This is a detailed note about the circulatory system and it talks about major components of the circulatory system, functions of the blood, and disorders regarding the circulatory system.
Main Functions of the Circulatory System
• The CIRCULATORY SYSTEM is the system that transports blood, nutrients, and waste
around the body. • The circulatory system has the following three main functions:
• It transports gases (from the respiratory system), nutrient molecules, and waste materials (from the
digestive system).
• It regulates internal temperature and transports chemical substances that are vital to health from one part
of the body to the other.
• It protects against blood loss from injury and against disease-causing microbes or toxic substances
introduced into the body.
Major Components of the Circulatory System
• The circulatory system has three major components: the heart, the blood vessels, and the blood. • The heart is
a muscular organ that continuously pumps the blood through the body and generates blood flow. • The blood
vessels are a system of hollow tubes through which the blood moves.
• Together, the heart and the blood vessels comprise the cardiovascular system.
• Blood is the fluid that transports nutrients, oxygen, carbon dioxide, and many other materials throughout the body.
The Two Types of Circulatory Systems
• Many invertebrates have an open circulatory system.
• An OPEN CIRCULATORY SYSTEM is a circulatory system in which vessels open into the animal’s
body cavity. It is called “open” because blood flows freely within the body cavity and makes direct contact with
organs and tissues. There is no distinction between the blood and the interstitial fluid. In invertebrates, such as
insects and crustaceans, the mixture of blood and fluids that surrounds the cells is called hemolymph.
• For example: In a grasshopper, hemolymph is pumped through a single vessel that runs from the head to
the abdomen. In the abdomen, the vessel divides into chambers that function as the grasshopper’s
heart. Tiny holes in the heart wall (called ostia) allow hemolymph to enter the heart chambers from the
body cavity. Nutrients and wastes are exchanged between the hemolymph and cells in the heart
chambers before the hemolymph passes back into the transporting vessel to be eliminated from the
grasshopper’s body.
• Other animals, such as vertebrates, earthworms, and octopus, need a system to circulate the blood, keep it
under pressure, and pump it at a speed sufficient to supply the metabolic needs of all parts of the body.
This type of system is called a closed circulatory system.
• A CLOSED CIRCULATORY SYSTEM is a circulatory system in which the circulating blood is
contained within vessels and kept separate from the interstitial fluid. The blood flows in a continuous fixed path
of circulation and is confined to a network of vessels.
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The Human Circulatory System: The
Heart
• Your heart is located slightly to the left of the middle of your chest and is about the size of your fist. •
The walls of the heart are made up of a unique type of muscle called cardiac muscle.
• Cardiac muscle cells are arranged in a network that allows the heart to contract and relax rhythmically and
involuntarily without becoming fatigued.
• In addition to pumping blood, a healthy heart also ensures that blood keeps flowing in one direction only, and that
oxygen-rich blood is kept separate from oxygen-poor blood.
• The human heart (like the heart of all mammals and birds) has four chambers – a top chamber and a bottom
chamber on both the right and left sides.
• The two top chambers are called the atria (singular: atrium) – fill with blood returning from the body or the lungs.
• The two bottom chambers are called the ventricles – they receive blood from the atria and pump it out to the
body or lungs.
• The atria and ventricles are separated from each other by a thick muscular wall called the septum. • The right
side of the heart receives blood that is coming back from the body, and then pumps this blood out to the lungs. Two
large vessels, called the vena cavae (singular: vena cava), open into the right atrium. The superior vena
cava collects oxygen-poor blood (called deoxygenated blood) coming from the tissues in the head, chest, and
arms. The inferior vena cava collects oxygen-poor blood coming from the tissues elsewhere in the body. •
The oxygen-poor blood flows from the right atrium into the right ventricle. The valve that separates the right atrium
and the right ventricle is called the tricuspid valve.
• From there, it enters the right and left pulmonary arteries – large blood vessels that carry blood from the
heart to the lungs. The pulmonary arteries are the only arteries that carry oxygen-poor blood. The valve between
the right ventricle and the pulmonary arteries is called the pulmonary semilunar valve.
• The pulmonary arteries bring the oxygen-poor blood to the lungs. Once at the lungs, gas exchange occurs. Now,
the blood is oxygen-rich (and has diffused its carbon dioxide out into the alveoli of the lungs). • The oxygen-rich
blood (called oxygenated blood) returns to the heart via the pulmonary veins – blood vessels that carry blood
from the lungs to the heart. The pulmonary veins bring the blood to the left atrium. The pulmonary veins are the
only veins in the body that contain oxygenated blood.
• The blood flows from the left atrium into the left ventricle. The valve that separates the left atrium and the left
ventricle is called the mitral valve (also known as the bicuspid valve).
• The left ventricle pumps blood through the largest vessel in the body called the aorta – the artery that carries
blood directly from the heart to the other arteries. This valve is called the aortic semilunar valve.
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