Transcript SUB 1.2 – CIRCULATORY SYSTEM_2009c

BIOLOGY FORM 5
THE HUMAN
HEART
LEARNING OUTCOMES
• Explain how blood is propelled through the human circulatory system,
• Explain briefly how blood pressure is regulated,
• Compare & contrast the circulatory systems in the following humans, fish
& amphibians.
• Conceptualise the circulatory system in humans.
The Structure & Function of the
Human Heart
• The 3rd component (PUMP) of circulatory
system
• Situated between the two lungs in the thoracic
cavity
• The organ responsible for generating the
pressure to pump blood through the network
of blood vessels
• A Dark red, cone-shaped, muscular organ
• Weight : 350-450 grams in an adult
• Made up of four chambers – two upper thinwalled atria, two lower thick-walled ventricles
SUPER VENA KAVA
SEMI-LUNAR VALVE
AORTA
RIGHT ATRIUM
PULMONARY ARTERY
PULMONARY VEIN
PULMONARY VEIN
TRICUSPID VALVE
RIGHT VENTRICLE
LEFT ATRIUM
LEFT VENTRICLE
SEMI-LUNAR VALVE
TRICUSPID VALVE
BICUSPID VALVE
• The valves in the heart ensure that blood
flows only in one direction
• Tricuspid valve – right atrium & right
ventricle
• Bicuspid valve – left atrium & left ventricle
• Semi-lunar valve – at the base of aorta &
pulmonary artery
• The heart is made up of cardiac muscle
which is myogenic (it contracts & relaxes
automatically throughout life)
• The rhythmic contractions are generated
within the cardiac muscle itself & are not
initiated by nerves.
• The heart functions like two pumps with
different pressure systems.
• Right pump  deoxygenated blood  lungs
• Left pump  oxygenated blood  body
• Sinoatrial (SA) node (specialised cardiac
muscle cell) located in the right atrial wall,
near the entrance of the superior vena cava.
• Function like a pacemaker  causing the
atria to contract simultaneously  force
blood into ventricles
• Atrioventricular (AV) node (lying at the
base of the right atrium)
• The excitatory waves stimulate the AV node
 generates its own electrical impulses
which are conducted by specialised muscle
fibres called bundle of His fibres &
Purkinje fibres to the walls of ventricles
 contract simultaneously & blood is
pumped out of the heart.
• Right ventricles which is less muscular pumps
the blood into the pulmonary artery 
lungs
• Left ventricle is thicker & more
muscular than the right ventricles 
generate a greater pressure to pump
blood through aorta & to other
arteries in the body.
How does the blood in the veins
flow back to the heart?
• Normal movement  contraction of skeletal
muscles squeezes the veins, increased
pressure pushes open the valves in the veins
to force the flow of blood towards the heart.
• The valves in the veins prevent back flow of
the blood
• The residual heart pressure
• Inhalation  the inspiratory movements
lower the thoracic pressure & helps to draw
the blood along the main veins towards the
heart.
• Gravity helps to return blood in those veins
above the heart.
FACTORS MODIFYING THE
HEART RATE
• SA node can initiate heartbeat on its
own. The heart rate may be modified
by certain other factors.
• (a) the sympathetic nerve carrying
impulses to the heart can increase the
heart rate
(b) the parasympathetic nerve
carrying impulses to the heart slows
down the heart rate.
• When excited  an increased
secretion of the hormone adrenaline @
epinephrine which causes the heart to
beat faster
(a) an increase in the partial pressure
of CO2 in the blood or a decrease in
pH increase the heart rate
(b) a fall in partial pressure of CO2 in
blood decreases the heart rate.
• Heart rate also increase when body
temperature is elevated @ when there
is a decrease in blood pressure
Regulatory Mechanism of blood
pressure
• Blood pressure – the force of the blood exerted by the
pumping heart on the walls of the arterial blood
vessels.
• Arterial blood pressure is highest during contraction of
the ventricles (ventricular systole) & lowest during
diastole.
• Normal human blood pressure – 120/80 mm Hg
• Can be measured by using a sphygmomanometer
• Blood pressure is regulated by a negative feedback
mechanism.
• Stretch-sensitive receptors @ baroreceptors – located
in the arch walls of the aorta & carotid arteries
(supply blood to the brain).
• Monitor the pressure of blood flowing to the brain &
to the body.
• An increase in blood pressure stretches the
baroreceptors  impulses are sent to the
cardiovascular centre in medulla oblongata to help
regulate blood pressure.
• Impulses sent via parasymphatetic nerve to the
heart  slow down the heartbeat  smooth muscles
of arteries relax, decrease the resistance of blood flow
in the blood vessels  blood pressure decrease
• The widening of blood vessels = vasodilation
• The weaker cardiac muscle contraction & lower
resistance of blood flow in blood vessels  blood
pressure , back to normal value
• If blood pressure low (in a state of shock) 
baroreceptors less stimulated  send nerve impulses
at a slower rate to the cardiovascular centre 
stimulation of SA node by the sympathetic nerve 
stronger cardiac muscle contraction as well as the
smooth muscles in the walls of arteries  increase
the resistance of blood flow in the blood vessels.
• Narrowing of blood vessel  vasoconstriction
CIRCULATORY SYSTEM IN FISH,
AMPHIBIANS & HUMANS
CIRCULATORY SYSTEM
CLOSED
SINGLE
OPENED
DOUBLE
FISH
COMPLETE :• HUMANS
INSECTS &
MOLLUSCS
INCOMPLETE :• REPTILES
• AMPHIBIANS
CIRCULATORY IN HUMANS, FISH
&AMPHIBIANS
CIRCULATORY SYSTEM IN FISH
• Single circulatory system – blood flows
through the heart only once for each
circulation.
• Heart  Deoxygenated blood  gills
oxygenated blood  body  heart
• Blood pressure drops  collected in sinuses
(large spaces)  atrium
CIRCULATORY SYSTEM IN HUMANS
• Double circulatory system – blood flows
through the heart only twice for each
circulation.
• The pulmonary circulation & systemic
circulation.
• Pulmonary circulation – right ventricle 
deoxygenated blood  lungs (via
pulmonary arteries)  pulmonary veins 
left atrium
CIRCULATORY SYSTEM IN HUMANS
• Systemic circulation – left ventricle 
oxygenated blood  body (via aorta) 
superior & inferior vena cava  right
atrium.
• Complete double circulatory system  heart
being divided into two  right pump (to
lungs) & left pump (to body parts).
• Advantages : oxygenated blood returns to
heart to be pumped again  increases the
pressure of the blood  speeding up the
delivery.
CIRCULATORY SYSTEM IN AMPHIBIANS
• Double circulatory system – blood flows
through the heart only twice for each
circulation.
• The pulmonary circulation & systemic
circulation.
• Three-chambered heart  two atria & one
ventricle.
• Some mixing of oxygenated & deoxygenated
blood in ventricle  enter the systemic
circulation (incomplete double circulatory
system)  less efficient.
CIRCULATORY SYSTEMS
ORGANISMS
CIRCULATORY
SYSTEM
CHARACTERISTICS
HUMANS
COMPLETE
DOUBLE CLOSED
CIRCULATORY
SYSTEM
• The blood enters the heart twice
during one complete cycle.
• The oxygenated & deoxygenated
blood not mixing together
FISH
SINGLE CLOSED
CIRCULATORY
SYSTEM
• An atrium & a ventricle
• The deoxygenated blood enters the
atrium & then the ventricle
• The blood enters the heart once
AMPHIBIANS
INCOMPLETE
DOUBLE CLOSED
CIRCULATORY
SYSTEM
• Two atria, one ventricle
• Mixing of oxygenated blood &
deoxygenated blood in the single
ventricle.
INSECTS
OPENED
CIRCULATORY
SYSTEM
•Blood flow in haemocoel
EXERCISE 1.2
1. The walls of the left side of the heart are
generally thicker than those of the right side
of the heart, and the wall of the ventricles
are thicker than those of the atria. Suggest
reasons for these differences.
2. Describe the similarities and differences
between the arteries and veins.