This week I thought I’d investigate different respiratory systems, and the problems that can occur with them, in two different species in particular after a topic I covered in biology. I wanted to see the differences between mammals and birds.
All mammals have the same respiratory system but adapted to their size and shape. The equation for respiration is:
oxygen + glucose -> carbon dioxide + water (+ATP)
Diffusion is when molecules move from an area of high concentration to an area of low concentration. The experiment below shows a container separated with a semipermeable membrane, dividing an area with a high concentration of a molecule (red dots) from an area with lower concentration. The membrane allows the molecules to move from one side to the other. Over time, the molecules will move from the area of high concentration to the area of low concentration. They do this until the overall net movement is equal. It reaches equilibrium.
This happens during respiration – oxygen and carbon dioxide are often highly concentrated on opposite sides of a cell membrane. Diffusion allows gas exchange to occurIn animals with a closed circulatory system (both chickens and sheep) – gas exchange takes place across capillaries.
Unlike humans who have two lungs – the left divided into two and the right into three lobes, sheep have highly segmented lungs consisting of two lobes in the left lung and four lobes in the right lung with the bronchus of the right cranial lobe coming directly from the trachea.
Sheep airways are similar to humans’ in terms of where epithelial cell and mast cells (type of white blood cell) are found. The sheep lungs have a significant population of pulmonary intravascular macrophages that are important in engulfing foreign particles and pathogens.
Inside the nasal cavity of mammals are ciliated epithelial cells, which are used to waft any bacteria and unwanted foreign bodies down the throat to the stomach. The stomach acid and digestive enzymes then kills them. There are also goblet cells, which are used to produce mucus. This catches the unwanted microbes; the moist and warm conditions make it even stickier to ensure the microbes do not enter and grow in the body.
Sheep are obligate nasal breathers, meaning they mostly breathe through their nose but do possess the ability to occasionally breathe through their mouths. It has been suggested that obligate nasal breathing is an adaptation very useful in prey species, as it allows an animal to keep eating, especially as sheep are grazing animals so preserve their ability to detect predators by scent. Humans however, breathe via either mouth or nose. The advantage of breathing through the nose is there is less chance of an infection as there are no ciliated epithelial cells or goblet cells, making mucus, in the mouth for protection.
In mammals, the trachea splits off into two bronchi, which then break off further to form bronchioles. The bronchioles have alveoli attached to the end of them which is where gas exchange occurs. There are many adaptations of the alveoli that allow it to exchange gases efficiently. These include:
- Lining of the lung and capillaries only one cell thick – so gases only have to diffuse over a short distance
- Large surface area – lots of oxygen can diffuse into blood stream at once
- Good blood supply – oxygen is carried away to the cells as soon as it has diffused
- Liquid surfactant – gases must diffuse in liquid to be able to diffuse into the blood
- Phagocytic white blood cells – engulfs bacteria to help protect the body
- High concentration gradient – gases can diffuse in and out of the alveoli quickly due to the concentration gradient (blood has high concentration of CO2 produced by respiration but not much O2; lungs have high concentration of O2 but not much CO2
Ventilation movements also maintain the concentration gradients because air is regularly moving in and out of the lungs.
- The diaphragm muscles contract and the diaphragm flattens and moves down
- The intercostal muscles contract, which makes the ribs move up and out
- The volume of the thorax increases. As the volume increases the pressure decreases
- Air flows in down a pressure gradient
- Oxygen diffuses into alveoli along the concentration gradient. In the alveoli, oxygen dissolves in liquid, which then diffuses the short distance into the blood capillaries
- The diaphragm muscles and the intercostal muscles relax
- Lungs return to normal size (recoils)
- Ribs move in and down and the diaphragm moves upwards
- Volume of the thorax decreases and the pressure increases
- Air moves out of the lungs and out of the body down a pressure gradient
The openings to the nasal cavity (nares) are on the top beak. Within the cavity are:
- Squamous epithelium – a single layer of flattened cells
- Ciliated epithelium – cube shaped cells with cilia or hairs that trap foreign bodies (dust/bacteria)
- Olfactory membrane – gives the sense of smell
The epithelium of the nasal cavity is covered in mucosal glands, which produce mucus that helps to keep foreign material from entering the body through the respiratory system.
Oropharynx (mouth and pharynx)
The oropharynx consists of the mouth and the pharynx (the cavity behind the nose and mouth, connecting them to the oesophagus). Behind the base of the tongue is the rima glottidis (opening into the larynx). There are no vocal cords, epiglottis or thyroid cartilages that are normally found in mammals.
Held open permanently by 108 to 125 cartilaginous rings. The trachea is lined with mucociliary epithelium where the hair-like cilia move foreign materials, such as dust, up and out of the trachea. Numerous mucous secreting glands are also found in the tracheal lining.
The trachea divides at the syrinx (found at the end of the trachea) into the left and right bronchi. The cartilaginous rings of the bronchi extend from the syrinx to where the bronchi enter the lungs. Ciliated epithelium with numerous mucous glands lines the bronchi. Some are lined with squamous epithelium.
Leading off from the bronchi in the lungs are a large number of extremely small air capillaries that are interlocked with the capillaries of the lung circulatory system. These interlocked capillaries are the lungs’ gas exchange system and are very thin. The cell layers that separate the two systems (the blood circulatory system and the air supply system of the lungs) are:
- Single cell epithelial wall of the air capillary
- Base membrane one cell thick
- Single cell epithelial wall of the blood capillary
Lungs and Air Sacs
Fowls do not actually have a diaphragm unlike the respiratory system in mammals. The chicken lung is also much denser than a mammals’. Unlike other species, the avian lung has very little elasticity.
The air sacs are very thin walled coming off of the bronchi (NOT FOUND IN THE LUNGS LIKE MAMMALS!). Some of these sacs also connect to many of the larger long bones to form the pneumatic bones. These make the bones lighter and are an advantage for flight.
It is very difficult to see the air sacs in a bird because they are so thin and transparent. It is believed the walls are lined with simple squamous epithelium although there is some ciliated epithelium in some areas. There are very few blood vessels present which indicates that these sacs play no part in gas exchange. The air sacs provide a very high volume of air for use by the lungs on inspiration and expiration. Therefore the lungs, with help from the air sac system, supply the oxygen necessary to fly. The air sac system permits the bird to change its center of gravity, which also aids flight.
Inspiration, Expiration and Movement of the Body Wall
Respiration is the result of movement of the body external wall that either causes an increase or decrease in the volume of the cavity – this is because, as I mentioned earlier, fowls do not have a diaphragm. These changes in volume cause changes in pressure inside of the lungs and this causes air to either enter or leave the system.
- Ribs are drawn forward
- Sternum pulled downwards
- Volume increases
- Pressure decreases
- Size of the cavity increases crosswise and from front to back
The increase in size and volume lowers the pressure in the lungs and air sacs and results in air from outside being drawn in to equalise the pressure. Inspiration occurs as the air moves into the lungs and air sacs.
- Muscles contract
- Ribs and sternum return to their original position
- Volume of the cavity reduces
- Air pressure in the system increases
This decrease in size and volume increases the pressure in the lungs and air sacs and results in air from inside being pushed out to equalise the pressure. Expiration occurs as the air moves out of the lungs and air sacs.
Chickens actually require two respiratory cycles!
(a flow diagram I made to try to understand how air travels through the respiratory system)
|Both have air sacs||Chicken’s air sacs are not inside the lungs, a sheep’s are|
|Air is drawn in and forced out of the lung due to change in pressure – by either movement of the body wall or by contraction of intercostal muscles||There are very few blood vessels in air sacs so probably aren’t used in gas exchange like in sheep|
|They are both obligate nasal breathers but can breathe through their mouths and tend to do so when stressed||There are no vocal cords, epiglottis or thyroid cartilages in a chicken like those that are found in sheep|
|Both produce mucus to catch bacteria and other foreign bodies – for protection||Chickens don’t have a diaphragm, sheep do|
|Both have single cell layers in capillaries to allow for quick diffusion||Chickens have very little elasticity in lungs like in sheep|
|Both have tracheas lined with epithelial cells||A chicken lung is also much denser than a sheep’s|
|Both have closed circulatory systems||Respiration in birds requires two respiratory cycles to move the air through the entire respiratory system. In mammals, only one respiratory cycle is necessary|
|When comparing birds and mammals of similar weight, birds have a slower respiratory rate|
|The respiratory system of birds is more efficient than that of mammals, transferring more oxygen with each breath|
|Holding a bird “too tight” can easily cause the bird to suffocate, with sheep this is harder to do|
Respiratory Diseases in Sheep and Chickens
|Gapeworm||Caused by the worm “Syngamus trachea” -the adult worms live in the chicken’s tracheaChickens acquire the disease by eating infected earthworms/snails||Coughing, head- shaking , ‘gaping’ mouth appearance||Wormer such as Flubenvet or Solubenol|
|Infectious Laryngotracheitis||Herpesvirus causes significant inflammation of the upper airways, and can have a high mortality in chicken flocks||Gasping, coughing of mucus and blood, drop in egg production,sinusitis||No definitive treatment (can only try to manage any secondary problems, such as secondary bacterial infections)|
|Infectious Bronchitis||Caused by a highly infectious virus (coronavirus) Mortality rate is higher in young birdsThere are vaccines available which are given to chicks||Depression, gasping, coughing, tracheal rales (crackling), nasal discharge, drop in egg production/ producing soft-shelled eggsWhen the kidneys are affected – increased water intake and scouring||No treatment (but if a secondary bacterial infection is suspected then antibiotics may help)|
I found the similarities and differences between the two animals very interesting and will find out more about different animal families and their respiratory systems. The diseases were also interesting to so I could learn more about why each species is more susceptible to certain diseases than the other. If anyone has seen any respiratory diseases perhaps on work experience or out practising, I would love to know more about what you saw and how it was resolved! I once saw a horse with Chronic Obstructive Pulmonary Disease (COPD)!