Human Circulatory System

Human Circulatory System

Table of Contents

Circulatory System

Imagine a complex network of highways and bylanes delivering essential supplies and removing waste throughout a city. That’s exactly what your circulatory system does for your body!

What it Circulatory System

The circulatory system is a network of organs and vessels that keeps your body functioning smoothly.

Vital Role

This system is like a lifeline, constantly ensuring:

  • Delivery of oxygen and nutrients: Every cell in your body needs oxygen and essential nutrients to function. The circulatory system acts like a delivery service, bringing these vital components to all corners of your body.
  • Waste removal: Cells produce waste products during their activities. The circulatory system acts like a sanitation service, collecting this waste and transporting it to organs like the kidneys and lungs for removal.

The Key Players

  • Heart: The tireless pump at the center of the system, the heart continuously pushes blood throughout the body.
  • Blood vessels: These are the highways and bylanes mentioned earlier. They come in different types:
    • Arteries: Strong, elastic vessels that carry oxygen-rich blood away from the heart to tissues.
    • Capillaries: Microscopic vessels where the exchange of oxygen, nutrients, and waste products takes place between blood and tissues.
    • Veins: Thinner-walled vessels that carry oxygen-depleted blood back to the heart.

Components of the Circulatory System

The Heart

Imagine your heart as a muscular pump, about the size of your fist. Its main job is to keep blood flowing throughout your body, delivering oxygen and nutrients to all your cells. Here’s a simplified breakdown of its structure and function:

Structure and Functions of Heart - GeeksforGeeks


Think of the heart as having two sides, each with two chambers:

  • Upper chambers:
    • Right atrium: Receives deoxygenated blood (low in oxygen) from your body.
    • Left atrium: Receives oxygenated blood (rich in oxygen) from your lungs.

Heart chamber anatomy PI - UpToDate

  • Lower chambers: These are the powerhouses that pump blood:
    • Right ventricle: Pumps deoxygenated blood to your lungs for oxygen pickup.
    • Left ventricle: Pumps oxygenated blood out to your entire body.


  • Doorkeepers: The heart has several valves that act like one-way doors, ensuring blood flows in the right direction:

Structural Heart Programme TAVI for Aortic Stenosis - CVSKL 

    • Tricuspid valve: Between the right atrium and ventricle (3 cusps or flaps).
    • Bicuspid valve (mitral valve): Between the left atrium and ventricle (2 cusps).
    • Semilunar valves: Guard the exits of the ventricles (aortic valve and pulmonary valve).

Pumping Action

  • Heartbeat: Your heart has a rhythmic contraction and relaxation cycle:

Diastole vs. systole: What is the difference? 

    • Systole (contraction): Muscles squeeze, pushing blood out of the ventricles (powerful pump).
    • Diastole (relaxation): Muscles relax, allowing chambers to refill with blood.

Blood Flow

1: (a) Blood flow through the heart; (b) The heart inside the whole... | Download Scientific Diagram

  • Two Circuits: Imagine two loops:
    • Pulmonary circulation:
      • Deoxygenated blood from the body enters the right atrium.
      • Flows through the tricuspid valve to the right ventricle.
      • Pumped to the lungs through the pulmonary artery for oxygenation.
      • Oxygenated blood returns to the left atrium through the pulmonary veins.
    • Systemic circulation:
      • Oxygenated blood from the lungs enters the left atrium.
      • Flows through the bicuspid valve to the left ventricle (strongest chamber).
      • Pumped out to the body through the aorta, the largest artery.
      • Deoxygenated blood from the body eventually returns to the right atrium, completing the cycle.

Blood Vessels

Imagine your circulatory system as a network of highways and tiny alleys:


These are like the highways that carry oxygen-rich blood away from your heart. They have thick, muscular walls to withstand the high pressure of the blood pumping out of the heart. Some major arteries include:

Arteries: Definition, Structure, Types, Functions, Diseases

  • Aorta: The largest artery, coming directly out of the heart.
  • Carotid arteries: Located on the neck, supplying blood to the brain.
  • Brachial artery: Located in the arm, supplying blood to the forearm and hand.


These are like the alleys that carry oxygen-depleted blood back to your heart. They have thinner walls than arteries and contain valves to prevent blood from flowing backward. Two major veins include:


  • Superior vena cava: Drains blood from the upper body.
  • Inferior vena cava: Drains blood from the lower body.


These are like the tiny connecting paths between arteries and veins. They are microscopic blood vessels with very thin walls that allow for the exchange of materials between the blood and tissues. This is where:

SEER Training: Classification & Structure of Blood Vessels

  • Oxygen and nutrients from the blood are delivered to the cells.
  • Carbon dioxide and waste products from the cells enter the bloodstream to be removed by the lungs and kidneys.

In simpler terms:

  • Arteries: Deliver fresh oxygen from the lungs to the body.
  • Veins: Collect used oxygen and waste products from the body to be disposed of.
  • Capillaries: Facilitate the exchange of oxygen, nutrients, and waste products between the blood and tissues.

Blood Composition:


Imagine blood as a watery soup. Plasma is the yellowish liquid part, making up about 55% of blood. It’s like the delivery truck, carrying vital supplies throughout your body:

Blood Plasma Components and Function

  • Nutrients: Minerals, vitamins, and sugars needed by your cells to function properly.
  • Hormones: Chemical messengers that control various body functions like growth, development, and metabolism.
  • Waste Products: Carbon dioxide and other substances removed by organs like the kidneys and liver.

Red Blood Cells (RBCs)

These are the most abundant cell type in blood, accounting for roughly 45% of its volume. They are the tiny red heroes responsible for oxygen delivery:

Red Blood Cell - The Definitive Guide | Biology Dictionary

  • Hemoglobin: A protein inside RBCs that acts like a taxi driver, specifically designed to pick up oxygen from your lungs and drop it off at your body tissues.

White Blood Cells (WBCs)

These are much fewer in number compared to RBCs, but they play a crucial role in your body’s defense system:

Blood cells and its types with functions

  • Imagine them as the army constantly patrolling your body, protecting you from invaders:
    • Neutrophils: The most common WBCs, like soldiers at the front line, attacking and engulfing bacteria.
    • Lymphocytes: Special forces that produce antibodies to fight specific infections, like remembering and targeting enemies they’ve encountered before.
    • Monocytes: Large warriors that gobble up foreign particles and debris.
    • Eosinophils: Battle against parasites and involved in allergic reactions.
    • Basophils: Release chemicals that trigger allergic reactions, like sounding the alarm when they sense danger.


These are tiny cell fragments, even smaller than WBCs, that play a vital role in preventing excessive blood loss:

File:Diagram of a platelet CRUK 407.svg - Wikimedia Commons

  • Imagine them as repair crew, patching up any holes in your blood vessels to stop bleeding. They clump together to form clots, sealing the wound and preventing you from leaking precious blood.

Function of the Circulatory System

Oxygen and carbon dioxide exchange:

  • Oxygen is picked up from the lungs by red blood cells.
  • As blood travels through arteries to body tissues, oxygen is delivered to cells for energy production.
  • Carbon dioxide, a waste product from cells, is then picked up by the blood.
  • In the lungs, carbon dioxide is released back into the air as you exhale.


  • Nutrients like glucose (sugar) and amino acids (building blocks for proteins) are absorbed from your digestive system.
  • The circulatory system transports these essential components throughout the body to fuel cells and keep them functioning.

Taking out Waste products

  • Cells constantly produce waste products like urea, a byproduct of protein breakdown.
  • The circulatory system collects this waste and transports it to the kidneys and liver for filtering and removal from the body through urine and feces.

Maintaining a healthy balance

Body temperature:

  • Blood acts like a mobile heater, carrying warmth from organs like the liver and muscles to the skin.
  • Sweating also helps regulate temperature by releasing excess heat.

Acid-base balance:

  • The body constantly works to maintain a slightly alkaline pH level (not too acidic, not too basic).
  • The circulatory system helps transport chemicals that neutralize acids and keep the body’s internal environment stable.

Hormone distribution:

  • Hormones are chemical messengers produced by glands.
  • The circulatory system acts like a postal service, delivering these hormones to various organs throughout the body to influence different functions.

Double Circulation

Imagine a highway system with only one lane. Cars carrying oxygenated blood (fresh air) would get stuck in traffic with those carrying used blood (waste). This is what happens in some animals with single circulation.

Our body is smarter! We have a double circulation system, like a two-lane highway:

  • Lane 1 (Pulmonary Circulation): Picks up oxygen from the lungs and delivers it to the heart.
  • Lane 2 (Systemic Circulation): Carries oxygen-rich blood from the heart to the entire body and returns used blood back to the lungs.

Benefits of Two Lanes

  • Keeps blood types separate: Oxygenated and used blood never mix, ensuring a constant supply of fresh oxygen to cells.
  • More efficient delivery: Higher blood pressure in one lane (systemic) allows for efficient distribution throughout the body.

Disorders of the Circulatory System

  • Heart Attack: Blocked blood flow due to a clot, damaging heart muscle.
  • Stroke: Clot or bleeding in the brain, affecting brain function.
  • High Blood Pressure: Persistently high force of blood against artery walls.

Maintaining a Healthy Highway

  • Exercise: Regular physical activity strengthens the heart and improves blood flow.
  • Balanced Diet: Eating nutritious foods provides essential nutrients for the heart and blood vessels.
  • Avoid Smoking: Smoking damages blood vessels and increases the risk of circulatory problems.


The human circulatory system stands as a testament to the remarkable engineering of the human body. This intricate network of heart, blood vessels, and blood acts as the body’s internal transport system, playing a critical role in:

  • Sustaining life: Delivering vital oxygen and nutrients to every cell, while removing waste products like carbon dioxide.
  • Maintaining homeostasis: Regulating body temperature, acid-base balance, and hormone distribution throughout the body.
  • Fighting off invaders: White blood cells within the circulatory system act as the body’s defense system against pathogens.


Think of your body as a kingdom. The circulatory system is its intricate network of highways, constantly delivering essential supplies and removing waste. It’s like a complex plumbing system that keeps everything running smoothly.

  • Heart: The powerful pump that keeps blood circulating.
  • Blood: The vital fluid carrying oxygen, nutrients, and waste products.
  • Blood vessels: The network of tubes (arteries, veins, and capillaries) that transport blood throughout the body.
  • Lymphatic system: A supporting network that helps drain excess fluid and fight infection.
  • Open Circulatory System: Found in simpler animals like insects. Blood flows freely in a cavity, not entirely contained within vessels.
  • Closed Circulatory System: Most common, including humans. Blood is enclosed within vessels, ensuring efficient delivery and separation of oxygenated and deoxygenated blood.
  • Double Circulatory System: A special type found in mammals and birds. Blood travels through two separate loops, one for the lungs and another for the rest of the body, maximizing efficiency.
  • Delivers oxygen: Fresh oxygen from the lungs is carried by red blood cells to fuel all your body’s functions.
  • Transports nutrients: Essential nutrients from food are absorbed and delivered to cells for growth and repair.
  • Removes waste: Carbon dioxide and other waste products are carried away from cells and eliminated from the body.
  • Regulates body temperature: Blood helps distribute heat throughout the body, maintaining a stable internal temperature.
  • Fights infection: White blood cells and other components of the immune system travel through the bloodstream to combat infections.


1. Which of the following is the main organ of the circulatory system? 

a) Liver
b) Heart
c) Brain
d) Kidneys
Answer: b) Heart

2. What is the function of red blood cells in the circulatory system? 

a) Fighting infections
b) Carrying oxygen
c) Producing hormones
d) Digesting food
Answer: b) Carrying oxygen

3. Which blood vessel carries oxygen-rich blood away from the heart to the body? 

a) Artery
b) Vein
c) Capillary
d) Bronchiole
Answer: a) Artery

4. What is the name of the largest artery in the human body? 

a) Aorta
b) Pulmonary artery
c) Jugular vein
d) Vena cava
Answer: a) Aorta

5. Which chamber of the heart pumps oxygenated blood to the rest of the body? 

a) Left atrium
b) Left ventricle
c) Right atrium
d) Right ventricle
Answer: b) Left ventricle

6. What is the liquid part of the blood called? 

a) Plasma
b) Platelets
c) White blood cells
d) Hemoglobin
Answer: a) Plasma

7. Which blood vessels carry blood back to the heart?

a) Arteries
b) Veins
c) Capillaries
d) Bronchioles
Answer: b) Veins

8. What is the average resting heart rate of an adult human? 

a) 60-80 beats per minute
b) 100-120 beats per minute
c) 20-40 beats per minute
d) 150-180 beats per minute
Answer: a) 60-80 beats per minute

9. What is the role of valves in the circulatory system? 

a) To regulate blood pressure
b) To control the flow of blood
c) To produce red blood cells
d) To store excess blood
Answer: b) To control the flow of blood

10. Which blood type is known as the universal donor? 

a) Type A
b) Type B
c) Type AB
d) Type O
Answer: d) Type O

11. What is the name of the condition where fatty deposits clog the arteries?

a) Aneurysm
b) Stroke
c) Atherosclerosis
d) Thrombosis
Answer: c) Atherosclerosis

12. Which organ produces most of the body’s blood cells? 

a) Liver
b) Lungs
c) Bone marrow
d) Spleen
Answer: c) Bone marrow

13. Which chamber of the heart receives blood from the body? 

a) Left atrium
b) Left ventricle
c) Right atrium
d) Right ventricle
Answer: c) Right atrium

14. What causes the sound of a heartbeat? 

a) Flow of blood through the veins
b) Closing of heart valves
c) Movement of the heart muscles
d) Expansion of blood vessels
Answer: b) Closing of heart valves

15. What is the name of the process by which blood clots to stop bleeding?

a) Hemorrhage
b) Coagulation
c) Inflammation
d) Circulation
Answer: b) Coagulation

16. Which blood vessels have the thinnest walls, allowing for the exchange of gases and nutrients? 

a) Arteries
b) Veins
c) Capillaries
d) Venules
Answer: c) Capillaries

17. What is the term for the force exerted by blood against the walls of blood vessels? 

a) Blood pressure
b) Heart rate
c) Pulse
d) Cardiac output
Answer: a) Blood pressure

18. Which component of blood is responsible for clotting? 

a) Red blood cells
b) White blood cells
c) Platelets
d) Plasma
Answer: c) Platelets

19. Which part of the brain controls the heartbeat and blood pressure? 

a) Cerebrum
b) Cerebellum
c) Medulla oblongata
d) Hypothalamus
Answer: c) Medulla oblongata

20. What is the term for the circulation of blood between the heart and lungs? 

a) Systemic circulation
b) Pulmonary circulation
c) Coronary circulation
d) Renal circulation
Answer: b) Pulmonary circulation

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