Avian Physiology

   

Avian Physiology:  Circulation, Respiration, and Digestion

Hummingbird heart rates can reach 1200 beats per minute, and their breath rate can be 250 times per minute.  Parrots need a mere 15 to 20 minutes to fully digest a meal.  In addition, all birds have a special organ called the ventriculus, commonly known as the gizzard, which grinds food that passes through the proventriculus (stomach).  Unlike other animals, many birds have a special part of their respiratory systems called air sacs, which serve a special function.  Birds have some of the most efficient and unique digestive, circulatory, and respiratory systems in the animal kingdom.

Avian digestion is special in the animal kingdom because the system uses two organs that most other animals do not have.  The organs are called the crop and the gizzard. The crop is the first organ in the digestive process, and the second organ is the proventriculus or the stomach.  The proventriculus starts the digestive process by breaking the food down with hydrochloric acid.  The acid softens the food for the gizzard.  Then, the gizzard grinds the food using strong muscles like a person’s mouth would chew food.  Some birds will eat small stones to help their gizzards to grind hard food.  Next, the food enters the small intestine.  The small intestine receives bile from the liver and the pancreas to counteract the acid.  The large intestine is a very small part of the digestive process and absorbs the remaining nutrients and water.  The final step is the cloaca where the waste and urine are mixed and excreted.  For most birds, the whole digestive process takes approximately 40 minutes. This is very fast compared to human digestion. Humans take about six to eight hours to digest a meal.

The avian respiratory system is different than humans and other mammals because of organs called air sacs. Birds are the only type of animal with these organs.  They help birds inhale and exhale because birds do not have a diaphragm muscle. The air sacs allow birds to collect more oxygen because less old air remains in the lungs.  The air sacs give a bi-directional airflow.  When the bird inhales, the air passes through the lungs and goes to the air sacs.  When it exhales, the air from the air sacs pass the lungs again and gives the bird more oxygen. Birds have small lungs compared to other animals.  One bird called the Bar-Headed Goose can fly as high as 23,000 feet above sea level where humans cannot breathe.

Bird circulatory systems are efficient because they are is strong enough for the birds to fly.  Birds are warm-blooded, and their hearts have four chambers:  two atria and two ventricles. The ventricles pump blood to the body while the deoxygenated blood enters the atria. The right ventricle pumps blood only to the lungs and the left ventricle pumps the blood to the rest of the body.  Birds’ hearts are relatively larger than mammalian hearts so they can move their wings fast enough to fly.  With larger hearts they can pump more blood and get more oxygen. There are five kinds of vessels in their circulatory system:  arteries, arterioles, capillaries, veins, and venules. The arteries carry blood from the heart. The arterioles distribute blood where it is needed in the body, including the more active tissues.  The capillaries exchange nutrients and waste.  Finally, the venules and veins take blood back to the heart.

Overall, birds have some of the most efficient and unique physiology to help them achieve the hard task of flying.  Birds have adapted to collect more oxygen and nutrients to survive. They can eat more food at a time because of their crop. They have air sacs which allow them to gain oxygen while breathing in and out.  They have bigger hearts so the heart can pump more blood.  With this physiology a bird could even be the next dominant species.