The circulatory system

When we humans exert ourselves, we breathe deeper, and our hearts beat faster, In order to get more oxygen and nutrients out into the body. Oxygen and nutrients are transported in the blood. All the blood vessels and the heart together make up the circulatory system. Deoxygenated blood is pumped from the right side of the heart… ...to the lungs… ...where the blood releases carbon dioxide and collects oxygen. Then it goes back to the heart -- to the left side.

This part of the circulatory system -- from the heart, to the lungs, and back -- is called the pulmonary circuit. Then from the left side of the heart, this oxygenated blood is pumped out into the entire body, where the blood releases the oxygen to the cells and collects carbon dioxide. The blood then travels back to the heart again, this time to the right side. This part of the circulatory system, from heart, to body, and back, is called the systemic circuit. When a body makes a physical effort, the blood flow has to increase... ...in order to transport the oxygen-rich blood to the muscles faster.

At the same time, the muscle cells have to get rid of waste that must be released from the body. Such as carbon dioxide. Which is carried by the blood back to the lungs, where it is vented out. The blood vessels that go from the heart are called arteries. The vessels going back toward the heart, are called veins.

The vessels that enter and leave the heart are the biggest ones in the body. On the way from the heart, they divide up and become smaller and smaller. The tiniest vessels - the capillaries - form a network in the body’s tissues. It is the capillaries that are in direct contact with the body’s cells. The walls of the capillaries are really thin!

They consist of a single layer of cells only. Here, oxygen and nutrients can easily move from the vessels and into the cells of the body, and waste products -- like carbon dioxide -- can move out from the cells and into the blood. In the capillary network, the arteries connect to the veins, that carry the blood back to the heart and then the lungs. When the heart is pumping, pressure builds up in the blood vessels. How much pressure depends - among other things - on the power and the frequency of the heartbeats.

But the vessels themselves also affect the pressure. If they contract - and get thinner - the pressure in them increases. If they widen, the pressure decreases. By widening some vessels, at the same time as others are contracted, the blood is moved around, so that some parts of the body get more blood than others. This happens for example when we get stressed.

Then, the vessels going to the large muscles are widened. At the same time, other vessels contract, so that other parts of the body have to get by on less oxygen and nutrients. This reaction in the body dates back to the time when we lived on the savanna. Back then, stress meant “now it’s time for either fight or flight.” And then the muscles require lots of blood. Nowadays, stress is mostly caused by other things.

But the body doesn’t know that. So it pumps up a pressure suitable for fight. Or for flight. The body has several ways to detect if the pressure needs adjustment. If need be, it will do it.

Here is a blood vessel called the aortic arch, which can detect the blood pressure… And which sends signals to the brain stem, which in turn sends signals to the heart, vessels and lungs, to adjust the heart rate, blood pressure and breathing. … and here, in the carotid artery, the amount of carbon dioxide in the blood is detected... ... the kidneys also detect if they don’t get enough blood, and need higher pressure. The nervous system is also involved. It notices if we are stressed, or working hard. It then affects how hard the heart is beating, and if the airways and blood vessels should contract or expand.

Isn’t it an amazing system?