Chicken Lungs

Anyone who knows me knows of my hate-hate relationship with running.  I have started running in the past year, and have decided that it is the most ridiculous activity known to man.  You can’t score goals, you can’t look cool, and you will NEVER make it to Sportcenter (Usain Bolt excluded….I mean….he IS Usain Bolt).

The main reason I hate running is because I suck at it.  I’m strong, but my aerobic capacity is lousy.  I wish I was a bird.  If I was a bird, my trachea (windpipe) would be 2.7 times as large, reducing air resistance.  My rate of breathing would be about 1/3 of what it is currently, and I would take much bigger breaths.  This is the first part of the system that makes the bird respiratory system much more efficient at gas exchange than mammals (especially this particular mammal).

Location of Air Sacs in a hen

Chickens also have structures called air sacs.  The way to think of them are a bit like bagpipes…..they act as reservoirs for air so that there is constantly fresh air passing through the hens lungs.  A bird’s respiratory cycle is much more complicated than ours….we inhale into a big, complicated balloon, pause, and then push the air out.  As the air sits in the tiny air sacs (called alveoli), oxygen diffuses into the blood, and CO2 diffuses out.   Our alveoli are like a bunch of grapes….blind ended sacs that expand and contract as air comes in and out.  During the pause between breaths, the oxygen concentrations of the gasses change, and diffusion becomes slower. In hens, it goes like this….INHALE – air goes into the lungs and the abdominal air sacs.  EXHALE – air leaves the cranial, clavicular and cervical air sacs.  PAUSE – air goes from lungs to front air sacs while air is travelling from abdominal air sacs to the lungs.  Repeat as necessary.The result of these airbags is that there is a constant, one way flow through the lungs, and every part of the lung is constantly filled with fresh, fully oxygenated air.  Chickens have no alveoli…they have a network of tiny tubes where the air never stops flowing.

Diagram of airflow….not simple, but effecive

Hens have other adaptations too.  Birds have hollow bones, and the front air sacs communicate with the wing bones and the clavicles.  Thus, chickens even use their bones to breathe!  At the microscopic level, the point at which the oxygen enters the blood (and CO2 leaves it) is different too.  Cross current exchange where blood travels at 90 degrees to the airflow in the tiny lung tubes….makes for much more efficient exchange because the same air crosses blood vessels several times, instead of just once as in us mammals.  Also, the thickness of the tissues between the blood and the air is less than half that in mammals of similar sizes.Chickens have no diaphragm, which is the muscle we use to expand our chest cavity downwards.  This has major implications if you are handling chickens, especially small ones.  Their keel bone (Breast bone) MUST be able to move, or they can’t inhale.  Holding or wrapping a chicken  too tightly will stop her from breathing.  This is really important when children are around the hens, since a hug that would work for Fido will not work for chickens.So, in summary…birds breathe slower, deeper and more efficiently.  They have one way flow of air through their lungs through the adaptation of air sacs that act as bellows to constantly supply fresh air to the blood, even when the hen is not inhaling.This is why birds have such an efficient respiratory system, and why aerobic exercise is so much easier for them.When I am about 2 miles into a run, I really hate chickens. Mike the Chicken Vet

Chicken Guts

As promised, I am starting a description of the different systems inside the modern chicken.  I thought I’d start with a description of the intestinal tract of a hen. From beak to butt, there are a lot of specializations that allow chickens to magically convert grubs, worms, corn, grains and calcium into eggs…one of the most nutritionally dense foodstuffs in nature.

Not a pretty picture, but shows the tongue and choanal slit well

First off, it is always amazing to me that people are surprised to find that chickens have triangular tongues. What other shape could it be? There is also a slit (called a choanal slit) in the roof of the top beak. It is here that many substances that are eaten and drank are exposed to the immune system. The triangular tongue is very poor at sensing taste. The human tongue has about 10,000 taste buds, cows have about 20,000, and chickens have 25. Not 25,000….25. Chickens, like most birds, rely heavily on their eyes to locate food, and don’t invest a lot of brain space to taste. It explains some of the things they are willing to eat!!  You will also notice the scarcity of teeth. (Yes, your grandmother was right….hens teeth ARE rare).   More on this later.

After the food passes through the beak mostly untasted, it passes through the esophagus, a 3″ muscular tube, until it is deposited into the crop.  The crop is basically a holding tank for food and water.  It is muscular, but weak.  If a 

chicken eats things that are difficult to pass through the crop (like long blades of grass or hay), or the stuff in the crop gets too dried out, you get impacted crops.  If the contents stay there too long and start to rot, sour crop can result.  To help with either of these problems, it is important first to get the offending material out of the crop, and allow the muscles to relax, and the interior of the crop to become normal.

I know, its gross, but I’m a vet…this is what I do. Plus it gives you a good idea how things really look.

The the food is now the consistency of gruel and enters the proventriculus.  This stomach is similar to ours.  It secretes acids and enzymes and starts actively breaking down the food.  The food now enters the gizzard, which is a very muscular organ that is lined with a tough substance called koilin.  Inside the gizzard, the hen stores small pebbles.  These pebbles get rolled around inside the gizzard through the muscular contractions, and the action is like a grist mill.  Remember the part about the lack of teeth?  This is the compensation.  Stones are lighter than teeth, because you don’t need too many of them, since the feed is already pre-softened by soaking in the crop.  In professional farms, the stones are provided as pieces of oyster shell or limestone, both of which also act as a source of calcium for egg-shell and bone strength.

Once through the gizzard, the gut contents are like a very moist paste, very uniform, and still smelling like chicken feed.  It enters the small intestine, where the pancreas and the gall bladder add their contributions of enzymes and bile acids respectively.  The food is now basically reduced into its components.  For the next 30 cm or so, the protein, fats, sugars and other nutrients are absorbed into the bloodstream.  This is the function of the small intestine.  You can imagine, if there is some problem with the intestine, absorption will be reduced, and the volume of manure will increase…..this is the root cause of many cases of diarrhea.  The gut contents are now a darker brown, and more dense and pasty…pretty much half-way between feed and poo.

The food now enters the two cecums (called cecae when plura….great word to remember for Scrabble).  The primary functions of the cecae are to absorb

Normal Cecal Dropping

Normal Dropping

water and to act as an immune organ.   The contents of these are fairly light….lighter than normal chicken poo.  Every couple of days, the cecae empty themselves, resulting in a cecal dropping, which is sticky, somewhat foamy, and lighter in colour.  These droppings are very distinct, and completely normal.  It also explains why anything that stimulates the immune system can result in diarrhea, since the same organ is responsible for water resorption from the gut contents.The final step in this process is the ejection of the manure (sometimes with a fair bit of velocity!).  The interesting part of this is that birds have “vents”.  That means that the urinary tract, reproductive tract and gastrointestinal tract all leave the body through the same orifice.  Meaning, everything comes out of the same hole…..sometimes at the same time.  That is why the normal manure has the white, uric acid cap on it…that is the bird’s version of urine.  Importantly, it is also a source of manure contamination of eggs, especially if the hens are sick and have loose manure.  I have talked about safe egg handling previously, but I would like to re-emphasize the importance of this.

All in all, it takes around 3 hours for a piece of chicken feed to become chicken manure.  For humans, gut passage time is around 2 days. This speedy transit time is important for….wait for it…..flight.  Birds will actually spend a fair bit of time with almost nothing in their guts….thus lighter and more able to fly.  But, this short passage time means that diets that are high in indigestible components will seriously increase the fecal volume, since there isn’t time to break down the more fibrous bits.

So, that is the story of  chicken feed becoming raw materials for eggs and the necessary body processes that support the chicken in everything she does.  Remember that bird digestion is quite different from mammalian digestion, and some of the gut problems you might see in a small (or large) flock will show certain symptoms because of the way the gut works.

Mike the Chicken Vet

What makes a chicken?

I am starting  a short series of posts that will describe some of the physiology, anatomy, and behaviour of the modern chicken.  This has been done many times before, by people more qualified than I am….in greater detail….likely more accurately….and with far more authority.  They are called textbooks.

The difference is, I am going to make it interesting, useful, and hopefully fun.  Oh, and it will be in the english that normal humans speak.  I also intend to explain some of the reasons why chickens are the way they are….how some of the attributes of “chicken-ness” are useful for the birds.  The aim of the series is to give the readers some tools to problem solve issues that may crop up in their flock (pardon the pun…..see, entertaining all ready!).  If you know how the guts of a hen works, you have an idea as to what might be going wrong when you see diarrhea or blood in the droppings.  At the very least, it will give you another way to think about the way a hen works.

To start, I will talk about some of the striking factors that make chickens the way they are, and why they are that way.

Birds are strange creatures….feathers, eggs, hollow bones, no teeth, don’t produce urine, different red blood cells, all of which I will discuss later.  All of these various oddities work together to allow one thing….flight.  It seems like the vast majority of adaptations that the successful dinosaurs made revolve around the ability to fly.  Feathers are lighter than hair, and insulate better.  Bone marrow is heavy, and provides little structural integrity.  Teeth are heavy, and carrying around a bag of urine inside your body definitely crimps your ability to soar.  Red blood cells that have nuclei (the difference between bird and mammal blood cells) are more efficient at carrying oxygen and are quicker to produces…making bird blood more able to support the high metabolic rate needed to stay aloft.  Getting the sugar shakes at 250 feet up is not an evolutionary advantage.

Chickens are omnivorous, able (and willing) to eat almost anything.  They have a very short digestive tract (again, lighter), that is very efficient at extracting nutrients from whatever they put in their beaks.

Birds have lung adaptations, great eyesight, and lay eggs…..all of which make flight possible.  (I am saying these like they are facts, but in reality, they might be adaptations that just followed along from their dinosaur heritage, and weren’t lost…..dinosaurs laid eggs and didn’t fly…..but it makes a good story, and is plausible).

Anyways….I will be developing the truly informative stuff over the next little while.  I hope it is useful to you, and gives you a different way to look at and evaluate the way your hens interact with the world.

Mike the Chicken Vet