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General Education
Lesson 8: Circulatory System
(Human Systems)
What is the Circulatory System?
The circulatory system is a network of organs and blood vessels that work together to transport blood throughout your body. Blood carries oxygen and nutrients to all your cells and picks up waste products to remove them. Your circulatory system is like a delivery and pickup system that keeps your whole body supplied with what it needs to survive.
- The circulatory system uses blood and blood vessels to deliver oxygen and nutrients to every part of your body.
Main Function of the Circulatory System
- Transport blood to other parts of the body. The circulatory system has one main job: to move blood continuously throughout your body so that every cell gets oxygen and nutrients while waste is removed.
The Three Main Parts of the Circulatory System
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| Image from MedicineNet |
1. Heart - Pumps blood: The heart is a muscular organ about the size of your fist that pumps blood throughout your body.
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| Image from Britannica |
- Contracts (squeezes) to push blood out into arteries
- Relaxes to allow blood to flow in from veins
- Pumps about 5 liters of blood per minute at rest
- Beats about 60 to 100 times per minute in adults
The heart has four chambers: two upper chambers called atria and two lower chambers called ventricles. Blood flows into the atria, then down into the ventricles, which contract to push blood out of the heart.
2. Veins - Bring blood back to the heart: Veins are blood vessels that carry blood BACK TO the heart.
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| Image from First Aid for Free |
- Thinner walls than arteries
- Less elastic than arteries
- Carry blood with low oxygen content (darker red or blue)
- Blood in veins moves more slowly
- No pulse (blood pressure is lower)
- Have valves inside to prevent blood from flowing backward
Bring blood back to the heart so it can be pumped to the lungs to get more oxygen.
3. Arteries - Bring blood away from the heart: Arteries are blood vessels that carry blood AWAY FROM the heart.
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| Image from Verywell Health |
- Thick, muscular walls
- Elastic (stretchy) to handle the pressure of blood being pumped
- Carry blood with high oxygen content (bright red)
- Blood in arteries moves quickly and under high pressure
- Pulses with each heartbeat (you can feel your pulse in arteries)
Bring blood away from the heart to deliver oxygen and nutrients to all body parts.
BLOOD
Blood - is not just a red liquid. It is made up of several important components:
Red Blood Cells (RBCs)
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| Image from Microbe Notes |
Cells that carry oxygen throughout your body.
Why they are red: Red blood cells contain a protein called hemoglobin that binds to oxygen. Hemoglobin is red, which makes red blood cells red.
What they do:
- Carry oxygen from lungs to body cells
- Pick up carbon dioxide from body cells
- Return to lungs to release carbon dioxide
Red blood cells are the delivery trucks of your circulatory system. They carry oxygen to all your cells.
White Blood Cells (WBCs)
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| Image from Microbe Notes |
Cells that fight infection and protect your body.
What they do:
- Identify and attack bacteria, viruses, and other pathogens
- Remove dead cells and foreign materials
- Part of your immune system
White blood cells are your body's security guards. They patrol and fight off invaders.
Platelets
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| Image from The Platelet Society |
Small cell fragments that help blood clot.
What they do:
- Form clots to stop bleeding when you are injured
- Stick together to plug holes in blood vessels
- Work with other factors to form stable clots
Platelets are like patches that seal up holes when you bleed.
Plasma
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| Image from News Medical |
The liquid part of blood that carries cells, nutrients, and waste.
What it contains:
- Water
- Proteins
- Nutrients (glucose, amino acids, fats)
- Hormones
- Waste products
- Clotting factors
Plasma is the liquid that carries everything in your blood.
Blood Clotting and How Your Body Stops Bleeding
When you get cut, you bleed, but then the bleeding stops. This happens through blood clotting. Blood clotting is a complex process that involves:
1. Platelets - Stick to the wound to form a plug
2. Fibrinogen - A protein in plasma that forms the blood clot. It creates a mesh that traps blood cells and stops bleeding.
3. Vitamin K - A vitamin needed for blood clotting to work properly
4. Calcium - A mineral needed for blood clotting
- When you bleed, platelets rush to the wound, fibrinogen creates a mesh to trap blood cells, and the clot stops the bleeding. Vitamin K and calcium are essential helpers.
Important Nutrients for Blood Health
Your blood needs certain nutrients to work properly:
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| Image from Ambar Lab |
Fibrinogen - a protein found in plasma. Forms the mesh structure of blood clots. When you bleed, fibrinogen polymerizes (connects together) to create a net that traps blood cells and stops bleeding. Without fibrinogen, your blood cannot clot and you would bleed to death from even a small cut.
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| Image from The Nutrition Source |
Vitamin K - fat-soluble vitamin that your body stores. Helps activate clotting factors so blood can clot. Without vitamin K, the clotting process cannot happen properly and you will bleed excessively.
Where you get it:
- Leafy green vegetables (spinach, kale, broccoli)
- Your intestinal bacteria also produce vitamin K
- Some from vegetable oils
Vitamin K is needed to activate the clotting process so your blood can form clots.
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| Image from Healthy Bones Australia |
Calcium - a mineral essential for many body functions, including blood clotting. Acts as a cofactor (helper) in the blood clotting cascade. Multiple steps of clotting require calcium. Calcium is needed for the chemical reactions that allow blood to clot. Even though calcium is common, without it your blood cannot clot properly.
Where you get it:
- Dairy products (milk, yogurt, cheese)
- Leafy green vegetables
- Fish with bones
- Fortified plant milks
Calcium is a mineral that helps activate the clotting process.
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| Image from swaeducation |
Vitamin B12 - a water-soluble vitamin important for many body functions, especially red blood cell production. Vitamin B12 is needed for your bone marrow to produce healthy red blood cells. Without vitamin B12, you can develop anemia (not enough red blood cells).
What it does:
- Needed to make DNA and RNA in red blood cells
- Essential for red blood cell production in bone marrow
- Helps maintain the health of nerve cells
Where you get it:
- Animal products (meat, fish, dairy, eggs)
- Some fortified cereals and plant milks
- Supplements
Vitamin B12 is needed to make new red blood cells. Without it, you do not have enough red blood cells to carry oxygen.
How the Circulatory System Works
1. Blood from the body enters the heart. Oxygen-poor blood returns from your body through veins. It enters the right atrium (upper right chamber) of the heart
2. Blood flows to the lungs. The right atrium contracts, pushing blood down into the right ventricle. The right ventricle contracts, pushing blood to the lungs through the pulmonary artery. In the lungs, blood picks up oxygen and releases carbon dioxide
3. Oxygen-rich blood returns from the lungs. Blood returns from the lungs through the pulmonary vein. It enters the left atrium (upper left chamber) of the heart
4. Blood is pumped to the body. The left atrium contracts, pushing blood down into the left ventricle. The left ventricle (the strongest chamber) contracts forcefully, pushing oxygen-rich blood out through the aorta. The aorta branches into smaller and smaller arteries that reach every part of your body
5. Blood delivers oxygen and picks up waste. Arteries branch into tiny capillaries (smallest blood vessels). In capillaries, oxygen moves from blood into cells. Carbon dioxide and other wastes move from cells into blood. Blood becomes oxygen-poor
6. Oxygen-poor blood returns to the heart. Capillaries merge into veins. Veins carry oxygen-poor blood back to the heart. The cycle repeats
Oxygen-poor blood comes back to the heart, gets pumped to the lungs to get oxygen, returns to the heart, gets pumped to the whole body to deliver oxygen, and then comes back again. This cycle repeats about 60 to 100 times per minute.
Blood Vessels
The circulatory system uses three types of blood vessels to transport blood:
1. Arteries
- Carry blood away from the heart
- Thick, muscular, elastic walls
- Carry oxygen-rich blood (bright red)
- High pressure
- Pulse with heartbeat
2. Capillaries
- Tiny blood vessels connecting arteries and veins
- Walls only one cell thick
- Allow exchange of oxygen, nutrients, and waste between blood and cells
- Very slow blood flow
- Connect billions of body cells to the circulatory system
3. Veins
- Carry blood back to the heart
- Thin walls with valves
- Carry oxygen-poor blood (darker red)
- Low pressure
- No pulse
Pulse and Heart Rate
Pulse: The feeling of blood being pushed through arteries with each heartbeat.
Where to feel your pulse:
- Wrist (radial artery)
- Neck (carotid artery)
- Temple
- Inside of elbow
How to take your pulse:
- Place two fingers (index and middle finger) on your wrist or neck
- Count the beats for 60 seconds
- This is your heart rate in beats per minute
- Normal resting heart rate for adults: 60 to 100 beats per minute
Your pulse is the beating of blood being pushed out of your heart. You feel a pulse every time your heart beats.
Circulatory System Quiz: click here
