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  • Blood circulates throughout the body in a system known as the circulatory system
  • The heart, blood and the blood vessel make up the circulatory system
  • The heart is a muscular organ that pumps blood to all parts of the body. Blood circulates around the body in blood vessels
  • Blood is a body fluid that is used in the transportation of substances within the body.
  • Blood vessels are tubes in which blood flows

          The main blood vessels are the arteries, veins and capillaries

Main Parts of Human Circulatory System

  • The circulatory system is important because it is involved in the transport of the following substances in the body.
    • Oxygen from the lungs to all parts of the body
    • Digested food from the small intestines to all parts of the body
    • Carbon dioxide from the body to the lungs where it is breathed out.
    • Heat from the liver to all parts of the body. This helps to regulate body temperature

Waste products from different parts of the body to organs such as kidneys to be removed from the body

Parts of the Heart and Their Functions

Parts of the heart and their functions

HEART – This is the organ that pumps blood throughout the body. It is muscular and placed between the lungs somewhere slightly to the left side of the body. The strong muscles of the heart can relax or contract when contracting the heart pumps blood with force. When the heart relaxes, the blood flows into chambers of the heart

  • The heart has 4 chambers.
  • The upper chambers are known as auricles and the lower chambers are known as


  • The heart has 2 auricles, the left auricle and the right auricle and 2 ventricles and the

right ventricle and the left ventricle.

  • Auricle have thinner walls than ventricles
  • The function of the heart is to pump blood to all parts of the body
  • Auricles receive blood from body organs and then empty it into the ventricles.
  • The ventricles then pump blood to the lungs and to other parts of the body
  • The heart has valves that prevent blood from flowing backwards

The heart is connected to blood vessels Pumping of the heart

  1. The right auricle receives blood from the body The blood then flows into the right ventricle

This blood does not have oxygen and is called deoxygenated blood

  • The right ventricle pumps the deoxygenated blood into the left into lungs through the pulmonary artery.

In lungs, the blood receives oxygen and becomes oxygenated blood.

  • The oxygenated blood flows from the lungs into the left auricle of the heart through the pulmonary vein

The blood then flows into the left ventricle

  • The left ventricle then pumps the oxygenated blood to all parts of the body(except the lungs) through the aorta

The left ventricle has thick muscular walls because they pump blood to all parts of the body

  • The heart has valves which prevent blood from flowing backwards

Types of blood vessels and their functions Blood vessels are tubes that carry blood around the body. The main blood vessels in the body are

  1. Arteries
  2. Capillaries
  3. Veins


  • Arteries have thick elastic walls
  • Arteries have a narrow lumen

Lumen is the space inside a tube such as a blood vessel

  • Arteries carry blood away from the heart to the parts f the body
  • Arteries carry oxygenated blood except the pulmonary artery which carries deoxygenated blood from the heart to the lungs
  • Blood in arteries flow under high pressure because it is pumped from the heart into the arteries at high pressure.

This high pressure can be felt in the arteries as a beat.

We can determine the number of times the heart beats in a minute by counting the beats in the arteries

The number of times the heart beat in a minute is called a pulse or a beat rate. The pulse is easiest to find on the wrist and on the neck.

To check the pulse at the wrist

Pupil’s activity Page 41-42

NamePulse at restPulse after jumping

NB: the pulse increased after jumping

  • The pulse rate of a healthy person at rest is between 60-100 beats per minute
  • Pulse increased during an activity

This because during activity, the body needs more oxygen and so the heart pumps blood faster to supply the body with more oxygen.

Grade 6 Science Notes-Human Circulatory System


  1. Veins have thin walls
  2. Veins have a wide lumen
  3. Veins have valves to ensure blood flows only in one direction
  4. Veins carry blood towards the heart
  5. Veins carry deoxygenated blood except the pulmonary vein that carries oxygenated blood from the lungs to the heart.

Valve A shows a vein with open valve to allow blood to pass through it while valve B shows a vein with closed valve to prevent blood from flowing backwards


  1. Capillaries have very thin walls
  2. Capillaries have no valves
  3. Capillaries reach every part of the body.

They allow movements of oxygen and food nutrients from blood into the body.

They also allow the movement of carbon dioxide and other wastes from the body into the blood

Difference between arteries, veins and capillaries

Have thick elastic walls,Have thin walls and elastic,Have thin walls
Have no valvesHave valvesInvolved in the exchange of substances between blood and body organs
Carry blood away from the heart Are very narrow,
  Form network in every organ and tissue

Components of blood and their functions

To find out the components of blood

Pupil’s activity Page 43-44

To study the components of blood

Pupil’s activity Page 44


The four main components of blood are:

  1. Plasma
  2. Red blood cells
  3. White blood cells
  4. Platelets

1.      Plasma

It is the liquid part that forms the main part of the blood It is mostly pale yellow.

It contains dissolved substances e.g. digested food, salts, amino acids and glucose.

Functions Plasma

Transportation of:

  1. Digested food from the ileum to all parts of the body.
  2. Oxygen from the lungs to all body tissues.
  3. Carbon dioxide from body cells to the lungs to be expelled.
  4. Waste products to the organs of excretion.
  5. Heat from the liver to all parts of the body.
  6. Hormones from the glands to where they are needed.
  7. Other blood components e.g. white blood cells, red blood cells and platelets to where they are needed

Red blood cells

-Biconcave in shape.

  • Contain haemoglobin (Red colouring matter).
  • Have nucleus which disappears on maturity.
  • Produced in red bone marrow
  • Destroyed (broken) in the liver/spleen.
  • Red blood cells carry oxygen from the lungs to the body tissues.
  • Oxygen combines with haemoglobin to form oxyhaemoglobin –oxygenated blood, which is bright red in colour (dark red)

White blood cells

  • Larger than red blood cells
  • Few in number compared to red blood cells i.e. ratio of white to red blood cells is 1:600
  • Have no definite shape- they change their shape easily
  • Have a nucleus at the centre
  • Produced in the yellow bone marrow and the lymph glands

N.B. White blood cells fight and kill germs by engulfing them.

d)   Platelets

  • They are tiny oval shaped cells
    • They are found in plasma
    • Help the blood to clot when injured.
    • They prevent further loss of the blood from the part that was injured. They help to stop bleeding from cuts and wounds.

Blood groups

  • Blood group is the type of blood a person has.
    • The ABO blood group system is one of the ways of grouping blood.
    • In the ABO blood group system, there are 4 main blood groups
    • These are
  • Blood group A
  • Blood group B
  • Blood group AB
  • Blood group O

Blood transfusion

  • It is the process by which blood from one person is added to another person
    • The person who gives blood is called donor.
    • The person who receives blood is known as recipient.
    • Blood transfusion is done to help restore blood in people who have lost a lot of blood due to injuries or disease
    • Before a blood transfusion is done it is important to know the blood group of both donor and the recipient.
    • This is to ensure that compatible, that is it can mix without the red blood calls clumping together (agglutination) in the receipt’s body.
    • Clumping together of red blood cells can be dangerous

Compatibility of blood group

Blood group ABlood group A and ABBlood group A and blood group O
Blood group BBlood group B and blood group ABBlood group B and Blood group O
Blood group ABBlood group ABAll groups
Blood group OAll groupsBlood group o

Compatibility of blood groups

  • A person with blood group O can donate blood to people of all the other blood groups.

People with blood group O are referred to as universal donor

  • A person with blood group AB can receive blood from all the blood groups and is therefore referred to as universal recipient.

To make models of different components of blood

Pupil’s activity Page 48-49


Living things have organs that enable them to reproduce. These organs form the reproductive system

This part presents two kinds of reproductive system, namely female and male reproductive systems.


Parts and functions of the female reproductive system

To discuss the parts and functions of the female productive system

Pupil’s activity Page 50-51

The system by which human beings are enabled to produce young ones is called female

reproductive system. The system consists of different parts. These include:

  1. Vagina
  2. Cervix
  3. Uterus
  4. Ovaries and
  5. Fallopian tubes.

Definition and functions of the major parts of the female reproductive system

1.      Ovary

  • They are mall oval shaped glands that are located on either side of the uterus
  • They produce egg cells called ova in a process called ovulation. When released the eggs enter into the oviduct
  • They produce hormones

1.      Oviduct

  • Also called fallopian tube
    • The oviduct is a tube that connects the ovary to the uterus
    • It is the place where fertilization of the egg by the sperm takes place

2.      Uterus

  • Also known as womb
  • The place where the fertilized egg develops into a foetus

3.      Cervix

  • It connects the vagina with the uterus
  • It produces mucus that facilitates the entry of sperms
  • It opens to allow passage of a baby from the uterus into the vagina during childbirth

4.      Vagina

  • Also known as birth canal
  • It is an elastic tube that extends from the vaginal opening (vulva) to the cervix
  • It receives semen during intercourse
  • It is the birth canal through which the body passes during birth

Parts and function of the male reproductive system

To discuss the parts and functions of the male reproductive system

Pupil’s activity Page 52-53

Male Reproductive System consists of various parts. These include

  1. Penis
  2. Testis
  3. urethra,
  4. Gland – Prostate gland, seminal vesicles and the Cowper’s gland

Definition and functions of the major parts of the male reproductive system

  1. Penis
  2. It is the male sex organ
  3. Tube like structure through which sperms are released
  4. It transfers sperm into the female reproductive system during intercourse
  5. Also in this structure urine is passed outside the body.

2.  Testicle or testis/testes

  • Testis is oval shaped and is enclosed within a structure called scrotum which hangs outside the body
  • Testis produces sperms and hormones

3.  Urethra

  • The urethra is a tube that runs through the penis
  • It is a passage of sperms and urine out of the body
  • Gland
  • The glands produce a fluid as seminal fluid
  • Sperm cells depend on seminal fluid to move and to keep them alive
  • The mixture of seminal fluid and sperm is called semen

5.  Sperm duct

  • A tube that allows the sperm to pass from testis to the urethra

Physical changes that occur during adolescence

  • The period in a persons’ life when developing from a child into an adult is referred to as adolescence.
  • The stage is usually between the age of 12 and 19 years.
  • During this time many changes take place in terms of growth and physical changes.
  • Physical changes are changes that are visible
  • The boy or girl who is undergoing this change is called adolescent

Physical Changes in Boys during adolescence

  1. Broader chest and shoulders
  2. Breaking voice to become deeper
  3. The penis, testes and scrotum enlarge
  4. Growing of hair in part of the body (around sex organs, pubic hair, chest hair, beards on the face and armpits.
  5. Sperms mature in the testis experiences ejaculation, which is release of sperms through penis. At times this can happen during the night and is called wet dreams.
  6. Boys eat more because height and weight are increasing and becoming muscular
  7. At times development of pimples on the face may occur

Physical Changes in Girls during adolescence

  1. Growth of breasts
  2. Hair grows in the armpits and around sex organs(pubic hair)
  3. Hips become broader,
  4. Release an egg by ovaries after 28 days (ovulation). This happens if the egg(ovum) is not fertilized. The lining that had been formed in the uterus along with the egg breaks down and flows out of the body through vagina as blood. This process is called menstruation. It occurs once a month and may last 4-5 days,
  5. Pimples may appear on the face
  6. Rapid increase of weight and heights and may cause them to eat more

NB: Apart from physical changes, adolescents undergo other changes that affect their feelings and behaviour towards other people.

These changes are known as emotional changes


Moods: Hormones lead to mood changes that an adolescent cannot explain.

This might bring misunderstanding between the adolescent and other people. They become unreasonably aggressive, angry, easily disappointed etc.

Shyness: Girls feel shy about their enlarging breasts or pimples on their face. Boys are shy about their cracking voice.

Embarrassment: Girls are embarrassed about their menstrual flow. Boys about their wet dreams

Unhappy: Boys and girls feel unhappy with the size and shape of their bodies.

Worry: Both tend to worry about their appearance, especially when pimples develop on the face, a condition known as Acne.

Girls who start their menstrual flow late or have small breasts tend to worry about themselves. They may feel abnormal.

Social implications of changes that occur during adolescence

  1. Development of new identity

This makes adolescent try out new clothing styles, listen to new music and develop new friendships all in a bid to behave like adults

  • Development of values

This makes adolescent question things.

They therefore seem like they are rebelling against established rules

  • Desire for independence

This makes adolescents want to make their own decisions like how to spend their free time or how to spend their money

  • Increased peer influence

This influences adolescent’s behaviour and mode of dressing Adolescents want to be important and recognised by their friends

  • Development of interest in the other gender leading to relationships
  • Increased influence from media. The internet greatly influences adolescents

Health implications of changes that occur during adolescents

  1. Sexually transmitted infections (STI) and diseases usually pass from one person to another through sexual contact

AIDS, Syphilis and Gonorrhoea are some of examples of sexually transmitted infections

  • Adolescents need to practise good hygiene fo their well being and for those around them
  • Teenage pregnancy can increased health risks for newborns as well as for the young mothers
  • The use of alcohol and other drugs can lead to addition, failure in school and poor judgement which may put adolescents at risk of accidents and suicide
  • Adolescents are encouraged to share their feelings when they feel overwhelmed.

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