GASEOUS EXCHANGE AND RESPIRATION

GASEOUS EXCHANGE AND RESPIRATION

Gaseous exchange

Gaseous exchange is the movement of oxygen and carbon dioxide across a respiratory surface. Unicellular organisms carry out gaseous exchange by diffusion across the cell membrane. Large organisms cannot carry out diffusion efficiently so they have developed specialized organs for gaseous exchange. These are called respiratory surfaces.

Table below shows examples of respiratory surfaces in various organisms. Respiratory surfaces in various organisms       

Organism	Respiratory surface
Amoeba	Cell membrane
Insects	Tracheal system
Spider	Book lung
Fish	Gills
Plants	Leaves, stems, roots
Amphibians	Skin, gills and lungs
mammals	Lungs
Birds	Lungs
Reptiles	Lungs

Characteristics of respiratory surfaces

1.  They are thin to reduce the diffusion distance.

2. They are moist to dissolve gases so that they diffuse in solution form.

3. They are highly branched, folded or flattened in order to increase the surface area for gaseous exchange,

4. They are close to an efficient transport and exchange system so that gases can be taken to and from the cells easily.

5. They are well ventilated so that gases can pass through them easily

The adaptations and functions of parts of the mammalian respiratory system

Part	Adaptive features	Functions
Nose and nasal cavity	Mucus lining and hairs (cilia)	Trap dust and microorganisms
Glottis	Presence of epiglottis	Closes the trachea during swallowing to prevent food from entering the respiratory system
Trachea, bronchus and bronchioles	Blood vessels near the surface	Warm the air
	Have rings of cartilage tissue along their length	Prevent collapse of the respiratory tract
	Mucus lining and cilia	Trap and filter dust and microorganisms
Lungs	Spongy with air spaces (alveoli)	Main organ of mammalian gaseous exchange Airspaces hold inhaled air
Alveoli (singular: alveolus)	Numerous in number	Provide large surface area for gaseous exchange
	Thin membranes	Reduce distance for diffusion of gases
	Moist surface	Enables gases to dissolve into solutions before diffusing
	Has dense network of capillaries	Transport oxygen from the alveoli to the tissues and carbon dioxide from the tissues to the alveoli
	Constantly contain air	Maintain shape to avoid collapsing
Pleural membrane	Contain pleural fluid	Lubricates the membranes so that the lungs can slide smoothly over the thoracic cavity during breathing
Ribs	Are made of hard bone tissue	Protect the lungs from injury
Intercostal muscles	Move antagonistically: when one muscle contracts the other relaxes and vice versa	Allow expansion and contraction of the thoracic cavity
Diaphragm,	Muscular sheet of tissue	Separates the thorax from the abdomen. Allows for gaseous exchange by becoming dome-shaped or flattens.

The mechanism of gaseous exchange in mammals

Gaseous exchange in mammals happens as a result of inhalation (or inspiration) and exhalation (or expiration). Inhalation is breathing in air into the lungs. Exhalation is breathing out air from the lungs

During inhalation the muscles of the diaphragm Contract, pulling the diaphragm downwards; As this happens, the external intercostal muscles contract and pull the ribcage upwards and outwards. The result of these movements is an increase in the volume and a decrease in the air pressure of the thorax. This makes air rush into the lungs through the nostrils, trachea and bronchioles.

During exhalation, the muscles of the diaphragm relax and the diaphragm resumes its dome shape. The external intercostal muscles relax, pulling the ribcage inwards and downwards. As a result, the volume of the thorax decreases and the pressure inside it increases. This forces air out through the bronchioles, trachea and nostrils

	Breathing   in (inhalation)		Breathing out (exhalation)
	External intercostal muscles contract		The external intercostal muscles relax
	Internal intercostal muscles relax		The internal intercostal muscle contract
	The ribcage is lifted outward and upward		The ribcage move inward and downward
	The diaphragm contracts and flattens		The diaphragm relaxes and become dome-shaped
5	The volume of thoracic cavity increase as pressure decrease This allows air to enter the thoracic cavity	5	The volume of thoracic cavity decrease as pressure increase
6	Air enter the alveoli through the nostrils, pharynx, glottis, trachea, bronchioles and finally alveoli	6	Air leaves the alveoli through the bronchioles, trachea, glottis, pharynx and finally nostrils

   

        

Gaseous exchange across the alveolus

The actual exchange of oxygen and carbon dioxide takes place in the alveoli. One mammalian lung has millions of alveoli. The alveoli are surrounded by network of capillaries.    

            
 

                            Gases exchange across alveolus

When we breathe in, air accumulates in the alveoli. There is a higher concentration of oxygen in the air in the alveoli than in the bloodstream.

Therefore, oxygen diffuses out the alveoli into the blood in the capillaries. It combines with haemoglobin to form oxyhaemoglobin            

The oxygen is then transported to the tissues. Once in the tissues, the oxyhaemoglobin breaks down to release oxygen and haemoglobin. The tissues use released oxygen and release carbon dioxide.

This causes the levels of carbon dioxide to become higher in the tissues than in the blood. Carbon dioxide therefore diffuses into the blood in the capillaries and combines with haemoglobin to form carbaminohaemoglobin. The capillaries transport carbon dioxide in this form to the alveoli.

The concentration of carbon dioxide is higher in lie blood in the capillaries than in the air in the

alveoli. Carbon dioxide therefore diffuses from the Capillaries into the alveoli. It is then transported through the bronchioles, trachea, glottis, pharynx and finally nostrils into the atmosphere

       Composition of inspired and expired air

gas	Inspired air	Expired air
Oxygen	20.95%	16.40%
Carbon dioxide	0.03%	4.00%

Factors affecting the rate of gaseous exchange

      1. Concentration of carbon dioxide

High concentration of carbon dioxide in the blood increases the rate of gaseous exchange. This provides the tissues with adequate amounts of oxygen and lower carbon dioxide concentration in the blood.

       2. Concentration of haemoglobin

Haemoglobin is responsible for the transportation of gases from the lungs to the tissues and back. Efficient transportation of gases takes place when the body has adequate amounts of haemoglobin.

When a person is anaemic, the body has a low concentration of haemoglobin. Only small amounts of oxygen can be transported at a time. As a result, the rate of gaseous exchange has to increase so that the tissues get adequate amounts of oxygen.

       3. Physical activity

A more active body requires more oxygen than a less active body. As a result, gaseous exchange takes place faster when there is increased body activity.

        4. Health status of the body

Generally, the rate of gaseous exchange increases when somebody is sick. This is as a result of increased metabolism by the liver in order to remove the toxins released by disease-causing microorganisms or break down the drugs taken. Certain diseases also make the body weak and cause slowing down of the breathing process.

       5.  Altitude

Altitude is the height above sea level. At high altitudes, the concentration of oxygen is lower compared to low altitudes. Breathing is therefore faster at high altitudes. At high altitudes, there is also decreased atmospheric pressure. This makes breathing difficult. Organisms therefore have to breathe in faster in order to get enough oxygen.

        6. Age

Young people are generally more active than old people. Also, a lot of growth processes take place in the bodies of young people. This increases the demand for oxygen and therefore increases the breathing rate.

Gaseous exchange in plants

In plants, gaseous exchange mostly takes place through the stomata on the leaves and lenticels on the stem. Some plants such as mangrove and ficus also carry out gaseous exchange through breathing roots.

Gaseous exchange in the leaves

Atmospheric air moves into and out of the leaf through the stomata. Gaseous exchange mostly takes place in the air spaces in the spongy mesophyll.         

     

    

During the day, guard cells that surround the stomata absorb water by osmosis.As a result, the cell sap of guard cells becomes hypertonic and draws in water from the neighbouring cells by osmosis.

The guard cells become turgid and the stomata open. Air from the atmosphere enters into the air spaces in the spongy mesophyll. The cells next to the air spaces have more oxygen (produced by the cells during photosynthesis) but less carbon dioxide (used up during photosynthesis).

On the other hand, carbon dioxide is more in the air within the air spaces but oxygen is less. Carbondioxide and oxygen diffuse in opposite directions depending on their concentration gradients. The carbon dioxide diffuses to neighbouring cells until it reaches the site for photosynthesis. Oxygen moves out through the open stomata into the atmosphere.

At night, there is no light, therefore photosynthesis ceases. No glucose is produced therefore the guard cells do not absorb water by osmosis. Hence, the stomata remain partially closed.

However, respiration takes place in plants at night. The partially open stomata allow in small amount of air which accumulate in the air spaces. There is more oxygen and less carbon dioxide in the air spaces compared to the plant cells.

Oxygen moves into the plant cells while carbon dioxide moves into the air spaces and eventually into the atmosphere through the partially open stomata. This explains why plants produce carbon dioxide at night and oxygen during the day.

Gaseous exchange through the lenticels

Lenticels made up of loosely packed cork cells located on the bark of woody stems and roots. They are small pores through which gaseous exchange occurs.

      
            

                    Gaseous exchange in the lenticels

The loose arrangement of the cells facilitates the movement of gases between them. The cells have a thin layer of moisture so that gases diffuse in and out while in solution form

At night, there is a higher concentration of oxygen in the air spaces between the cork cells than in the ells themselves. Oxygen therefore diffuses into the cells surrounding the lenticels. The cells use oxygen far respiration and release carbon dioxide in the process. Thus, the concentration of carbon dioxide in the cells becomes higher than in the air spaces. Carbon dioxide therefore diffuses out through the cells into the air spaces and then out through the lenticel. The opposite happens during the day.

Gaseous exchange through the roots

This occurs through breathing roots. Plants with breathing roots have a very thin epidermal layer which enables the root to carry out gaseous exchange.

            

                           Breathing roots