The Circulatory System — Biology of the Chicken, Part 6

By Thomas L. Fuller, New York

The circulatory or transport system of the chicken is very similar to that of our own cardiovascular system. Throughout this series about the chicken’s biological systems, a common influence has evolved. Hank and Henrietta, as birds, need special physiological adaptations for their inherent need of flight. The circulatory system of the chicken, with this same distinction, must provide a more efficient manner of retrieving oxygen from our atmosphere. In other words, flight muscles need a lot of oxygen.

The primary purpose of the circulatory system is to provide every living cell of the bird with oxygen and food while removing carbon dioxide and wastes from those same cells. In addition, this system plays an important role in maintaining the chicken’s body temperature of more than 104°F. The circulatory system consists of the heart, the blood vessels, the spleen, the bone marrow, and the blood and lymph vessels. The beginnings of this specialized transport system starts after only one hour of incubation in the fertile egg. It is clearly operating after only two days and the beating heart can be seen with the naked eye on the third day.

Hank and Henrietta, like you and I, have a four-chambered heart. It is located in the thoracic cavity (chest area) between and in front of the two lobes of the liver. The purpose of a four-chambered heart is to divide the oxygenated blood (that leaving the heart with oxygen for the cells) from the deoxygenated blood (that coming from the cells with more carbon dioxide in it to be expelled in the lungs).

The left and the right atrium are located on the top of the heart and act as receiving chambers for the blood coming from the lungs and body, respectively. Atria are a thin-walled muscle that pushes blood to the true pumps of the heart, the ventricles.

The muscle wall of the right ventricle is lesser than that of the left ventricle. The right side of the heart is only pushing blood a short path to the lungs while the left side ventricle has to push blood from the tip of the comb to the tip of the toes. A chicken’s heart pumps more blood per minute (cardiac output) than that of mammal’s of the same body mass. Birds also tend to have larger hearts (relative to body size) than mammals. These physiological adaptations precipitate in them having a higher blood pressure and resting heart beat than humans (180/160 BP and 245 bpm heart beat).

As we have mentioned before, the high-energy demands of flight have influenced this unique cardiac muscle, the chicken heart. As wonderful of an organ as the chicken heart is, it would not be effective without its plumbing. The circulatory system of the chicken is a closed circulatory system. That is to say, the life giving blood of the system is always contained in a vessel. The vessels we are talking about are arteries, veins and capillaries. Arteries carry the bright red oxygenated blood away from the heart to the capillaries. There is no exchange of gases or food in the arteries. Arteries are a network of elastic like tubes, squeezing blood pushed from the heart. Beginning at the largest artery, the aorta, and ending in the smallest arteries, arterioles, they then connect to the capillaries. Here capillaries, only one cell in diameter, interact with the tissues exchanging gases and nutrients and obtaining wastes. The other end of the capillary is then connected to another network of vessels called veins for a trip back to the heart.

Veins carry all blood back to the heart. After the exchange in the capillaries, the darkened blood with less oxygen works its way back to the right atrium of the heart. From the end of the capillary, small veins called “venules” flow to larger sized veins called “vena cavae.” Veins tend to be thin-walled compared to arteries and contain small check valves to assist in blood flow by not allowing it to flow backwards in the system. Once to the right atrium, blood flows to the right ventricle and is pushed to the lungs for gas exchange, and then takes a ride to the left atrium. From the left atrium, blood travels to the left ventricle, and from there, to the aorta and to the body.

The design of our vascular system in the chicken also considers its need to conserve heat. Birds’ arteries and veins are designed so they lie next to each other. As warm blood leaves the heart via the arteries and goes to the extremities it warms the cooled blood returning in veins from the extremities. The placement of the vessels tends to then conserve heat from the body core.

The spleen assists the circulatory system by filtering the blood and removing aging red blood cells and antigens. It also stores some red blood cells and platelets. As a secondary lymphoid organ, it contributes to the chicken’s immune system.

Blood is the transport vehicle for the body. We are all familiar with the four most common components of the blood, red blood cells, white blood cells, platelets and plasma. Red blood cells called “erythrocytes” are large oval and flat. Their red coloring is caused by the presence of hemoglobin, which is an iron compound that carries oxygen. The function of the red blood cells is the transport of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. They are formed in the bred bone marrow.

White blood cells or leucocytes are irregularly shaped cells with colorless cytoplasm. They are formed in the spleen, lymphoid tissue and in the bone marrow. These cells play an important role in the chicken’s defense against bacterial invasion.

The third component and what we associate with blood clotting would be platelets. In the chicken, however, thrombocytes replace mammalian blood platelets and are less involved with their blood clotting.

Plasma is the liquid or noncellular portion of the blood. It may include, but is not limited to, blood sugar, proteins, products from metabolism (wastes), hormones, enzymes, antibodies and nonprotein nitrogen substances.

The lymph system is also connected to our circulatory system. The lymphatic system has the function of draining the body systems of fluid that is left behind by the blood vessels. Chickens do not have lymph nodes, as we do. Rather, they have an intertwining of very small lymph vessels to do that filtering.

Hank and Henrietta truly have an efficient mode of transport or circulation. Being animals of flight, their body demands more oxygen and energy for that adaptation. Take note the next time you think your heart is beating too fast after chasing that chicken around the yard. The chicken’s heart is still beating faster.

Thomas Fuller is a retired biology teacher and lifelong poultry owner. Look for the next part in his series on the biology of a chicken in the next Backyard Poultry.