Did you know that if we stretch out all the blood vessels in the human body, it can be as long as 106,000 kilometers and can circle the earth 2.5 times?!
The human circulatory system is a feat of engineering, routing to every tissue and organ, ensuring that they get the needed nutrients and oxygen.
Let’s say you just had a big lunch. How do all the nutrients from the food get to where it is needed?
If you’re wondering how the nutrients get to the different parts of the body, the answer is through the circulatory system. When the food you eat is digested and converted into transportable nutrients, they are transferred to the bloodstream via the capillaries of the small intestine. Now that the nutrients are in the bloodstream, they are ready to be delivered to various places.
We’ll look at the nitty-gritty and see the exact routes around the heart and body that the blood takes. But first, let’s look at what is in the blood.
It’s In The Blood
It’s the thing rushing through the arteries, veins, and capillaries of the body, and around 7-8% of our body weight. The blood is composed of plasma, red blood cells, white blood cells, and platelets.
Plasma is a mixture of water, salt, sugar, proteins, enzymes, and antibodies. It comprises 55% of the blood and contains the nutrients and building blocks needed by organs and tissues.
Red blood cells on the other hand are specially constructed to carry life-giving oxygen to the tissues and organs, exchanging it for carbon dioxide. Did you know that mature RBCs do not have a nucleus and therefore do not contain DNA? Researchers speculate that the absence of the nucleus creates more space for hemoglobin, the protein that carries oxygen.
White blood cells, for their part, play a vital role in defense and help the body fight infection. There is not just one type of white blood cell. There are 8 major types, all of which have varying roles to play. You have the macrophages, dendritic cells, B-cells, T-cells, Natural Killer Cells, Neutrophils, Eosinophils, and Basophils. Whew! That was a mouthful, but the main thing to remember is that white blood cells keep us healthy by keeping infections at bay.
Finally, those platelets. When a blood vessel is damaged, a signal is sent to the blood platelets. They help the blood clot by clumping and forming plugs in the gap. Thanks to platelets, we don’t bleed out.
The Heart of the Matter
Now the human heart is one of the most hardworking organs of the body, beating over 100,000 per day. Every heartbeat expels around 2 ounces of blood or around 208 million liters over the course of a lifetime (70 years).
The heart has 4 major parts: the right and left atrium, and the right and left ventricle. It also has 4 major valves. They open and close to ensure that blood flows at the right moment and in the right direction, safeguarding against blood backflow.
When it comes to the heart, it’s important to remember that there are two types of blood cycling through it: the oxygenated blood and the de-oxygenated blood. Oxygenated blood is carrying oxygen, ready for delivery to the rest of the body. It’s usually the arteries that carry oxygenated blood from the heart.
On the other hand, blood coming into the heart is de-oxygenated. They’ve already made their oxygen deliveries and are now full of carbon dioxide. It’s usually the veins that convey returning blood to the heart.
(Some congenital heart conditions involve an atrial septal defect, which means there’s a hole that causes the mixing of the oxygenated and deoxygenated blood. This results in an abnormal heartbeat and increases the risk of stroke.)
Traces of Blood (Deoxygenated Pathway)
Let’s say we have blood coming from the leg. This blood will enter the heart through the (inferior) vena cava, the largest vein in the human body. (Blood coming from the upper portions of the body like the head, neck, arms, and chest, will enter through the superior vena cava.)
From the vena cava, blood is pushed to the right atrium and then to the tricuspid valve.
From there, it goes to the right ventricle, then to the pulmonary valve, and then the pulmonary artery.
From the pulmonary artery...guess where this blood is going.
Right! To the lungs.
Remember that this is deoxygenated blood. It is carrying carbon dioxide.
The lungs are where red blood cells give up their carbon dioxide and take in oxygen.
All of a sudden, this blood is now oxygenated. It will return to the heart so it can be pumped through to the rest of the body.
(For comparison, oxygenated blood is bright red. De-oxygenated blood is dark red.)
Traces of Blood (Oxygenated Pathway)
From the lungs, oxygenated blood enters the heart through the pulmonary vein, which leads to the left atrium. From the left atrium, it flows through the bicuspid valve, to the left ventricle.
The left ventricle is the heart’s main pumping chamber and pumps blood through to the aorta. (The term we call “blood pressure” measures the force applied to the arteries when the heart beats.)
The aorta is the major artery that supplies oxygenated blood to the rest of the body. It branches off in many places to supply the different regions. For example, near the pelvic area, the aorta branches off into two arteries, supplying both legs.
The general rule is that the farther the blood vessels are from the heart, the smaller they get. Next in size to the arteries, and are a little further from the heart, are the arterioles. These blood vessels are quite flexible and can expand and contract to maintain the body’s blood pressure.
With a diameter one-tenth that of the human hair, the tiniest blood vessels are the capillaries. They are the terminals or endpoints for oxygen delivery. The capillaries are where the exchange of oxygen & carbon dioxide, and nutrients & waste products, happen. These blood vessels have very thin walls to allow the trade of materials easily.
From the capillaries, the now deoxygenated blood, (having traded oxygen for carbon dioxide), starts its journey back to the heart, moving through the venules and then to the veins.
But, have you ever wondered, in places like the leg, how do veins fight gravity and move the blood back up to the heart? Unlike arteries which move blood with pressure provided by the pumping heart, the veins do not carry such highly pressurized blood.
So, how does the blood climb back to the heart? First, the veins have valves that open and close to ensure that blood moves in the right direction, back to the heart. Second, the muscles in the leg contract and squeeze the blood along the way, keeping the pressure and momentum to move the blood along the final leg of its journey.
In conjunction, these mechanisms ensure the smooth flow of blood back up. But sometimes, the valves do not function properly, so blood pools back down at the lower veins. This results in swollen veins in the legs, which we know as varicose veins, and is common among elderly adults.
But if everything goes well, the blood from the leg enters the inferior vena cava once again and we have just completed the full cycle of blood!
Do you want to know just how healthy the blood that’s traveling through your veins is? Bloodworks Lab is your one-stop shop for all your blood test needs. We do comprehensive blood workups and offer packages that check on your cholesterol, lipid profile, urine, etc.
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