Body Fluids and Circulation

Blood

• All living cells require nutrients and oxygen for metabolism and need continuous removal of waste products.
• Different organisms have evolved different transport mechanisms.
• Simple organisms (like sponges) circulate water through their bodies to meet these needs.
• More complex organisms use specialised transport fluids such as blood and lymph.

“Blood is the most common transport fluid in higher organisms, while lymph assists in transport of certain substances.”

Blood

• Blood is a mobile connective tissue composed of a fluid matrix called plasma and cellular components known as formed elements.

“Blood is a slightly alkaline fluid with a pH around 7.4.”

1. Plasma

• Plasma is a straw-colored, thick fluid making up 55% of the total blood volume.
• It is composed of about 90–92% water and 6–8% plasma proteins.
• Major plasma proteins include:
  1. Fibrinogen: Required for blood clotting.
  2. Globulins: Involved in body defence.
     • “Immunoglobulins present in plasma play an important role in immune responses.”
  3. Albumins: Help in osmotic balance.
• Plasma also contains minerals such as Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ and Cl⁻.
• Nutrients like glucose, amino acids and lipids remain dissolved in plasma while being transported to different body parts.
• Coagulation factors are present in plasma in an inactive form.
• Plasma without clotting factors is called serum.

“Plasma is slightly alkaline in nature and provides a medium for transport of nutrients, gases and waste products.”

2. Blood: Formed Elements

• Blood has three main types of cells: erythrocytes, leucocytes, and platelets.
• These cells make up about 45% of the blood.

a. Erythrocytes (Red Blood Cells)

• Most abundant blood cells.
• Healthy adult males have 5 to 5.5 million RBCs per cubic millimeter of blood.
• Formation: Made in the red bone marrow.
• Structure:
  • No nucleus (enucleated), biconcave shape.
  • This shape increases surface area for gas exchange.
• Function:
  • Contain haemoglobin which transports oxygen.
  • Normal haemoglobin content is about 12–16 g per 100 mL of blood.
• Life Span:
  • About 120 days.
  • Old RBCs are destroyed mainly in the spleen (graveyard of RBCs).

“Formation of RBCs is called erythropoiesis and requires iron, proteins, vitamin B₁₂ and folic acid.”

“An abnormal increase in RBC count is called polycythemia, while a decrease causes erythrocytopenia, leading to reduced oxygen supply and anaemia.”

b. Leucocytes (White Blood Cells)

• Characteristics: Colorless, have a nucleus (nucleated), less abundant than RBCs.
  • Normal count range – 6000-8000 WBCs per cubic millimeter of blood.
  • Leucocytes are mainly involved in body defence.
• Types of WBCs:
• a. Granulocytes:
  1. Neutrophils: Most abundant (60–65%), destroy foreign organisms by phagocytosis.
  2. Eosinophils: 2–3%, involved in allergic reactions and defence against parasites.
  3. Basophils: Least abundant (0.5–1%), release histamine and other inflammatory chemicals.
• b. Agranulocytes:
  1. Lymphocytes: 20–25%, include B and T cells responsible for immune responses.
  2. Monocytes: 6–8%, largest WBCs, phagocytic in nature.

Formation of leucocytes is called leucopoiesis.

“Decrease in WBC count is called leukopenia, while an abnormal increase occurs in leukemia.”

c. Platelets (Thrombocytes)

• Formation: Platelets are small, colourless cell fragments formed from megakaryocytes in the bone marrow.
• Normal Count: 1,50,000 to 3,50,000 per cubic millimeter of blood.
• Function:
  • Release substances essential for blood clotting.
  • Prevent excessive blood loss during injury.

“Platelets have a short life span of about one week, and a reduced platelet count can lead to bleeding disorders.”

Blood Groups

• Although blood appears similar in all humans, it exists in different types based on the presence or absence of specific antigens on red blood cells.
• ABO system and Rh system are the 2 most important blood grouping systems in humans.

ABO Grouping

“ABO blood grouping was first described by Karl Landsteiner in 1900.”

• Criteria: The ABO blood group system is based on the presence or absence of two surface antigens, antigen A and antigen B, on red blood cells.
• Blood plasma contains antibodies that react against the antigens absent on an individual’s RBCs.
• Interaction between matching antigens and antibodies can cause clumping (agglutination) of RBCs, which is dangerous during blood transfusion.
• Four major ABO blood groups:
  1. Group A:
     • Antigen present on RBCs: A
     • Antibody present in plasma: Anti-B
  2. Group B:
     • Antigen present on RBCs: B
     • Antibody present in plasma: Anti-A
  3. Group AB:
     • Antigens present on RBCs: A and B
     • Antibodies present in plasma: None
     • Can receive blood from all groups, hence called the universal recipient
  4. Group O:
     • Antigens present on RBCs: None
     • Antibodies present in plasma: Anti-A and Anti-B
     • Can donate blood to all groups, hence called the universal donor

“ABO blood groups are genetically controlled by a single gene with multiple alleles.”

• Blood Transfusion:
  • Before blood transfusion, it is essential to match the blood group of the donor and recipient.
  • Incorrect transfusion can lead to agglutination and destruction of red blood cells, which may be fatal.

Rh Blood Grouping

• Another important blood grouping system is based on the Rh antigen.
• This antigen is present on the RBCs of about 80% of humans.
  • Rh⁺ (positive): Rh antigen present.
  • Rh⁻ (negative): Rh antigen absent.
• Antibodies Formation: An Rh-negative individual does not naturally possess antibodies against the Rh antigen. However, exposure to Rh-positive blood can stimulate antibody formation.

Rh Incompatibility During Pregnancy

• In an Rh-negative mother carrying an Rh-positive fetus:
  • First Pregnancy: Usually safe, as maternal and fetal blood remain separated by the placenta.
  • During delivery: Small amounts of fetal Rh-positive blood may enter the mother’s circulation, triggering antibody formation.
  • Subsequent Pregnancies: Maternal Rh antibodies can cross the placenta and destroy fetal RBCs, leading to severe anaemia and jaundice.
    • This condition is called erythroblastosis foetalis.
  • Prevention: Administration of anti-Rh antibodies to the mother immediately after the first delivery prevents antibody formation.

Coagulation of Blood

• When a blood vessel is injured, bleeding does not continue indefinitely because blood undergoes clotting, also known as coagulation.
• Blood clotting prevents excessive loss of blood from the body.

What is Blood Clotting?

• Blood clotting is the process by which liquid blood changes into a semi-solid mass at the site of injury.
• Clot Formation: The blood clot appears as a dark reddish-brown scab and is mainly composed of fibrin threads that trap blood cells.

Mechanism of Blood Clotting

1. Conversion of fibrinogen
   • Inactive fibrinogen present in plasma is converted into fibrin.
2. Role of thrombin
   • The enzyme thrombin catalyses the conversion of fibrinogen into fibrin.
3. Formation of thrombin
   • Thrombin is produced from an inactive plasma protein called prothrombin.
4. Role of thrombokinase
   • Thrombokinase (prothrombinase) is an enzyme complex formed during a series of reactions that activates prothrombin.

Activation of Clotting

• Platelets role: Platelets release clotting factors at the site of injury.
• Tissue Factors: Damaged tissues also release substances that aid coagulation.
• Calcium Ions: Calcium ions are essential for multiple steps in the clotting process.

“Vitamin K is essential for blood clotting as it is required for prothrombin synthesis in the liver.”

“Anticoagulants like heparin prevent blood clotting.”

Lymph (Tissue Fluid)

What is Lymph?

• As blood flows through capillaries, some plasma leaks out into the spaces between body cells, forming tissue fluid or interstitial fluid.
• This fluid lacks RBCs, platelets and most plasma proteins.
• Lymph is the tissue fluid that enters the lymphatic vessels.
• Composition & Characteristics of Lymph:
  • Colourless fluid
  • Contains lymphocytes
  • Fewer proteins than blood plasma
  • Erythrocytes and platelets are absent

Function of Lymph

• Acts as a medium for exchange of nutrients and gases between blood and tissues
• Drains excess tissue fluid back into major veins
• Transports hormones and nutrients
• Absorbs fats through lacteals present in intestinal villi
• Plays a crucial role in immune responses, as lymph nodes destroy invading microorganisms

“Lymph acts as a connecting link between blood and body tissues.”

Circulatory Pathways

Living organisms show two main types of circulatory systems based on how blood flows in the body.

• Open Circulatory System:
  • In this system, blood pumped by the heart flows into open spaces or body cavities called sinuses instead of remaining confined within blood vessels.
  • This system is found in arthropods and most molluscs.
  • Blood comes in direct contact with tissues, but regulation of blood flow is less precise.
• Closed Circulatory System:
  • In this system, blood always circulates through a closed network of blood vessels.
  • This allows efficient and regulated transport of substances.
  • Closed circulatory system is found in annelids and chordates.

Heart Chambers in Vertebrates (Evolutionary Trend)

• Fishes:
  • Heart is two-chambered (1 atrium and 1 ventricle).
  • Pumps only deoxygenated blood to the gills.
  • Oxygenated blood flows directly to body tissues.
  • Shows single circulation.
• Amphibians and Reptiles (except crocodiles):
  • Heart is three-chambered (2 atria and 1 ventricle).
  • Oxygenated and deoxygenated blood partially mix in the ventricle.
  • Shows incomplete double circulation.
• Crocodiles, Birds, and Mammals:
  • Heart is four-chambered (2 atria and 2 ventricles).
  • Oxygenated and deoxygenated blood remain completely separate.
  • Shows complete and efficient double circulation.

Human Circulatory System

The human circulatory system is also called the blood vascular system.
It consists of three main components:

1. Blood
2. Blood vessels
3. Blood

Blood Vessels

There are three types of blood vessels:

a. Arteries

• Carry blood away from the heart.
• Usually carry oxygenated blood.
• Pulmonary artery is an exception; it carries deoxygenated blood.

b. Veins

• Carry blood towards the heart.
• Usually carry deoxygenated blood.
• Pulmonary vein is an exception; it carries oxygenated blood.
• Veins contain valves to prevent backflow of blood.

c. Capillaries

• Thin-walled vessels connecting arteries and veins.
• Exchange of gases, nutrients and wastes occurs here.

Walls of Arteries and Veins

• Blood vessel walls consist of three layers:
  1. Tunica interna – inner endothelial layer
  2. Tunica media – smooth muscle and elastic fibres
  3. Tunica externa – outer connective tissue layer

Human Heart

Location and Protection

• Located in the thoracic cavity between the lungs.
• Slightly tilted to the left.
• Enclosed in a double-walled membranous sac called pericardium.
• Pericardial cavity contains pericardial fluid which reduces friction.

Structure of the Heart

• About the size of a clenched fist.
• Four chambers:
  – Two upper atria
  – Two lower ventricles

Septa of the Heart

• Inter-atrial septum separates right and left atria.
• Inter-ventricular septum separates right and left ventricles.
• Atrioventricular septum separates atria and ventricles.

Valves of the Heart

• Tricuspid valve – between right atrium and right ventricle
• Bicuspid (mitral) valve – between left atrium and left ventricle
• Semilunar valves – present at the openings of pulmonary artery (right ventricle) and aorta (left ventricle).
• Function: Valves ensure one-way blood flow and prevent backward flow.

Chordae tendineae are fibrous cords that connect atrioventricular valves to papillary muscles and prevent valve inversion.

Heart Muscles and Nodal Tissue

• The heart wall is made of cardiac muscle, which is involuntary and striated.
• Ventricular walls are thicker than atrial walls because ventricles pump blood with greater force.

Nodal Tissue (Conducting System of Heart)

• Sino-atrial Node (SAN)
  • Located in the right atrium near the opening of superior vena cava.
  • Generates 70–75 action potentials per minute.
  • Acts as the pacemaker of the heart.
• Atrio-ventricular Node (AVN)
  • Located in the right atrium near the septum.
  • Transmits impulses from atria to ventricles.
• AV Bundle (Bundle of His)
  • Extends from AVN.
  • Divides into right and left bundles.
  • Further branches into Purkinje fibres in ventricles.

Function of Nodal Tissue

• Shows auto-excitability (can generate impulses without external stimulus).
• Coordinates rhythmic contraction of atria and ventricles.
• AV node acts as a pacesetter by transmitting impulses efficiently.

Type of Heart Beat

• Neurogenic Heart Beat
  • Initiated by nerve impulses.
  • Found in some annelids and most arthropods.
• Myogenic Heart Beat
  • Initiated by specialised cardiac muscle.
  • Found in molluscs and vertebrates including humans.

Cardiac Cycle

• The cardiac cycle is the sequence of events that occur in the heart during one complete heartbeat.
• It includes alternate contraction (systole) and relaxation (diastole) of atria and ventricles.

Stages of Cardiac Cycle

• Joint Diastole:
  • All four chambers of the heart are relaxed.
  • Atria and ventricles are in diastole.
  • Tricuspid and bicuspid valves are open.
  • Semilunar valves are closed.
  • Blood flows from pulmonary veins into left atrium and from vena cava into right atrium, then passively into ventricles.
  • No blood flows into aorta or pulmonary artery at this stage.
• Atrial Systole:
  • Sino-atrial node (SAN) generates an action potential.
  • Both atria contract simultaneously.
  • About 30% additional blood is pushed into ventricles.
  • Ventricles are still relaxed.
• Ventricular Systole:
  • Action potential reaches ventricles via AV node, AV bundle (bundle of His) and Purkinje fibres.
  • Ventricles contract while atria relax.
  • Ventricular pressure rises sharply.
  • Tricuspid and bicuspid valves close to prevent backflow into atria.
  • Closure of these valves produces the first heart sound (“lub”).
  • Semilunar valves open.
  • Blood is pumped into pulmonary artery (right ventricle) and aorta (left ventricle).
• Ventricular Diastole:
  • Ventricles relax.
  • Ventricular pressure falls.
  • Semilunar valves close to prevent backflow of blood into ventricles.
  • Closure of semilunar valves produces the second heart sound (“dub”).
  • As pressure further drops, tricuspid and bicuspid valves open again.
  • Blood starts flowing into ventricles and the heart returns to joint diastole.

Summary of the Cardiac Cycle

• Sequential Events: Cardiac cycle consists of repeated systole and diastole.
• Duration: Duration of one cardiac cycle: about 0.8 seconds.
  – Atrial systole: ~0.1 s
  – Ventricular systole: ~0.3 s
  – Joint diastole: ~0.4 s
• Heart rate: about 72 beats per minute in a healthy adult.
• Stroke volume: about 70 mL of blood pumped by each ventricle per beat.
• Cardiac output:
  Cardiac output = Heart rate × Stroke volume
  ≈ 72 × 70 mL ≈ 5000 mL (5 litres per minute).
• Athletes generally have higher cardiac output.

Heart Sounds

• First heart sound (“lub”):
  – Produced by closure of tricuspid and bicuspid valves.
• Second heart sound (“dub”):
  – Produced by closure of semilunar valves.
• Diagnostic Significance:
  • Heart sounds are heard using a stethoscope.
  • Abnormal sounds indicate valve defects or cardiac disorders.

Electrocardiograph (ECG)

What is ECG

• Electrocardiograph is the instrument used to record heart activity.
• ECG (Electrocardiogram): is a graphical record of electrical activity of the heart during a cardiac cycle.

How ECG is Taken

• Standard ECG uses three leads attached to:
  – Right wrist
  – Left wrist
  – Left ankle
• For detailed examination, multiple chest leads are used.

ECG Waves and Their Significance

• P-Wave:
  • Represents depolarisation (electrical excitation) of atria.
  • Leads to atrial contraction.
  • Caused by activation of SA node.
• QRS Complex:
  • Represents depolarisation of ventricles.
  • Initiates ventricular contraction.
  • Ventricular systole begins shortly after the Q wave.
  • Atrial repolarisation occurs simultaneously but is masked by QRS complex.
• T-Wave:
  • Represents repolarisation (relaxation) of ventricles.
  • Marks the end of ventricular systole.

Clinical Significance of ECG

• Heart Rate: Heart rate can be calculated by counting QRS complexes per minute.
• Shape Consistency: Shape and duration of ECG waves are normally consistent for everyone.
• Diagnosing Abnormalities: Deviations from normal ECG pattern indicate cardiac abnormalities such as arrhythmia, myocardial damage, or conduction defects.
• ECG is an essential diagnostic tool used in hospitals.

Double Circulation

• Double circulation is the passage of the same blood twice through the heart in one complete cycle.
• It consists of two separate pathways: pulmonary circulation and systemic circulation.
• This type of circulation ensures efficient separation of oxygenated and deoxygenated blood.

a. Pulmonary Circulation
• Right ventricle pumps deoxygenated blood into the pulmonary artery.
• Blood travels to the lungs for oxygenation.
• Oxygenated blood returns to the left atrium through pulmonary veins.

b. Systemic Circulation
• Left ventricle pumps oxygenated blood into the aorta.
• Oxygenated blood is supplied to all body tissues through arteries.
• Deoxygenated blood returns to the right atrium through veins.

Special Circulatory Pathways

1. Hepatic Portal System:
   • Blood from the digestive tract passes through the liver via the hepatic portal vein before entering systemic circulation.
2. Coronary Circulation:
   • A special system of blood vessels supplies oxygenated blood to the heart muscles themselves.

Regulation of Cardiac Activity

• The heart functions continuously throughout life and its activity is finely regulated.
• Myogenic Heart:
  • Human heart is myogenic in nature.
  • Its activity is self-regulated by specialised cardiac muscles called nodal tissue.
• Neural Control:
  • A regulatory centre in the medulla oblongata influences heart activity through the autonomic nervous system.
  • Sympathetic Nerves:
    • Increase heart rate
    • Increase force of ventricular contraction
    • Increase cardiac output
  • Parasympathetic Nerves:
    • Decrease heart rate
    • Decrease conduction of action potential
    • Reduce cardiac output
• Hormonal Control: Adrenal medullary hormones (adrenaline and noradrenaline) increase heart rate and cardiac output during stress or excitement.

Disorders of the Circulatory System

• Hypertension (High Blood Pressure):
  • Normal blood pressure: 120/80 mm Hg.
  • Hypertension: Blood pressure equal to or above 140/90 mm Hg.
  • Systolic pressure: Pressure during ventricular contraction.
  • Diastolic pressure: Pressure during ventricular relaxation.
  • Pulse pressure: Difference between systolic and diastolic pressure.
  • Effects: Can damage heart, brain, kidneys and eyes.
  • Blood pressure is measured using a sphygmomanometer.
• Coronary Artery Disease (CAD)/ Atherosclerosis:
  • Caused by deposition of lipids (cholesterol), calcium and fats on arterial walls.
  • These deposits form atherosclerotic plaques.
  • Plaques narrow or block arterial lumen, reducing blood supply.
  • If coronary arteries are affected, it can lead to heart attack or stroke.
• Arteriosclerosis:
  • Hardening and thickening of arteries due to calcium and cholesterol deposition.
  • Arteries lose elasticity and may rupture, causing thrombosis.
• Angina (Angina Pectoris):
  • Characterised by acute chest pain.
  • Occurs due to insufficient oxygen supply to heart muscles.
  • Common in middle-aged and elderly individuals.
  • Triggered by conditions affecting blood flow to the heart.
• Heart Failure:
  • Condition in which the heart cannot pump blood effectively to meet body needs.
  • Also called congestive heart failure.
  • Congestion of lungs is a common symptom.
  • Heart failure is NOT the same as heart attack or cardiac arrest.
• Cardiac Arrest:
  • Sudden stoppage of heart beating.
  • Requires immediate medical intervention.
• Heart Attack:
  • Occurs when heart muscle is suddenly damaged due to inadequate blood supply.
  • Commonly caused by blockage of coronary arteries.

Chapter Summary:

• Blood transports essential substances like oxygen, nutrients and hormones to body cells and removes carbon dioxide and other metabolic wastes.
• Lymph (tissue fluid) also participates in transport, especially of nutrients, fats and immune cells.

• Blood is a connective tissue composed of plasma and formed elements.
• Plasma is the fluid matrix that carries proteins, nutrients, minerals and clotting factors.
• Formed elements constitute about 45% of blood and include red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes).
• Erythrocytes transport oxygen using haemoglobin, leucocytes provide defence against infections, and platelets help in blood clotting.

• Human blood groups are classified mainly under the ABO system (A, B, AB and O) based on the presence or absence of antigens A and B on red blood cells.
• Another important grouping is based on the Rhesus (Rh) factor present on RBCs.
• Rh incompatibility between mother and fetus can cause erythroblastosis foetalis, which can be prevented by timely medical intervention.

• Lymph is a colourless tissue fluid formed from plasma leaking out of capillaries.
• It contains fewer proteins than blood and lacks RBCs and platelets but is rich in lymphocytes.
• Lymph helps in exchange of materials between blood and tissues, absorption of fats from intestinal villi, and immune defence.

• Circulatory systems may be open or closed.
• Vertebrates and some invertebrates possess a closed circulatory system in which blood flows through a closed network of vessels.
• The human circulatory system consists of the heart, blood vessels and blood.

• The human heart is a four-chambered muscular organ with two atria and two ventricles.
• Cardiac muscles are auto-excitable.
• A specialised nodal tissue is present in the heart, and the sino-atrial node (SAN) acts as the pacemaker by generating 70–75 action potentials per minute.
• These impulses regulate the rhythmic contraction (systole) and relaxation (diastole) of atria and ventricles.

• The cardiac cycle is a sequence of repeated events occurring during one heartbeat.
• A healthy person has about 72 cardiac cycles per minute.
• Each ventricle pumps approximately 70 mL of blood per beat, known as stroke volume.
• Cardiac output is the volume of blood pumped by each ventricle per minute and is about 5 litres in a healthy adult.

• Electrocardiogram (ECG) is a graphical recording of the electrical activity of the heart and is an important diagnostic tool in clinical practice.

• Humans show double circulation, consisting of pulmonary and systemic circulation.
• In pulmonary circulation, the right ventricle pumps deoxygenated blood to the lungs and oxygenated blood returns to the left atrium.
• In systemic circulation, the left ventricle pumps oxygenated blood to the body and deoxygenated blood returns to the right atrium.
• Cardiac activity is regulated by neural and hormonal mechanisms to meet the body’s changing demands.