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Circulatory system.

posted Jan 27, 2015, 9:43 PM by Ranmini Perera   [ updated Sep 14, 2017, 12:49 PM by Upali Salpadoru ]
Heart and Circulation


Fig. 1   The four chambered heart.

 It doesn't miss a beat,       until death. 

  It has to go on beating at least once in every     second. 

 Mammals have developed a four chambered heart which is highly efficient. 


The heart  is 12 cm long, 8-9 cm wide and 6 cm from front to back, and is roughly the size of your fist. 

The heart has 3 layers. 

1.The smooth inside lining of the heart is called the endocardium. 

2.The middle layer of heart muscle is called the myocardium. 

3.It is surrounded by a fluid filled sac called the pericardium.

It has a double circulation system where the oxygenated blood does not mix with the de- oxygenated blood. 

Everyone is familiar with the  beeps of the heart. ”Lub-- dub, Lub-- dub” we can hear, if our ear is on the chest of a buddy.

What causes these peculiar sounds?




Systole                                      .


Ventricles  relaxed.               

Blood flows Into the heart.

Ventricles contract.                 

Blood flows out of the heart


Relax and fill up.

Contracts at the end pushing blood into ventricles.



Relax and blood flows from Atria.

Lowest pressure ,maximum volume.

R Ventricle contract.

Blood pushed into pulmonary artery.

AV. valves


Close - First heart sound (S1).

SV  valves.

Close- Second heart sound (S2)


.        AV-Atrioventricular- (Mitral and bicuspid valves) ,                 SV- Semilunar valves (Aortic and pulmonary valves)

The Pulse

Every time the ventricles contract and sends the blood into the aorta, a tidal wave runs along the arteries. This has come to be known as pulse. The table given below shows some variation of pulse according to age.    Variation of pulse with age.

                    Age / yr           0         0.5       1          2       3      4 to 7         8          12        Adult
                    Pulse /m        130      140    130       120    115      100         90         85      60 - 100

Fig.  An Electro cardiogram.   ECG.

Figure shows the variation of electrical potential across the heart during the heart beat  is shown here. The electrical pulses are identified by letters starting from P to T. From T to P  the heart rests..

  •  P wave - coincides with the spread of electrical activity over the Atria and   the beginning of it's contraction.
  •  QRS  - coincides with the spread of electrical activity over the ventricles and the beginning of it's contraction.
  •  T wave - coincides with the recovery phase of the ventricles 

Blood Pressure.

Blood Circulation

The heart, just like any other organ, requires oxygen. The heart does not extract oxygen and other nutrients from the blood flowing inside it. If Coronary arteries are blocked a heart attack results The heart gets its blood from coronary arteries that carry blood within the heart muscle. Approximately 4-5% of the blood output of the heart goes to the coronary arteries 225 ml/min).   

An average heart pumps 70 cm3 / beat. So according to your age it may be interesting to calculate the amount of blood you will pump in one hour.

Blood supply to Heart

There are two main coronary arteries (Fig.) - The Left Main Coronary artery (1) and the Right Coronary Artery (2) which arise from the Aorta. Each Artery supplies blood to different parts of the heart muscle and the electrical system. The heart also has veins that collect oxygen poor blood from the heart muscle and opens into the Right Atrium. Coronary artery disease is caused by a blockage in one of the coronary arteries. When a coronary artery is partially blocked that artery cannot supply enough blood to the heart
muscle to meet its needs during exertion. When someone with coronary artery disease exerts himself, it causes chest pain. This is due to lack of blood and oxygen to that part of the heart muscle. This is called angina. 

Fig.   The Blood supply to the body (Diagrammatic)

Blood vessels

The vessels that carry blood from the heart have been named arteries and those that bring blood back to the heart are called veins. The capillaries connect the arteries and the veins. The three types of blood vessel are compared in this chart below. The artery walls have to withstand a high pressure as the ventricles contract. (Systole) This may amount to 16 kPa (120 mm of Hg) in a healthy heart. The blood pressure in the veins may not exceed 80mm of Hg in a normal person.Both pressures depend on the heart output and the resistance offered by the vessels.

Narrowing or hardening of arteries (arteriolosclerosis)  may  lead to high blood pressure.  (hyper tension) Thrombosis is the clogging of blood vessels. These can be fatal if they occur in the brain or the heart. (cerebral thrombosis or coronary thrombosis)

Composition of Blood

Blood  Plasma

Blood Consists of differentiated cells floating or swimming in a viscous liquid called plasma. The cells make up 45% of the volume. Plasma has a pale yellowish colour and consists of 90% water. It has many plasma proteins such as albumin, globulin, fibrinogen and anti bodies. The ions of  Na+ , K+,Ca++ , Cl- , PO4---, and HCO3-are always present . The food materials such as amino acids, glucose and fats, (alcohol in drunkards) along with excretory products  such as urea, and some pigments are present in varying amounts. The ‘sodium hydrogen carbonate’ maintains the pH level of the blood.

Red Blood Cells (Erythrocytes)

These are cells that have sacrificed self control and ability to reproduce by the dis-integration of nuclei giving more space to oxygen carrying substance haemoglobin. They also lack mitochondria which makes it impossible for them to use up any oxygen they are carrying. They have to respire anaerobically.. They have a biconcave shape exposing a large area for diffusion while being flexible to squeeze through capillaries. Although the main function of these is to transport oxygen, an enzyme called ‘carbonic
anhydrase’ plays a role in carbon dioxide removal.

Fig. Blood cells and their functions.

White Blood Cells (Leucocytes)

These are larger than erythrocytes but fewer in number.  About 7000/mm3. There are two main types called granulocytes, which appear to be granular and agranulocytes which are clear. They have nuclei and capable of amoeboid movement as independent organisms. They even squeeze through the capillary walls in search of disease causing microbes. 

  • Phagocytes are cells that squeeze out of the blood and devour bacteria and worn out cell fragments and digest.
  • Polymorphs’, ‘Macrophages’ are also names used to describe phagocytic cells.(Cellsthat engulf organic matter; amoeboid fashion)
  • A normal man has about 5 litres of blood.
  • There are 5,000,000. /mm3 of blood. 
  • Life span of a red cell is about 3 months. Up to 10 million red cells are destroyed
  • in a person every second This massacre is mainly carried out by  the liver and the spleen.
  • The red cells cannot divide and multiply. So they are made in the red marrow of
  • certain bones such as the sternum, ribs and vertebrae
  • Red cells are unable to use the O2 it is carrying due to lack of mitochondria.
  • Anaemia is the condition caused by a shortage of red cells.
  • Serum is the name given to plasma when the fibrinogen has been removed.
  • A cell completes a circuit in approximately 45 seconds.
Fig.  White cell creeping through a  Capillary and the fluids entering the Lymphatic   system.

Blood Groups

In 1900 Karl Landsteiner discovered that there are 4 different blood groups. They have been named as type O, typeA, type B, and type AB.  Blood type and the antibodies present are given here.

Donors and Recipients.

  •   O    is a universal donor.
  •   AB is a Universal recipient.

Blood type

Antibodies present


Anti A and anti B antibodies present


Only anti B bodies


Only anti A bodies


No anti bodies.

Lymphatic System

Due to the blood pressure, which is higher than the fluid pressure in the tissues, a lot of water along with the solutes  get out of the capillaries. Some of this re enter the capilleries in the region where the pressure is low. The rest of the fluid enter the lymphatic capillaries. This fluid is called ‘lymph’. The lymph is driven through the vessels by muscular contractions and ultimately drain into the subclavian vein. The back flow is prevented by valves as in veins.

Spleen is the largest organ in the lymphatic system. It is a deep red body,12cm long, and lies in the left side of the abdomen. The phagocytes there not only destroy bacteria but also ingest the worn out red cells. The haemoglobin in them are converted into ‘bilirubin’ and ‘ferritin’. Ferrritin returns to the bone marrow for making new cells while the yellow pigment, bilurubin is excreted in the bile.

Blood Clotting

When a capillary is pierced, the platelets can plug the hole, but if the wound is large they
secrete a substance that can convert fibrinogen in blood to fibrin. This substance form a
net work of fibres that can trap the red cells. A clot such formed stops bleeding as well as the entry of germs.

Lymph Glands

  • These are the check points guarded by the lymphocytes that ingest any harmful  bacteria.
  • There are important glands in the neck,groin and arm pits.
  • ‘Thymus gland’ is placed over the heart and is responsible for the development of the spleen in babies.Even in adults, though it becomes smaller is responsible for stori white cells and the multiplication of lymphocytes.

Steps in Clotting

  1.    Platelets release the enzyme ‘thrombokinase’.
  2.    This converts ‘prothrombin’ into ‘thrombin’.
  3.    Thrombin converts fibrinogen into fibrin in the presence of Ca++.
  4.     A mesh of fibrin fibres along with blood cells forms a clot.

Rhesus Factor

Majority of the population has an antigen called Rhesus positive. 85%. Rh+ Those who lack it are called Rhesus negative. Rh_   If Rh+  is introduced into Rh- blood , antibodies are formed. When a Rh- mother carrries the child of a Rh+ father, the child will be Rh+. The fetal blood usually leak into mothers blood, which will start the production of anti bodies. In a subsequent pregnancy the mothers antibodies go into the foetus and make the child badly ill. A complete blood transfusion will have to done in order to save the child.

Blood Cancer.      

Leukemia is a cancer of the white blood cells. This cancer starts in the  bone marrow but can then spread to the blood, lymph nodes, the spleen,  other organs. Leukemia is divided into two types: acute (rapidly growing) and chronic (slowly growing), with the vast majority of childhood leukemia being the acute form. The bone marrow is the place where the blood cells are mainly produced. The production of the different types of blood cells is a carefully controlled activity. When cancer affects the bone marrow, it interferes with the  cell production. White blood cells, do not mature into fully functioning, normal cells. These abnormal cells grow in numbers and crowd out the normal cells of the blood, and weaken the body’s defense mechanism and the other functions of the blood. Lymphocytic leukemia develops in the lymph tissue.