The formed elements are erythrocytes, leucocytes and platelets, constituting nearly 45 per cent of the blood.

If only three components are wanted: RBCs for transport of respiratory gases, WBCs for defence and immunity, and platelets for blood clotting.

Proteins contribute 6–8 per cent of plasma, and fibrinogen, globulins and albumins are the major proteins. Each has a distinct importance:

• Fibrinogens are needed for clotting or coagulation of blood. They are the precursor that thrombin converts into the fibrin threads of the clot, so without them a wound would continue to bleed. Note that plasma without the clotting factors is called serum — and cannot clot.
• Globulins are primarily involved in defense mechanisms of the body, forming the antibody fraction of the plasma proteins and so underlying immunity.
• Albumins help in osmotic balance. Being the most abundant and smallest of the three, they hold water within the vessels; if albumin falls, fluid leaks into the tissue spaces and swelling results.

A further importance worth adding: the plasma proteins also give plasma its viscosity, which helps maintain blood pressure, and they act as carriers for many substances in transit.

Because blood satisfies the defining criteria of a connective tissue. Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

The reasons, set out:

• It has cells suspended in an extracellular matrix — the defining structural feature of a connective tissue. Here the formed elements (erythrocytes, leucocytes and platelets) are the cells and plasma is the matrix.
• It is mesodermal in origin, like all connective tissues.
• Its cells are not in direct contact with one another but are separated by the matrix — again characteristic of connective tissue, and unlike epithelium, where cells are packed tightly together.
• It performs a connective function, linking every tissue of the body to every other by transporting nutrients, O₂ and other essential substances to the cells and carrying waste substances away.

As the chapter's summary puts it, lymph is almost similar to blood except for the protein content and the formed elements. In detail:

And their relationship. Lymph is derived from blood: as the blood passes through the capillaries in tissues, some water along with many small water soluble substances move out into the spaces between the cells of tissues, leaving the larger proteins and most of the formed elements in the blood vessels. That single sentence explains every difference in the table — lymph lacks RBCs, platelets and the larger proteins simply because those were too large to leave the capillary.

Double circulation means that two separate circulatory pathways are present, so that blood passes through the heart twice in one complete circuit of the body. We have a complete double circulation, i.e., two circulatory pathways, namely pulmonary and systemic, are present.

The two pathways

• Pulmonary circulation. The blood pumped by the right ventricle enters the pulmonary artery. The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs, from where the oxygenated blood is carried by the pulmonary veins into the left atrium.
• Systemic circulation. The left ventricle pumps blood into the aorta. The oxygenated blood entering the aorta 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 vena cava and emptied into the right atrium.

Its significance

• No mixing of blood. Oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides, and the ventricles pump it out without any mixing up. Tissues therefore receive fully oxygenated blood, not the mixed blood that a three-chambered heart delivers.
• A steep oxygen gradient at the tissues. Because arterial blood is fully oxygenated, the partial pressure difference driving O₂ into the tissues is as large as possible — which is what allows a high rate of oxygen delivery.
• Support for a high metabolic rate. This is why the arrangement is found in crocodiles, birds and mammals, the animals with the highest and most sustained energy demands.
• Two circuits at appropriate pressures. The systemic circuit needs high pressure to reach the whole body; the pulmonary circuit needs gentler pressure so as not to damage the delicate alveolar capillaries. Separate pumps — the thick-walled left ventricle and the thinner right — allow each circuit its own pressure.
• Efficient and complete re-oxygenation. Every unit of blood returning from the tissues must pass through the lungs before being sent out again.

(a) Blood and Lymph

(b) Open and Closed system of circulation

(c) Systole and Diastole

(d) P-wave and T-wave

All vertebrates possess a muscular chambered heart, but the number of chambers rises through the series, and with it the degree to which oxygenated and deoxygenated blood are kept apart.

Fishes — two chambers, single circulation

Fishes have a 2-chambered heart with an atrium and a ventricle. In fishes the heart pumps out deoxygenated blood, which is oxygenated by the gills and supplied to the body parts, from where deoxygenated blood is returned to the heart — single circulation. Blood passes through the heart only once per complete circuit, and only deoxygenated blood ever enters the heart.

Amphibians and reptiles — three chambers, incomplete double circulation

Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. 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 — incomplete double circulation. The advance over fishes is the division of the atrium into two; the limitation is the undivided ventricle.

Crocodiles, birds and mammals — four chambers, complete double circulation

Crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles. Oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms; hence these animals have double circulation.

The trend, in one sentence: progressive separation of the oxygenated and deoxygenated streams, from no separation at all, through separate atria with a shared ventricle, to complete separation — which allows the tissues to receive fully oxygenated blood and supports the high metabolic rates of birds and mammals. Note the exception: crocodiles are reptiles yet have four chambers, so the progression is not a simple ladder.

Because the heartbeat originates in the heart's own muscle and not in any nerve. Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.

The evidence and mechanism:

• Cardiac musculature is auto-excitable. The nodal musculature has the ability to generate action potentials without any external stimuli.
• The SAN generates the maximum number of action potentials, 70–75 per minute, and is therefore responsible for initiating and maintaining the rhythmic contractile activity of the heart.
• Consequently, the heart continues to beat even if its nerve supply is cut, and an excised heart kept in a suitable medium goes on beating for some time — which is what makes heart transplantation possible.

Because it is the fastest of the self-exciting tissues, and therefore sets the rhythm for all the rest. The SAN can generate the maximum number of action potentials, 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.

The full reasoning:

• The SAN is a patch of nodal tissue present in the right upper corner of the right atrium.
• The whole nodal musculature is autoexcitable — it can generate action potentials without any external stimuli — so in principle any part of it could set the pace.
• But the number of action potentials that could be generated in a minute varies at different parts of the nodal system, and the SAN's rate is the highest.
• Since its impulse reaches the other parts of the nodal system and triggers them before they can fire on their own, the SAN's rhythm is imposed on the entire heart. Our heart normally beats 70–75 times in a minute (average 72 beats min⁻¹) — exactly the SAN's own rate.

Their significance is that they form the only electrical route from the atria to the ventricles, and they ensure the ventricles contract in the right way at the right moment.

Their structure and position

• The AVN is a mass of nodal tissue seen in the lower left corner of the right atrium, close to the atrio-ventricular septum.
• A bundle of nodal fibres, the atrio-ventricular bundle (AV bundle), continues from the AVN, passes through the atrio-ventricular septa to emerge on the top of the inter-ventricular septum, and immediately divides into a right and left bundle. These branches give rise to minute fibres throughout the ventricular musculature of the respective sides and are called purkinje fibres.

Their significance

• They conduct the impulse from atria to ventricles. 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, causing ventricular systole. This matters because the atrio-ventricular septum is a thick fibrous tissue that does not itself conduct — without the AV bundle crossing it, the impulse could not reach the ventricles at all.
• They introduce a necessary delay. Conduction through the AVN is relatively slow, so the ventricles contract only after the atria have finished emptying into them. Without this pause, atrial and ventricular systole would overlap and the ventricles would contract while still filling.
• They make the ventricle contract as one unit. Through the right and left bundles and the purkinje fibres throughout the ventricular musculature, the impulse reaches the whole ventricle almost simultaneously, so it squeezes as a single coordinated pump rather than in a slow wave — which is what allows it to generate enough pressure to open the semilunar valves.
• The AVN can act as a reserve pacemaker. Being nodal tissue it too is autoexcitable, so if the SAN fails the AVN takes over, at a slower rate.

Cardiac cycle. The sequential event in the heart which is cyclically repeated is called the cardiac cycle, and it consists of systole and diastole of both the atria and ventricles. Its stages in order are joint diastole → atrial systole → ventricular systole → ventricular diastole, after which the SAN fires again. The heart beats 72 times per minute, i.e., that many cardiac cycles are performed per minute; from this it could be deduced that the duration of a cardiac cycle is 0.8 seconds.

Cardiac output. The 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. It is obtained as follows: during a cardiac cycle, each ventricle pumps out approximately 70 mL of blood, which is called the stroke volume, and the stroke volume multiplied by the heart rate (no. of beats per min.) gives the cardiac output.

One point to add: the body has the ability to alter the stroke volume as well as the heart rate, and thereby the cardiac output — which is why 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.

Points worth noting.

• Both sounds are produced by valves closing — not by muscle contracting or blood flowing. The sound is the vibration of the closing cusps and the blood behind them.
• The interval between lub and dub is therefore the period during which blood is being ejected from the ventricles; the interval from dub to the next lub is ventricular diastole.
• These sounds are of clinical diagnostic significance. A valve that fails to close properly produces an abnormal sound, a murmur, at precisely the moment that valve should have shut — so a doctor listening with a stethoscope can tell which valve is faulty and whether it is failing to open or failing to close.
• The first sound is longer and lower-pitched, the second shorter and sharper, because the semilunar valves are smaller and snap shut more quickly.

What an ECG is. ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle, obtained with an electro-cardiograph. 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. Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.

Its clinical uses.

• 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 a great clinical significance.