Body Fluids and Circulation

1. Chapter at a Glance

All living cells need nutrients, oxygen and other essential substances, and the waste or harmful substances they produce must be removed continuously. Simple organisms like sponges and coelenterates circulate water from their surroundings through body cavities. More complex organisms use special fluids. Blood is the most commonly used body fluid in higher organisms including humans; lymph also helps in the transport of certain substances. This chapter covers the composition of blood, blood groups, coagulation, lymph, the circulatory pathways, the human heart, the cardiac cycle, the ECG, double circulation, regulation of cardiac activity and disorders of the circulatory system. NEET asks about two questions every year, usually on formed elements, blood groups, the cardiac cycle and ECG waves.

2. Blood

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

Plasma

• Plasma is a straw-coloured, viscous fluid forming nearly 55% of the blood.
• 90–92% of plasma is water, and proteins contribute 6–8% of it.
• Fibrinogen, globulins and albumins are the major plasma proteins. Fibrinogen is needed for clotting or coagulation of blood. Globulins are primarily involved in the defence mechanisms of the body, and albumins help in osmotic balance.
• Plasma also contains small amounts of minerals such as Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ and Cl⁻. Glucose, amino acids and lipids are also present, as they are always in transit in the body.
• Factors for coagulation or clotting of blood are also present in plasma in an inactive form.
• Plasma without the clotting factors is called serum.

Formed elements

Erythrocytes, leucocytes and platelets are collectively called formed elements. They constitute nearly 45% of the blood.

A reduction in platelet number can lead to clotting disorders, causing excessive loss of blood from the body.

3. Blood Groups

Blood of humans differs in certain aspects though it looks similar. Various types of grouping of blood have been done; two such groupings — the ABO and the Rh — are widely used all over the world.

ABO grouping

ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the RBCs, namely A and B. Similarly, the plasma of different individuals contains two natural antibodies (proteins produced in response to antigens).

Group O blood can be donated to persons with any other blood group, so O persons are called universal donors. Persons with AB group can accept blood from persons with AB as well as the other groups, so they are called universal recipients. During blood transfusion, any blood cannot be used: the blood of a donor has to be carefully matched with the blood of a recipient before transfusion to avoid severe problems of clumping (destruction of RBCs).

Rh grouping

• Another antigen, the Rh antigen, similar to one present in Rhesus monkeys (hence Rh), is also observed on the surface of RBCs of the majority (nearly 80%) of humans. Such individuals are called Rh positive (Rh+ve); those without it are Rh negative (Rh−ve).
• An Rh−ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens. Therefore Rh group should also be matched before transfusion.
• A special case of Rh incompatibility (mismatching) has been observed between the Rh−ve blood of a pregnant mother and the Rh+ve blood of the foetus.
• Rh antigens of the foetus do not get exposed to the Rh−ve blood of the mother in the first pregnancy, as the two bloods are well separated by the placenta. However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the Rh+ve blood from the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood.
• In subsequent pregnancies, the Rh antibodies from the mother (Rh−ve) can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby. This condition is called erythroblastosis foetalis.
• It can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

4. Coagulation of Blood

Blood exhibits coagulation or clotting in response to an injury or trauma. This is a mechanism to prevent excessive loss of blood from the body.

• A dark reddish-brown scum formed at the site of a cut or injury over a period of time is a clot or coagulum. It is formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped.
• Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
• Thrombins, in turn, are formed from another inactive substance present in the plasma called prothrombin. An enzyme complex, thrombokinase, is required for this reaction.
• This complex is formed by a series of linked enzymic reactions (a cascade process) involving a number of factors present in the plasma in an inactive state.
• An injury or a trauma stimulates the platelets in the blood to release certain factors that activate the mechanism of coagulation. Certain factors released by the tissues at the site of injury can also initiate coagulation.
• Calcium ions play a very important role in clotting.

5. Lymph (Tissue Fluid)

• As blood passes through the capillaries in tissues, some water along with many small water-soluble substances moves out into the spaces between the cells of tissues, leaving the larger proteins and most of the formed elements in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid.
• It has the same mineral distribution as that in plasma. Exchange of nutrients, gases and so on between the blood and the cells always occurs through this fluid.
• An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. The fluid present in the lymphatic system is called lymph.
• Lymph is a colourless fluid containing specialised lymphocytes, which are responsible for the immune responses of the body.
• Lymph is also an important carrier of nutrients, hormones and so on. Fats are absorbed through lymph in the lacteals present in the intestinal villi.

6. Circulatory Pathways

The circulatory patterns are of two types — open and closed.

• Open circulatory system: present in arthropods and molluscs. Blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.
• Closed circulatory system: present in annelids and chordates. Blood pumped by the heart is always circulated through a closed network of blood vessels. This pattern is considered more advantageous, as the flow of fluid can be more precisely regulated.

7. The Human Heart

• The heart, the mesodermally derived organ, is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist.
• It is protected by a double-walled membranous bag, the pericardium, enclosing the pericardial fluid.
• It has four chambers: two relatively small upper chambers called atria and two larger lower chambers called ventricles.
• A thin, muscular wall called the inter-atrial septum separates the right and left atria; a thick-walled inter-ventricular septum separates the left and right ventricles. The atrium and ventricle of the same side are separated by a thick fibrous tissue called the atrio-ventricular septum, each with an opening.
• The opening between the right atrium and right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve. A bicuspid or mitral valve guards the opening between the left atrium and left ventricle.
• The openings of the right and left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves.
• The valves in the heart allow the flow of blood only in one direction — from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.
• The entire heart is made of cardiac muscles. The walls of the ventricles are much thicker than those of the atria.

Nodal tissue

• A specialised cardiac musculature called the nodal tissue is also distributed in the heart.
• A patch of this tissue is present in the right upper corner of the right atrium, called the sino-atrial node (SAN). Another mass of this tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum, called the atrio-ventricular node (AVN).
• A bundle of nodal fibres, the atrio-ventricular bundle (AV bundle), continues from the AVN, passes through the atrio-ventricular septa, emerges on the top of the inter-ventricular septum and immediately divides into right and left bundles. These branches give rise to minute fibres throughout the ventricular musculature called Purkinje fibres. These fibres along with the right and left bundles are known as the bundle of His.
• The nodal musculature has the ability to generate action potentials without any external stimuli; it is autoexcitable. The number of action potentials varies in different parts of the nodal system: the SAN can generate the maximum number, i.e. 70–75 min⁻¹, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore it is called the pacemaker.
• Our heart normally beats 70–75 times in a minute (average 72 beats min⁻¹).

8. Cardiac Cycle

1. To begin, all four chambers are in a relaxed state (joint diastole). The tricuspid and bicuspid valves are open, so blood from the pulmonary veins and vena cava flows into the left and right ventricles respectively through the atria. The semilunar valves are closed at this stage.
2. The SAN now generates an action potential, which stimulates both atria to undergo a simultaneous contraction — the atrial systole. This increases the flow of blood into the ventricles by about 30%.
3. The action potential is conducted to the ventricular side by the AVN and AV bundle, from where the bundle of His transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract (ventricular systole); the atria undergo relaxation (diastole), coinciding with ventricular systole.
4. Ventricular systole increases the ventricular pressure, causing the closure of the tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
5. As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing blood in the ventricles to flow through these vessels into circulation.
6. The ventricles now relax (ventricular diastole) and the ventricular pressure falls, causing the closure of the semilunar valves, which prevents backflow of blood into the ventricles.
7. As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood being emptied into them by the veins. Blood once again moves freely to the ventricles. The ventricles and atria are again in a relaxed (joint diastole) state, as earlier.

This sequential event in the heart, which is cyclically repeated, is called the cardiac cycle. It consists of systole and diastole of both the atria and ventricles. As the heart beats about 72 times per minute, the duration of a cardiac cycle is 0.8 seconds.

• During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood, which is called the stroke volume.
• The stroke volume multiplied by the heart rate (number of beats per minute) gives the cardiac output. Cardiac output can be defined as the volume of blood pumped out by each ventricle per minute, and averages 5000 mL or 5 litres in a healthy individual.
• The body has the ability to alter the stroke volume as well as the heart rate and thereby the cardiac output. For example, the cardiac output of an athlete will be much higher than that of an ordinary man.
• During each cardiac cycle two prominent sounds are produced, which can be easily heard through a stethoscope. The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves, whereas the second heart sound (dub) is associated with the closure of the semilunar valves. These sounds are of clinical diagnostic significance.

9. Electrocardiograph (ECG)

The ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.

• By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.
• Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence it is of great clinical significance.

10. Double Circulation

• Blood flows strictly by a fixed route through blood vessels — the arteries and veins. Basically, each artery and vein consists of three layers: an inner lining of squamous endothelium, the tunica intima; a middle layer of smooth muscle and elastic fibres, the tunica media; and an external layer of fibrous connective tissue with collagen fibres, the tunica externa. The tunica media is comparatively thin in the veins.
• The deoxygenated blood pumped into the pulmonary artery by the right ventricle is passed on to the lungs, from where the oxygenated blood is carried by the pulmonary veins into the left atrium. This pathway constitutes the pulmonary circulation.
• The oxygenated blood entering the aorta from the left ventricle is carried by a network of arteries, arterioles and capillaries to the tissues, from where the deoxygenated blood is collected by a system of venules, veins and the vena cava and emptied into the right atrium. This is the systemic circulation. It provides nutrients, O₂ and other essential substances to the tissues and takes CO₂ and other harmful substances away for elimination.
• A unique vascular connection exists between the digestive tract and the liver, called the hepatic portal system. The hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation.
• A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.

11. Regulation of Cardiac Activity

• Normal activities of the heart are regulated intrinsically, i.e. auto-regulated by specialised muscles (nodal tissue). Hence the heart is called myogenic.
• A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system (ANS).
• Neural signals through the sympathetic nerves (part of the ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output.
• Parasympathetic neural signals (another component of the ANS) decrease the rate of heart beat, the speed of conduction of action potential and thereby the cardiac output.
• Adrenal medullary hormones can also increase the cardiac output.

12. Disorders of the Circulatory System

13. Master Summary Table

14. Mind Map

15. Tips and Tricks

16. Important One-liners

• Blood is a fluid connective tissue.
• Albumins maintain osmotic balance; globulins help in defence; fibrinogen helps in clotting.
• Mammalian RBCs are biconcave and lack a nucleus.
• The spleen is called the graveyard of RBCs.
• Basophils secrete histamine, serotonin and heparin.
• About 80% of humans are Rh positive.
• Fats are absorbed through lymph in the lacteals of intestinal villi.
• Crocodiles have a four-chambered heart.
• The human heart is myogenic.
• The SAN is called the pacemaker of the heart.
• Each ventricle pumps about 70 mL of blood per beat (stroke volume).
• Coronary artery disease is often referred to as atherosclerosis.

17. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

18. Quick Revision Summary