biology module 3 exchange surfaces

why do single celled organisms have a high sa:v ratio

allows for the exchange of substances to occur via simple diffusion

what does the large surface area in single celled organisms allow for

maximum absorption of nutrients and gases and secretion of waste products

what does the small volume in single celled organisms allow for

the diffusion distance to all organelles is short

what happens to the sa:v ratio when an organism increases in size

their sa:v ratio decreases

why do organisms require a specialised system for gas exchange for the supply of oxygen

organisms require ATP in order to carry out the biochemical processes needed to survive, ATP is produced through aerobic respiration which requires oxygen

why do organisms require a specialised system for gas exchange for the removal of carbon dioxide

carbon dioxide is a waste product of aerobic respiration and if it accumulates in cells or tissues it alters the pH

what is the metabolic rate of an organism

the energy expended by that organism within a given period of time

how can the metabolic rate of an organism be measured

• oxygen consumption (respirometers)

• carbon dioxide production (carbon dioxide probe)

• heat generation (calorimeter)

how is the sa:v ratio of an organism related to its metabolic rate

• because of the relationship between sa:v and heat

• Heat is lost to the environment at the body's surface, so having a large body surface in relation to volume will allow more heat to be lost

• This means that:

  • small animals, with a higher SA:V ratio, will lose more heat to their surroundings, meaning that they need a relatively high metabolic rate to maintain body temperature

  • large animals, with a lower SA:V ratio, will lose less heat, meaning that they can maintain body temperature at a relatively low metabolic rate

Effective exchange surfaces in organisms have a:

Large surface area

Short diffusion distance (thin)

Good blood supply

Ventilation mechanism

root hair cells

specialised cells found in the roots of plants. They play an important role in the absorption of water and mineral ions from the soil

Root hair cells have a root hair that increases the surface area so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)

where does the exchange of oxygen and carbon dioxide occur in the lungs

between the alveoli and the capillaries in the lungs

short diffusion distance in the alveoli

The air in the alveoli contains a high concentration of oxygen. The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration

The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide. The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled

The walls of the alveoli are only one cell thick and these cells are flattened

This means that gases have a very short diffusion distance so gas exchange is quick and efficient

extensive capillary network in the alveoli

The walls of the capillaries are only one cell thick and these cells are flattened, keeping the diffusion distance for gases short

The constant flow of blood through the capillaries means that oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them

This maintains the concentration gradient necessary for gas exchange to occur

good blood supply in fish gills

In order for the diffusion of a substance across an exchange site to continue for a prolonged period of time, the concentration gradient must be maintained

An adequate blood supply helps to maintain a concentration gradient as it is continuously flowing, bringing substances that have just entered the blood away from the exchange site

Fish gills are adapted to directly extract oxygen from water as they have a large capillary network

The extensive capillary system that covers the gills ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system

The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary

Water continues to supply the blood with oxygen along the whole gill arch and ends with water with the lowest oxygen concentration adjacent to the most deoxygenated blood with continued diffusion occurring

Ventilation mechanism in mammalian lungs

• A ventilation mechanism also helps to maintain a concentration gradient across an exchange surface

• Ventilation (mass flow of gases) in the lungs helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood

• The movements involved in breathing causes the air in the alveoli to change. Breathing removes air with low amounts of oxygen and high amounts of carbon dioxide and replaces it with air that has high amounts of oxygen and low amounts of carbon dioxide

cartilage

Cartilage is a strong and flexible tissue found in various places around the body

One place is in rings along the trachea, called Tracheal rings

These rings help to support the trachea and ensure it stays open while allowing it to move and flex while we breathe

Ciliated epithelium

• Ciliated epithelium is a specialised tissue found along the trachea down to the bronchi

  • Each cell has small projections of cilia which sweep mucus, dust and bacteria upwards and away from the lungs and the epithelium itself

Goblet cells

• Goblet cells can be found scattered throughout the ciliated epithelium in the trachea

• They are mucus-producing cells that secrete viscous mucus which traps dust, bacteria and other microorganisms and prevents them from reaching the lungs

• The mucus is then swept along by the cilia of the ciliated epithelium upwards and is swallowed

• The mucus and any microorganisms will then be destroyed by the acid in the stomach

Squamous epithelium

The alveoli have a lining of thin and squamous epithelium, that allows for gas exchange

The squamous epithelium forms the structure of the alveolar wall and so is very thin and permeable for the easy diffusion of gases

smooth muscle

Smooth muscle can be found throughout the walls of the bronchi and bronchioles

It helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed

elastic fibres

Elastic fibres are present in all lung tissues. They are very important as they enable the lung to stretch and recoil. This ability to recoil is what makes expiration a passive process

capillaries

Each alveolus is surrounded by an extensive network of capillaries

Carbon dioxide diffuses out of the capillaries and into the alveoli to be exhaled, while oxygen diffuses the other way from alveoli and into the capillaries to be carried around the body

These capillaries have a diameter of around 3-4µm, which is only wide enough for one red blood cell to travel through at any one time

This ensures that there is sufficient time and opportunity for gas exchange to occur

Trachea

The trachea is the channel that allows air to travel to the lungs

C-shaped rings of cartilage ensure that this air channel remains open at all times

They are C-shaped to prevent any friction from rubbing with the oesophagus located close behind

The trachea is lined with ciliated epithelium

There is a substantial covering of mucus inside the trachea (produced by goblet cells and mucous glands) that helps to trap dust and bacteria to prevent them from entering the lungs

The wall of the trachea contains smooth muscle and elastic fibres

Bronchi

Bronchi have a similar structure to the trachea but they have thinner walls and a smaller diameter

The cartilage in the bronchi does not form a c-shape, but can form full rings, and can also form irregular blocks

bronchioles

Bronchioles are narrow self-supporting tubes with thin walls

They are not usually supported by cartilage, though a few bronchioles may contain some cartilage

A large number of bronchioles are present in the gas exchange system

Bronchioles are lined with ciliated epithelium in the same way as the trachea and bronchi, though the usually do not contain any goblet cells

Bronchioles vary in size and structure, getting smaller as they get closer to the alveoli

The larger bronchioles possess elastic fibres and smooth muscle that adjust the size of the airway to increase or decrease airflow

The smallest bronchioles do not have any smooth muscle but they do have elastic fibres

alveoli

Groups of alveoli are located at the ends of the bronchioles

The alveolar wall consists of a single layer of epithelium

Elastic fibres are located in the extracellular matrix

There is an extensive capillary network

A watery fluid lines the alveoli, facilitating the diffusion of gases

what does ventilation do

ensure that there is always a higher concentration of oxygen in the alveoli than in the blood

passage of air

1. Nose / mouth

2. Trachea (windpipe)

3. Bronchi

4. Bronchioles

5. Alveoli

process of breathing in (inhalation)

1. external intercostal muscles contract

2. ribcage moves up and out

3. diaphragm contracts and flattens

4. volume of thorax increases

5. pressure inside thorax decreases

6. air is drawn in

process of breathing out (exhalation)

1. external intercostal muscles relax

2. ribcage moves down and in

3. diaphragm relaxes and becomes dome shaped

4. volume of thorax decreases

5. pressure inside thorax increases

6. air is forced out

what is vital capacity

this is the maximum volume of air that can be breathed in or out in one breath

what is tidal volume

this is the volume of air that is breathed in or out during normal breathing (at rest)

what is breathing rate

the number of breaths taken in one minute

what is oxygen uptake

the volume of oxygen used up by someone in a given time

how can you take breathing measurements

using a spirometer

process of using a spirometer

The person (subject) being examined breathes in and out through the spirometer

Carbon dioxide is absorbed from the exhaled air by soda lime in order to stop the concentration of carbon dioxide in the re-breathed air from getting too high, as this can cause respiratory distress

As the subject breathes through the spirometer, a trace is drawn on a rotating drum of paper or a graph is formed digitally, which can be viewed on a computer

From this trace, the subject's vital capacity, tidal volume and breathing rate can all be calculated

how to calculate oxygen uptake using a spirometer

Carbon dioxide is removed from the exhaled air, meaning that the total volume of air available in the spirometer gradually decreases, as oxygen is extracted from it by the subject's breathing

This change in volume is used as a measure of oxygen uptake

what are spiracles

openings in the exoskeleton of an insect which allow air to flow into the internal system of tubes known as the tracheal system

what are tracheae

tubes within the insect respiratory system which lead to narrower tubes known as tracheoles

• Rigid rings of chitin keep the tracheae open

tracheoles in insects

carry oxygen into the muscle fibres of the insect, where gas exchange takes place

• The ends of the tracheoles are filled with tracheal fluid; gases can dissolve in this fluid before diffusing to the cells for gas exchange

  • The large number of tracheoles that are in contact with the muscle cells provides a large surface area for gas exchange

ventilation mechanism in insects

• When insects are at rest their energy requirements are low and diffusion alone is fast enough to supply oxygen to the cells

  • In this state insects may close their spiracles to reduce water loss by evaporation

• Active insects need a rapid supply of oxygen; this can be achieved as follows:

  • contracting and relaxing the muscles of the thorax and abdomen alters the volume, and therefore pressure, inside the tracheae, drawing air in and out

    • during flight the tracheal fluid at the narrow ends of the tracheoles is drawn into the respiring muscle; removing fluid from the tracheoles reduces the diffusion distance between the air and the muscle cells, speeding up diffusion

structure of gills in bony fish

Series of gills on each side of the head

Each gill arch is attached to two stacks of filaments

On the surface of each filament, there are rows of lamellae

The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

ventilation mechanism in fish

The ventilation mechanism in fish constantly pushes water over the surface of the gills and ensures they are constantly supplied with water rich in oxygen (maintaining the concentration gradient)

When the fish open their mouth they lower the floor of the buccal cavity. This causes the volume inside the buccal cavity to increase, which causes a decrease in pressure within the cavity

The pressure is higher outside the mouth of the fish and so water flows into the buccal cavity

The fish then raises the floor of the buccal cavity to close its mouth, increasing the pressure within the buccal cavity

Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure)

As water enters pressure begins to build up in the gill cavity and causes the operculum (a flap of tissue covering the gills) to be forced open and water to exit the fish

The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle