Overview of the Circulatory System

Overview of the Circulatory System

Overview

Revised for Bio 101, Human Circulatory System, Errors in MC questions fixed.

By the end of this section, you will be able to:

  • Describe an open and closed circulatory system
  • Describe interstitial fluid and hemolymph
  • Compare and contrast the organization and evolution of the vertebrate circulatory system.

In all animals, except a few simple types, the circulatory system is used to transport nutrients and gases through the body. Simple diffusion allows some water, nutrient, waste, and gas exchange into primitive animals that are only a few cell layers thick; however, bulk flow is the only method by which the entire body of larger more complex organisms is accessed.

Circulatory System Architecture

The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity. In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figurea. As opposed to a closed system, arthropods—including insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figureb. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid (tissue fluid or fluid located between cells making up body tissues). As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy as a closed system to operate or to maintain; however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outcompeted by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects.

Illustration A shows the closed circulatory system of an earthworm. Dorsal and ventral blood vessels run along the top and bottom of the intestine, respectively. The dorsal and ventral blood vessels are connected by ring-like hearts. Hearts are also associated with the dorsal blood vessel. These hearts pump blood forward, and the ring-like hearts pump blood down to the ventral vessel, which returns blood to the back of the body. Illustration B shows the open circulatory system of a bee. The dorsal blood vessel, which contains multiple hearts, runs along the top of the bee. Blood exits the dorsal blood vessel through an opening in the head, into the body cavity. Blood reenters the blood vessels through openings in the hearts called ostia.

In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods like this bee and most mollusks have open circulatory systems.

 

For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved. Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups.  Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation). In amphibians, gas exchange also occurs through the skin.

  

The Pulmonary and Systemic Circuits.  Credit, Diagram by Tina B. Jones modified from Kamiille via Wikimedia Commons [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

 

Amphibians have a three-chambered heart that has two atria and one ventricle. The two atria (superior heart chambers) receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart’s ventricle (inferior heart chamber), which reduces the efficiency of oxygenation. 

Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits. The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence, for instance, while the animal waits for prey.

In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. For the remainder of this module, we will focus exclusively on the human circulatory system.

 
The Mammalian Heart.  Credit:  Wapcaplet [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

Section Summary

In most animals, the circulatory system is used to transport blood through the body. Some primitive animals use diffusion for the exchange of water, nutrients, and gases. However, complex organisms use the circulatory system to carry gases, nutrients, and waste through the body. Circulatory systems may be open (mixed with the interstitial fluid) or closed (separated from the interstitial fluid). Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptions during evolution and associated differences in anatomy. 

Review Questions

Why are open circulatory systems advantageous to some animals?

A.  They use less metabolic energy.

B.  They are more complex.

C.  They do not need a heart.

D.  They help large insects develop.

Hint:

A

The circulatory system is used to transport _____ through the body.

A.  undigested materials

B.  food

C.  lymph

D.  blood

Hint:

D

Blood flow that is directed through the lungs and back to the heart is called ________.

A.  systemic circulation

B.  gill circulation

C.  pulmonary circulation

D.  cutaneous circulation

Hint:

C

Free Response

Describe a closed circulatory system.

Hint:

A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and then returns to the heart.

Describe systemic circulation.

Hint:

Systemic circulation flows through the systems of the body. The blood flows away from the heart to the brain, liver, kidneys, stomach, and other organs, the limbs, and the muscles of the body; it then returns to the heart.

Additional Credits

Opening Image:  BodyParts3D/Anatomography [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)]