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 thin-

walled atria, two lower thick-walled ventricles

SUPER VENA KAVASUPER VENA KAVA

RIGHT ATRIUMRIGHT ATRIUM

TRICUSPID VALVETRICUSPID VALVE

RIGHT VENTRICLERIGHT VENTRICLE LEFT VENTRICLELEFT VENTRICLE

LEFT ATRIUMLEFT ATRIUM

PULMONARY VEINPULMONARY VEIN

PULMONARY ARTERYPULMONARY ARTERY

AORTAAORTA

SEMI-LUNAR VALVESEMI-LUNAR VALVE

PULMONARY VEINPULMONARY VEIN

TRICUSPID VALVETRICUSPID VALVE

SEMI-LUNAR VALVESEMI-LUNAR VALVE

BICUSPID VALVEBICUSPID 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 OPENED

SINGLE DOUBLE INSECTS & MOLLUSCS

COMPLETE :-• HUMANS

INCOMPLETE :-• REPTILES

• AMPHIBIANSFISH

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 SYSTEMSORGANISMS CIRCULATORY

SYSTEMCHARACTERISTICS

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 1.21. 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.