The pathway of blood and oxygen
On this page, I will be highlighting how blood is passed around are body and how oxygen gets to the muscles.
Pathway of Blood in the Heart
The blood arrives to the heart in the two vana cavas.
It starts in the right atrium as deoxygenated blood, often shown as blue. It is then pushed into the right ventricle through the tricuspid valve when the atrium contracts. The tricuspid valve prevents the blood from flowing back into the atrium.
When the right ventricle contracts, the blood is forced through the right semilunar valve (also known as the pulmonary valve) into the pulmonary artery where the blood can be carried to the lungs to be oxygenated.
The now oxygenated blood is then brought back to the heart via the pulmonary vein where it is pushed into the left atrium. When the left atrium contracts, the blood is pushed through the bicuspid (mitral) valve to the left ventricle.
Finally, when the left ventricle contracts, the blood is forced through the left semilunar valve (aortic valve) to the aorta where the blood can be transported to the body.
The air is first brought into the body, through either the mouth or nose, when we inhale. The air is filtered here to get rid of dust particles.
The air then passes through the pharynx which leads to the oesophagus and trachea. Air is passed through the trachea as the oesophagus is designed for food and liquids.
Following the pharynx, the air goes to the larynx which opens and closes depending on whether it is food or air.
As previously mentioned, the air then travels through the trachea into the bronchi (singular- bronchus) which are inside the lungs. There are one of these for each lung.
Each bronchus divides into several bronchioles which are the final passageway.
The air finishes up in the alveoli, which are surrounded in capillaries, ready for gaseous exchange. There are roughly 480 million of these alveoli in the lungs!
Pathway of Air
Gaseous Exchange
In the alveoli, a process called gaseous exchange occurs which is the transference on gasses between the alveoli and the blood vessels.
The alveoli are surrounded by capillaries and both the alveoli and capillaries have walls that are only 1 cell thick making them semi permeable which allows gasses to pass through.
Due to the fact that there is far more oxygen in air than carbon dioxide, there is a relatively higher concentration of oxygen in the alveoli than the capillaries. This means that through a process called diffusion, the oxygen will flow into the capillaries from the alveoli.
This is because naturally, molecules want to fill up a space if there are less of them in the other area. It is like when you put a hole in the bottom of a bottle and because there is a lower concentration of water on the outside of the bottle than the inside, the water flows out.
At the same time as this, carbon dioxide is going the other way because there is a higher concentration of carbon dioxide in the blood (because the muscles have got rid of it into the bloodstream after they break down the oxygen) than in the alveoli.
This means that the capillaries will have oxygen in the blood ready for the heart to distribute to the body and the alveoli will have carbon dioxide which it will get rid of when we breath out.
After the blood leaves the heart through the aorta, the blood will be forced through a series of arteries which carry the oxygenated blood away from the heart. The contraction of the left ventricle pushes all of the blood in the body around meaning that there are no other big contractions involving the blood.
These arteries branch of into vessels called arterioles which are like smaller versions of arteries and then these break of into capillaries.
These capillaries sit very close to the fibres of the muscles meaning that diffusion can occur in the same way as in the gaseous exchange previously described.
This means that the muscle fibres have the oxygen required.
The blood then leaves the capillaries which branch back into slightly larger vessels called venules which are like small veins.
These then branch back into veins which carry the blood back to the heart via the biggest vein called the vena cava.
How blood and oxygen gets to the muscles
The blood vessels
There are five different types of blood vessels each designed for different purposes.
Arteries-
Arteries are the the blood vessels which carry the blood away from the heart. All of them (apart from the pulmonary artery) carry oxygenated blood.
They have the thickest walls out of any of the blood vessels due to having the highest blood pressure inside them.
They have a relatively small lumen for their size which is the space inside the vessel where the blood travels through.
Arterioles-
Arterioles have two main functions.
The fist function is that they act as links between the arteries and the capillaries.
The second function is that they are responsible for around 80% of your body's blood flow regulation. This means that they undergo a process called vasoconstriction to reduce the blood flow increasing the blood pressure. They can then undergo a process called vasodilation to increase the blood flow reducing the blood pressure.
Vasoconstriction is when the lumen shrinks.
Vasodilation is where the lumen grows in size.
Capillaries-
Capillaries are the smallest of the blood vessels. They have walls that are only 1 cell thick.
Their function is to allow diffusion between the blood and the alveoli, organs, or muscles.
Venules-
Venules have only one function. It is to act as a connector between the capillaries and the veins.
Veins-
Veins are the blood vessels which carry the blood back to the heart. All of them (apart from the pulmonary vein) carry deoxygenated blood.
They have relatively thinner walls but have a much wider lumen than the other vessels.
They are the only blood vessel to contain valves. Valves are gate-like structures which only allow blood to go through one way. They open to let blood push through one way but then close to prevent a backflow of blood.