Circulation is the transport of nutrients, oxygen, CO2 and excretory products to the concerned tissues or organs.
For circulation, simple organisms (sponges, coelenterates etc) use water from their surroundings. Complex organisms use body fluids (blood & lymph) for circulation.
Circulatory system is 2 types- Open and Closed.
§ Open circulatory system: Here, the blood pumped by the heart passes through large vessels into open spaces or cavities called sinuses. E.g. Arthropods and mollusks.
§ Closed circulatory system: Here, the blood pumped by the heart is always circulated through blood vessels. This system is more advantageous as the flow of fluid can be more precisely regulated. E.g. Annelids and chordates.
All vertebrates have a muscular chambered heart.
· Fishes: 2-chambered heart (an atrium + a ventricle).
· Amphibians: 3-chambered heart (2 atria + a ventricle).
· Reptiles (except crocodiles): 3-chambered heart (2 atria + a ventricle). Ventricle is incompletely partitioned.
· Crocodiles, birds & mammals: 4-chambered heart.
- Single circulation in fish: In fishes, heart receives impure blood only (venous heart).
Deoxygenated blood from heart → oxygenated by gills → supplied to body parts → deoxygenated blood → to heart.
- Incomplete double circulation in amphibians & reptiles: The left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood.
- Double circulation in birds & mammals: Oxygenated and deoxygenated blood received by the left and right atria respectively passes on to ventricles of the same sides. The ventricles pump it out separately without any mixing up.
HUMAN CIRCULATORY SYSTEM
It consists of Blood vascular system & Lymphatic system.
BLOOD VASCULAR SYSTEM
(Heart, Blood vessels & Blood)
Formed of plasma (55%) & formed elements (45%).
Straw-coloured and slightly alkaline (pH 7.4) fluid matrix.
Constituents of plasma and their functions
· Water (90-92%): It is a good solvent. Transports vitamins, hormones, enzymes, nutrients etc.
· Plasma proteins (6-8 %): Include
o Fibrinogen: For blood coagulation.
o Globulins: Act as antibodies (for defense of the body).
o Albumins: For osmotic balance. Regulate blood pressure.
· Glucose, amino acids, lipids & cholesterol: For energy production and growth.
· Inorganic constituents: Na+, K+, Mg2+, Cl-, HCO3- etc regulates osmosis etc. Ca2+ ions help in blood clotting and muscle contraction.
· Gases like CO2, O2, N2 etc: For transport.
Plasma without clotting factors is known as Serum.
b. Formed elements (RBC, WBC & platelets)
§ Erythrocytes or Red Blood Cells (RBC):
- Count (mm-3): 5 - 5.5 millions.
- Average lifespan: 120 days.
- Formed in: Red Bone marrow.
- Other features: No nucleus, mitochondria etc. Biconcave in shape. Haemoglobin gives red colour to RBC.
- Normal Hb level: 12-16 gm/ 100 ml. Worn-out RBCs are destroyed in spleen (graveyard of RBCs).
- Function: CO2 and O2 transports.
§ Leucocytes or White Blood Cells (WBC):
- Count (mm-3): 6000-8000.
- Average lifespan: Generally short lived (1- 15 days).
- Formed in: Bone marrow, lymph glands, spleen.
- Other features: Colourless. Nucleated. Different types.
- Function: Part of immune system.
§ Platelets (Thrombocytes):
- Count (mm-3): 1.5 - 3.5 lakhs.
- Average lifespan: 7 days.
- Formed in: Megakaryocytes in Bone marrow.
- Other features: Colourless non-nucleated cell fragments.
- Function: Blood clotting.
Types of WBC: Granulocytes & Agranulocytes
1. Granulocytes: 3 types
a. Neutrophils (Heterophils): 60-65%. Soldier of the body.
b. Eosinophils (Acidophils):2-3%. Resist infections. Cause allergic reactions.
c. Basophils (Cyanophils):0.5-1%. Secrete histamine, serotonin, heparin etc. Cause inflammatory reactions.
2. Agranulocytes: 2 types
a. Lymphocytes (20-25%): Smallest WBC with largest nucleus. Includes B- lymphocytes & T- lymphocytes. Cause immune responses. Secrete antibodies.
b. Monocytes (6-8%): Largest WBC.
BLOOD GROUPS (ABO grouping & Rh Grouping)
Blood groups are discovered by Carl Land Steiner.
1. ABO grouping
- It is based on presence or absence of 2 surface antigens (chemicals that induce immune response) on RBCs namely A & B. Similarly, plasma contains 2 antibodies (proteins produced in response to antigens) namely anti-A & anti-B.
Can donate blood to
blood from (Donor’s group)
A & AB
B & AB
A, B, AB & O
AB & O
- Antigen A reacts with anti-A. Antigen B reacts with anti-B.
- If bloods with interactive antigens & antibodies are mixed together, it causes clumping (agglutination) of RBCs.
- Persons with O Group are called Universal donors because they can donate blood to persons with any other blood group. Persons with AB group are called Universal recipients because they can accept blood from all groups.
2. Rh grouping
- Rhesus (Rh) factor is another antigen found on RBC.
- Rh+ve means the presence of Rh factor and Rh-ve means absence of Rh factor. Nearly 80% of humans are Rh+ve.
- Anti-Rh antibodies are not naturally found. So Rh-ve person can receive Rh+ve blood only once but it causes the development of anti-Rh antibodies in his blood. So a second transfusion of Rh+ve blood causes agglutination. Therefore, Rh-group should be matched before transfusion.
- It is an Rh incompatibility between the Rh-ve blood of a pregnant mother and Rh+ve blood of the foetus.
- Rh antigens do not get mixed with maternal blood in first pregnancy because placenta separates the two bloods.
- But at the time of first delivery, there is a possibility of exposure of the maternal blood to small amounts of the Rh+ve blood from the foetus. This induces the formation of Rh antibodies in maternal blood.
- In case of her subsequent pregnancies, the Rh antibodies from the mother leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs. This is fatal to the foetus or cause severe anaemia and jaundice to the baby. This condition is called Erythroblastosis foetalis.
- This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of first child.
It is a mechanism for haemostasis (prevention of blood loss through injuries). It involves the following events:
Clumped platelets & tissues at the site of injury release thromboplastin → Thromboplastin form an enzyme, thrombo-kinase (Prothrombinase) → Thrombokinase hydrolyses prothrombin to thrombin in presence of Ca2+ →Thrombin converts soluble fibrinogen to insoluble fibrin → Fibrin traps dead & damaged formed elements to form clot (coagulum).
Blood vessels are 3 types
§ Arteries: They carry blood from heart to other tissues. They contain oxygenated blood (except pulmonary artery). 3-layered. Their smaller branches are called arterioles.
§ Veins: They carry blood towards heart. They contain deoxygenated blood (except pulmonary vein). 3-layered. Their smaller branches are called venules.
§ Capillaries: In tissues, arterioles divide into thin walled single layered vessels (capillaries). They unite into venules.
STRUCTURE OF HEART
- Heart is a mesodermally derived organ located in mediastinum. It is protected by double-layered pericardium.
- The pericardial space (between pericardial membranes) is filled with pericardial fluid. It reduces the friction between the heart walls, and surrounding tissues.
- The heart is 4 chambered, two upper atria (auricles) and two lower ventricles. The walls (cardiac muscles) of the ventricles are much thicker than that of the atria.
- The atria are separated by an inter-atrial septum and the ventricles are separated by inter-ventricular septum.
- In between atrium and ventricle there is a thick fibrous atrio-ventricular septum with an opening.
- A tricuspid valve (3 muscular flaps or cusps) guards the opening between right atrium and right ventricle. A bicuspid (mitral) valve guards the opening between left atrium and left ventricle. These valves allow the flow of blood only in one direction, i.e. from atria to ventricles.
- The openings of right and left ventricles into pulmonary artery and aorta respectively are provided with the semi-lunar valves. They prevent backwards flow of blood.
CONDUCTING SYSTEM OF HEART
- Human heart is myogenic, i.e. normal activities of heart are auto regulated by nodal tissues (a specialized cardiac musculature present in heart wall). It consists of
o Sino-atrial node (SAN) in the right upper corner of the right atrium.
o Atrio-ventricular node (AVN) in the lower left corner of the right atrium close to the atrio-ventricular septum.
- From the AVN, a bundle of fibrous atrio-ventricular bundle (AV bundle) passes through atrio-ventricular septa and divides into a right & left branches. Each branch passes through the ventricular walls of its side. In the ventricular wall, it breaks up into minute fibres (Purkinje fibres). These fibres along with the bundles are known as bundle of His.
- Nodal tissues generate action potential without any external stimuli, i.e. it is autoexcitable. SAN initiates and maintains contraction of heart by generating action potentials (70-75/min). So it is called the pacemaker.
· Joint diastole: Firstly, all chambers of heart are in relaxed state (joint diastole). When the tricuspid and bicuspid valves open, blood from pulmonary vein and vena cava flows into left & right ventricles respectively through left and right atria. Semilunar valves are closed at this stage.
· Atrial (Auricular) systole: The SAN generates an action potential which stimulates both the atria to undergo contraction (atrial systole). This increases the flow of blood into the ventricles by about 30%.
· Ventricular systole: The action potential is conducted to ventricular side by AVN & AV bundle from where bundle of His transmits it through the ventricular musculature. This causes the contraction of ventricles (ventricular systole). During this, the atria undergo diastole. Ventricular systole increases the ventricular pressure causing
* Closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
* Semilunar valves open. So deoxygenated blood enters the pulmonary artery from right ventricle and oxygenated blood enters the aorta from left ventricle.
The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing
* The closure of the semilunar valves which prevents the backflow of blood into the ventricles.
* The tricuspid and bicuspid valves are opened by the pressure in the atria.
The ventricles and atria again undergo joint diastole and the above processes are repeated. This is called cardiac cycle. A cardiac cycle (atrial systole + ventricular systole + diastole) is completed in 0.8 seconds.
· One heartbeat = a cardiac cycle. So number of normal heartbeat: 70-75 times/min (average: 72/min).
· Stroke volume: It is the volume of blood pumped out by each ventricle during a cardiac cycle. It is about 70 ml.
· Cardiac output: It is the volume of blood pumped out by each ventricle per minute, i.e. stroke volume x heart rate (70 x 72). It is about 5000 ml (5 litres).
Cardiac output of an athlete is very high.
· Heart sounds: During each cardiac cycle, 2 prominent sounds are produced. The first sound (lub) is due to the closure of tricuspid and bicuspid valves. The second sound (dub) is due to the closure of the semilunar valves.
One heartbeat = a lub + a dub.
In man, blood flows through the heart twice for completing its circuit. This is called double circulation. It includes,
1. Pulmonary circulation: Circulation b/w lungs and heart. The deoxygenated blood pumped into the pulmonary artery is passed on to lungs from where oxygenated blood is carried by pulmonary veins into the left atrium.
2. Systemic circulation: Circulation b/w heart and various body parts. The oxygenated blood is passed through aorta, arteries, arterioles and capillaries and is reached the tissues. The deoxygenated blood collected from the tissues by venules, veins and vena cava is carried to the right atrium. The systemic circulation provides nutrients, O2 and other essential substances to the tissues and takes CO2 and other harmful substances away for elimination.
· Hepatic portal system: It is a system which includes the hepatic portal vein that carries blood from intestine to the liver before it is delivered to the systemic circulation.
· Coronary circulatory system: A system of coronary vessels that circulate blood to and from the cardiac musculature.
(Lymph, Lymph vessels & Lymph nodes)
- As the blood passes through the capillaries in tissues, some water and soluble substances are filtered out from plasma to the intercellular spaces, to form tissue (interstitial) fluid. It has the same mineral distribution as that in plasma.
- Exchange of nutrients, gases, etc between the blood and the cells occur through this fluid.
- Some tissue fluid enters lymphatic system (a system of lymph vessels and lymph glands) and the tissue fluid in them is called lymph. It drains back to the major veins.
Functions of lymph
§ It is the middleman between blood and tissues.
§ It carries plasma proteins synthesized in liver to the blood.
§ Transports digested fats (through lacteals in the intestinal villi), fat soluble vitamins, hormones etc.
§ Filtration of bacteria and foreign particles.
§ Lymph nodes produce WBC (lymphocytes) & antibodies.
§ It helps in the defensive mechanism of the body.
- It is an instrument used to obtain electrocardiogram (graphical representation of the electrical activity of the heart during a cardiac cycle).
- To get an ECG, a patient is connected to the machine with 3 electrical leads (one to each wrist and to left ankle) that monitor heart activity. For a detailed evaluation of heart’s function, multiple leads are attached to the chest region.
- An ECG consists of the following waves:
o P-wave: Represents the excitation (depolarization) of atria which causes atrial systole.
o QRS-complex: Represents depolarization of ventricles (Ventricular systole).
o T-wave: Represents the repolarisation of ventricles.
Deviation in the ECG indicates the abnormality or disease.
So ECG has great clinical significance.
REGULATION OF CARDIAC ACTIVITY
- Normal activities of heart are auto regulated by nodal tissues (myogenic heart).
- Medulla oblongata regulates cardiac activity through ANS.
- Sympathetic nerves of ANS increase the rate of heartbeat, the strength of ventricular contraction and cardiac output.
- Parasympathetic nerves of ANS decrease the heart beat, conduction of action potential and the cardiac output.
- Adrenal medullary hormones increase the cardiac output.
DISORDERS OF CIRCULATORY SYSTEM
· Hypertension (High Blood Pressure): Here, the blood pressure is higher than normal systolic (pumping) pressure (120 mm Hg) and normal diastolic (resting) pressure (80 mm Hg), i.e. above 120/80 mm Hg. If the BP is 140/90 or above, it is hypertension. It leads to heart diseases and also affects vital organs (brain, kidney etc).
· Coronary Artery Disease (CAD) or Atherosclerosis: Here, deposition of Ca, fat, cholesterol and fibrous tissue occurs in coronary arteries which makes the lumen of arteries narrower and thereby affect the blood supply.
· Angina (angina pectoris): An acute chest pain due to O2 deficiency to heart muscles. It occurs due to improper blood flow. It is common among middle-aged and elderly.
· Heart Failure (congestive heart failure): It is the condition in which heart is not pumping blood enough to meet the needs of the body. Congestion of the lungs is the main symptom. Heart failure is not same as cardiac arrest (heart stops beating) or a heart attack (sudden damage of heart muscle due to inadequate blood supply).