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Body Fluids and Circulation

Blood

Blood is a specialised fluid connective tissue that circulates throughout the body. It is the primary medium of transport for various substances and plays crucial roles in maintaining homeostasis.

Blood consists of two main components:

  1. Plasma: The fluid matrix.
  2. Formed elements: Blood cells suspended in the plasma.

In a healthy adult human, blood constitutes about 7-8% of the body weight. The volume of blood is typically around 5-6 litres.

Plasma

Plasma Proteins:

These are major components of plasma (6-8% of plasma). They are produced in the liver and perform various functions:

  • Fibrinogen: Involved in blood clotting (coagulation).
  • Albumins: Help maintain osmotic balance (colloid osmotic pressure) of the blood.
  • Globulins: Involved in defence mechanisms (antibodies or immunoglobulins) and also transport substances.

Serum is plasma without the clotting factors (fibrinogen).

Other Components of Plasma:
  • Electrolytes: Ions like $Na^+, K^+, Ca^{2+}, Mg^{2+}, Cl^-, HCO_3^-$, etc., important for osmotic balance and nerve/muscle function.
  • Nutrients: Absorbed products of digestion like glucose, amino acids, fatty acids, vitamins.
  • Waste products: Urea, uric acid, creatinine, etc.
  • Gases: Small amounts of $O_2, CO_2, N_2$ dissolved.
  • Hormones: Transported from endocrine glands to target tissues.

Formed Elements

The formed elements are the blood cells suspended in the plasma. They constitute about 45% of the total blood volume. They are produced in the bone marrow (haemopoiesis).

There are three main types of formed elements:

  1. Erythrocytes (Red Blood Cells - RBCs):
    • Most abundant blood cells (about 5-5.5 million per cubic millimetre of blood in healthy adults).
    • Biconcave in shape, lack a nucleus in most mammals (anucleated). This shape and lack of nucleus increases surface area for $O_2$ transport and flexibility to squeeze through capillaries.
    • Contain the red-coloured iron-containing protein haemoglobin, which transports oxygen and some carbon dioxide. The colour of blood is due to haemoglobin.
    • Life span is about 120 days. Old RBCs are destroyed in the spleen ('graveyard of RBCs') and liver.
  2. Leucocytes (White Blood Cells - WBCs):
    • Colourless, nucleated cells.
    • Lesser in number compared to RBCs (about 6,000-8,000 per cubic millimetre).
    • Involved in the body's defence mechanisms.
    • Short-lived compared to RBCs.
    • WBCs are broadly classified into two main types:
      • Granulocytes: Possess granules in their cytoplasm. Include Neutrophils, Eosinophils, Basophils.
        • Neutrophils: Most abundant WBCs (60-65%), phagocytic, defend against bacterial infection.
        • Eosinophils: (2-3%), associated with allergic reactions and parasitic infections.
        • Basophils: (0.5-1%), release histamine, serotonin, heparin (involved in inflammatory reactions).
      • Agranulocytes: Lack granules in their cytoplasm. Include Lymphocytes and Monocytes.
        • Lymphocytes: (20-25%), involved in immune response (B lymphocytes produce antibodies, T lymphocytes are involved in cell-mediated immunity).
        • Monocytes: (6-8%), phagocytic, differentiate into macrophages in tissues.
  3. Thrombocytes (Platelets):
    • Cell fragments produced from megakaryocytes (special cells in bone marrow).
    • Lack a nucleus.
    • Number is about 150,000-350,000 per cubic millimetre.
    • Involved in blood clotting (coagulation). They release substances that promote clotting.
    • A decrease in platelet count can lead to excessive bleeding disorders.
Diagram showing different types of formed elements in blood: RBC, various WBCs (Neutrophil, Eosinophil, Basophil, Lymphocyte, Monocyte), Platelets

*(Image shows microscopic views or illustrations of the different types of blood cells and platelets)*

Blood Groups

Blood of humans differs in certain aspects, especially regarding specific proteins (antigens) present on the surface of RBCs. These antigens determine a person's blood group. The two major blood grouping systems are ABO and Rh.

ABO Grouping:
  • Based on the presence or absence of two surface antigens on RBCs: A and B.
  • Also involves antibodies (proteins) in the plasma against antigens NOT present on their own RBCs.
  • There are four main blood groups: A, B, AB, and O.
Blood Group Antigen on RBCs Antibodies in Plasma Can Donate Blood To Can Receive Blood From
A A anti-B A, AB A, O
B B anti-A B, AB B, O
AB A and B Neither anti-A nor anti-B AB A, B, AB, O (Universal Recipient)
O Neither A nor B anti-A and anti-B A, B, AB, O (Universal Donor) O

During blood transfusion, blood of a donor must be compatible with the blood of the recipient. Mismatched transfusion can cause agglutination (clumping) of RBCs, leading to severe complications.

Rh Grouping:
  • Another antigen, the Rh antigen (similar to the one found in Rhesus monkeys), is also present on the surface of RBCs in a majority of humans (about 80%).
  • Individuals with the Rh antigen are called Rh positive ($Rh^+$).
  • Individuals without the Rh antigen are called Rh negative ($Rh^-$).
  • An $Rh^-$ person, if exposed to $Rh^+$ blood (e.g., during transfusion or pregnancy), will develop antibodies against the Rh antigen.
  • Subsequent exposure to $Rh^+$ blood can cause a severe transfusion reaction.
  • Erythroblastosis foetalis: A special case arising from Rh incompatibility during pregnancy. If an $Rh^-$ mother carries an $Rh^+$ foetus, the mother's immune system can produce antibodies against the foetus's Rh antigen. In subsequent pregnancies with an $Rh^+$ foetus, these antibodies can cross the placenta and destroy the foetal RBCs, causing severe anaemia and jaundice in the baby. This can be prevented by administering anti-Rh antibodies to the mother immediately after the first delivery and in subsequent pregnancies.

Coagulation Of Blood

Coagulation or clotting of blood is a mechanism to prevent excessive blood loss from injured blood vessels. It involves the formation of a clot or thrombus at the site of injury.

Mechanism of Blood Clotting (Simplified):
  • At the site of injury, platelets in the blood release various factors (along with damaged tissue cells) that initiate the process of clotting.
  • These factors activate a cascade of enzymatic reactions involving several clotting factors present in the plasma in an inactive state.
  • A key step is the formation of the enzyme thrombokinase (or thromboplastin).
  • Thrombokinase, in the presence of Calcium ions ($Ca^{2+}$), converts the inactive plasma protein prothrombin into the active enzyme thrombin.

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

  • Thrombin then converts the soluble plasma protein fibrinogen into insoluble fibrin threads.

    $ \text{Fibrinogen} \xrightarrow{\text{Thrombin}} \text{Fibrin} $

  • Fibrin threads form a network that traps blood cells (especially RBCs), forming the blood clot.
  • Clotting factors are essential for this process. Deficiency of certain clotting factors (e.g., Factor VIII or IX) causes haemophilia, a bleeding disorder.
Diagram illustrating the simplified cascade of blood clotting (Prothrombin -> Thrombin, Fibrinogen -> Fibrin)

*(Image shows a simplified flow chart: Injury/Platelets $\rightarrow$ Thrombokinase $\rightarrow$ Prothrombin $\rightarrow$ Thrombin $\rightarrow$ Fibrinogen $\rightarrow$ Fibrin network trapping cells)*

Blood contains anti-coagulants (like heparin) in the plasma which prevent clotting in the intact blood vessels.


Lymph (Tissue Fluid)

Lymph is a fluid connective tissue that is closely associated with the blood circulatory system. It is also called tissue fluid or interstitial fluid.

Formation of Lymph:
  • As blood flows through the capillaries in tissues, some water and small soluble substances (like minerals, glucose, amino acids) from the plasma filter out through the capillary walls into the intercellular spaces.
  • This fluid, now called tissue fluid or interstitial fluid, surrounds the tissue cells and acts as a medium for exchange of nutrients, gases, and waste products between the blood and the cells.
  • Larger proteins and most formed elements (RBCs, most WBCs) remain in the blood vessels.
  • Some of the tissue fluid is reabsorbed back into the blood capillaries, but a portion enters the lymphatic vessels. Once inside the lymphatic vessels, it is called lymph.
Diagram illustrating the formation of tissue fluid from blood capillaries and its entry into lymphatic vessels

*(Image shows blood capillaries in tissues, showing fluid filtering out (tissue fluid) and some of it entering a lymphatic vessel (becoming lymph))*

Composition of Lymph:
  • Lymph is a colourless fluid (because it lacks RBCs).
  • It is similar in composition to plasma but contains fewer proteins and usually more lymphocytes.

Functions of Lymphatic System:
  • Drains excess tissue fluid: Returns interstitial fluid to the bloodstream, preventing tissue swelling (oedema).
  • Transport of absorbed fats: Lymphatic vessels (lacteals) in the intestinal villi absorb fatty acids and glycerol (in the form of chylomicrons) from the small intestine and transport them to the bloodstream.
  • Immune response: Lymph nodes (part of the lymphatic system) are sites where lymphocytes proliferate and initiate immune responses. Lymph transports lymphocytes and antibodies throughout the body.
  • Transport of proteins: Returns leaked plasma proteins back to the blood.

The lymphatic system is a parallel system to the blood circulatory system, consisting of lymphatic vessels, lymph nodes, and lymphoid organs (spleen, thymus). Lymph flows in one direction, towards the heart, and eventually drains into major veins.


Circulatory Pathways

Circulatory pathways are the routes through which blood (or other circulatory fluid) flows within an organism. Animals have evolved different types of circulatory systems.

Types of Circulatory Systems:

Human Circulatory System

Humans have a closed circulatory system, also known as the blood vascular system. It consists of the heart, blood vessels, and blood.

The Heart:
  • A muscular organ, located in the thoracic cavity between the lungs, slightly tilted to the left.
  • Covered by a double-walled membranous sac called the pericardium, enclosing pericardial fluid (reduces friction).
  • Heart is four-chambered in humans (and other mammals and birds): two upper smaller chambers called atria (singular: atrium) and two lower larger chambers called ventricles.
  • A septum separates the right and left atria (interatrial septum) and the right and left ventricles (interventricular septum). The atrial and ventricular chambers are separated by the atrioventricular septum.
  • Valves: Prevent the backward flow of blood.
    • Tricuspid valve: Between the right atrium and right ventricle (three cusps).
    • Bicuspid valve (Mitral valve): Between the left atrium and left ventricle (two cusps).
    • Semilunar valves: At the opening of the pulmonary artery (from right ventricle) and the aorta (from left ventricle).
  • The walls of the ventricles are thicker than the walls of the atria. The wall of the left ventricle is thicker than the right ventricle (as it pumps blood to the entire body).
Diagram showing the structure of the human heart with four chambers and major blood vessels

*(Image shows a diagram of the human heart highlighting the four chambers, atria, ventricles, septa, and major vessels like vena cava, pulmonary artery, pulmonary veins, aorta, and indicating the location of valves)*

Blood Vessels:
  • Arteries: Carry blood away from the heart. Generally carry oxygenated blood (except the pulmonary artery, which carries deoxygenated blood from the right ventricle to the lungs). Have thick, elastic walls to withstand high pressure.
  • Veins: Carry blood towards the heart. Generally carry deoxygenated blood (except the pulmonary veins, which carry oxygenated blood from the lungs to the left atrium). Have thinner walls and often contain valves to prevent backflow of blood, especially in limbs.
  • Capillaries: Very thin-walled, narrow vessels that connect arteries to veins. Form networks in tissues, facilitating the exchange of substances between blood and tissue cells. Walls are one cell thick (endothelium).

Cardiac Cycle

The sequence of events that occurs from the beginning of one heartbeat to the beginning of the next is called the cardiac cycle. It involves the contraction (systole) and relaxation (diastole) of the atria and ventricles.

Steps of the Cardiac Cycle (Approx. 0.8 seconds at rest):
  1. Joint Diastole: All four chambers are relaxed. Blood flows from the veins (superior vena cava, inferior vena cava, pulmonary veins) into the atria and passively into the ventricles through the open AV valves. Semilunar valves are closed.
  2. Atrial Systole: The atria contract, pushing the remaining blood into the ventricles.
  3. Ventricular Systole: The ventricles contract. The AV valves close (producing the first heart sound, 'lub'). When ventricular pressure exceeds the pressure in the pulmonary artery and aorta, the semilunar valves open, and blood is pumped into these vessels.
  4. Ventricular Diastole: The ventricles relax. The semilunar valves close (producing the second heart sound, 'dub'), preventing backflow of blood into the ventricles. The AV valves remain closed initially, then open to allow ventricular filling from the atria (joint diastole starts again).

The heartbeat is initiated and maintained by a specialised muscle tissue called the nodal tissue.

  • Sinoatrial Node (SA node): Located in the wall of the right atrium. It generates the electrical impulse that initiates the heartbeat. It is the 'pacemaker' of the heart.
  • Atrioventricular Node (AV node): Located in the AV septum. It receives the impulse from the SA node, delays it slightly, and then transmits it to the ventricles.
  • Bundle of His: A bundle of fibres originating from the AV node, passing through the interventricular septum.
  • Purkinje fibres: Fine fibres branching from the Bundle of His into the ventricular walls.

The electrical impulse from the SA node spreads through the atria (causing atrial contraction), then to the AV node, Bundle of His, and Purkinje fibres, which rapidly spread the impulse throughout the ventricular muscle, causing ventricular contraction.

Cardiac Output:

Cardiac output is the volume of blood pumped by each ventricle per minute.

$ \text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate} $

  • Stroke Volume: Volume of blood pumped by one ventricle in a single beat (about $70 \text{ mL}$ at rest).
  • Heart Rate: Number of heartbeats per minute (about 70-75 per minute at rest).

Cardiac Output $\approx 70 \text{ mL/beat} \times 72 \text{ beats/min} \approx 5040 \text{ mL/min} \approx 5 \text{ litres/min}$ at rest. Cardiac output can significantly increase during exercise.

Electrocardiograph (ECG)

An Electrocardiograph (ECG or EKG) is a graphic representation of the electrical activity of the heart during a cardiac cycle. It is obtained using an electrocardiogram machine.

Recording an ECG:
  • A standard ECG is recorded by attaching three electrical leads to the patient (one to each wrist and one to the left ankle) to monitor heart activity. For detailed evaluation, multiple leads (12-lead ECG) are used on the chest.

Interpretation of a Standard ECG:

A standard ECG tracing shows a series of waves corresponding to the electrical events of the cardiac cycle:

  • P wave: Represents the electrical excitation (depolarisation) of the atria. This leads to atrial contraction.
  • QRS complex: Represents the electrical excitation (depolarisation) of the ventricles. This is a large wave due to the larger mass of ventricular muscle. It leads to ventricular contraction. (Atrial repolarisation occurs simultaneously but is masked by the QRS complex).
  • T wave: Represents the electrical relaxation (repolarisation) of the ventricles. The end of the T wave marks the end of systole.

The ECG tracing represents the sum of electrical potentials generated by millions of heart muscle cells. Deviations from the normal pattern can indicate various cardiac abnormalities (e.g., changes in rhythm, damage to heart muscle).

By counting the number of QRS complexes in a given time period, the heart rate can be determined.

Diagram showing a typical ECG tracing with P, QRS, and T waves labelled

*(Image shows a standard ECG trace on grid paper, with labels for P wave, QRS complex, and T wave)*


Double Circulation

In humans (and other mammals and birds), the blood flows through the heart twice during each complete cycle of circulation. This type of circulation is called double circulation.

Double circulation consists of two separate pathways:

  1. Pulmonary Circulation:
    • Transports deoxygenated blood from the right ventricle to the lungs for oxygenation.
    • From the lungs, oxygenated blood is carried back to the left atrium.
    • Pathway: Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary veins $\rightarrow$ Left atrium.
  2. Systemic Circulation:
    • Transports oxygenated blood from the left ventricle to all parts of the body tissues.
    • From the body tissues, deoxygenated blood is carried back to the right atrium.
    • Pathway: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Arteries $\rightarrow$ Arterioles $\rightarrow$ Capillaries (in tissues) $\rightarrow$ Venules $\rightarrow$ Veins $\rightarrow$ Superior and Inferior vena cava $\rightarrow$ Right atrium.
Diagram illustrating double circulation in humans showing pulmonary and systemic circuits

*(Image shows a diagram of the human heart and the two circulatory loops: pulmonary loop (heart to lungs and back) and systemic loop (heart to body and back), using different colours to indicate oxygenated and deoxygenated blood)*

Significance of Double Circulation:

Other Vertebrate Circulatory Systems:

The evolution of a four-chambered heart and double circulation is an adaptation for efficient oxygen transport in higher vertebrates.


Regulation Of Cardiac Activity

The rhythmic activity of the heart (cardiac cycle) is intrinsically regulated by the SA node (pacemaker). However, the rate and strength of the heartbeat can be modified or regulated by other factors to meet the varying demands of the body.

Cardiac activity is regulated by:

  1. Neural Regulation: Controlled by the nervous system.
  2. Hormonal Regulation: Influenced by certain hormones.

Neural Regulation:

Diagram illustrating the neural regulation of heart rate by sympathetic and parasympathetic nerves from the medulla

*(Image shows a simplified diagram of the brainstem (medulla) connected to the heart via sympathetic and vagus (parasympathetic) nerves, indicating their effects on heart rate/force)*

Hormonal Regulation:

Neural and hormonal signals work together to adjust cardiac activity in response to the body's needs, such as during exercise (increased demand for oxygen and nutrients, requires increased cardiac output) or rest.


Disorders Of Circulatory System

Various conditions can affect the functioning of the circulatory system, leading to cardiovascular disorders. These are a major health concern globally.

Common Disorders of the Circulatory System:

Risk factors for circulatory disorders include unhealthy diet (high in saturated fats, salt), lack of physical activity, smoking, excessive alcohol consumption, stress, obesity, diabetes, and a family history of heart disease.