South Tuen Mun Government Secondary School
Biology Revision Note 4
The Human Breathing System:
Nose
Trachea +
bronchus
Pleural
membrane
Structure
Functions
hair – forms a net
Both are used to trap foreign particles and prevent their
mucus – sticky
entrance into the trachea.
mucus – sticky
trap foreign particles and prevent their entrance into the lung
cilia
beat to move the sputum out and prevent the entrance of
foreign particles
incomplete ring of cartilage
to make the trachea remain open all the time
secretes pleural fluid
reduces the friction during breathing.
Bell Jar Model
When the rubber sheet is pulled down, the volume of the bell jar increases and the air pressure inside the bell jar
decreases. When the air pressure in the atmosphere is higher than the air pressure inside the balloon, air is
pushed air into the balloon. The balloon inflates.
When the rubber sheet is pushed up, the volume of the bell jar decreases and the air pressure inside the bell jar
increases. When the air pressure inside the balloon is higher than the air pressure in the atmosphere, air is pushed
out of the balloon. The balloon deflates.
Differences between the Bell Jar Model and the breathing system :
Bell Jar Model
Breathing system
The bell jar is rigid and cannot be moved.
The rib cage can be moved by the contraction of
intercostal muscles and diaphragm.
When the balloon is filled up with air, the rubber When the lung is filled up with air, the diaphragm
sheet is pushed down to become curved down.
contracts to become a flat shape.
When the balloon is not inflated, the rubber sheet is When the lung is not inflated, the diaphragm relaxes
not pushed and become flat.
and become curved upward.
Ventilation :
Gaseous exchange :
Characteristics to increase the rate of diffusion :
There are millions of air sacs / alveolus, circular / folded to increase the surface area for diffusion.
The air sac is one-celled thick to reduce the distance of diffusion.
The air sac is richly supplied of blood capillary. The blood carries oxygen away and brings carbon dioxide to
maintain a higher concentration gradient for diffusion.
Transport of oxygen
Oxygen is carried mainly by haemoglobin in red blood cells. Haemoglobin can bind with oxygen reversibly.
Under high oxygen concentration (in the lung):
Haemoglobin + oxygen  oxyhaemoglobin (bright red in colour)
Under low oxygen concentration (in the organ):
Oxyhaemoglobin  haemoglobin (dull red / dark red in colour) + oxygen
Transport of carbon dioxide
A small amount of carbon dioxide dissolves in plasma.
A small amount of carbon dioxide is carried by haemoglobin inside the red blood cells: oxygen + haemoglobin 
carbamino-haemoglobin
Most carbon dioxide is carried by dissolving in plasma in form of hydrogencarbonate ions.
Under high carbon dioxide concentration (in the organ):
Carbon dioxide enters the red blood cell by diffusion, carbon dioxide + water  carbonic acid  hydrogen ions +
hydrogencarbonate ions, this chemical reaction is catalysed by an enzyme, carbonic anhydrase. Hydrogencarbonate
ions diffuse out into the plasma of the blood and carried by the plasma of blood to the lung.
Under low carbon dioxide concentration (in the lung):
Hydrogen carbonate ions enter the red blood cells by diffusion. Hydrogen carbonate ions + hydrogen ions 
carbonic acid  carbon dioxide + water, this chemical reaction is catalysed by carbonic anhydrase. Carbon dioxide
diffuses from the red blood cells to the plasma in the blood, and then to the air sac.
The composition of blood:
 Blood is made up plasma, white blood cells (leucocytes) and red blood cells (erythrocytes).
The functions of plasma:
to transport chemical substances that dissolves in water. e.g. mineral salts, glucose, amino acids, urea, carbon
dioxide.
*Difference between red blood cells and white blood cells :
Red blood cells
White blood cells
smaller
larger
thin, biconcave in shape
round or irregular in shape
no nucleus
has lobed or spherical nucleus
contains haemoglobin for carrying oxygen
do not contains haemoglobin
cannot kill pathogens
to kill pathogens
made by bone marrow of long bones
made by bone marrow of long bones or lymph nodes
Red blood cells are thin and biconcave to provide a larger surface area to volume ratio to provide a short distance
for diffusion of oxygen into the centre of the cell. The larger surface area also increases the rate of diffusion.
Blood platelets are used for blood clotting. Blood clotting prevents excess bleeding and the entrance of foreign
particles.
The circulatory system - double circulation
The Advantage of double circulation : no mixing of oxygenated blood and deoxygenated blood. The
concentration gradient of oxygen and carbon dioxide at the lung is higher for a faster rate of diffusion.
Pulmonary artery has the highest concentration of carbon dioxide and the lowest concentration of oxygen.
Pulmonary vein has the lowest concentration of carbon dioxide and the highest concentration of oxygen.
Hepatic portal vein has the highest concentration of glucose and amino acids after meals. It is the only vein that
has capillary on both sides.
Hepatic vein has the highest concentration of glucose between meals / when starved.
Hepatic vein has the highest concentration of urea. Renal vein has the lowest concentration of urea
Structure of the Heart
Atrial systole (contraction of heart muscle) + ventricular diastole (relaxation of heart muscle)  blood pressure in
the atria is higher  blood moves from atria into the ventricles [valve prevents the backflow of blood from aorta,
pulmonary artery into the ventricles]
Atrial diastole + ventricular systole  blood flows from vena cava into right atrium, from pulmonary vein into left
atrium, from right ventricle into pulmonary artery, from left ventricle into aorta. [bicuspid valve and tricuspid valve
prevents the backflow of blood into the artria]
Special adaptations of the heart for its function:
 The left heart and the right heart are separated by a septum (muscle wall), so that there is no mixing of
oxygenated blood and deoxygenated blood. The concentration gradient of oxygen and carbon dioxide at the
lung is higher for a faster rate of diffusion.
 Thinner atria muscle wall – a smaller blood pressure to push blood from atria to ventricle [a very short
distance].
 Thicker ventricular muscle wall – a larger blood pressure is required to push blood to the lung (right ventricle)
and around the body (left ventricle) because the distance is much longer.
 Left ventricle has a thicker wall than the right ventricle because it is a longer distance for left ventricle to push
blood to the whole body as compared with for right ventricle contracts to push blood to the lung.
The heart tendons are used to hold the bicuspid valve and tricuspid valve in position. The tendons prevent the
valves from turning inside out during the powerful contraction of the ventricles.
Heart attack /coronary heart disease
Cholesterol, fat or blood clots can deposit in the blood capillary of the coronary artery. The blood supply to the
heart muscles is reduced. Less oxygen and glucose is transported to the heart muscle. Thus the heart stops beating.
The blood supply to the brain, the person will faint and then may die.
Artery and Vein
Artery :
Artery
Vein
Vein
brings blood away from heart to organ
brings blood from organs to heart
 Thicker wall to withstand (抵抗) the higher blood  thinner wall as the blood pressure is smaller
pressure of the ventricle contraction
 more elastic wall to push blood forward
 less elastic wall


more muscular wall to control the amount of blood 
flowing into an organ
no valve

less muscular wall
valve is present to prevent the backflow of blood
higher blood pressure because it is close to the pumping lower blood pressure because it is very far away from
force of the ventricles
the pumping force of the ventricle
higher concentration of oxygen except the pulmonary lower concentration of oxygen except the pulmonary
artery
vein
lower concentration of carbon dioxide except the higher concentration of carbon dioxide except the
pulmonary artery
pulmonary vein
blood moves forward by the pumping pressure of Blood moves forward by the contraction of skeletal
ventricle contraction.
muscles near the veins and the change of air pressure in
the thorax during breathing movement.
Valve is present to prevent the backflow of blood.
Exchange of materials:
Formation of tissue fluid :
(1) The higher blood pressure near the arterial end of the capillary will push small molecules in blood (plasma
without red blood cells, large blood proteins, blood platelets) to form the tissue fluid.
(2) Useful substances will be taken up by the body cells by diffusion from the tissue fluid into the cells or from the
blood capillary into the body cells. Waste products will also diffuse from the body cells into the tissue fluid or
from the body cells into the blood capillary.
(3) Continuous entry of blood content will form a hydrostatic pressure to push the tissue fluid back into the blood
capillary at the venous end. Water will also enter the blood by osmosis.
(4) Some tissue fluid enters the lymph capillary. The tissue fluid is now called lymph.
Lymphatic system :
Lymph is moved forward by the contraction of skeletal muscles near the lymph vessels and the change of air
pressure in the thorax during breathing movement. Valves in the lymph vessel prevents the backflow of blood.
Function of the lymphatic system :
(a) Lymphatic vessel collects excess tissue fluid back into the blood through the vein in the neck.
(b) Lacteal (lymphatic vessels in the villus, small intestine) transports fat.
(c) Lymph nodes make lymphocytes (white blood cells). Lymphocytes kill bacteria.
Differences between lymphatic system and circulatory system:
(a) Lymphatic system is not a circulatory system.
(b) Lymphatic system has no pumping organ while in circulatory system, the heart acts as a pump.
(c) Lymph vessels have blind ends while blood vessels do not have.
(d) Lymph does not have red blood cells, plasma protein and blood platelet while the blood has.