The main transport system of human is the circulatory system ie system of blood vessels with the heart.


  • To transport nutrients and oxygen to the cells.
  • To remove waste and carbon dioxide from the cells.
  • To provide for efficient gas exchange.

The circulatory system

  • Animals are made up of numerous body cells. Each of these cells needs oxygen and nutrients to
    carry out life processes. They also excrete waste materials that need to be taken away.
  • To achieve this, animals have a transport system called the circulatory system that carries
    substances from one place to another.
  • There are two types of the circulatory system

Open circulatory system

  •  The open circulatory system is found in small animals such as insects.
  • Animals with an open circulatory system do not contain blood vessels but have a fluid
    cavity where organs are suspended.
  • Oxygen and nutrients diffuse directly from the fluid to body cells and waste materials from
    the body cells to the fluid.

Closed circulatory system

  • A closed circulatory system is found in large animals, such as humans.
  • Animals with a closed circulatory system contain blood vessels where blood flows
    continuously and is not in contact with body cells.
  • Blood is transported in a single direction carrying oxygen, nutrients and waste materials.
  • Substances move in and out of blood vessels by diffusion.
  • The closed circulatory system is further divided into two types of pathways

Single circulatory pathway

  • A single circulatory pathway is found in fish.
  • It consists of a single circuit, so blood passes once in the heat in each complete circuit.
  • The pathway consists of a two-chambered heart, with one atrium and one ventricle.
  • The atrium receives deoxygenated blood from all parts of the body.
  • The ventricle pumps deoxygenated blood to the gills to be oxygenated.
  • Oxygenated blood from the gills is supplied to the entire body, where it is used and returned to the heart as deoxygenated blood.

2. Double circulatory pathway

  • A double circulatory pathway is found in most animals.
  • The pathway consists of 2 circulations.
    1. Pulmonary circuit (also called pulmonary circulation): transport deoxygenated blood from the
    heart to the lung to be oxygenated, and back to the heart.
    2. Systemic circuit (also called systemic circulation): transport oxygenated blood from the heart the
    all parts of the body and returned to the heart as deoxygenated.
  • Animals with a double circulatory pathway contain a four-chambered heart, with two atria and two
  •  The right atrium receives deoxygenated blood from the entire body.
  • The right ventricle pumps deoxygenated blood to the lungs to be oxygenated.
  • The left atrium receives oxygenated blood from the lungs.
  •  The left ventricle pumps oxygenated blood to the entire body

The Heart

Structure and function

  • the heart has four chambers;
  • two atria and two ventricles
  • atria receive blood and ventricle pumps blood
  • the left and right side are separated by septum ie it prevents the oxygenated and deoxygenated blood from mixing
  • Deoxygenated blood from the rest of the body enters the heart through the vena cava
  • blood enters the right atrium, then through tricuspid valve into right ventricle
  • then via semi-lunar valves to pulmonary artery to the lungs
  • Oxygenated blood from the lungs enters the heart through pulmonary vein
  • It enters the left atrium of the heart, then through bicuspid valve into left ventricle
  • Then via semi-lunar valves to aorta which takes oxygenated blood to the body
  • a branch of the aorta ie coronary artery supplies blood to the heart
  • the valve inside the heart prevents the backflow of blood
  • the muscle of the left ventricle is thicker than those of the right ventricle to prevent it from bursting since this pumps blood under high pressure to the body
  • Atria receive blood;
  • the right Actium receive deoxygenated blood
  • left atrium receive oxygenated blood
  • Ventricles pump blood.
  • the right ventricle pump deoxygenated blood
  • the left ventricle pump oxygenated blood
  • Remember this Left Oxygenated | Right Deoxygenated [LO | RD]!!.

Cardiac Cycle 

Cardiac diastole: all chambers are relaxed, and blood flows into the heart Atrial systole, ventricular diastole: atria contract, pushing blood into the ventricles Atrial diastole, ventricular systole: after atria relax, ventricles contract, pushing blood out of heart

Coronary Heart Disease

  • Coronary artery becomes blocked, interrupting the supply of blood to the heart muscle.
  • The heart muscle cells are deprived of oxygen & glucose, and poisonous wastes such as lactic acid build up.
  • Part of the heart muscle stops contracting, causing a heart attack
  • Caused by stress, smoking, poor diet, poor lifestyle & genetically
  • Can be prevented by not smoking, avoiding fatty food and exercising regularly
  • Treated by aspirin and surgery (stents, angioplasty and by-pass)

Blood Circulation

  • Oxygenated and deoxygenated blood are separate
  • Remember this Left Oxygenated | Right Deoxygenated [LO | RD] !!.
  • blood pressure in systemic circulation is higher than the pulmonary circulation
  • the left ventricle is thicker as it pumps oxygenated blood under high pressure to the rest of the body

Blood vessels

  • They are tubular structures that extends throughout the body transporting blood to and from the heart.
  • Main types are.
    • Artery
    • Veins
    • Capillaries

Vessel Function Structure
Artery Transport high pressure blood away from heart Elastic walls expand and relax as blood is forced out; causes pulse
Thick walls withstand high pressure
Rings of muscle narrow or widen artery to control blood flow.
Vein Transport low pressure blood to the heart Valves prevent backflow of blood.
Blood is at low pressure, but nearby muscles squeeze veins and help push blood to the heart
Large diameter and thin walls reduce resistance to flow of blood
Capillary Allow substances to diffuse into cells One cell thick wall for easy diffusion
Highly branched; large surface area
Capillary beds constantly supplied with fresh blood, so diffusion occurs

The artery

  • carry blood away (to the rest of the body) from the heart.
  • they have thick muscles, elastic fibres and small lumen.
  • the elastic fibres allow them to stretch under pressure.

The vein

  • carry blood towards to the heart
  • they have thin muscle, elastic fibres and large lumen.
  • they have valves which prevents the back flow of blood
  • when the body muscles around the veins contract, they also squeeze the veins and push the blood along the vessels

The capillary

  • they link arteries with veins
  • they exchange materials between the blood and other body cells
  • they have one cell thickness

Artery Vein Capillary
Wall thickness Thick Thin Single cell thick
Lumen Narrow Wide Relatively large
Elasticity Elastic Inelastic No elastic fibres
Valves Absent Present Absent
Direction of blood flow Away from heart Towards heart From artery to vein
Pressure Higher Lower Relatively low
  • Semi-lunar valves.
  • prevent the back flow of blood
  • ensure one way blood movement

Causes of high blood pressure

  • Cigarette smoking causes deposits in arteries which reduce the smooth flow of blood
  • Diet: animal fats eg cholesterol is deposited in the walls of the arteries and reduce their diameter- atherosclerosis. The walls become hard and less elastic
  • Genetic factors

Effects of coronary heart diseases- CHDs

CHDs occurs when the coronary artery which supplies the heart muscle with blood is blocked and this leads to heart failure.

Causes of CHDs

  • Stress
  • Smoking- nicotine increases the changes of blood clotting
  • Obesity
  • Poor diet eg high fats intake
  • Lack of exercise

The Blood

A liquid connective tissue found circulating in the blood vessels. It consists of plasma and blood cells

Functions of blood

  • Red blood cells transport oxygen, nutrients, wastes, hormones etc.
  • phagocytes engulf pathogens
  • lymphocytes release antibodies to fight against pathogens
  • Blood platelets help the blood to clot to prevent the entry of pathogens

Components of plasma

  • is a pale-yellow fluid consisting of 90-95% water.
  • dissolved substances
  • glucose, amino acids, lipids, salts and other nutrients
  • hormones, fibrinogen, albumen proteins
  • antibodies, some enzymes, suspended cells
The functions of plasma
  • transport of red blood cells (carry oxygen)
  • transport dissolved food substances
  • transport of metabolic wastes
  • transport hormones
  • regulation of pH

Blood cells

Red blood cells (erythrocytes)

  • Made in the bone marrow of long and flat bones
  • Transport O2 from lungs to all respiring tissues
  • Contain hemoglobin (Hb), a red iron-containing pigment which can carry O2.
  • Have no nucleus
  • Have a biconcave disc shape -increases the surface area and makes the diffusion of oxygen into and out of the cell easier

White blood cells (leukocytes)

  • Made in the bone marrow and in the lymph nodes
  • Have a nucleus, often large and lobed.
  • move around and squeeze out through the walls of blood capillaries into all parts of the body
  • mainly two types- lymphocytes and phagocytes
  • lymphocytes release antibodies to fight against pathogens
  • phagocytes engulf and digest pathogens

Immune system

Phagocyte Lymphocyte
Phagocyte has lobed nucleus and vesicles containing digestive enzymes. Lymphocytes are found in blood and in lymph nodes
Phagocytosis: engulf pathogen, vesicles fuse with vacuole, enzymes digest bacteria. Large nucleus and they produce antibodies,
Antigen: protein/ carbohydrate on surface of pathogen which provokes immune system Antibodies: Y-shaped protein, bind to label pathogens.
Then either destroyed by being ingested by phagocytes, or the antibodies may do it.

Platelets (thrombocytes)

  • Small fragments of cells, with no nucleus
  • Made in the red bone marrow
  • Involve in blood clotting, prevent the entry pathogens and loss of blood

Formation of Tissue Fluid

  • It is the fluid that surrounds all body cells.
  • It supplies cells with all the requirements eg oxygen nutrients and takes away metabolic eg carbon dioxide.
  • it is similar in composition with plasma but with less protein.
  • It does not have red blood cells. lymphocytes, platelets and other proteins


January 15, 2024

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