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This unit is about transport in animals in detail. Animals have special structures and pathways through which they they transfer materials throughout the Body. In this unity we dive in deep to understand these processes and structures.


This refers to the movement of materials from one part of the organism to another.

Smaller organisms (protozoa) that have large surface area to volume ratio carry out transport by simple diffusion. Transport system is important in large organisms (multicellular) because the increased size of the organisms and the great distance over which materials are supposed to move makes diffusion rate slow which in turn make it inadequate for the distribution of these materials.

To overcome the physical limitation on size placed by diffusion, multicellular animals have the major adaptations. They have organs that provide a large surface area for absorption of nutrients such as small intestines and exchange of gases such as lungs/ gills, without a great increase in total body volume. They have a transport (circular) system within the body, so that substances can be carried to cells that need them and waste products removed more quickly than in diffusion.


Requirements of transport system

  • The materials to be transported
  • The medium of transport
  • The channels of transport
  • Energy

Materials to be transported:

In animals, they include respiratory gases oxygen and carbon dioxide, nitrogenous excretory products e.g. uric acid, nutrients e.g. glucose, amino acid, etc. In plants, they include oxygen and carbon dioxide.

The medium of transport:

The medium of transport in plants and lower animals is water and it is blood in vertebrates and in a few invertebrates like arthropods, annelids (earth worm).

The channels of transport:

In most animals, these are blood vessels, in others like earth worms, it is the body cavity (coelom). In higher plants, there is a vascular system or system of xylem and phloem.

Earth worms                     Most Animals             Plants



Circulation of blood in animals requires energy supplied from respiration used in pumping of the heart and muscle contractions.


Substances like nutrients and excretions move in and out of the cell by:

  1. Active transport
  2. Phagocytosis
  3. Pinocytosis
  4. Diffusion
  5. Osmosis

Movement of substances depends on the permeability of the cell membrane or cell wall.


This is the process by which animal cells take in liquid materials into their bodies. Thus it is said to be cell-drinking.


This is the process by which animal cells take in solid materials. The cell engulfs/invaginates or takes in solid materials and form a food vacuole where the food is digested.

Importance of phagocytosis

  • Used by amoeba during feeding
  • White blood cells destroy pathogens by phagocytosis
  • Unicellular animals egest undigested material by phagocytosis

Active transport

This is the movement of molecules from the region of low concentration to the region of higher concentration against concentration gradient using energy.

Examples of active transport

  • Up take of mineral salts from soil by plant roots
  • Absorption of some food molecules e.g. glucose
  • Selective re absorption of molecules e.g. glucose

Importance of active transport

  • Used by plant roots or root hairs to absorb minerals from the
  • Used in the absorption of food materials from the ileum into the blood stream
  • Used in the reabsorption of minerals in the kidney during urine formation
  • Used in the secretion and active uptake of ions in the fish gills from fresh water


This is the movement of molecules of gases and liquids from a region of high concentration to a region of low concentration. Diffusion occurs because small molecules are in constant random motion. Molecules of gases and liquids by random motion tend to distribute themselves evenly, throughout the available space, unlike in solids where molecules are closely packed together and have no freedom of movement. Diffusion only takes place where there is a difference in concentration i.e. where there is a concentration gradient and continues until there is even distribution of molecules.

Experiment to demonstrate diffusion in gases


  • Wet red litmus paper,
  • cotton wool,
  • glass tube,
  • ammonium solution,
  • glass rod


Some strips of wet red litmus papers are stuck on the walls of a glass tube as indicated below.

The glass tube is corked as one end and a piece of cotton wool is soaked in ammonium solution and is introduced at the other end which is also plugged.


Squares of wet red litmus paper were pushed with a glass rod or wire into a wide glass tube so that they stick to the side and are evenly spaced out. The glass tube is corked at one end the other end is closed with a cork carrying a plug of cotton wool, soaked in ammonia


The alkaline ammonia gas, diffused along the glass tube, turning the litmus papers blue in succession from 1to 5, showing that the ammonia gas was diffusing from one end to the other.

NB: If the experiment is repeated using more dilute solution of ammonia, the rate of diffusion would be seen to be slower.



Experiment to demonstrate diffusion in liquids


  • Glass beaker
  • Water
  • Potassium permanganate crystals


Fill a glass beaker with about 50cc of water

  • spatula

Place a few crystals of potassium permanganate at the base of the beaker in the water. Leave the set up for about 30 minutes.


After 30-40 minutes, the potassium permanganate color will have spread first at the bottom and later upward to color all the water in the beaker.


Diffusion occurs in liquids.


Factors affecting the rate of diffusion

Concentration gradient

Concentration gradient is the difference in concentration between the 2 regions where diffusion takes place. The higher the concentration gradient between the two regions, the faster is the rate of diffusion.


The higher the temperature of the substances (molecules), the faster the rate of diffusion, because temperature increases the kinetic energy of molecules.

Size/density of molecules

The smaller the molecules, the faster the rate of diffusion. The denser the particle, the lower the rate of diffusion.

Distance over which diffusion occurs

The shorter the distance between the two regions of different concentration, the greater is the rate of diffusion like the alveoli of lungs or the epithelial linings of the ileum are thin to provide a short distance for diffusion thus increasing the rate of diffusion.

Surface area over which diffusion occurs

The larger the surface over which diffusion is to take place, the faster is the rate of diffusion e.g. diffusion surfaces like the ileum have numerous villi to increase the rate of diffusion.


Types of diffusion

Simple diffusion

This is the type of diffusion where molecules or ions move freely across the cell membrane without being aided.

Facilitated diffusion

This is where molecules or ions move across the cell membrane by being aided by protein carriers using energy.

Significance of diffusion to organisms

  • It helps substances to move in and out of cells
  • Plant root hairs take up some salts by diffusion
  • Unicellular microorganisms like amoeba, take in oxygen and pass out carbon dioxide through the cell membrane by diffusion.
  • Digested food e.g. simple sugars, amino acids, enter the blood from the gut by diffusion.
  • Once dissolved in blood, the food substances diffuse out of the blood into the cells where they are needed.
  • Oxygen diffuses into blood and CO2 out of blood in the lungs of mammals and gills of fish by diffusion.
  • Waste products of metabolisms e.g. nitrogen containing substances like urea, diffuse out of the animal cells into blood.


This is the movement of water/solvent molecules from a dilute solution to a concentrated solution across a semi permeable membrane.

It is the movement water/solvent molecules from a solution of low solute concentration to a solution of high solute concentration across a semi permeable membrane.

A semi/partially/selectively permeable membrane is one which can allow the passage of some materials to occur and prevent other materials from passing across it.

When 2 solutions are separated by a semi permeable membrane having small pores, water molecules continue to move from a dilute solution to a concentrated solution through it.

Experiment to demonstrate osmosis in an artificial membrane


  • Cellophane /visking tube,
  • Capillary tube,
  • Beaker,
  • Syrup or sugar solution
  • Thread,
  • Clamp


  • Tie one end of the visking tube using a thread.
  • Make a sugar solution and pour it into the tube
  • Tie the open end of the tubing to the capillary tube using a thread
  • Pour some water in the beaker half way
  • Insert the capillary tube with the visking tube into water in the beaker.
  • Note the level of the solution in the capillary tube and that of water in the beaker.
  • Clamp the capillary tube on a retort stand and leave the set up for 30 minutes.


In a few minutes, the level of the solution is seen to rise up the capillary tube


  • Water molecules are passed through the cellophane tubing into the sugar solution by osmosis, thus increasing its volume and forcing it up the capillary
  • Water acts as a dilute solution
  • Sugar solution acts as a concentrated solution
  • Membrane of the visking tubing acts as the semi permeable


Experiment to demonstrate osmosis in a living tissue


  • Fresh Irish potatoes,
  • knife,
  • Petri dishes,
  • sugar or salt
  • water


  • 3 fresh Irish potatoes are peeled and their ends sliced flat. The interiors are scooped out to form a ‘cup’ with walls of uniform thickness. The potato cups are labelled A, B and C
  • In A, some grains of sugar are placed in the cup, while the other potato B is left empty as a control
  • The third potato ‘cup’ C is boiled to kill or destroy the tissues and also some sugar grains are put in it.
  • All the potato cups are placed in water in Petri dishes. The experiment is let to run for 2-6


End of experiment (2-6 hours)

The liquid in the cup potato A had risen to form a sugar solution and in the Petri dish, the level water had fallen.

In potato B and in the boiled potato, the cups were still empty and the water level in the Petri dishes remained the same.


Osmosis takes place in living tissues and does not take place in boiled tissues. This is because, by boiling, the tissues are destroyed and loose semi permeability


Living tissues have cell membrane or cell walls acting as semi permeable membrane and allow water to move through by osmosis while boiling a living tissue makes it impermeable.

Terms used in osmosis

Osmotic potential:

This is the capacity of a solution to allow in water molecules by osmosis. Therefore a concentrated solution has a higher osmotic potential than a dilute one.

Osmotic pressure:

This is the force that must be applied to stop water molecules from entering that solution, i.e. a dilute solution has a higher osmotic pressure than a concentrated solution.

Water potential of a cell:

This is the ability of water molecules to move out of a cell by osmosis. It is the concentration of water in a solution. A dilute solution has a higher water potential than a concentrated one.

Solute potential:

It is a measure of the amount of solute in the solution. It is also defined as the degree of lowering the water potential.

Pressure potential:

This is a force extended on the cell contents by the cell wall as a result of reaching the cell wall after water absorption.

Hypotonic solutions:

This is a solution which is dilute compared to another solution. A hypotonic solution has a lower osmotic pressure and is generally termed as less concentrated.

Isotonic solutions:

These are solutions with the same concentration.

Hypertonic solution:

This is a solution which is more concentrated than the other. A hypertonic solution has a higher osmotic pressure and is generally termed as more concentrated solution.

Osmosis in red blood cells

Unlike plant cells, animal cells like red blood cells lack a cell wall and only have a cell membrane which is weak and non-resistant to high internal pressure.

When red blood cells are placed in a dilute solution (hypotonic solution) i.e. distilled water, the cells swell up and eventually burst (haemolyse). This is because water moves from the surrounding solution (distilled water) via the semi permeable cell membrane into cells.

Hemolysis in red blood cells

When the red blood cells are placed in a more concentrated solution (hypotonic solution) e.g. a strong sugar solution, water moves out of the cells to the surrounding solution by osmosis. As a result, the cells shrink, the process called crenation.

However, when red blood cells are placed in isotonic solution they neither gain nor lose water.

Plants do not need a circulatory system because:

  • The oxygen requirement of the plant is very low as compared to
  • Plants have a continuous series of air spaces throughout the body opening to the atmosphere by the stomata and
  • In plants oxygen from the air diffuses through the stomata opening in to the air spaces and from the air spaces in to the cells by diffusion. And the oxygen dissolved in the soil water also diffuses through the root hairs in to the plant
  • The carbon dioxide produced during respiration is used up during


Closed circulatory system:

Closed circulatory system e.g. in earthworm, fish and mammals have blood enclosed in tubes. Here blood is pumped by the heart to tissues through the arteries and return to the heart through the veins. The arteries and veins are connected by capillaries which are thin walled.

The body cells do not come in to direct contact with blood but are bathed in the tissue fluids. Substances diffuse out of the blood which is confined to blood vessels into the tissue fluid and then across to cell membrane into the cell.

Advantages of closed circulatory system

  • Distribution of blood/materials is easily
  • Blood moves or flows very fast leading to quick supply of
  • Blood flows at a high pressure leading to an effective

Demerits of closed circulatory system

  • It requires a special heart whose pumping action provides pressure for movement of
  • Blood movement meets a high resistance within

Open circulatory system e.g. in mollusks and arthropods

Here the artery that leaves the heart is very short and blood empties in a large blood filled space called haemocoel. Then blood from these spaces return to the heart through the short veins.

The organism cells are directly bathed in blood and materials diffuse out of the blood into each cell across the cell membrane.


Advantages of open circulatory system

  • Easy diffusion of materials due to absence of vessel
  • It does not require special pumping hearts since blood is flowing through cavities with less

Disadvantages of open circulatory system

  • Blood flows sluggishly/slowly leading to slow supply of
  • Blood flows at a low
  • There is little control over distribution of materials.


Single circulatory system:

This is the type of circulation where blood from the body cells flows once through the heart and goes back to the body cells. It has a heart with only two chambers i.e. one atrium and one ventricle e.g. in fish.

The demerit of single circulation is that blood moves very slowly leading to slow supply of materials. Blood pressure is also greatly reduced by gill capillaries.

Double circulatory system

In a double circulatory system, blood is pushed out in the heart in to a series of capillaries and the blood passes through the heart twice in each circulation. It involves two separate circulation i.e.

  • Pulmonary circulation to the lungs
  • Systemic circulation to the rest of the body

That is, blood from the right ventricle is pumped into the lungs through the pulmonary artery and return to the left atrium via the pulmonary vein and this is called pulmonary circulation.

Blood from the left ventricle is pumped through the aorta to the rest of the body and returns to the right atrium through the vena cava and this is called systemic circulation

Double circulation is further divided into 2;

  • Incomplete double circulation
  • Complete double circulation

Incomplete double circulation:

This is a system in which blood flows through the heart twice for every complete cycle through a three-chambered heart. The heart has one ventricle through which both oxygenated and deoxygenated blood from the two atria flow.

Mixing of oxygenated and deoxygenated bloody is prevented by ridges present in the ventricle. This system of blood circulation is found in amphibians like frogs.

Diagram of incomplete double circulation

Complete double circulation

This is a type of circulation where blood flows through the heart twice within a four-chambered heart for every complete cycle of circulation. Mixing of oxygenated and deoxygenated blood is prevented by a wall called septum. It is found in birds, reptiles and mammals.

Diagram showing complete double circulation


Advantages of double circulatory system

High blood pressures required for fast flow of blood is reached than in open circulation. Gives more rapid circulation since blood is returned rapidly to the heart for pumping.

There is complete separation of oxygenated and deoxygenated blood which improves efficiency of oxygen distribution and can therefore sustain the high metabolic rate required by such animals that possess it.

Blood is pumped directly to where it’s needed

Note: The amount of blood flowing to a certain organ can be regulated by changing the diameter of the blood vessel.


The continual circulation of blood in mammals is due to the pumping action of the heart. The circulation of blood in mammals is divided into two. That is;

  1. The pulmonary circulation; this is the circulation of blood from the heart to the lungs and from the lungs back to the heart. It is the simplest circulation where blood moves a very short distance. This type of circulation involves the pulmonary artery and pulmonary
  2. The systemic circulation; this is the circulation of blood from the heart to the rest of the body apart from the lungs and from the rest of the body back to the




These are the tubes that carry blood throughout the body and they include: arteries, veins, and capillaries

Arteries and veins both have three layers in their walls but the layer of the muscles (elastic tissue).is much greater in arteries than in the veins.


These carry blood from the heart to the body capillaries. Arteries divide into smaller vessels called arterioles which then divide repeatedly to form capillaries.

Characteristics of arteries

  • Has three layered wall. These are strong to withstand the higher pressure as resulting from the pumping action of the
  • They have fibrous outer wall so as to withstand high pressure
  • They are found deeply in the
  • Their walls are elastic to allow stretching due to high blood
  • They have no valves except at the base of the pulmonary artery and
  • They have narrow lumen than veins which maintains blood flow at high pressure.
  • They carry oxygenated blood except the pulmonary artery and umbilical
  • They all carry blood from the heart to other parts of the body.



These are the smallest blood vessels with thin walls to allow diffusion of materials between blood and the tissue fluid. They connect arterioles to venules.

They pass very close to the cells taking to the cells food, oxygen, and mineral salts etc. as well as taking a way carbon dioxide, urea and other waste products from the cells.

They are responsible for the exchange of materials between blood and cells, because their walls are permeable allowing water, dissolved food substances to pass through except proteins because they have large molecules.

Blood pressure reduces in them as a result of their resistance, and blood flows in them slowly without pulse. The capillaries network is so dense and the capillaries unite to form large vessels called venules which join to form veins.

Adaptations of capillaries to its functions

  • They have a large surface area for exchange of materials.
  • They have very thin walls for faster diffusion of materials.
  • They have a high diffusion gradient leading to rapid diffusion of
  • Slow movement of blood in capillaries makes exchange of materials

Characteristics of capillaries

  1. They carry both deoxygenated and oxygenated blood.
  2. They have a small lumen
  3. They have permeable thin walls to allow diffusion of materials.
  4. They have no valves
  5. Blood flows slowly
  6. There is a decrease in pressure

Cross-section through a capillary


These carry blood from tissues to the heart. The pressure in them is steady and less than in arteries. All veins carry de-oxygenated blood except pulmonary vein. Blood in the veins flows slowly after losing pressure in the capillaries; however the sluggish flow of blood is maintained by:

  • Possession of valves which prevent back
  • Having a wide lumen that offers a low resistance to blood
  • Action of skeletal muscles against veins as they contract during movement increases blood pressure in

Inhaling lowers the pressure in thoracic cavity leading to flow of blood towards the heart

Characteristics of veins / Adaptations

  • They have wide lumen to encourage flow of blood at low
  • They have thinner walls than arteries which are adequate to withstand low
  • They have valves at intervals along their length which prevent blood from flowing backwards / maintain flow of blood in one
  • They are not capable of
  • They transport deoxygenated blood except the pulmonary vein and umbilical
  • They have less elastic
  • They are found near the body surface.

Cross section through a vein

Differences between arteries, veins and capillaries


Have     thick     walls    with    smooth musclesHave thin walls with smooth musclesHave thinner walls with smooth muscles
have more elastic fibresHave few elastic fibresDo not have elastic fibres
Have smaller lumen relative to diameterHave a wider lumen relative to diameterHave largest lumen relative diameter
Have no valves except at the base of aortaHave valves throughout their lengthHave no valves
Can constrictCan’t constrictCan’t constrict
Walls not permeableWalls not permeableWalls permeable



Carry blood away from the heartCarry blood towards the heartCarry blood to and from the heart
Carry oxygenated blood except pulmonary artery and  umbilical arteryCarry deoxygenated blood except pulmonary vein and umbilical veinCarry       both       oxygenated      and deoxygenated blood
Blood flow at high pressure( flow in pulse)Blood flow at low pressureBlood flow at intermediate pressure
Blood flow in pulseBlood does not flow in pulseBlood does not flow in pulses


Its function is to pump blood around the body. The whole heart is surrounded by the pericardium which has two layers between which is the pericardial fluid that reduce friction between them.The heart is made of tissues called cardiac muscles which have the potential to contract rapidly.

It’s divided in to four chambers. The upper chambers are called atrium / auricle and the lower chambers are each called ventricle.

The heart is divided in to sections i.e. left and right by a muscular septum whose function is to prevent mixing of oxygenated and deoxygenated blood

Movement of blood in the heart is maintained in a single direction i.e. from the auricle to ventricle and then to blood vessels.Blood flow in one direction in the heart is maintained by the presence of valves.

The auricles receive blood from all parts of the body while the ventricles pump blood to the body e.g. the left atrium receives oxygenated blood from the pulmonary vein and pump it to the left ventricle through the bicuspid valve.

The right atrium receives deoxygenated blood from the rest of the body from the vena cava and pumps it to the right ventricle via the tricuspid valve.

The ventricle walls are more muscular (have thicker walls) than those or the auricles because the auricle pump blood to shorter distance i.e. to the ventricle while the ventricles pump blood longer distances i.e. to body and lungs.

The walls of the left ventricle that pump blood in to the systemic circulation are thicker than those of the right ventricle which pump blood to pulmonary circulation.

Flow of blood through the heart:

Blood flows in to the heart from the rest of the body via the vena cava to the right atrium which pumps it to the right ventricle via the tricuspid valve.

The right ventricle pumps blood to the pulmonary artery to the lungs and blood flows back to the left atrium via the pulmonary vein which pumps it to the left ventricle via the bicuspid valve and then finally pumped to the rest of the body via the aorta.

Longitudinal section of the heart




This refers to the sequence of events by which the heart pumps and is refilled with blood. The cardiac cycle involves two phases:

  • Re-filling of the heart with blood
  • Pumping of blood

The pumping action of the heart consists of alternate contraction and relaxation of cardiac muscles in the walls of the heart. Contraction of cardiac muscles is called systole while relaxation is called diastole.

During diastole, the cardiac muscles in the walls of the atria relax and expand; blood from the vena cava and pulmonary vein enter the atria and becomes filled with blood. The walls of the ventricles relax and expand while those of the atria contract, forcing blood from the atria into ventricles via bicuspid and tricuspid valves as semilunar valves remain closed.

During systole, cardiac muscles of the ventricles contract, forcing blood out of the heart via the semi lunar valves into the aorta and pulmonary artery. At this time, the atria relax and expand in order to be re-filled with blood. The cuspid valves close against high blood pressure to prevent the back flow of blood into the auricles. The closure of the valves produces the heart sound termed as ‘lub’.

After expelling blood, ventricles relax and their pressure lowers compared to aorta and pulmonary artery pressure. This would cause back flow of blood to the heart but is prevented by sudden closure of the semi lunar valves. The closure of the semi lunar valves causes a second heart sound called ‘dub’.

The 2 sounds ‘lub’ and ‘dub’ are so close and often described as ‘lub-dub’ and they form a single heartbeat.

Initiation and control of the heart beat

Contraction of the heart is initiated by heart, heart muscles/cardiac muscles themselves. Therefore the heart muscles are myogenic i.e. the rhythmic contraction a rise from within the tissue itself.

Heart beat is controlled by collection of cells in the right atrium called pacemakers located in the sino-atrio node (SAN) which are controlled by nervous impulse from the medulla oblongata of the brain that change the rate of heart beat.

Factors affecting the heart beat rate

  • Body size e. it is faster in small organisms than
  • Lack of hormones in the body e.g. adrenaline
  • State of health and diseases e.g. malaria
  • Age i.e. it’s faster in infants than large
  • Sex i.e. faster in female than in

NB: In normal adults at rest, heart contracts about 70 to 72 times per minute.

Blood pressure

This is the force with which blood flows from one part of the body to another. The blood pressure is due to the pumping action of the heart as experienced by the blood vessels. The narrow blood vessels experience high blood pressure and wide vessels experience low blood pressure. Sometimes fats accumulate in the blood vessels making their rumens narrow. This increases blood pressure and it is the major cause of high blood pressure in fat people, however small people also experience high blood pressure. This is due to conditions like stress, anxiety, fear, etc. These conditions tend to increase the rate of heartbeat and more blood is pumped to the blood vessels causing high pressure in them.


Blood is a connective tissue made up of cells suspended in a fluid matrix called plasma. There are two types of cells in blood i.e. White blood cells (leucocytes) and red blood cells (erythrocytes). The platelets (thrombocytes) are fragments of cells.

In an adult human being, there are five to six liters of blood with blood making up approximately 10% of the body weight.

Main components of blood

  • Red blood cells/erythrocytes
  • White blood cells/leucocytes
  • Platelets/thrombocytes
  • Plasma

General importance of blood in the bodies of animals

  • It transports oxygen from the lungs to all parts of the body
  • It transports digested food from the ileum to other parts of the body for use
  • It transports Carbon dioxide from the tissues to the lungs
  • It transports nitrogenous wastes from the liver to the kidney where they are excrete
  • It transports hormones from their site of production to where they perform their functions
  • It distributes heat and aids in temperature control
  • It prevents infection by transportation of white blood cells


 Characteristics of Red Blood Cells

  • They have hemoglobin molecules which carry oxygen from the lungs to the tissues
  • They lack nuclei
  • They have thin cell membranes which thinness reduces the diffusion distance for gaseous exchange.
  • They are manufactured from the red bone marrow
  • On average, red blood cells last for four month after which they are destroyed by the liver to form bile pigment and the iron in haemoglobin is stored in the liver
  • They have a biconcave disk shape
  • They are approximately 5 million/mm3 of blood.

Importance of Red Blood Cells

  • They transport oxygen from gaseous exchange surfaces to the tissues
  • They transport carbon dioxide from tissues to the gaseous exchange surfaces

Adaptation of Red Blood Cells to carry out their function

  • They are biconcave in shape so as to avail a large surface area to volume ratio for absorption of oxygen.
  • They have hemoglobin molecules that bind to oxygen and transport it from the lungs to the tissues.
  • They have a thin membrane which reduces the diffusion distance for the respiratory gases in and out of the cells.
  • They lack nuclei which provides enough space for packaging of haemoglobin
  • They lack mitochondria and generate their ATP exclusively by anaerobic respiration to prevent them from using the oxygen they are carrying.
  • They are numerous per mm3 to increase surface area for transportation of oxygen.
  • They have flexible membranes which make them able to squeeze through capillary networks as they exchange materials they transport with the surrounding tissues.

NB: The concentration of red blood cells increases as one climbs up a mountain because the concentration of oxygen in the air reduces with increase in height above sea level. So the body adopts by producing more red cells to increase the available total surface area to bind and carry oxygen to the tissues regardless the reducing oxygen concentration main.

Red blood cells are made from the red bone marrow of short bones in adults and in the fetus, red blood cells are made in the liver. They last for approximately four months after which they are taken to the liver or spleen for their destruction. They are more numerous than any other cells in the blood.


These are blood cells made from the white bone marrow of long bones. They are also made in the spleen and lymphatic system. They are responsible for defense of the body against infection. They are fewer in blood than the red blood cells.

Characteristics of white blood cells

  • They have no definite shape (they are amoeboid)
  • They have a nucleus even at maturity.
  • They are relatively few in blood but their number increases when the body is attached by an infection
  • They lack haemoglobin
  • They feed on foreign particles by Phagocytes

Structure of a white blood cell


White blood cells are divided into two major categories. These are;

Phagocytes: These are white blood cells with a lobed nucleus. They ingest and destroy germs by phagocytes.

Lymphocytes: These are white blood cells, which defend the body by producing antibodies.

Production of red and white blood cells

The red blood cells are manufactured form the red bone marrows in adults. Old red blood cells are taken to the liver for destruction.

White blood cells are manufactured from the white bone marrows of long bones. Some white blood cells are manufactured from the lymph nodes. Worn out white blood cells are also taken to the liver for destruction. In the fetus, the liver manufactures blood cells.

Action of white blood cells on the foreign particles

Some white blood cells attack and destroy the foreign particles directly by themselves. These are called phagocytes and they destroy the foreign particles by Phagocytosis. In this process the white blood cells form pseudopodia, which they use to engulf the foreign particle by Phagocytosis.

After engulfing the foreign particle, a food vacuole is formed into which digestive enzymes are produced. The enzymes break down the particle and the important materials are absorbed by the white blood cell while the wastes are excreted out of the cell through the contractile vacuole.

Some white blood cells destroy foreign particles by releasing antibodies, which destroy the particles. White blood cells, which produce antibodies, are called lymphocytes. There are four types of antibodies produced.

  • Opsonins; these attach to the outer surface of the foreign particle and make it easier for phagocytic white blood cells to ingest them.
  • Agglutinins; these cause the foreign particles to stick together. In this condition the foreign particles cannot invade the tissues.
  • Lysins; these destroy bacteria by dissolving their outer coats.
  • Anti-toxins; these combine with and so neutralize the toxins produced by foreign particles.



These are blood cells formed as fragments in the bone marrows during the formation of red blood cells. They are responsible for blood clotting.

Characteristics of platelets

  • They are cell fragments.
  • They are spherical in shape.
  • They do not have a nucleus.
  • They do not have haemoglobin.


They play a role in blood clotting which protects the body against excessive loss of blood and entry of pathogens through the injured part. Blood clotting is the process by which blood stops oozing out of a cut or wound. It is important because of the following reasons.

  • It prevents excessive loss of blood from the body.
  • It is a step towards healing of cuts and wounds.
  • The blood clot creates a barrier to prevent entry of bacteria and other pathogens in the body.

The Process of Blood Clotting:

When blood is exposed to air as a result of a cut or wound, the platelets in the blood at the damaged tissue stimulate the release of a chemical called Thromboplastin (thrombokinase). In the presence of calcium ions and vitamin K, thromboplastin stimulates the conversion of prothrombin to thrombin enzyme. Thrombin then catalyzes the conversion of soluble blood protein fibrinogen to the insoluble form fibrin. Fibrin forms fibers, which form a mesh and trap blood cells and proteins. This mesh dries to form a scab, which is called the blood clot.



This is the fluid part of blood. It is made up of;

  • A soluble protein called fibrinogen that plays a role in blood
  • Serum, a watery fluid containing a variety of substances transported from one part of the body to another e.g. hormones, lipids, enzymes, urea, carbon dioxide, plasma, proteins, amino acids

Functions of blood plasma

  • To transport hormones from gland producing them to the target sites.
  • To transport food nutrients from the gut to the other parts of the body.
  • To transport antibodies to the infected parts of the body.
  • To transport Urea from the liver to the Kidneys for excretion.
  • To transport carbon dioxide from the body muscles to gaseous exchange system.
  • To transport heat from the liver and body muscles to other body parts hence maintaining a constant body temperature range.
  • To transport platelets to injured sites on the body so as to initiate blood clotting.
  • To distribute salts around the body so as to maintain the body’s electrolytes balance.

Capillary exchange, formation of tissue fluid and later lymph.

As blood flows from arterioles into blood capillaries. Pressure builds up in the capillaries forcing small molecules like food materials and the fluid part of blood to leave the capillaries and enter the intercellular spaces, leaving behind large molecules like proteins in plasma and cells.

Once the fluid is in the intercellular spaces of tissues, it is no longer called blood but tissue fluid.

Once formed, the tissue fluid surrounds the cells. Body cells then get their requirements e.g. glucose, oxygen, etc. from the tissue fluid and they add excretory materials to the fluid.

Some of the fluid returns in to the capillaries and the other is drained in to a system of narrow channels called lymph vessels. The fluid in these vessels is now called lymph. Lymph is therefore, tissue fluid in the lymph vessels.


This is part of the vascular system. It forms the second type of circulation. Most of the tissue fluid as explained above goes back into the blood capillaries and the remainder enters the lymphatic system and becomes lymph fluid. The lymph fluid is transported through lymph vessels. The lymph vessels are similar to veins but they have more valves than the veins. The movement of the lymph fluid through the lymph vessels is due to the contractions of the surrounding muscles. As they contract and relax, they squeeze the lymph vessels to gain the force by which lymph moves. The walls of the lymphatic vessels have pores, which allow the entry of cell, wastes and bacteria. Before reaching the blood, lymph passes through the lymph nodes where the wastes and bacteria are removed.


Functions of the lymphatic system

  • It transports fatty acids and glycerol from the ileum to the heart where they join the blood
  • It carries excretory substances from tissues to the blood
  • It produces white blood cells, which assist in defense of the
  • It filters out bacteria before they reach the blood
  • Transports hormones from glands to other body
Blood circulatory systemLymphatic system
Has a heart which acts as a pumpHas no pump
Blood flow is two way, i.e. from heart to body and

back to the heart.

Lymph flow is one way, i.e. from body tissues to the


Blood travels at high speed.Lymph travels at a very slow speed
Valves are only found in veinsHave valves in all its vessels
Contains blood cells and proteinsOnly white blood cells present. Proteins are lacking
Does not contain emulsified fatsContains and transports fatty acids and glycerol.
Have no nodesHave nodes that produce lymphocytes

Similarities between blood system and lymphatic system

  • Both have valves in their vessels.
  • Both are means of transporting materials in the body
  • In both a selected muscle provides a force by which substances are removed
  • Both have vessels through which materials are transported


There are 4 main blood groups i.e.

  • Blood group A
  • Blood group B
  • Blood group AB
  • Blood group O

When one has got less blood than necessary, blood transfusion is carried out. The one who gives blood to a patient is called a donor and the one receiving is known as a recipient. Doctors have to match the blood of the donor to that of the recipient because when incompatibles blood is mixed, the red blood cells stick together (agglutinate) and blood clots. This is a fatal situation.

Agglutination is caused by the presence of proteins called antigens on the surface of cells being mixed with specific antibodies, which work against them. Blood groups are determined by the type of antigens one has in blood. This means that one having antigen A belongs to blood group A. Those with antigen B belong to blood group B. Those with antigens A and B belong to blood group AB while those without antigens belong to blood group O. Each blood produces particular antibodies, which work against particular antigens when introduced into the body. For example, blood group A produces antibody b. This means that blood group A is anti (against) blood containing antigen B (blood group B).

The table below shows the blood groups, the antigens they carry and the antibodies they produce.

Blood groupAntigen presentAntibody produced
ABA and BNone
ONo antigena and b


Antibodies are represented by small letters while antigens are represented by capital letters. Before doctors can carry out transfusion, they carry out tests to make sure that the patient’s and donor’s blood are compatible (the recipient’s blood must not contain antibodies that act on the antigens in the donor’s blood. For example antigen A would agglutinate if mixed with blood containing antibody a. i.e. blood group B.

Table of compatibility



Blood group AB can receive blood from all other blood groups because it has no antibodies and it is therefore called a universal recipient.

Blood group O can donate blood to all blood groups because it has no antigens and it is therefore called a universal donor.

“Rhesus factor” System

Rhesus factor is a protein (antigen) also found on the cell membranes of the red blood cells.

Many individuals have the Rhesus factor and are said to be rhesus positive (Rh+) while a few do not have the Rhesus factor and are said to be Rhesus negative (Rh-).

A person who is Rhesus factor positive can receive a successful blood donation without agglutination from a person of Rhesus positive and a person of Rhesus negative.

However, a person who is Rhesus negative can only receive a successful blood donation without agglutination from his fellow Rhesus negative person though he can be transfused with blood which is Rhesus positive quite successfully only once and after this transfusion, his body produces antibodies against the Rhesus factor. Such antibodies attack the Rhesus factor with subsequent transfusion of Rhesus positive blood leading to agglutination.


Immunity is the ability of an organism to resist infection. The immune response is based upon recognition of a foreign particle and the release of chemicals that destroy it. The foreign particle may be an antigen, bacteria, virus or any other pathogen. The substance that destroys these particles can be a white blood cell or antibodies produced by white blood cells.

Active (Antibodies made by the human immune system, long term acting due to memory cells)Passive (Given-Antibodies, short term acting)
Natural–  Response to disease

–  Rejecting transplant

– Acquired antibodies (via placenta, breast milk)


– Vaccination (Injection of the antigen in a weakened form)– Injection of antibodies from an artificial source,

e.g. anti-venom against snake bite

Differences–  Antibody in response to antigen

–  Production of memory cells

–  Long lasting

–  Antibodies provided

–  No memory cells

–  Short lasting









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