Summary Heart and Circulatory System
With each heartbeat, blood is sent throughout our bodies, carrying
oxygen and nutrients to every cell. Every day, the approximately 10 pints
(5 liters) of blood in your body travel many times through about 60,000
miles (96,560 kilometers) of blood vessels that branch and cross, linking
the cells of our organs and body parts.
About the Heart and Circulatory System
The circulatory system is composed of the heart and blood vessels,
including arteries, veins, and capillaries. Our bodies actually have two
circulatory systems: The pulmonary circulation is a short loop from the
heart to the lungs and back again, and the systemic circulation (the
system we usually think of as our circulatory system) sends blood from
the heart to all the other parts of our bodies and back again.
The heart is the key organ in the circulatory system. As a hollow,
muscular pump, its main function is to propel blood throughout the body.
It usually beats from 60 to 100 times per minute,but can go much faster
when necessary. It beats about 100,000 times a day, more than 30 million
times per year, and about 2.5 billion times in a 70-year lifetime.
The heart gets messages from the body that tell it when to pump more or
less blood depending on an individual’s needs. When we’re sleeping, it
pumps just enough to provide for the lower amounts of oxygen needed by
our bodies at rest. When we’re exercising or frightened, the heart pumps
faster to increase the delivery of oxygen.
The heart has four chambers that are enclosed by thick, muscular walls.
It lies between the lungs and just to the left of the middle of the chest
cavity. The bottom part of the heart is divided into two chambers called
the right and left ventricles, which pump blood out of the heart. A wall
called the interventricular septum divides the ventricles.
The upper part of the heart is made up of the other two chambers of the
heart, the right and left atria. The right and left atria receive the blood
entering the heart. A wall called the interatrial septum divides the right
and left atria, which are separated from the ventricles by the
atrioventricular valves. The tricuspid valve separates the right atrium from
the right ventricle, and the mitral valve separates the left atrium and the
left ventricle.
, Two other cardiac valves separate the ventricles and the large blood
vessels that carry blood leaving the heart. These are the pulmonic valve,
which separates the right ventricle from the pulmonary artery leading to
the lungs, and the aortic valve, which separates the left ventricle from the
aorta, the body’s largest blood vessel.
Arteries carry blood away from the heart. They are the thickest blood
vessels, with muscular walls that contract to keep the blood moving away
from the heart and through the body. In the systemic circulation, oxygen-
rich blood is pumped from the heart into the aorta. This huge artery
curves up and back from the left ventricle, then heads down in front of the
spinal column into the abdomen. Two coronary arteries branch off at the
beginning of the aorta and divide into a network of smaller arteries that
provide oxygen and nourishment to the muscles of the heart.
Unlike the aorta, the body’s other main artery, the pulmonary artery,
carries oxygen-poor blood. From the right ventricle, the pulmonary artery
divides into right and left branches, on the way to the lungs where blood
picks up oxygen.
Arterial walls have three layers:
1. The endothelium is on the inside and provides a smooth lining for
blood to flow over as it moves through the artery.
2. The media is the middle part of the artery, made up of a layer of
muscle and elastic tissue.
3. The adventitia is the tough covering that protects the outside of the
artery.
As they get farther from the heart, the arteries branch out into arterioles,
which are smaller and less elastic.
Veins carry blood back to the heart. They’re not as muscular as arteries,
but they contain valves that prevent blood from flowing backward. Veins
have the same three layers that arteries do, but are thinner and less
flexible. The two largest veins are the superior and inferior vena cavae.
The terms superior and inferior don’t mean that one vein is better than the
other, but that they’re located above and below the heart.
A network of tiny capillaries connects the arteries and veins. Though tiny,
the capillaries are one of the most important parts of the circulatory
system because it’s through them that nutrients and oxygen are delivered
With each heartbeat, blood is sent throughout our bodies, carrying
oxygen and nutrients to every cell. Every day, the approximately 10 pints
(5 liters) of blood in your body travel many times through about 60,000
miles (96,560 kilometers) of blood vessels that branch and cross, linking
the cells of our organs and body parts.
About the Heart and Circulatory System
The circulatory system is composed of the heart and blood vessels,
including arteries, veins, and capillaries. Our bodies actually have two
circulatory systems: The pulmonary circulation is a short loop from the
heart to the lungs and back again, and the systemic circulation (the
system we usually think of as our circulatory system) sends blood from
the heart to all the other parts of our bodies and back again.
The heart is the key organ in the circulatory system. As a hollow,
muscular pump, its main function is to propel blood throughout the body.
It usually beats from 60 to 100 times per minute,but can go much faster
when necessary. It beats about 100,000 times a day, more than 30 million
times per year, and about 2.5 billion times in a 70-year lifetime.
The heart gets messages from the body that tell it when to pump more or
less blood depending on an individual’s needs. When we’re sleeping, it
pumps just enough to provide for the lower amounts of oxygen needed by
our bodies at rest. When we’re exercising or frightened, the heart pumps
faster to increase the delivery of oxygen.
The heart has four chambers that are enclosed by thick, muscular walls.
It lies between the lungs and just to the left of the middle of the chest
cavity. The bottom part of the heart is divided into two chambers called
the right and left ventricles, which pump blood out of the heart. A wall
called the interventricular septum divides the ventricles.
The upper part of the heart is made up of the other two chambers of the
heart, the right and left atria. The right and left atria receive the blood
entering the heart. A wall called the interatrial septum divides the right
and left atria, which are separated from the ventricles by the
atrioventricular valves. The tricuspid valve separates the right atrium from
the right ventricle, and the mitral valve separates the left atrium and the
left ventricle.
, Two other cardiac valves separate the ventricles and the large blood
vessels that carry blood leaving the heart. These are the pulmonic valve,
which separates the right ventricle from the pulmonary artery leading to
the lungs, and the aortic valve, which separates the left ventricle from the
aorta, the body’s largest blood vessel.
Arteries carry blood away from the heart. They are the thickest blood
vessels, with muscular walls that contract to keep the blood moving away
from the heart and through the body. In the systemic circulation, oxygen-
rich blood is pumped from the heart into the aorta. This huge artery
curves up and back from the left ventricle, then heads down in front of the
spinal column into the abdomen. Two coronary arteries branch off at the
beginning of the aorta and divide into a network of smaller arteries that
provide oxygen and nourishment to the muscles of the heart.
Unlike the aorta, the body’s other main artery, the pulmonary artery,
carries oxygen-poor blood. From the right ventricle, the pulmonary artery
divides into right and left branches, on the way to the lungs where blood
picks up oxygen.
Arterial walls have three layers:
1. The endothelium is on the inside and provides a smooth lining for
blood to flow over as it moves through the artery.
2. The media is the middle part of the artery, made up of a layer of
muscle and elastic tissue.
3. The adventitia is the tough covering that protects the outside of the
artery.
As they get farther from the heart, the arteries branch out into arterioles,
which are smaller and less elastic.
Veins carry blood back to the heart. They’re not as muscular as arteries,
but they contain valves that prevent blood from flowing backward. Veins
have the same three layers that arteries do, but are thinner and less
flexible. The two largest veins are the superior and inferior vena cavae.
The terms superior and inferior don’t mean that one vein is better than the
other, but that they’re located above and below the heart.
A network of tiny capillaries connects the arteries and veins. Though tiny,
the capillaries are one of the most important parts of the circulatory
system because it’s through them that nutrients and oxygen are delivered
