Topic 3A - Exchange and Transport Systems - complete

- the smaller an animal is, the higher the SA:V ratio
- for example, a mouse has a bigger surface area relative to its volume than a hippo

- e.g. the hippo could be represented by a block measuring 2cm x 4cm x 4xm
~ its volume would be 2 x 4 x 4 = 32cm³
~ its surface area would be 2(4x4) + 4(2x4) = 64cm²
~ therefore, the surface area:volume ratio is 64:32 = 2:1

- the mouse could be represented by a cube measuring 1cm x 1cm x 1cm
~ its volume is 1 x 1 x 1 = 1cm³
~ its surface area is 6 x 1 x 1 = 6cm²
~ so its SA:V ratio is 6:1

- an organism needs to supply every one of its cells with substances like glucose and oxygen (for respiration)
- is also needs to remove waste products from every cell to avoid damaging itself

- in single-celled organisms, these substances can diffuse directly into (or out of) the cell across the cell-surface membrane
- the diffusion rate is quick because of the small distances the substances have to travel

- in multicellular animals, diffusion across the outer membrane is too slow, for two reasons:
- some cells are deep within the body, there's a big distance between them and the outside evironment
- larger animals have a low surface area to volume ratio & its difficult to exchange enough substances to supply a large volume of animal through a relatively small outer surface 

- rather than using straightforward diffusion to absorb and excrete substances, multicellular animals need specialised exchange organs 

- they also need an efficient system to carry substances to and from their individual cells, this is mass transport
- in mammals, 'mass transport' normally refers to the circulatory system, which uses blood to carry glucose and oxygen around the body
- it also carries hormones, antibodies and waste like CO₂ 
- mass transport in plants involves the transport of water and soluted in the xylem and phloem

- as well as creating waste products that need to be transported away, the metabolic activity inside cells creates heat 
- staying at the right temperature is difficult, and its heavily influenced by the size and shape of the organism

Size
- the rate of heat loss from an organism depends on its surface area
- if an organism has a large volume, e.g. a hippo, its surface area is relatively small
- this makes it harder for it to lose heat from its body 
- if an organism is small, e.g. a mouse, its relative surface area is large, so heat is lost more easily
- this means smaller organisms need a relatively high metabolic rate, in order to generate enough heat to stay warm 

Shape
- animals with a compact shape have a small surface area relative to their volume (minimising heat loss from their surface)
- animals with a less compact shape (those that are a bit gangly or have sticky outy bits) have a larger surface area relative to their volume which increases heat loss from their surface
- whether an animal is compact or not depends on the temperature of its environment

- not all organisms have a body size or shape to suit their climate & some have other adaptations

- animals with a high SA:V ratio tend to lose more water as it evaporates from their surface
- some small desert mammals have kidney structure adaptatios so that they produce less urine to compensate

- to support their high metabolic rates, small mammals living in cold regions need to eat large amounts of high energy food such as seeds and nuts

- smaller mammals may have thick layers of fur or hibernate when the weather gets really cold 

- larger organisms living in hot regions find it hard to keep cool as their heat loss is relatively slow
- elephants have developed large flat ears to increase their surface area, allowing them to lose more meat
- hippos spend much of the day in water, a behavioural adaptation to help them lose heat 

- most gas exchange surfaces have two things in common
- they have a large surface area
- they're thin (often just one layer of epithelial cells) which provides a short diffusion pathway across the gas exchange surface
- the organism also maintains a steep concentration gradient of gases across the exchange surface

- single-celled organisms absorb and release gases by diffusion through their outer surface 
- they have a relatively large surface, a thin surface and a short diffusion pathway so there's no need for a gas exchange system

- there's a lower concentration of oxygen in water than in air so fish have special adaptations to get enough of it

- water, containing oxygen, enters the fish through its mouth and passes out through the gills
- each gill is made of lots of thin plates called gill filaments, which give a big surface area for exchange of gases 
- the gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more 
- the lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion
- blood flows through the lamellae in one direction and water flows over in the opposite direction
- this is called a counter-current system & it maintains a large concentration gradient between the water and the blood
- the concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood

- insects have microscopic air-filled pipes called tracheae which they use for gas exchange
- air moves into the tracheae through pores on the surface called spiracles
- oxygen travels down the concentration gradient towards the cells
- the tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells
- this means that oxygen diffuses directly into the respiring cells & the insect's circulatory system doesn't transport O₂ 
- carbon dioxide from the cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere
- insects use rhythmic abdominal movements to move air in and out of the spiracles 

- plants need CO₂ for photosynthesis, which produces O₂ as a waste gas
- they need O₂ for respiration, which produces CO₂ as a waste gas 

- the main gas exchange surface is the surface of the mesophyll cells in the leaf
- they're well adapted for their function as they have a large surface area

- the mesophyll cells are inside the leaf
- gases move in and out through special pores in the epidermis called stomata

- the stomata can open to allow exchange of gases, and close if the plant is losing too much water 
- guard cells control the opening and closing of stomata

- exchange gases tend to make you lose water meaning there's a sort of trade-off between the two
- luckily for plants and insects though, they've evolved adaptations to minimise water loss without reducing gas exchange too much

- if insects are losing too much water, they close their spiracles using muscles
- they also have a waterproof, waxy cuticle all over their body and tiny hairs are around their spiracles, both of which reduce evaporation

- plants' stomata are usually kept open during the day to allow gaseous exchange
- water enters the guard cells, making them turgid, which opens the stomatal pore
- if the plant starts to get dehydrated, the guard cells lose water and become flaccid, which closes the pore 

- some plants are specially adapted for life in warm, dry or windy habitats, where water loss is a problem
- these plants are called xerophytes

- stomata sunk in pits that trap moist air, reducing the concentration gradient of water between the lead and the air
- this reduces the amount of water diffusing out of the leaf and evaporating away 

- a layer of 'hairs' on the epidermis which trap moist air round the stomata

- curled leaves with the stomata inside, protecting them from wind as windy conditions increase the rate of diffusion and evaporation

- a reduced number of stomata, so there are fewer places for water to escape

- a waxy, waterproof cuticles on leaves and stems to reduce evaporation

- humans need to get oxygen into the blood (for respiration) and they need to get rid of carbon dioxide (by respiring cells)

- as you breathe in, air enters the trachea
- the tracheae splits into two bronchi with one bronchus leading to each lung
- each bronchus then branches off into smaller tubes called bronchioles
- the bronchioles end in small 'air sacs' called alveoli
- the ribcage, intercostal muscles and diaphragm all work together to move air in and out

- the external intercostal and diaphragm muscles contract

- this causes the ribcage to move upwards and outwards and the diaphragm to flatten, increasing the volume of the thoracic cavity (the space where the lungs are)

- as the volume of the thoracic cavity increases, the lung pressure decreases to below atmospheric pressure 

- air will always flow from an area of higher pressure to an area of lower pressure (i.e. down a pressure gradient) so air flows down the trachea and into the lungs

- inspiration is an active process meaning it requires energy

- the external intercostal and diaphragm muscles relax

- the ribcage moves downwards and inwards and the diaphragm becomes curved again

- the volume of the thoracic cavity decreases, causing the air pressure to increase (to above atmospheric pressure)

- air is forced down the pressure gradient and out of the lungs

- normal expiration is a passive process meaning it doesn't require energy

- expiration can be forced e.g. if you want to blow out candles on your birthday cake

- during forced expiration, the external intercostal muscles relax and internal intercostal muscles contract, pulling the ribcage further down and in
- during this time, the movement of the two sets of intercostal muscles is said tp be antagonistic (opposing)

- lungs contain millions of microscopic air sacs where gas exchange occurs called alveoli
- each alveolus is made from a single layer of thin, flat cells called alveolar epithelium

- there is a huge number of alveoli in the lungs, which means there's a big surface area for exchanging oxygen and carbon dioxide
- the alveoli are surrounded by a network of capillaries
- oxygen diffuses out of the alveoli, across the alveolar epithelium and the capillary endothelium (a type of epithelium that forms the capillary wall) and into haemoglobin in the blood
- carbon dioxide diffues into the alveoli from the blood, and is breated out

- oxygen from the air moves down the trachea, bronchi and bronchioles into the alveoli
- this movement happens down a pressure gradient
- once in the alveoli, the oxygen diffuses across the alveolar epithelium, then the capillary endothelium, ending up in the capillary itself
- this movement happens down a diffusion gradient

- alveoli have features that speed up the rate of diffusion so gases can be exchanged quickly 
- a thin exchange surface - the alveolar epithelium is only one cell thick which means there's a short diffusion pathway (which speeds up diffusion)
- a large surface area - the large number of alveoli means there's a large surface area for gas exchange

- there's also a steep concentration gradient of oxygen and carbon dioxide between the alveoli and the capillaries, which increases the rate of diffusion
- this is constantly maintained by the flow of blood and ventilation

- lung diseases affect both ventilation and gas exchange in the lungs
- doctors can carry out tests to investigate lung function and diagnose a lung disease

- tidal volume = the volume of air in each breath (usually between 0.4dm³ and 0.5dm³)
- ventilation rate = is the number of breaths per minute (about 15 for a healthy person)
- forced expiratory volume₁ (FEV₁) = the maximum volume of air that can be breathed out in 1 second
- forced vital capacity (FVC) = the maximum volume of air it is possible to breathe forcefully out of the lungs after a really deep breath in 

- you can figure out tidal volume, ventilation rate and other measures of breathing from the graph produced by a spirometer

- when someone becomes infected with TB bacteria, immune system cells build a wall around the bacteria in the lungs
- this forms small, hard lumps known as tubercles

- infected tissue within the tubercles dies and the gaseous exchange surface is damaged, so tidal volume is decreased

- TB also causes fibrosis, which further reduces the tidal volume

- a reduced tidal volume means less air can be inhaled with each breath
- in order to take in enough oxygen, patients have to breathe fastser, i.e. ventilation rate is increased 

- common symptoms include a persistent cough, coughing up blood and mucus, chest pains, shortness of breath and fatigue

- fibrosis is the formation of scar tissue in the lungs
- this can be the result of an infection or exposure to substances like asbestos or dust

- scar tissue is thicker and less elastic than normal lung tissue

- this means that the lungs are less able to expand and so can't hold as much air as nomal 
- tidal volume is reduced, and so if FVC as a smaller volume of air can be forcefully breathed out

- there's a reduction in the rate of gaseous exchange; diffusion is slower across a thicker scarred membrane

- symptoms of fibrosis include shortness of breath, a dry cough, chest pain, fatugue and weakness

- fibrosis sufferers have a faster ventilation rate than normal so they can get enough air into their lungs to oxygenate their blood

- asthma is a respiratory condition where the airways become inflamed and irritated
- the causes vary from case to case but it's usually because of an allergic reaction to substances such as pollen and dust

- during an asthma attack, the smooth muscle lining for the bronchioles contracts and a large amount of mucus is produced

- this causes constriction of the airways, making it difficult for the sufferer to breathe properly
- air flow in and out of the lungs is severely reduced, so less oxygen enters the alveoli and moves into the blood
- reduced air flow mans that FEV₁ is severely reduced (i.e. less air can be breathed out in 1 second)

- symptoms include wheezing, a tight chest and shortness of breath
- during an attach the symptoms come on very suddenly
- they can be relieved by drugs (often in inhalers) which cause the muscle in the bronchioles to relax, opening up the airways

- emphysema is a lung disease caused by smoking or long-term exposure to air pollution - foreign particles in the smoke (or air) become trapped in the alveoli

- this causes inflamation, which attracts phagocytes to the area
- the phagocytes produce an enzyme that breaks down elastin (a protein found in the walls of the alveoli

- elastin is elastic which means it helps the alveoli to return to their normal shape after inhaling and exhaling air

- loss of elastin means the alveoli can't recoil to expel air as well (it remains trapped in the alveoli)

- it also leads to destruction of the alveoli walls, which reduces the SA of the alveoli, so the rate of gaseous exchange decreases

- symptoms of emphysema include shortness of breath and wheeing
- people with emphysems have an increased ventilation rate and they try to increase the amount of air containing oxygen reaching their lungs

- TB, fibrosis, asthma and emphysema all reduce the rate of gas exchange in the alveoli
- less oxygen is able to diffuse into the bloodstream, the body cells receive less oxygen and the rate of aerobic respiration is reduced
- this means less energy is released and sufferers often feel tired and weak

- all diseases have factors that will increase a person's chance of getting that disease
- these are called risk factors
- for example, it's widely known that if you smoke you're more likely to get lung cancer (smoking is a risk factor for lung cancer)

- this is an example of correlation - a link between two things
- however, a correlation doesn't always mean that one thing causes the other
- smokers have an increased risk of getting cancer but that doesn't necessarily mean smoking causes the disease
- there are lots of factors to take into consideration

Describe the data
- the graph on the left shows that the number of adult males in GB who smoke decreased between 1990 and 2012
- the graph on the right shows that the male lung cancer mortality rate decreased between 1990 and 2012 in the UK

Draw Conclusions
- there's a correlation between the number of males who smokes and the mortality rate for male lung cancer
- however, one did not cause the over
- there could be other reasons for the trend
- for example, deaths due to lung cancer may have decreased because less asbestos was being used in homes (not because fewer people were smoking)

Other Points to Consider
- the graph on the right shows mortality rates
- the rate of cases of lung cancer may have been increasing but medical advances may mean more people were surviving

Responses to Data
- medical studies in 1950s and 1960s documented the link between smoking and various forms of cancer, particularly lung cancer
- the evidence prompted the first voluntary agreement between the UK government and tobacco companies in 1971, which stated that tobacco products and adverts should carry a health warning label
- as of october 2008, picture health warnings were made compulsory on all UK boxes of cigarettes after studies suggested they were more effective than written warnings alone

- the first graph shows the number of new cases of asthma per 100,000 of the population diagnosed in the UK from 1996 to 2000
- the second graph shows the emissions (in millions of tonnes) of sulfur dioxide (an air pollutant) from 1996 to 2000 in the UK

Describe the Data
- the top graph shows that the number of new cases of asthma in the UK fell between 1996 and 2000, from 87 to 62 per 100,000 people
- the bottom grapgh shows that the emissions of sulfur dioxide in the UK fell between 1996 and 2000, from 2 to 1.2 million tonnes

Conclusions
- there is a link between the number of new cases of asthma and emissions of sulfur dioxide in the UK
- the rate of new cases of asthma has fallen as sulfur dioxide emissions have fallen
- you can't say that one causes the other though because there could be other reasons for the trend
- for example, the number of new cases of asthma could be falling due to the decrease in the number of people smoking

Other Points to Consider
- the top grapgh shows new cases of asthma
- the rate of new cases may be decreasing but existing cases may be becoming more severe
- the emissions were for the whole of the UK but air pollution varies from area to area, e.g. cities tend to be more polluted
- the asthma data doesn't take into account any other factors that may increase the risk of developing asthma, e.g. allergies, smoking etc.

Responses to Data
- in response to studies connecting air pollution to various diseases, the EU adopted the National Emissions Ceilings Directive
- this set upper limits of the total emissions of four major pollutants in the atmosphere, to be achieved by 2010
- new limits are being agreed on for 2020
- the EU also introduced the Clean Power for Transport Package to promote cleaner fuels for vehicles, and the UK taxes car owners according to their car's emissions

- you should be wearing a lab coat & your tools should all be clean, sharp and free from rust as blunt tools don't cut well and can be dangerous
- lay the lungs on a cutting board, you should be able to see the trachea and two bronchi going into the lungs
- to see the lungs inflate, attach a piece of rubber tubing to the trachea and pump air into the lungs using a foot or bicyle pump
the lungs will deflate by themselves because of the elastin in the walls of the alveoli
- once you've seen the lungs inflate, you can examine the different tissue types in the lungs
- the trachea is supported by C-shaped rings of cartilage
- cartilage is tough, so if you want to open up the trachea, it's best to cut it lengthways, down the gap in the C-shaped rings
- use dissecting scissors or a scalpel to make the cut & if using the scalpel, cut down and don't apply too much pressure
- continue cutting down one of the bronchi & you should be able to see the bronchioles branching off
- cut off a piece of the lung & the tissue will feel spongy because of the air trapped in all the alveoli
- lungs from a butcher are safe for humans to handle, but they could still contain bacteria that cause food poisoning & that's why its important to wash your hands and clean work surfaces

- make sure you're wearing an apron or lab coat

- place your fish (like a perch or salmon) in a dissection tray or on a cutting board

- gills are located on either side of the fish's head
- they're protected on each side by a bony flap called an operculum and supported by gill arches

- to remove the gills, push back the operculum and use scissors to carefully remove the gills
- cut each gill arch through the bone at the top and bottom

- if you look closely, you should be able to see the gill filaments

- big insects like grasshoppers or cockroaches are usually best for dissecting because they're easier to handle
- for dissection, you need to use an insect that's been humanely killed fairly recently

- first fix the insect to a dissecting board
- you can put dissecting pins through its legs to hold it in place

- to examine the trachea, you'll need to carefully cut and remove a piece of exoskeleton from along the length of the insect's abdomen

- use a syringe to fill the abdomen with saline solution
- you should be able to see a network of very thin, silvery-grey tubes (the tracheae)
- they look silver because they're filled with air

- you can examine the tracheae under an optical microscope using a temporary mount slide
- again, the tracheae will appear silver or grey
- you should also be able to see rings of chitin in the walls of the tracheae which are there for support (like rings or cartilage in a human trachea)

- dissecting animals can give you a better understanding of their anatomy
- however, there are some ethical issues involved

- some people argue that it is morally wrong to kill animals just for dissections, as it is unnecessary killing
- however many dissections that are carried out in schools involve animals that have already been killed for their meat
- some people disagree with killing animals altogether though

- there are concerns that the animals used for dissections are not always raised in a humane way
- they may be subject to overcrowding, extremes of temperature or lack of food
- they may not be killed humanely either
- if animals are raised in school for dissection, it's important to make sure they are looked after properly and killed humanely to minimise any suffering or distress