All living cells require continuous supply of nutrients, oxygen, and other essential substances. Simultaneously, waste and harmful substances produced by cells must be efficiently removed. Various animal groups have evolved different mechanisms for transporting these substances.

Simple organisms like sponges and coelenterates rely on circulating water from their surroundings through their body cavities for exchange of substances directly with cells.

More complex organisms utilize specialized body fluids for transport. Blood is the most common transport fluid in most higher organisms, including humans. Another body fluid, lymph (tissue fluid), also aids in transport.

This chapter covers the composition and properties of blood and lymph, and the mechanism of blood circulation.

Blood

Blood is a specialized connective tissue consisting of a fluid matrix called plasma and cellular components collectively called formed elements.

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Plasma

Plasma is the straw-colored, viscous fluid component of blood, making up approximately 55% of blood volume.

Composition of plasma:

• Water: 90-92%
• Proteins: 6-8% of plasma, include major types:
  • Fibrinogen: Essential for blood clotting (coagulation).
  • Globulins: Involved in defense mechanisms (e.g., antibodies).
  • Albumins: Help maintain osmotic balance of blood.
• Minerals: Small amounts of ions like Na$^+$, Ca$^{++}$, Mg$^{++}$, $\textsf{HCO}_3^-$, Cl$^-$, etc.
• Other substances: Glucose, amino acids, lipids, vitamins, etc., which are transported in the plasma.
• Clotting factors: Present in plasma in an inactive form, involved in coagulation.

Serum: Plasma minus the clotting factors. This is the fluid left after blood clots.

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Formed Elements

The cellular components of blood are collectively called formed elements and constitute nearly 45% of blood volume. They include erythrocytes, leucocytes, and platelets (Figure 18.1).

1. Erythrocytes (Red Blood Cells - RBCs):
   • Most abundant cells in blood.
   • Average count: 5-5.5 million cells per mm$^3$ of blood in a healthy adult man.
   • Formed in the red bone marrow in adults.
   • Shape: Biconcave (in most mammals, including humans), increasing surface area.
   • Nucleus: Devoid of nucleus in most mammals at maturity.
   • Contain haemoglobin: A red-colored, iron-containing protein. Gives RBCs their color and name. Plays a significant role in the transport of respiratory gases ($\textsf{O}_2$ and $\textsf{CO}_2$).
   • Haemoglobin level: 12-16 grams per 100 ml of blood in a healthy individual.
   • Life span: Average 120 days, after which they are destroyed in the spleen (often called the "graveyard of RBCs").
2. Leucocytes (White Blood Cells - WBCs):
   • Colourless due to lack of haemoglobin.
   • Nucleated cells.
   • Relatively fewer in number: Average 6,000-8,000 cells per mm$^3$ of blood.
   • Generally short-lived.
   • Main categories:
     • Granulocytes: Have granules in the cytoplasm. Types: Neutrophils, Eosinophils, Basophils.
     • Agranulocytes: Lack granules. Types: Lymphocytes, Monocytes.
   • Specific types and functions:
     • Neutrophils: Most abundant (60-65% of total WBCs). Phagocytic (engulf and destroy foreign organisms).
     • Basophils: Least abundant (0.5-1%). Secrete histamine, serotonin, heparin; involved in inflammatory reactions.
     • Eosinophils: (2-3%). Resist infections and associated with allergic reactions.
     • Monocytes: (6-8%). Phagocytic; differentiate into macrophages.
     • Lymphocytes: (20-25%). Two main types (B and T lymphocytes) responsible for the immune responses of the body.
3. Platelets (Thrombocytes):
   • Cell fragments (not complete cells) produced from megakaryocytes (special cells in bone marrow).
   • Normal count: 150,000-350,000 per mm$^3$ of blood.
   • Play a crucial role in coagulation or clotting of blood by releasing various factors.
   • Reduced platelet count can lead to excessive blood loss due to clotting disorders.

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Blood Groups

Human blood is categorized into different groups based on the presence or absence of specific antigens on the surface of RBCs and antibodies in the plasma. The two most widely used grouping systems are ABO and Rh.

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ABO grouping

Based on the presence or absence of two surface antigens on RBCs: Antigen A and Antigen B. Plasma contains two natural antibodies: anti-A and anti-B. There are four blood groups: A, B, AB, and O (Table 18.1).

Blood Group	Antigens on RBCs	Antibodies in Plasma	Donor’s Group
A	A	anti-B	A, O
B	B	anti-A	B, O
AB	A, B	nil	AB, A, B, O
O	nil	anti-A, B	O

Compatibility: Careful matching of donor and recipient blood is vital before transfusion to avoid clumping (agglutination) and destruction of RBCs. Clumping occurs if the recipient's plasma antibodies react with the donor's RBC antigens.

• Universal Donor: Group 'O' blood lacks A and B antigens on RBCs, so it can be donated to persons of any ABO blood group.
• Universal Recipient: Persons with 'AB' group have both A and B antigens on RBCs and no anti-A or anti-B antibodies in plasma, so they can receive blood from persons of any ABO blood group.

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Rh grouping

Based on the presence or absence of the Rh antigen on the surface of RBCs. About 80% of humans are Rh positive ($\textsf{Rh}^+$) (possess the Rh antigen), and the rest are Rh negative ($\textsf{Rh}^-$) (lack the Rh antigen).

$\textsf{Rh}^-$ person will form specific antibodies against the Rh antigen if exposed to $\textsf{Rh}^+$ blood. Therefore, Rh compatibility must be matched for transfusions.

Erythroblastosis foetalis: A special case of Rh incompatibility during pregnancy. Occurs when an $\textsf{Rh}^-$ mother carries an $\textsf{Rh}^+$ foetus.

• First pregnancy: Generally no problem, as the foetal blood is separated by the placenta.
• At delivery: Small amounts of foetal $\textsf{Rh}^+$ blood may enter the mother's circulation, causing the mother to develop anti-Rh antibodies.
• Subsequent pregnancies: If the foetus is again $\textsf{Rh}^+$, the mother's anti-Rh antibodies can cross the placenta, enter the foetal blood, and destroy foetal RBCs. This can cause severe anaemia, jaundice, brain damage, or even be fatal to the foetus.

Prevention: Erythroblastosis foetalis can be prevented by administering anti-Rh antibodies to the mother immediately after the delivery of the first $\textsf{Rh}^+$ child, which destroys any foetal $\textsf{Rh}^+$ cells that may have entered her circulation, preventing her from developing her own antibodies.

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Coagulation Of Blood

Blood clotting or coagulation is a protective mechanism that prevents excessive blood loss from an injured blood vessel. It forms a plug or clot at the site of injury.

Clot formation:

1. A dark reddish-brown clot (coagulam) forms at the injury site.
2. The clot is mainly a network of protein threads called fibrins.
3. Dead and damaged formed elements of blood are trapped within the fibrin network.

Mechanism of clotting (a cascade process involving many clotting factors present in inactive form in plasma):

1. Injury or trauma stimulates platelets to release certain factors that initiate the coagulation mechanism. Factors released by injured tissues also contribute.
2. A complex enzymatic pathway is activated, leading to the formation of the enzyme complex thrombokinase.
3. Thrombokinase catalyzes the conversion of the inactive plasma protein prothrombin into the active enzyme thrombin. This reaction requires calcium ions ($\textsf{Ca}^{2+}$).

Prothrombin $\xrightarrow{\textsf{Thrombokinase, Ca}^{2+}}$ Thrombin

4. Thrombin catalyzes the conversion of the soluble inactive plasma protein fibrinogen into insoluble active fibrins.

Fibrinogen $\xrightarrow{\textsf{Thrombin}}$ Fibrins

Calcium ions ($\textsf{Ca}^{2+}$) are essential for the clotting process.

Lymph (Tissue Fluid)

As blood flows through capillaries in tissues, some water and small water-soluble substances from the plasma filter out into the spaces between the tissue cells. Larger proteins and formed elements remain within the blood vessels.

This fluid that moves out is called interstitial fluid or tissue fluid. It has a mineral composition similar to plasma but is protein-poor.

Function: Exchange of nutrients, gases, waste products, etc., between the blood and tissue cells occurs through this tissue fluid.

Lymphatic system: An extensive network of vessels (lymphatic vessels) collects the tissue fluid and drains it back into the major veins of the circulatory system.

Lymph: The fluid present within the lymphatic system. It is essentially tissue fluid that has entered the lymphatic vessels.

Characteristics of lymph:

• Colourless fluid.
• Contains specialized lymphocytes, which are responsible for the body's immune responses.
• Important carrier for certain substances, including nutrients and hormones.
• Plays a crucial role in the absorption of fats. Fats are absorbed from the intestine into the lymph in the lacteals (lymph vessels) within the intestinal villi.

Circulatory Pathways

Circulatory patterns in animals are of two main types:

• Open Circulatory System: Blood pumped by the heart flows through large vessels into open spaces or body cavities called sinuses. Tissues and organs are bathed directly in the blood (haemolymph). Found in arthropods and molluscs.
• Closed Circulatory System: Blood is always circulated within a closed network of blood vessels (arteries, veins, capillaries). Found in annelids and chordates (including vertebrates). This system allows for more precise regulation of blood flow.

Vertebrates possess a muscular, chambered heart, but the number of chambers varies:

• Fishes: 2-chambered heart (one atrium, one ventricle). Pumps deoxygenated blood to gills for oxygenation, then distributed to the body (single circulation).
• Amphibians and most Reptiles: 3-chambered heart (two atria, one ventricle). Oxygenated blood from lungs/skin (left atrium) and deoxygenated blood from the body (right atrium) mix in the single ventricle (incomplete double circulation).
• Crocodiles, Birds, and Mammals: 4-chambered heart (two atria, two ventricles). Oxygenated and deoxygenated blood are kept separate (double circulation).

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Human Circulatory System

The human circulatory system (blood vascular system) is a closed system consisting of:

• A muscular, four-chambered heart.
• A network of closed blood vessels (arteries, veins, capillaries).
• The circulating fluid, blood.

Heart:

• Mesodermally derived organ, located in the thoracic cavity between the lungs, slightly tilted to the left. About the size of a clenched fist.
• Protected by a double-walled membranous sac, the pericardium, which contains pericardial fluid.
• Four chambers: Two upper, smaller chambers called atria (left and right) and two lower, larger chambers called ventricles (left and right).
• Separating walls: Thin inter-atrial septum (between atria), thick inter-ventricular septum (between ventricles). Atrio-ventricular septum separates atrium and ventricle on the same side, with openings between them.
• Valves: Guard the openings in the heart to ensure unidirectional blood flow and prevent backflow.
  • Tricuspid valve: Guards the opening between the right atrium and right ventricle (formed by 3 cusps).
  • Bicuspid valve (Mitral valve): Guards the opening between the left atrium and left ventricle (formed by 2 cusps).
  • Semilunar valves: Guard the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta.
• Heart muscle: Entire heart is made of cardiac muscles. Ventricular walls are thicker than atrial walls.
• Nodal tissue: Specialized cardiac musculature that is autoexcitable (generates action potentials spontaneously).
  • Sino-atrial node (SAN): Located in the upper right corner of the right atrium. Generates the maximum number of action potentials (70-75 per minute). Initiates and maintains the heart's rhythmic beat, hence called the pacemaker.
  • Atrio-ventricular node (AVN): Located in the lower left corner of the right atrium, near the atrio-ventricular septum.
  • AV bundle (Bundle of His): Bundle of nodal fibers extending from the AVN, passing through the septum and dividing into right and left branches.
  • Purkinje fibres: Minute fibers branching from AV bundles throughout the ventricular musculature.
• Heart rate: Average human heart beats 70-75 times per minute (resting heart rate).

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Cardiac Cycle

The cardiac cycle is the sequence of events that occur cyclically in the heart, consisting of systole (contraction) and diastole (relaxation) of both atria and ventricles.

Average duration: A heart beating 72 times per minute completes a cardiac cycle in approximately $\frac{60 \textsf{ s}}{72} \approx 0.8$ seconds.

Events in a cardiac cycle (starting from joint diastole):

1. Joint Diastole: All four chambers are relaxed. Tricuspid and bicuspid valves are open, allowing blood to flow from pulmonary veins (into left atrium) and vena cavae (into right atrium) into the respective ventricles. Semilunar valves are closed.
2. Atrial Systole: SAN generates an action potential, stimulating both atria to contract simultaneously. This pushes about 30% more blood into the ventricles.
3. Ventricular Systole: Action potential is conducted through AVN, AV bundle, and Purkinje fibers, stimulating ventricular muscles to contract. Atria relax (atrial diastole). Ventricular pressure rises, closing tricuspid and bicuspid valves to prevent backflow into atria. As ventricular pressure increases further, semilunar valves are forced open, and blood is pumped into the pulmonary artery (from right ventricle) and aorta (from left ventricle).
4. Ventricular Diastole: Ventricles relax, and ventricular pressure falls. Semilunar valves close to prevent backflow from pulmonary artery and aorta into ventricles. As ventricular pressure drops further, tricuspid and bicuspid valves open (due to pressure from blood filling the atria), allowing blood flow into ventricles, returning to the joint diastole state.

Stroke volume: Volume of blood pumped out by each ventricle during a single cardiac cycle ($\sim 70$ mL in a healthy individual).

Cardiac output: Volume of blood pumped out by each ventricle per minute. Cardiac Output = Stroke Volume $\times$ Heart Rate. Averages $\sim 5$ liters (5000 mL) per minute in a healthy individual. Can be altered by changing stroke volume or heart rate.

Heart sounds: Two prominent sounds are produced during each cardiac cycle, audible with a stethoscope:

• First heart sound ('lub'): Due to closure of the tricuspid and bicuspid valves at the start of ventricular systole.
• Second heart sound ('dub'): Due to closure of the semilunar valves at the beginning of ventricular diastole.

These sounds are clinically significant for diagnosing heart conditions.

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Electrocardiograph (ECG)

An Electrocardiograph is a machine used to record the electrical activity of the heart. The graphical representation of this electrical activity over time is an Electrocardiogram (ECG) (Figure 18.3).

A standard ECG is obtained using three leads (one to each wrist, one to left ankle). Multiple leads on the chest are used for detailed evaluation.

Key waves in a standard ECG:

• P-wave: Represents electrical excitation (depolarisation) of the atria, leading to atrial contraction (systole).
• QRS complex: Represents the depolarisation of the ventricles, which triggers ventricular contraction. The contraction starts shortly after the Q wave, marking the beginning of ventricular systole.
• T-wave: Represents the repolarisation (return to normal state) of the ventricles. The end of the T-wave marks the end of ventricular systole.

Clinical significance: Counting the number of QRS complexes in a given time provides the heart rate. Deviations from the normal ECG shape indicate potential abnormalities or diseases in the heart's electrical activity, making ECG a valuable diagnostic tool.

Double Circulation

In humans and other mammals and birds, blood flows through specific pathways of blood vessels (arteries and veins). Arteries and veins have three layers: inner tunica intima (squamous endothelium), middle tunica media (smooth muscle and elastic fibers), and outer tunica externa (fibrous connective tissue). Tunica media is thinner in veins than arteries (Figure 18.4 - diagram shows vessel structure).

Humans exhibit double circulation, meaning there are two separate circulatory pathways:

1. Pulmonary Circulation: Starts from the right ventricle, which pumps deoxygenated blood into the pulmonary artery. The pulmonary artery carries blood to the lungs, where it gets oxygenated. Oxygenated blood is then carried back to the left atrium by the pulmonary veins. This pathway circulates blood between the heart and the lungs.
2. Systemic Circulation: Starts from the left ventricle, which pumps oxygenated blood into the aorta (the largest artery). The aorta branches into a network of arteries, arterioles, and capillaries that distribute oxygenated blood to all body tissues. From the tissues, deoxygenated blood is collected by a system of venules, veins, and vena cavae (superior and inferior) and returned to the right atrium. This pathway circulates blood between the heart and the rest of the body (except the lungs).

Significance of Systemic Circulation: Provides tissues with nutrients, $\textsf{O}_2$, and other essential substances, while removing $\textsf{CO}_2$ and other harmful substances for elimination.

Special circulations:

• Hepatic portal system: A unique vascular connection where the hepatic portal vein carries blood from the digestive tract (intestine) to the liver before it is delivered to the systemic circulation via the hepatic vein. This allows the liver to process absorbed nutrients and remove toxins.
• Coronary system: A specialized network of blood vessels (coronary arteries and veins) that exclusively supply blood to and from the heart muscle itself.

Double circulation ensures efficient separation of oxygenated and deoxygenated blood, allowing for a higher supply of oxygenated blood to the body tissues compared to single or incomplete double circulation.

Regulation Of Cardiac Activity

The heart's normal activity is intrinsically regulated by the specialized nodal tissue (SAN and AVN). Because the heart beat originates within the heart muscle itself, it is described as myogenic.

However, cardiac function can be moderated by the autonomic nervous system (ANS) through a special neural center in the medulla oblongata.

• Sympathetic nerves (part of ANS): Neural signals through sympathetic nerves increase the heart rate, the strength of ventricular contraction, and consequently, the cardiac output.
• Parasympathetic nerves (part of ANS): Neural signals through parasympathetic nerves decrease the heart rate, the speed of conduction of action potentials, and consequently, the cardiac output.

Hormonal control: Hormones from the adrenal medulla (adrenaline and noradrenaline) can also increase cardiac output by increasing heart rate and contractility.

Disorders Of Circulatory System

Common disorders affecting the circulatory system include:

• High Blood Pressure (Hypertension): Blood pressure is consistently higher than normal (normal is around 120/80 mmHg). Hypertension is diagnosed if repeated checks show blood pressure of 140/90 mmHg or higher. It increases the risk of heart diseases and damage to vital organs like the brain and kidneys.
• Coronary Artery Disease (CAD) / Atherosclerosis: Affects the blood vessels supplying blood to the heart muscle (coronary arteries). Caused by the buildup of deposits (calcium, fat, cholesterol, fibrous tissues) within the artery walls, which narrows the lumen (opening) and reduces blood flow.
• Angina (Angina Pectoris): A symptom of acute chest pain resulting from insufficient oxygen reaching the heart muscle. Occurs when blood flow to the heart is reduced. More common in middle-aged and elderly individuals.
• Heart Failure: The state where the heart is unable to pump blood effectively enough to meet the body's metabolic needs. Often called congestive heart failure due to lung congestion being a common symptom. It is distinct from cardiac arrest (heart stops beating) and heart attack (sudden damage to heart muscle due to blocked blood supply).

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