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Gas Exchange and Circulation. Diversity amongst humans, fish and insects. Gas Exchange.

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gas exchange and circulation

Gas Exchange and Circulation

Diversity amongst humans, fish and insects.

gas exchange
Gas Exchange

Cells need energy in order to continue living. Sugars and other substances that are broken down metabolically in cellular respiration release energy into the cells. During this process gases need to be exchange between the environment and the cells. CO2 and O2 are the gases exchanged in most organisms with gas exchange surfaces in multi-cellular organisms providing a way for gases to come into and exit the body. Some organisms use their whole body surface as an exchange system while many organisms have developed special gas exchange structures such as lungs or gills.

lung structure
Lung Structure

The nasal passage warms and moistens the air that travels into the body through the nostrils with the nostril hairs filtering out unwanted particles.

The air that enters the body through the mouth mixes with the air from the nasal passages in the pharynx.

In front of the oesophagus lies the trachea which extends into the thorax. C shaped bands of cartilage strengthen it with a layer of ciliated epithelium.

The trachea splits into two Bronchi which are also supported by cartilage bands.

Bronchioles stem off from the Bronchi and continue to divide into smaller and smaller branches. Less and less cartilage is used on each new branch.

lung structure continued
Lung structure continued

Terminal Bronchiole structure

Lungs: Are internal sac-like organs which play a major role in respiration and are where gas exchange occurs. They are found in most amphibians and all reptiles, birds and mammals.

Ciliated epithelium: Ciliated, mucus secreting epithelium lines the trachea, bronchi and Bronchioles trapping and removing dust and pathogens before they reach the gas exchange surfaces.

Terminal Bronchiole: the Bronchiole end in the terminal Bronchiole which have 2-11 alveolar ducts attached to them which in turn have many alveoli attached to them. This creates an extremely large surface area (70m²) for gas exchange to take place.

gas exchange in humans
Gas Exchange In Humans

In the human lung gas exchange occurs by diffusion between the alveoli and the blood in the capillaries. Gases move freely across the respiratory membrane which is a junction between the alveolar cells and the cells of the capillary. Elastic connective tissue gives the alveoli their ability to expand and recoil. There is an estimated 300 million alveoli which have a huge surface area for gas exchange to take place; 70m². This gas exchange of oxygen and carbon-dioxide results in the reoxygenation of the blood coming from the heart (by the pulmonary artery). This oxygenated blood returns to the heart by the pulmonary veins and is then distributed around the rest of the body.

Respiratory membrane.

gas exchange in fish
Gas Exchange in Fish

Fish have to obtain the oxygen they need from water using gills. Gills are membranous structures supported by cartilaginous or bony struts. The gills have a big surface and as the water flows over this the respiratory gases are exchanged between the blood and the water. There is a far less percentage of oxygen in water than there is in the air. Air is 21% oxygen while the same volume of water is 1% dissolved oxygen. An active aquatic organism such as a fish need to extract high rates of oxygen from the water in order to survive. This is achieved by pumping water across the gills or swimming continuously with the mouth open. The gills of a fish have lots of folds which are supported and kept apart by the water. This creates a high surface area for gas exchange. Gas exchange occurs by diffusion between the water and the blood vessels inside the gill across the gill membrane and capillaries. The operculum also known as the gill cover allows for the exit of water and acts as a pump drawing water past the gill filaments. Fish gills are highly efficient and achieve an oxygen extraction rate of 80% which is three times our own from air.

gas exchange in insects
Gas Exchange in Insects

Tracheal Systems

Insects in general are small terrestrial animals, so they have a large surface areato volume ratio. In terrestrial arthropods tracheal systems are the most common type of gas exchange organs.

Most body segments of the insect have spiracles (max 20) down their sides (the entrance/exits of the gas exchange system). Filtering devices prevent small particles found in the air from clogging the respiration system and a valve controls the degree to which a spiracle is open. Large, active insects such as locusts tracheal systems include air sacs which compress and expand like a bellows to aid the movement of air through the tubules.

The air containing oxygen moves through the spiracles down the tracheal tubes that branch off into the tracheoles. The gases in the air move by diffusion across the moist lining directly to and from tissues. The end of the tubes contain small amounts of fluid where the respiratory gases are dissolved. The fluid gets drawn into the muscle tissue during contraction and is released back into the tracheole when the muscle rests. Insects make rhythmic body movements to help move air in and out of the tracheae.

the circulatory system
The Circulatory System

Large complex organisms require circulatory systems to transport materials around their bodies as diffusion would be too slow and inefficient and would inconsistently supply the cells in the body with the materials required e.g. oxygen. The main parts of the circulatory system are blood, a heart and blood vessels this system transports nutrients, oxygen, carbon dioxide, wastes and hormones. The circulatory system also maintains fluid balance, regulates body temperature and can aid in the body’s resistance to harmful microorganisms.

circulatory systems
Circulatory Systems

There are two basic types of circulatory systems that have evolved in animals; open circulatory systems and closed circulatory systems. Many invertebrates such the arthropods have open circulatory systems which are not only used as transport systems but are important to hydraulic movements of the whole body. Humans and other vertebrates have closed circulatory systems also known as cardiovascular systems because they contain a heart and a network of tube-like vessels. Closed circulatory systems can be can be single circuit systems or double circuit systems.

open circulation system
Open Circulation system

Arthropods and most molluscs except for squid and octopus all have open circulatory systems. These organisms all have tube or sac-like hearts. The heart pumps blood through short vessels into the sizable spaces in the body cavity. The organisms cells are then soaked in the blood before the blood reenters the heart through holes called ostia. The circulation of the blood is aided by muscle movements.

Insects have open circulatory systems

Ostium are holes for the uptake of blood

The tubular heart is stretched across the top (dorsal surface) of the animal and circulating fluids get pumped towards the head.

One way valves in the heart ensure the blood moves in a forward circulation.

Body fluids flow freely within the body cavity.

closed single circuit systems
Closed, Single Circuit Systems

Blood in closed circulation systems are held in blood vessels and the blood is returned to the heart after every circulation of the body. The fluids which bath the cells exchange substances with the blood by diffusion across capillaries. In fish the blood goes directly from the gills to the body with the blood losing pressure at the gills and flowing at low pressure around the body.

Oxygen moves into the blood

Deoxygenated blood

oxygenated blood

Oxygen moves into the tissues

Fish have closed single circuit systems

closed double circuit systems
Closed, Double Circuit Systems

All vertebrates besides fish have closed double circuit systems. The blood gets pumped through a pulmonary circuit to the lungs where it is oxygenated the blood then returns to the heart which pumps the oxygenated blood through a systematic circuit through the body. In amphibians and most reptiles there is some mixing of oxygenated and deoxygenated blood in the heart as it is not completely divided. In birds and mammals there is no mixing.

Deoxygenated blood

oxygenated blood

the heart
The Heart

The heart is the centre of the cardiovascular system. Insects have a tubular heart (see the open circulatory system slide), fish have linear hearts and humans and mammals have fully partitioned hearts.

the human heart
The human Heart

The human heart is a hollow muscular organ, it beats over 100 000 times a day to pump 3780 litres of blood through our 100 000km of blood vessels. The heart is made up of four muscular chambers which alternate between filling and emptying of blood and act as a double pump. The left side pumps oxygenated blood to the body tissues while the right pumps deoxygenated blood to the lungs. The upper chambers are the atria (right and left) and the lower chambers are the ventricles (right and left). Both upper and lower chambers are separated by a partition or septum. Coronary arteries branch off from the aorta and provide circulation for the heart muscle itself.

the fish heart
The Fish Heart

Fish have linear hearts, which are made up of a sequence of three chambers in a series. (in some cases the conus/bulbus arteriosus may be included as a fourth chamber.) Blood from the fishes’ body enters the heart through its sinus venosus, then passes into the atrium and the ventricle. A series of one way valves situated between the chambers prevents blood from flowing in reverse.


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Helpful books: Year 12 Biology Biozone 2008