Transcript (for teacher reference only)
Many organisms, like bacteria, yeast, or amoebas, consist of just one cell each. But human beings (like other animals, and like plants) are multicellular organisms. The trillions of cells in a human body work together in a cooperative whole. They cooperate successfully by being organized into tissues, which are organized into organs, which are organized into organ systems that support each other.
The whole organism can be seen as a system of organ systems.
Each organ system takes care of some broad part of what an organism has to do to stay alive.
The integumentary system protects the body from the outside world. It contains sense receptors for temperature, pain, and touch.
The muscular system moves the body, moves materials within the body, and generates heat.
The skeletal system provides shape, support, and protection to the body, while allowing it to move.
And so on.
Let’s focus on the cardiovascular system.
The cardiovascular system keeps blood flowing throughout the body. Here are just a few of the ways this system works with the body’s other organ systems…
The cardiovascular system picks up oxygen that’s been breathed in by the respiratory system. Blood flowing through the arteries (shown here in red) carries the oxygen to cells throughout the body. Then the blood flows back through the veins (shown here in blue), carrying waste carbon dioxide back to the respiratory system to be breathed out.
The blood cells that carry oxygen and carbon dioxide through the cardiovascular system are born in the marrow of the skeletal system’s bones.
Red blood cells stay inside the cardiovascular system, but some of the clear fluid they are floating in leaks out of the arteries and veins as the blood circulates. This fluid, called lymph, is collected by the lymphatic system. This system scans lymph for signs of infectious disease, then eventually channels the lymph back into the cardiovascular system.
The urinary system gets rid of waste and helps control blood volume and blood chemistry. The kidneys filter the blood, forming urine that gets sent to the bladder, where it’s stored until urination.
Those are some of the ways the cardiovascular system works with other system. Now let’s look more closely inside the cardiovascular system itself. The main parts of the cardiovascular system are the blood, the blood vessels, and the heart.
The body’s complex, branching network of blood vessels can be represented schematically as a figure eight. The blood leaves the heart to deliver oxygen to cells throughout the body and pick up carbon dioxide and other waste. Then the blood returns to the heart.
Next, the blood is sent on a shorter trip to the lungs to get rid of carbon dioxide and pick up a new batch of oxygen. The oxygen-rich blood returns to the heart again, and starts the figure eight all over.
The heart is a pump with four chambers. Those chambers squeeze in a repeating sequence, and they have one-way valves that keep blood flowing in the right direction.
Like other organs, the heart is made up of various kinds of tissues.
A cross section of the heart reveals several different kinds of tissues in the wall of the heart. There are three main layers of tissue in the heart wall. From the outside to the inside, they are the epicardium, the myocardium, and the endocardium.
The epicardium has a thin layer of simple squamous epithelial cells over a layer of loose connective tissue and fat cells that help cushion the heart.
The myocardium is made of cardiac muscle cells that do the hard work of contracting and relaxing 60 or more times a minute, 24 hours a day, year after year.
One difference between cardiac muscle cells and other muscle cells in the body is that cardiac muscle cells are able to pass electrical signals to one another, kind of like nerve cells. This ability helps all the cardiac muscle cells in each chamber of the heart to contract at precisely the same time.
The endocardium is made mostly of simple squamous epithelial cells that make the inner lining of the heart nice and smooth, to help blood flow easily.
Let’s retrace our steps:
These and other cells make up the tissues…
…that make up the heart…
…which, along with the blood vessels and blood, form the cardiovascular system.
Through this complex hierarchy of systems within systems, about 200 kinds of cells in the human body—trillions of cells in all—contribute to the well being of the whole. They exchange goods and services, in a vast, decentralized, cooperative community of specialists.
And that’s how trillions of cells team up to make one big multicellular organism.