impregnated with lignin which helps to waterproof the walls -> prevents collapse cells are dead -> end walls spiralled or rings in the xylem mean that the plant can remain flexible.  pits or bordered pits allows water to leave one vessel to an adjacent one.	carries water and minerals from roots to plants -> moves in one direction dead cells aligned from end to end in a column narrow tubes so water column doesn’t break easily and capillary action is effective.  pits in the lignified walls allows water to move sideways no end walls no cell contents no nucleus or cytoplasm
PHLOEM TISSUE	transports sugars from one part of the plant to another made up of sieve tube elements and companion cells  Sieve Tubes: contain little cytoplasm and no nucleus, lined end to end. contains cross walls, they also have sieve plates, have thin walls, sieve tube elements Companion Cells: small cells, large nucleus w/ dense cytoplasm, numerous mitochondria, produces ATP, also has plasmodesmata for communication and flow of minerals

PLANT CELLS & WATER 

Water potential: 0 (highest value)

Cells have a  negative water potential because they contained dissolved sugars and salts

Water move from less negative regions to more negative regions 

apoplast: space between the cell walls is filled with water and water cos move through these spaces .

symplast pathway: water enters the cytoplasm through the plasma membrane and passes through plasmodesmata. linked cytoplasm 

vacuolar: water is not confined to the cytoplasm of the cells but also through the vacuoles. 

MOVING UP THE STEM:

Root pressure 

the action of the endodermis moving minerals into the xylem  by active transport moves the water into the xylem by osmosis. It forces water into the xylem and pushes water up the xylem. can only push water up a few metres, 

Transpiration pull

water lost from transpiration must be replaced, water molecules are attracted to each other by cohesion, strong enough to hold the molecules in one long column. so as molecules are lost, they get pulled up -> transpiration stream, cohesion-tension theory states that relies on the plant making sure that the column of water remains unbroken. if it is broken it can move to another vessel via the pits. 

Capillary action 

same forces that hold water molecules together also attract the water molecules together this is called adhesion, these forces can also help to pull the water up the sides of the vessel.