Gas exchange in animals

Providing for the cell’s needs

The cell theory states that all living organisms are made up of cells or substances produced by cells.

All cells need nutrients eg glucose and oxygen in order to function properly, be active and grow.

The cells use the nutrients and produce wastes eg carbon dioxide that must be removed as they can be toxic and damage the cells.

The process that provides energy for the activity of cells is cellular respiration. The formula for cellular respiration is:

glucose + oxygen carbon dioxide + water + ENERGYC6H12O6 + O2 CO2 + H2O + Energy (ATP + waste heat)

Why we need organ systemsOrganisms which are single-celled or very

small and thin (less than 1 mm) can exchange materials through their body surfaces.

Because most multi-cellular organisms are too large for exchange of nutrients and wastes to occur through their body surfaces, they need organs to allow this exchange. Most multi-cellular organisms have organs arranged in systems to do this:

• A transport system - called the circulatory system to bring these nutrients to the cells, and to carry away wastes,

• A system to allow exchange of oxygen and carbon dioxide - called the respiratory system

• A system to obtain nutrients - called the digestive system

• A system to deal with and remove wastes from the body - called the excretory system

Respiratory Surfaces• Gases are exchanged

with the environment at the respiratory surface.

• Gas movement is by diffusion.

• Gases are dissolved in water, so respiratory surfaces must be moist

• Respiratory surfaces are usually thin and have large areas as well as adaptations to facilitate the exchange.

• Different organisms have different adaptations, based on their size, body covering and habitat.

Gas exchange in aquatic animals

• Oxygen is already dissolved in the water

• There is no problem with drying out of surfaces, only in preventing physical damage

• Oxygen has low solubility in water

• The challenge is to provide sufficient surface area for exchange

Gills

Gills greatly increase the surface area for gas exchange. They occur in a variety of animal groups including arthropods (including some terrestrial crustaceans), annelids, fish, and amphibians. Gills typically are folded outgrowths containing blood vessels covered by a thin epithelial layer. Typically gills are organized into a series of plates and may be internal (as in crabs and fish) or external to the body (as in some amphibians).Gills are very efficient at removing oxygen from water: there is only 1/20 the amount of oxygen present in water as in the same volume of air. Water flows over gills in one direction while blood flows in the opposite direction through gill capillaries. This counter-current maximizes oxygen transfer.

Gas exchange in terrestrial animals

• Oxygen is not already dissolved in water so moist surfaces must be provided for it to dissolve

• Surfaces need to be protected from drying out, as well as preventing physical damage

• Most animals have internal surfaces for gas exchange, although smaller animals living in moist conditions eg amphibians, worms can exchange gases through their skin

• The challenge is to provide protection and sufficient surface area for exchange

Tracheal systems

Many terrestrial animals and some of their aquatic larvae have their respiratory surfaces inside the body and connected to the outside by a series of tubes.

Tracheae are these tubes that carry air directly to cells for gas exchange.

Spiracles are openings at the body surface that lead to tracheae that branch into smaller tubes known as tracheoles.

Body movements or contractions speed up the rate of diffusion of gases from tracheae into body cells.

However, tracheae will not function well in animals whose body is longer than 5 cm.

Lungs

Lungs are found inside of the body wall and connect to the outside by as series of tubes and small openings.

Lungs are found mainly in vertebrates, but some terrestrial snails have a gas exchange structures similar to those in frogs.

The human respiratory system1 = nasal conchae2 = nostril3. Hard palate4 = mouth5. Vocal cords6 = larynx

7 = trachea8 = tracheal cartilage/ring9 = oesophagus10 = epiglottis11 = pharynx12 = uvula

1 = larynx 8 = alveoli2 = tracheal cartilage 9 = pleura/pleural membranes3 = trachea 10 = ribs4 = lung 11 = pleural fluid5 = bronchus 12 = internal intercostal muscles6 = heart 13 = external intercostal muscles7 = diaphragm 14 = bronchiole

Special features• Nose Warms, moistens and filters air, allows sense of smell, allows

resonance of sounds (speech)and has the following special features:Hairs, cilia and mucus trap small particles, mucous membranes

produce mucus for filtering and moisten the air, large folds (conchae) increase the surface area, large blood supply (capillary network) helps warm the air

• Airways – Larynx, Trachea and Bronchi have cartilage to keep them open, airways have cilia and mucus to trap small particles, mucous membranes produce mucus for filtering and moistening air

• Lungs – are elastic – can stretch to let air in & relax to original size

• Muscles – diaphragm and intercostals pull air in and push them out

Breathing

During inspiration, the external intercostals contract, the diaphragm contracts down, the internal intercostals relax, the volume of the thorax increases, the pressure in the thorax decreases, and air moves in to the lungs.

During expiration, the external intercostals relax, the diaphragm relaxes and moves upwards, the internal intercostals contract, the volume of the thorax decreases, the pressure in the thorax increases, and air moves out of the lungs.

Diffusion of O2 and CO2

Exchange at the alveoli

Features of the lungs that help increase gas exchange include:

• large surface area due to lots of alveoli - this increases the amount of exchange that can occur

• large blood supply due to lots of capillaries - this brings lots of blood to be oxygenated

• thin membranes of alveoli so that diffusion can happen easily

• lungs positioned inside so that they can’t be damaged or dried out easily

• lung volume can be changed so that high oxygen and low carbon dioxide levels are maintained – this increases diffusion