What Is the Circulatory System?
Picture yourself running on a cool fall day. The sun is shining and the leaves are changing color. You can probably feel the blood pumping through your body, powering you through the day's workout. While you're running, your heart is working hard to pump blood, which carries oxygen and nutrients all over your body for you to make energy. Your heart, blood, and the vessels that carry it are collectively referred to as the circulatory system. All animals have a circulatory system, and today we're going to look at the structure and function of bird circulatory systems, which are surprisingly like our human ones.
Structure of the Bird Circulatory System
To begin, let's start with the heart, which is like the boss of the circulatory system. The heart in birds and mammals is divided into four parts, called chambers. The job of the heart is to deliver oxygen and nutrients to the body through the blood. When the blood is in the body, the oxygen is used up to make energy.
The blood then flows back to the heart into the first chamber, or the right atrium, through blood vessels called veins, which bring all blood to the heart. From the right atrium, the deoxygenated blood goes to the right ventricle. The right ventricle is a larger chamber that pumps the blood to the lungs.
At the lungs, the blood picks up oxygen and is moved to the left atrium of the heart. The left atrium pumps blood to the left ventricle, the strongest chamber of the heart. The left ventricle's job is to pump blood through the arteries to the entire body, so it needs to have a thick wall of muscle to do such a big job. From there, the process repeats again with each heartbeat.
Only birds and mammals have four chambers to their heart. The purpose of this is to divide the oxygenated and deoxygenated blood efficiently. Other animals have two chambers, or no chambers in their heart, so all the blood mixes together. To be the most efficient at delivering oxygen, we want to keep the oxygenated blood moving towards the body and the deoxygenated blood coming back to the heart to get more oxygen.
Picture a street. The cars on the street are like the blood in our body. Our streets usually have two or more lanes. One you can imagine going into the city, and the other leads out to the country. With two lanes, the traffic can easily flow in two directions. However, if there were no lanes, all the traffic would be mixing together, going both directions. There would probably be a lot of accidents and people wouldn't get where they are going as efficiently. This is why a 4-chambered heart is helpful to an animal needing a lot of energy. Next, let's look at some other adaptations of the bird circulatory system.
Size of Bird Hearts
The bird's heart, although similar to mammals, is structured slightly differently for their lifestyle. Birds have proportionately larger hearts compared to mammals. A heart of a human is about 0.4% of our body weight, whereas a bird can have a heart weighing up to 4% of its body weight!
Small birds have especially large hearts compared to their size, as flight requires a lot of energy. Hummingbirds are known for their large hearts and incredibly fast heart rate needed for hovering flight. Hummingbird hearts beat up to 1,200 beats per minute, compared with an average human resting heart rate of 60 to 100 beats per minute.
Capillaries in Birds
Another adaptation for flight involves the struggles of high altitude. As altitude increases, the concentration of oxygen decreases. Since flying requires so much energy to begin with, birds must have special adaptations to get the extra energy needed in such low-oxygen conditions. For example, bar-headed geese migrate over the Himalayan Mountains, where they fly over five miles high.
These geese have an increased number of blood vessels in their muscle tissue compared to birds that live at low altitudes, like sea birds. These blood vessels, called capillaries, are very small and are the end point for delivering oxygen and nutrients to the rest of the body. More capillaries means more blood, and thus more oxygen can be delivered to the muscle. More oxygen means more energy and the stamina needed for flight.
Efficiency of Blood Pumping
Birds' hearts also look different on the inside compared to mammals'. Bird hearts fill more fully and empty more completely than mammalian hearts due to something called stroke volume. This leads to a more efficient heart, able to deliver loads of oxygen needed for the energy of flight.
Picture filling a jug of water and carrying it to water your plants. In mammalian hearts, you fill the jug about 3/4 full then take it to the plant, but don't totally empty it. You can see this is quite inefficient compared to bird hearts, which fill the jug to the brim and them dump every last drop out onto the plant.
In summary, the circulatory system is made of the heart, blood vessels, and blood, which carry oxygen and nutrients to the body to make energy. Birds have 4-chambered hearts made of the right atrium, right ventricle, left atrium and left ventricle. The division helps for efficient flow of oxygenated and deoxygenated blood. Birds also have specific adaptations for flight, including a larger heart relative to body weight and a faster heart rate. They also have more capillaries in the muscle designed to deliver oxygen at high altitudes. Their hearts are more efficient to meet the energetic demands of flight, having a larger stroke volume compared to humans.