Body Fluids and Circulation

Blood

• All living cells need nutrients, oxygen, and removal of waste.
• Different animals have evolved various methods for transport.
• Simple organisms (like sponges) circulate water through their bodies.
• Complex organisms use special fluids like blood and lymph.

Blood

• Blood is a connective tissue with a fluid matrix (plasma) and formed elements discussed below.

1. Plasma

• Plasma is a straw-colored, thick fluid making up 55% of blood.
• Composed of 90-92% water and 6-8% proteins.
  • Major proteins: Fibrinogen, globulins, and albumins.
  • Fibrinogen: Needed for blood clotting.
  • Globulins: Involved in body defense.
  • Albumins: Help in osmotic balance.
• Contains minerals: Na+, Ca++, Mg++, HCO3–, Cl–.
• Contains glucose, amino acids, lipids (in transit).
• Coagulation factors (inactive form) are present.
• Plasma without clotting factors is called serum.

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 cells in blood.
• 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, biconcave shape.
• Function: Contain haemoglobin which carries oxygen.
  • 12-16 grams of haemoglobin per 100 ml of blood.
• Life Span: Live for about 120 days, then destroyed in the spleen.

b. Leucocytes (White Blood Cells)

• Characteristics: Colorless, have a nucleus, less abundant than RBCs.
  • 6000-8000 WBCs per cubic millimeter of blood.
• Types of WBCs:
  • Granulocytes: Neutrophils, eosinophils, basophils.
  • Agranulocytes: Lymphocytes, monocytes.
• Functions:
  • Neutrophils: Most common (60-65%), destroy foreign organisms.
  • Eosinophils: Fight infections, involved in allergies (2-3%).
  • Basophils: Least common (0.5-1%), release histamine and other chemicals during inflammation.
  • Lymphocytes: 20-25%, includes ‘B’ and ‘T’ cells, important for immune response.
  • Monocytes: 6-8%, destroy foreign organisms.

c. Platelets (Thrombocytes)

• Formation: Cell fragments from megakaryocytes in the bone marrow.
• Normal Count: 1,50,000 to 3,50,000 per cubic millimeter of blood.
• Function: Release substances for blood clotting.
  • Important to prevent excessive bleeding.

Blood Groups

• Blood looks similar in everyone but has different types.
• The main blood grouping systems are ABO and Rh.

ABO Grouping

• Based on Antigens: Blood groups are identified by two surface antigens, A and B, on RBCs.
• Antibodies in Plasma: Blood also contains antibodies that react against these antigens.
• Four Blood Groups:
  • Group A: Has A antigens, B antibodies.
  • Group B: Has B antigens, A antibodies.
  • Group AB: Has A and B antigens, no antibodies. Universal recipient.
  • Group O: No antigens, A and B antibodies. Universal donor.
• Blood Transfusion:
  • Important to match donor and recipient blood to prevent clumping of RBCs.
  • Group O can donate to anyone; Group AB can receive from anyone.

Rh Grouping

• Rh Antigen: Another antigen found in 80% of humans.
  • Rh+ (positive): Has Rh antigen.
  • Rh- (negative): No Rh antigen.
• Antibodies Formation: Rh- person exposed to Rh+ blood makes antibodies against Rh antigen.
• Pregnancy Concerns:
  • First Pregnancy: Usually no problem as the placenta separates the mother’s and baby’s blood.
  • Delivery Risk: Mother may be exposed to Rh+ blood from baby, starts making antibodies.
  • Subsequent Pregnancies: Mother’s Rh antibodies can harm an Rh+ baby, causing severe issues like anaemia or jaundice (erythroblastosis foetalis).
  • Prevention: Giving anti-Rh antibodies to the mother after the first delivery.

By knowing these blood groupings, we can ensure safe blood transfusions and manage Rh incompatibility during pregnancy.

Coagulation of Blood

• When you cut yourself, blood doesn’t keep flowing for long.
• This is because blood clots to prevent excessive bleeding.

What is Blood Clotting?

• Clotting Mechanism: Blood coagulates or clots to stop bleeding.
• Clot Formation: You see a dark reddish-brown scab over a wound. This is the blood clot.

How Clots are Formed

• Fibrins: The clot is mainly made of fibrins, thread-like structures.
• Trapping Elements: Dead and damaged blood cells get trapped in these fibrins.

Steps in Clotting

1. Inactive Fibrinogens: Fibrins form from fibrinogens in the plasma.
2. Role of Thrombin: Thrombin enzyme converts fibrinogen to fibrin.
3. Prothrombin: Thrombin is made from prothrombin, another inactive substance in plasma.
4. Thrombokinase: An enzyme complex needed for these reactions, formed through a series of steps.

Activation of Clotting

• Platelets: Injury stimulates platelets to release factors that start clotting.
• Tissue Factors: Injured tissues also release factors that help in coagulation.
• Calcium Ions: Very important for the clotting process.

Lymph (Tissue Fluid)

What is Lymph?

• Formation: As blood moves through capillaries, some water and small substances leak out into spaces between cells, forming interstitial fluid or tissue fluid.
• Components: This fluid has minerals like plasma but leaves behind larger proteins and blood cells in the vessels.

Function of Lymph

• Exchange of Nutrients and Gases: Nutrients and gases are exchanged between blood and cells through this fluid.
• Lymphatic System: An elaborate network of vessels collects this fluid and drains it back into major veins.
• Lymph Fluid: Fluid in the lymphatic system, called lymph, is colorless and contains lymphocytes.
• Immune Response: Lymphocytes are crucial for immune responses.
• Transport: Lymph carries nutrients and hormones. Fats are absorbed through lymph in the intestinal villi.

Circulatory Pathways

Types of Circulatory Systems

• Open Circulatory System: Found in arthropods and molluscs. Blood is pumped by the heart into open spaces or body cavities (sinuses).
• Closed Circulatory System: Found in annelids and chordates. Blood circulates through a closed network of vessels, allowing precise regulation of blood flow.

Heart Chambers in Vertebrates

• Fishes: 2-chambered heart (1 atrium, 1 ventricle). Heart pumps deoxygenated blood to gills for oxygenation, then to body (single circulation).
• Amphibians and Reptiles (except crocodiles): 3-chambered heart (2 atria, 1 ventricle). Blood mixes in the ventricle, leading to mixed blood being pumped out (incomplete double circulation).
• Crocodiles, Birds, and Mammals: 4-chambered heart (2 atria, 2 ventricles). Oxygenated and deoxygenated blood are kept separate, leading to efficient double circulation.

Human Circulatory System

Overview

• Components: Muscular heart, network of blood vessels, and blood.
• Location: Heart is in the thoracic cavity, between the lungs, tilted slightly to the left.
• Protection: Enclosed in a double-walled membranous bag called the pericardium, which contains pericardial fluid.

Structure of the Heart

• Size: About the size of a clenched fist.
• Chambers: Four chambers – two upper atria and two lower ventricles.
• Septa:
  • Inter-atrial septum: Separates right and left atria.
  • Inter-ventricular septum: Separates right and left ventricles.
  • Atrio-ventricular septum: Separates atria and ventricles on each side.

Heart Valves

• Tricuspid Valve: Between right atrium and right ventricle.
• Bicuspid/Mitral Valve: Between left atrium and left ventricle.
• Semilunar Valves: At the openings of the pulmonary artery (right ventricle) and aorta (left ventricle).
• Function: Valves ensure one-way blood flow and prevent backward flow.

Heart Muscles and Nodal Tissue

• Cardiac Muscles: Make up the heart walls; ventricles have thicker walls than atria.
• Nodal Tissue:
  • Sino-atrial Node (SAN): In the right atrium, generates 70-75 action potentials per minute, acting as the pacemaker.
  • Atrio-ventricular Node (AVN): In the right atrium near the septum.
  • AV Bundle (Bundle of His): Runs from AVN, divides into right and left bundles, and branches into Purkinje fibers in the ventricles.

Function of Nodal Tissue

• Auto-excitability: Generates action potentials without external stimuli.
• Heart Rate: SAN controls the heartbeat, which is usually 70-75 beats per minute (average 72).

Cardiac Cycle

How the Heart Functions

• Joint Diastole: All four heart chambers are relaxed. Blood flows from pulmonary veins and vena cava into the ventricles through the atria. Semilunar valves are closed.
• Atrial Systole: SAN generates an action potential, causing atria to contract and push more blood into ventricles (about 30% more).
• Ventricular Systole: Action potential reaches ventricles via AVN, AV bundle, and bundle of His, causing ventricles to contract. Atria relax. Increased pressure closes tricuspid and bicuspid valves and opens semilunar valves, allowing blood to flow into pulmonary artery and aorta.
• Ventricular Diastole: Ventricles relax, pressure falls, semilunar valves close to prevent backflow. Tricuspid and bicuspid valves open again, and blood flows into ventricles.

Summary of the Cardiac Cycle

• Sequential Events: This cycle of systole (contraction) and diastole (relaxation) repeats with each heartbeat.
• Duration: Each cardiac cycle lasts about 0.8 seconds.
• Heart Rate: Average is 72 beats per minute, meaning 72 cardiac cycles per minute.
• Stroke Volume: Each ventricle pumps out about 70 mL of blood per cycle.
• Cardiac Output: Stroke volume multiplied by heart rate, averaging 5000 mL or 5 liters per minute in a healthy individual. Athletes have higher cardiac output.

Heart Sounds

• “Lub”: Closure of tricuspid and bicuspid valves.
• “Dub”: Closure of semilunar valves.
• Diagnostic Significance: These sounds help doctors assess heart function using a stethoscope.

Electrocardiograph (ECG)

What is an ECG?

• Electrocardiograph: A machine used to obtain an ECG.
• ECG (Electrocardiogram): A graphical representation of the heart’s electrical activity during a cardiac cycle.

How is an ECG Taken?

• Standard ECG:
  • Three electrical leads attached to each wrist and the left ankle.
  • Continuously monitors heart activity.
• Detailed Evaluation: Multiple leads attached to the chest region.

Understanding ECG Waves

• P-Wave: Represents electrical excitation (depolarization) of the atria, leading to atrial contraction.
• QRS Complex: Represents depolarization of the ventricles, initiating ventricular contraction. The contraction starts shortly after the Q wave, marking the beginning of systole.
• T-Wave: Represents repolarization (return to normal state) of the ventricles, marking the end of systole.

Clinical Significance

• Heart Rate: Count the number of QRS complexes in a given time period to determine heart rate.
• Shape Consistency: The shape of ECGs is roughly the same for everyone with the same lead configuration.
• Diagnosing Abnormalities: Deviations from the normal ECG shape indicate possible heart abnormalities or diseases.

An ECG is a crucial tool in hospitals to monitor and diagnose heart conditions by tracking the electrical activity of the heart.

Double Circulation

• Right Ventricle: Pumps deoxygenated blood into the pulmonary artery.
• Pulmonary Circulation: Blood goes to the lungs, gets oxygenated, and returns to the left atrium via pulmonary veins.
• Left Ventricle: Pumps oxygenated blood into the aorta.
• Systemic Circulation: Oxygenated blood travels through arteries to tissues; deoxygenated blood returns to the right atrium via veins.
• Hepatic Portal System: Blood from the digestive tract goes to the liver via the hepatic portal vein before entering systemic circulation.
• Coronary System: Special blood vessels for the heart’s own blood supply

Regulation of Cardiac Activity

• Myogenic Heart: Heart’s activity is self-regulated by nodal tissue.
• Neural Control:
  • Sympathetic Nerves: Increase heart rate and strength of contraction.
  • Parasympathetic Nerves: Decrease heart rate and action potential conduction.
• Hormonal Control: Adrenal medullary hormones can increase cardiac output.

Disorders of the Circulatory System

• Hypertension (High Blood Pressure):
  • Normal BP: 120/80 mm Hg.
  • Hypertension: 140/90 mm Hg or higher.
  • Effects: Can lead to heart diseases, affects brain and kidney.
• Coronary Artery Disease (CAD):
  • Also known as atherosclerosis.
  • Caused by deposits of calcium, fat, cholesterol, narrowing artery lumen.
• Angina:
  • Acute chest pain due to insufficient oxygen to heart muscle.
  • Common in middle-aged and elderly.
• Heart Failure:
  • Heart doesn’t pump blood effectively.
  • Congestive heart failure involves lung congestion.
  • Not the same as cardiac arrest (heart stops) or heart attack (sudden damage due to inadequate blood supply).

Chapter Summary:

• Vertebrates circulate blood to transport essential substances to cells and remove waste.
• Lymph (tissue fluid) also helps in transporting certain substances.
• Blood consists of plasma and formed elements.
• Formed elements include RBCs (erythrocytes), WBCs (leucocytes), and platelets (thrombocytes).
• Human blood groups are A, B, AB, and O, based on antigens A and B on RBCs.
• Another blood grouping is based on the Rhesus factor (Rh) on RBCs.
• Tissue fluid, called lymph, is similar to blood but has different protein content and formed elements.
• Vertebrates and some invertebrates have a closed circulatory system.
• The human circulatory system includes the heart, blood vessels, and blood.
• The heart has two atria and two ventricles.
• Cardiac muscles are auto-excitable.
• The Sino-atrial node (SAN) sets the heart’s pace and is called the Pacemaker.
• The SAN generates 70-75 action potentials per minute.
• Action potentials cause atria and ventricles to contract (systole) and relax (diastole).
• Systole moves blood from atria to ventricles and to pulmonary artery and aorta.
• The cardiac cycle consists of repeated sequential events in the heart.
• A healthy person has 72 cardiac cycles per minute.
• Each ventricle pumps out 70 mL of blood per cycle (stroke or beat volume).
• Cardiac output is the volume of blood pumped by each ventricle per minute (about 5 liters).
• ECG (electrocardiogram) records the heart’s electrical activity and is clinically important.
• Humans have double circulation: pulmonary and systemic.
• Pulmonary circulation: right ventricle pumps deoxygenated blood to lungs, oxygenated blood returns to left atrium.
• Systemic circulation: left ventricle pumps oxygenated blood to the body, deoxygenated blood returns to right atrium.
• Heart functions can be moderated by neural and hormonal mechanisms.