Question

1-what is partial pressure? Explain it’s role in gas exchange?

2- Describe the factors affecting gas exchange. Apply these factors to the efficiency of a tracheal system, gills, lungs in mammals, lungs in birds?

Answer 1

Q1) Partial pressure is the pressure exerted by an individual gas in a mixture of gases.

Gas exchange is the movement of oxygen into the blood and movement of carbon dioxide out of the blood. Oxygen and carbon dioxide move across the respiratory membrane,by the help of alveolis and pulmonary capillaries.Gas exchange takes place in the lungs between alveoli and blood plasma and throughout the body and between plasma and interstitial fluids .

Q 2) There are certain factors which help in diffusion of O₂ and CO₂ in these sites, they are

i) Partial pressures and solubilities:-  Poor solubility can be offset by a high partial pressure (or vice versa).The partial pressure of O ₂ in the lungs is high (air is 21percent O ₂), but it has poor solubility properties.but in case of CO₂ it is just the opposite , the partial pressure of CO₂ in air is extremely low (air is only 0.04 percent CO₂), but its solubility in plasma is more, about 24 times than that of O_2.

ii) Partial pressure gradients:- A gradient is the change in some quantity from one region to another. According to Henry's Law, the greater the partial pressure of a gas, the greater is the diffusion of the gas into the liquid.

Diffusion of a gas into a liquid (or the reverse) takes place down a partial pressure gradient—that is, from a region of higher partial pressure to a region of lower partial pressure. For example, the strong partial pressure gradient for O₂ (pO ₂) from alveoli to deoxygenated blood (105 mm Hg in alveoli versus 40 mm Hg in blood) helps in rapid diffusion.

Surface area for gas exchange:- The larger expanding surface area of the lungs helps in extensive diffusion.

Diffusion distance:- This is the distance travelled by the gas, rate of diffusion increases when distance decreases. thin alveolar and capillary walls increases the rate of diffusion.

Tracheal Systems:-

In Insects respiration takes place by Tracheal system. it is not dependent on its circulatory system, so here blood do not play a direct role in oxygen transport. The tracheal system, consists of a network of small tubes which carries oxygen to the entire body. here gas passes directly to the required tissues, it is the most direct and efficient respiratory system to get oxygen for the respiratory sites. The tubes in the tracheal system are made up of a polymeric material called chitin.

Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, and they allowes oxygen to pass into the body and regulate the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system by spiracles.

Gills :-

In organisms which live in water they get oxygen from water. Oxygen found in dissolved condition in water has a lower concentration than the atmospheric oxygen. The atmosphere has around 21 percent of oxygen. But In water, the oxygen concentration is much smaller than that. Fish and many other aquatic organisms have developed gills (outgrowths of the body required for gas exchange) ,by this they take the dissolved oxygen from water. Gills are made up of thin tissue filaments which are highly branched and folded. When water passes over the gills, the dissolved oxygen in water quickly diffuses across the gills into the bloodstream. The circulatory system then carries the oxygenated blood to the other parts of the body. Due to constant flow of gas across the gas-exchange membrane and the constant partial pressure differences, gills are the most efficient respiratory system in exchanging gases. In animals having coelomic fluid instead of blood, there also oxygen diffuses across the gill surfaces into the coelomic fluid and gas exchange occurs.

The folded surfaces of the gills gives a large surface area to the fish to get sufficient oxygen. Diffusion is the process by which materials travel from regions of high concentration to low concentration till they reach equilibrium. Here, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuses from water (high concentration) to blood (low concentration). Like that carbon dioxide of the blood diffuses from the blood (high concentration) to water (low concentration).

Lungs in Mammals:-

In mammals, pulmonary ventilation takes palce by inhalation (breathing) taking air into the lungs. During inhalation, air enters the body by the nasal cavity. When air passes through the nasal cavity, it gets warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. After air crosses the surfaces of the mucous membranes, it takes water. This processes helps air to get equilised to the body conditions, and reduces the damage which cold,or dry air can make to the body. Particulate matteres which are floating in the air are also removed in the nasal passages by mucus and cilia.

The processes of warming, humidifying, and removing particles are important protective mechanisms to prevent damage to the trachea and lungs. From the nasal cavity, air passes through the pharynx (throat) and the larynx(voice box), and reaches trachea. The main function of the trachea is to take the inhaled air into the lungs and the exhaled air back out of the body. Trachea is made up of incomplete rings of hyaline cartilage and smooth muscle. The trachea is lined with mucus-producing goblet cells and ciliated epithelia. The cilia propels foreign particles trapped in the mucus to pharynx. The cartilage gives strength and support to the trachea to keep the passage open. The smooth muscle can contract, decreasing the trachea's diameter, by this the expired air rush upwards from the lungs at a great force. The forced exhalation helps expel mucus when we cough. Smooth muscles contract or relax, based on the stimuli from the external environment or nervous system.

In the lungs, air goes into smaller passages, or bronchi. Like the trachea, the bronchi are also made up of cartilage and smooth muscles. At the bronchioles, the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems which controls muscle contraction .

Bronchioles end in alveloies, many alveoli and alveolar sacs surround the alveolar ducts.Gas exchange takes place in alveoli.  Alveoli are in direct contact with capillaries of the circulatory system. This intimate contact helps oxygen to diffuse from alveoli into the blood and get distributed in the cells of the body, besides that the carbon dioxide produced by cells as a waste product also gets diffuses from the blood into alveoli to get exhaled.

within each alveolar sac there are many sacs at the end of each alveolar duct, lungs have a sponge-like structure. They creates a large surface area for gas exchange. This large surface area, and the thin-walled alveolar parenchymal cells, helps gases to get diffused easily across the cells.

Lungs in Birds:-

As Birds are the flying organisms they face a unique challenge related to breathing, when they fly, flying consumes lots of energy, as a result they require lots of oxygen for their metabolic processes. So in Birds a special respiratory system develops ,which supplies them the required oxygen for flying. Like mammals, birds also have lungs, oxygenated ai is taken in at the time of inhalation, and diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during exhalation.

Besides lungs, birds have air sacs in their body. When air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs, the flow of air is in the opposite direction from blood flow, and gas exchange takes place more efficiently. This type of breathing helps the birds to get the required oxygen, at higher altitudes. Where the oxygen concentration is low. This direction of airflow requires two cycles of air intake and exhalation to completely take out the air from the lungs.

These are the various types of gaseous exchanges seen in animals and birds.