What Type of Circulatory System Do Spiders Have?

The Heart

• A spider's heart, which is shaped like a thin tube, is located in a cavity in the rear of the abdomen. It is protected by the pericardium, which also contains the muscles that contract the heart to pump the blood. There are valves inside the heart that allow blood to flow in only one direction. The heart is not dependent on the brain to function, but they are connected. The heart is able to receive messages from the brain and can adjust the heart rate as needed.

The Blood

• Like humans, a spider's blood is responsible for moving nutrients and hormones to the rest of the body. While hemoglobin is used to move oxygen through human bodies, a spider uses hemocyanin to complete this task. This protein moves within their blood, rather than being contained within a cell. Both hemoglobin and hemocyanin contain hydrogen, carbon, and nitrogen, but hemoglobin has iron at the core, while hemocyanin has a copper core. The copper, when combined with oxygen, gives the blood a blue tint, as opposed to a red one.

Open Circulatory System

• A human's circulatory system is closed, meaning the blood travels through the body via tubes such as veins. A spider's circulatory system is open, meaning the blood flow is not as restricted. Vital organs receive blood via the blood vessels, but the movement is based on available free space between the organs, not a preplanned route. The blood travels back to the heart via the same open system. The lack of structure (veins) means a spider's blood travels slowly unless the spider increases its blood pressure, thus increasing how fast the blood moves. A detrimental effect of the spider's open circulatory system is that it will bleed to death quickly if the skin is punctured.

Molting

• Much like a snake sheds its skin, a spider sheds its exoskeleton on a regular basis. This is necessary in order for a spider to grow. A spider's ability to regulate its blood pressure plays a key part in the molting process.

  When a spider is ready to molt, it will become darker in color and new skin will grow beneath the old skin. Because the nerves stay connected, the body can still receive messages from the brain. The spider hangs upside down and the abdomen gets smaller. The blood pressure around the head doubles and allows the spider to crack the skin near the mouth. Once the head area is free, the increased blood pressure moves to the abdomen. The spider is then able to crack the remainder of the old skin and leave it behind.