Grade 12 Zoology Note



It is the flow of fluid connective tissue through definite vessel either blood vessels or lymph vessels.

Functions of the Circulatory System

1.Respiration - delivers oxygen to the cells and removing carbon dioxide from them.

2.Nutrition - carries digested food substances to the cells of the body.

3.Waste Removal -  disposes of waste products and poisons that would harm the body if they accumulated.

4.Immunity - helps protect the body from disease.

5.Cellular Communication - the circulatory system provides a mode of transport for hormones.

6.Thermoregulation - the circulatory system transports heat (can both warm and cool body).

Types of Circulatory system:

Animals possess two type of circulatory system. They are

a) Open type       and            b) Closed type



Open Type:

In this type of circulatory system the blood may be present in the blood vessels for some time but finally it comes out of the blood vessels. The internal organs are directly bathed in blood. The blood flows from the heart into the arteries. The artery open into large spaces called sinuses. From the sinus the blood is carried by the veins to the heart. There are no inter connecting vessels or capillaries between the arteries and the veins, as the blood comes out of blood vessels. This type of circulatory system is called open type. It occurs in annelids like leeches, arthropods, most of the molluscs and ascidians.


Closed Type:

In the closed type of circulatory system, the blood remains inside the blood vessels and does not come out. The blood flows from arteries to veins through small blood vessels called capillaries. The closed type of circulatory system occurs in most of the Annelids, Cephalopods and Vertebrates including man.

Types of circulation:

1.Single circulation:

1.Blood flows only once through the heart in a complete cycle.

2.Heart pumps deoxygenated blood only.

3.Blood get oxygenated by gills.

4.Blood flow is slow and of low pressure.


2.Double circulation:

1.Blood flows twice through the heart in a complete cycle.

2.Heart pumps deoxygenated blood to lungs and oxygenated blood to the body.

3.Blood get oxygenated by lungs.

4.Blood flow is fast and of high pressure.




Structure of heart

External structure:


                       Fig. External structure of heart


A human heart is muscular hollow pumping organ which can maintain continuous flow of blood circulation inside the body. It is roughly triangular in shape. It is about the size of a person’s fist and weighs about 300 grams. It is reddish brown in color. It is situated ventrally in the middle of thoracic cavity in between two lungs. The narrow end of the heart is slightly displaced to the left side. It can be heard towards the left side of the chest.


The heart is enclosed in a double walled membranous sac called pericardium. The inner membrane is attached to the heart. In between two membranes, there is presence of a pericardial fluid. The fluid is shock absorbing so protect heart from any shock and minor injuries. It also allows free movement to the heart.


Anterior broad part is called auricular part and posterior narrow part is called ventricular part. Towards right side of auricular part there is superior and inferior venacavae carrying impure blood from different parts of body except lungs. Towards the left side of auricular part, the pulmonary veins carry pure blood from lungs. The auricular part receives blood. The lower ventricular part sends out blood. The arch of Aorta and Pulmonary trunk carries blood out of the heart.


Internal structure:

                                  Fig. Internal structure of heart

The human heart is four chambered. The upper two are right and left atria or auricles. The lower two are right and left ventricles. Auricles are thin walled chambered separated by inter auricular septum. There are presence of muscular ridges called musculi pectinati. Right auricle is having openings for superior and inferior venacavae to receive impure or venous blood. Left auricle is having openings for pulmonary veins to receive pure blood from lungs. The opening of inferior venacava is guarded by valve of Eustachius while the opening coronary sinus is guarded by valve of Thebesius. Right and left auricle open into respective ventricle through an auriculo ventricular apertures. These AV apertures are guarded by valves.

The sinus venosus is completely merged into right auricle. So caval veins directly open into auricles. Truncus arteriosus has split into systemic (aortic) and pulmonary trunk in mammals.


The two ventricles are separated by inter ventricular septum. The septum comes right side from the apex. Ventricles are thick walled than atria. The left ventricle is thicker than right ventricle.

Valves  Bicuspid valve is also known as mitral valve. It is situated between the left auricle and left auricle and left ventricle. It allows unidirectional flow of oxygenated blood from left atrium to left ventricle. It consists of two flaps or cusps. Tricuspid valve is the right AV valve. It consists of three flaps or cusps. It allows impure blood to flow from right atrium to right ventricle.


Both valves are provided with tendons or chords made up of tough strands of connective tissue called chordae tendinae. The chordae tendinae arise from papillary muscle present in the wall of ventricles. Their contractions bring the tightening of chordae tendinae which in turn prevent the valves from turning inside out or from being forced upward during contraction of ventricles.

Semi lunar valves:At the base of pulmonary trunk and aortic arch, there are pulmonary semi lunar and aortic semi lunar valves. Such valve is made up of three flaps attached to inside of arterial wall. These valves allow only unidirectional flow of blood from ventricle to artery and prevent back ward flow.



Heart beat and origin and conduction of heart beat:

The rhythmic contraction and relaxation of the heart is known as heart beat or cardiac cycle. The cardiac cycle consists of two parts: 

systole (contraction of the heart muscle) and diastole (relaxation of the heart muscle).

1.Atria contract while ventricles relax.

2.The pulse is a wave of contraction transmitted along the arteries.

3.Valves in the heart open and close during the cardiac cycle.

4.Heart muscle contraction is due to the presence of nodal tissue in two regions of the heart.

5.The SA node (sinoatrial node) initiates heartbeat.

6.The AV node (atrioventricular node) causes ventricles to contract.

7.The SA node is sometimes called the pacemaker since it keeps heartbeat regular.

8.Heartbeat is also controlled by the autonomic nervous system.

The cardiac cycle.

Diastole is the filling of the ventricles with blood. Ventricular systole opens the SL valves, forcing blood out of the ventricles through the pulmonary artery or aorta. The sound of the heart contracting and the valves opening and closing produces a characteristic "lub-dub" sound. Lub is associated with closure of the AV valves, dub is the closing of the SL valves.

Origin and Conduction of heart beat:

                                  Fig. Origin and Conduction of heart beat

The heart beat starts in a tiny island of tissues present in the upper region of right auricle, called the sino-auricular node or sino-atrial node (S.A.). It is located near the opening of large vein. S.A. acts as the pace maker of the heart. The heart impulses generated here radiate out from the heart as a wave and cause each muscle fibre, through which they pass, to contract. A similar node is present in the partition dividing the right and left au­ricles. A bundle of fibres originates from the auriculoventricular node (AV). This is known as Bundle of His or Purkinje fibres.

The impulse after passing through the AV node passes along bundle of HIS, which happens to be the only direct muscular communication be­tween auricles and ventircles. High in the interventricular septum the bundle of HIS divides into right and left bundle branches which are distributed respectively to the right and left ventricle. The bundle branches terminate in thin fibres called the Purkinje fibres through which the impulses reach the ventricular muscles.

The sino-auricular node sends out intermittent impulses. Each impulse initiates a wave of contraction, which passes through both the auricles and causes their contraction. When this wave of contraction alongwith the impulse reaches the auriculo-ventricular node, it is also'stimulated. The impulses from it make the ventricles contract.

While the ventricles con­tract the auricles relax. This sequence is repeated again and again. The whole system- the SA node, the AV node, bundle of HIS and the purkinje fibre systems constitute the conducting system of the heart. These are composed of modified cardiac muscle cells. The stimulus for the con­traction of auricles and ventircles is electrical.

This sequnece of electrical discharge and the phases of the cardic cycle may be studied with the help of an instrument called the Electrocardiogram. This instrument can mea­sure the electrical impulses of the heart muscles and give out the wave forms on a strip of paper and this is called the Electrocardiograph.


A pacemaker is a small device that's placed in the chest or abdomen to help control abnormal heart rhythms. This device uses electrical pulses to prompt the heart to beat at a normal rate. The sinu-auricular node (SA node ) is called as natural pacemaker.


Artificial pacemaker:

A device that uses electrical impulses to regulate the heart rhythm or reproduce it. An internal pacemaker is one in which the electrodes to the heart, the electronic circuitry, and the power supply are all implanted internally, within the body. Although there are different types of pacemakers, all are designed to treat a heart rate that is too slow (bradycardia). Pacemakers may function continuously and stimulate the heart at a fixed rate, or they may function at an increased rate during exercise. A pacemaker can also be programmed to detect an overly long pause between heartbeats and then stimulate the heart.


Blood pressure

It is a force exerted by blood against the wall of arteries, veins and capillaries. It is called BP in short. It is of two types. Systolic blood pressure is maximum pressure during contraction. Diastolic blood pressure is minimum pressure during the relaxation. The systolic and diastolic blood pressure of a normal adult is 120/80 mm of Hg. The range of systolic blood pressure is 100 to 140. 100 is lower limit and 140 is upper limit. The range of diastolic blood pressure is 55 to 80.


Problems related to BP


Hypertension or high BP

It is the condition when the systolic and diastolic blood pressure is 150/90 or above persistently.  Causes 

1.Unbalanced diet, cholesterol rich food 


3.Alcohol consumption

4.Stress, anxieties 

5.Loss of elasticity of arteries, arterosclerosis



Hypotension or low BP

It is the condition when the systolic and diastolic blood pressure is below the lower limit persistently. Causes 

1.Unbalanced diet, less nutrient food.

2.Loss of blood or less volume of blood



Heart sounds

The first heart sound is lubb. It is produced due the sharp closing of atrioventricular valves like the bicuspid and tricuspid valves. At the beginning of ventricular systole, these valves are closed. 

The second heart sound is dup. It is produced due the closing of semilunar valves at the end of the aortic arch and pulmonary aorta. It occurs at the end of ventricular systole. 



The pulse is a decidedly low tech and high yield and antiquated term still useful at the bedside in an age of computational analysis of cardiac performance. Claudius Galen was perhaps the first physiologist to describe the pulse.[1] The pulse is an expedient tactile method of determination of systolic blood pressure to a trained observer. Diastolic blood pressure is non-palpable and unobservable by tactile methods, occurring between heartbeats.

Pressure waves generated by the heart in systole moves the arterial walls. Forward movement of blood occurs when the boundaries are pliable and compliant. These properties form enough to create a palpable pressure wave.





An electrocardiogram (ECG) records the electrical activity of the heart. The heart produces tiny electrical impulses which spread through the heart muscle to make the heart contract. These impulses can be detected by the ECG machine. You may have an ECG to help find the cause of symptoms such as palpitations or chest pain. Sometimes it is done as part of routine tests - for example, before you have an operation. The ECG test is painless and harmless.

Some terms related to Circulation system of Heart

Heart beat

Heart beat consists of one contraction phase and one relaxation phase. During the contraction phase, there is maximum point(peak period) which is known as systole. During the relaxation phase also, there is minimum point which is known as diastole. One heart beat is completed in about 0.8 second time. 


Stroke volume 

It is the volume of blood pumped out in every heart beat. It comes to be about 70 ml of blood. It is denoted by SV.


Heart rate 

It is the number of heart beats per minute time. In the normal adult, it is about 70 to 80 times. In an average, it comes to be 72 times. This is called heart rate. It is denoted by HR.


Cardiac output

It is the volume of blood pumped out in one minute time. It is given by the product of stroke volume and heart rate. It is denoted by CO





Arterial System



Blood supplies through aortic system and pulmonary system.There are many arteries in the body. They distribute blood to different parts of body. All these arteries together make arterial system. This system supply pure blood to body organs and impure blood to lungs.  Pulmonary aorta arises from right ventricle. It divides to two branches. They are pulmonary arteries. These arteries supply impure blood to two lungs. 

Aortic arch arises from the left ventricle. It gives one branch called innominate on the right side. It divides into right common carotid and right subclavian. On the left side, aortic arch gives left common carotid and left subclavian. The right and left common carotids ascend up through neck. In the head region, they divide into internal and external carotids. The internal carotids supply pure blood to brain. The external carotids supply to outer part of head region such jaw, tongue, face etc. The right and left subclavians enter into the fore limbs(hand). Subclavians divide into radial and ulnare arteries. They all supply pure blood to different parts of hand. There are vertebral arteries also to supply blood to neck region. 


The aortic arch runs down and forms the dorsal aorta. It gives many branches. In the chest, there are intercostal arteries to supply blood to intercostal muscles. One pair of inferior phrenic arteries supply blood to the lower part of diaphragm. One coliac artery comes out from the dorsal aorta. It divides into the hepatic and leinogastric arteries. Hepatic artery supplies blood to liver. The leinogastric supplies blood to stomach and spleen. The superior mesenteric arteries divides into many branches and supply blood to duodenum, jejunum and ileum. The right and left renal arteries supply blood to the kidneys. The genital arteries supply blood to testes in male and ovaries in female. The lumbar arteries supply blood to the wall of abdomen. The inferior mesenteric artery supplies blood to the large intestine like ascending, transverse, descending colon, pelvic colon and rectum. 


Then the dorsal aorta divides into two branches called common iliacs. They enter into two legs. The right and left iliacs give small internal iliacs to supply blood to urinary bladder. The external iliacs give deep femoral which supply blood to thigh. The external iliacs run down as popliteal. These divide into anterior and posterior tibials. They supply blood to lower part of leg.



Venous System:


                                          Fig.Venous Nervous System


All veins in the body together make venous system. This system is for collection of pure blood from lungs and impure blood from rest of the parts of body. Four pulmonary veins collect pure blood form lungs and open into left atrium. Superior venacava collect blood from anterior part of body. It is made by different veins. Right and left innominate veins join to form superior venacava. Each innominate is made by right and left external and internal jugular veins collecting blood from head and brain. The right and left subclavians collect from hand. The subclavian is again made by axillary and cephalic veings. Azygous vein collects from chest region in the right side. Hemiazygous collect from left side . both open into superior venacava. Inferior venacava is made by different veins. Two short and thick hepatic veins collect blood from liver. 


Two common iliac veins join to make the inferior venacava. Each iliac vein is made by internal iliac from urinary bladder, large saphenous from thigh, anterior and posterior tibial from lower part of leg.


The blood collection form stomach and intestine has different arrangement. It is called Portal system. The blood is not directly carried to the heart. It is taken to liver first. Gastric vein, duodenal vein, intestinal veins etc join to form hepatic portal vein. It enters into liver to form hepatic portal system. 


Importance of hepatic portal system

1.Stores excess glucose in the form of glycogen

2.Converts highly poisonous ammonia into less poisonous urea

3.Kills Bacteria, germs etc and protect from diseases

4.Removes impurities

5.Regulate body temperature etc.


Blood Types

There are a number of different blood group systems, with the International Society of Blood Transfusion recognizing up to 30 major group systems. The two main blood group systems are ABO antigens and RhD (Rhesus) antigens. Most antigens are protein molecules that are situated on the surface of the red blood cells, it is with these two antigens that blood types are classified. 

The ABO system is used to determine the different types of antigens in the red blood cells and antibodies in the plasma. These differences mean that there are four ABO groups, these are:

Group A - red blood cells contain A antigens and the plasma has anti-B antibodies

Group B - B antigens are found in the red blood cells and anti-A antibodies in the plasma

Group AB - the red blood cells have both A and B antigens, however there are no antibodies.

Group O - this time the plasma contains both types of antibodies but neither type of antigen.

As some red blood cells have an additional antigen (known as the RhD antigen), there is a further type of classification. If the red blood cells contain this antigen then they are classed as RhD +ve, and if they don't, are classed as RhD -ve. This means that there eight main blood types, these are:

A positive (A+)

A negative (A-)

B positive (B+)

B negative (B-)

AB positive (AB+)

AB negative (AB-)

O positive (O+)

O negative (O-)


Rhesus (Rh) factor:

Rhesus (Rh) factor is an inherited trait that refers to a specific protein found on the surface of red blood cells. If your blood has the protein, you're Rh positive — the most common Rh factor. If your blood lacks the protein, you're Rh negative.

Although Rh factor doesn't affect your health, it can affect pregnancy. Your pregnancy needs special care if you're Rh negative and your baby's father is Rh positive.

If you're pregnant, your health care provider will recommend an Rh factor test during your first prenatal visit. The Rh factor test is a basic blood test that indicates whether you're Rh positive or Rh negative.

Role of Rh factor:

a.During blood transfusion:

When Rh-negative persons are given transfusions of Rh-positive blood, more than 50 per cent are sensitized to the Rh0 factor. Such sensitization of female children may be the cause of hemolytic disease in their offspring many years later, while severe hemolytic reactions may follow a second transfusion of Rh-positive blood in either sex.

The gross hemolysis of transfused blood may be entirely asymptomatic, however. In one case a pint of blood was completely hemolyzed within two hours without producing symptoms. The only signs were hemoglobinuria, low grade jaundice, urobilinogenuria and a rising Rh antibody titer. The patient had previously been sensitized by a single pint of Rh-positive blood.

b.During  pregnancy in woman:

The Rh factor is written as either positive (present) or negative (absent). Most people are Rh positive. This factor does not affect your health except during pregnancy.

A woman is at risk when she has a negative Rh factor and her partner has a positive Rh factor. This combination can produce a child who is Rh positive. While the mother's and baby's blood systems are separate there are times when the blood from the baby can enter into the mother's system. This can cause the mother to create antibodies against the Rh factor, thus treating an Rh positive baby like an intruder in her body. If this happens the mother is said to be sensitized. A sensitized mother's body will make antibodies. These antibodies will then attack an Rh positive baby's blood, causing it to breaking down the red blood cells of the baby and anemia will develop. In severe cases this hemolytic disease can cause illness, brain damage and even death.

Sensitization can also occur during a blood transfusion, miscarriage, abortion, ectopic pregnancy and even during some procedures, like amniocentesis. Since the antibodies do not disappear and rarely cause a problem in first pregnancies, it is very important to be screened thoroughly and give an accurate medical history to your doctor or midwife.

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