Chapter Notes

Body Fluids and Circulation

All living cells require a constant supply of nutrients, oxygen ($O_2$), and other essential substances to function. At the same time, waste products must be continuously removed. This transport of substances to and from the cells is handled by the circulatory system. While simple organisms like sponges circulate water from their surroundings, more complex animals use specialized body fluids. The two main body fluids in humans are blood and lymph.

Blood

Blood is a specialized connective tissue that consists of a fluid part called plasma and cellular components called formed elements.

Plasma

Plasma is the straw-coloured, viscous fluid that makes up about 55% of the blood. Its main components are:

• Water: Constitutes 90-92% of plasma.
• Proteins: Make up 6-8% of plasma. The major proteins are:
  • Fibrinogens: Essential for the clotting or coagulation of blood.
  • Globulins: Primarily involved in the body's defense mechanisms (immunity).
  • Albumins: Help maintain the osmotic balance of the blood.
• Minerals: Small amounts of minerals like $Na^+$, $Ca^{++}$, $Mg^{++}$, $HCO_3^-$, and $Cl^-$ are present.
• Other Substances: Glucose, amino acids, and lipids are also found in plasma as they are transported throughout the body.

Plasma also contains inactive factors required for blood clotting. When these clotting factors are removed from plasma, the remaining fluid is called serum.

Formed Elements

Constituting about 45% of the blood, the formed elements include erythrocytes (RBCs), leucocytes (WBCs), and platelets.

Erythrocytes (Red Blood Cells or RBCs)

• Abundance: They are the most numerous cells in the blood, with a healthy adult male having about 5 to 5.5 million RBCs per cubic millimeter ($\text{mm}^{-3}$) of blood.
• Formation: In adults, RBCs are formed in the red bone marrow.
• Structure: In most mammals, RBCs are biconcave in shape and lack a nucleus. This shape increases the surface area for gas exchange.
• Haemoglobin: They contain a red, iron-containing protein called haemoglobin, which gives blood its colour. A healthy person has 12-16 grams of haemoglobin per 100 mL of blood.
• Function: Haemoglobin plays a crucial role in transporting respiratory gases, primarily oxygen.
• Lifespan: RBCs have an average lifespan of 120 days, after which they are destroyed in the spleen, often called the "graveyard of RBCs."

Leucocytes (White Blood Cells or WBCs)

• Characteristics: WBCs are colourless because they lack haemoglobin. They are nucleated and are far less numerous than RBCs, averaging 6,000-8,000 per $\text{mm}^{-3}$ of blood. They are generally short-lived.
• Types: WBCs are categorized into two main groups:
  1. Granulocytes: These have granules in their cytoplasm.
     • Neutrophils (60-65%): The most abundant WBCs. They are phagocytic, meaning they engulf and destroy foreign organisms.
     • Eosinophils (2-3%): Resist infections and are involved in allergic reactions.
     • Basophils (0.5-1%): The least common WBCs. They secrete histamine, serotonin, and heparin and are involved in inflammatory reactions.
  2. Agranulocytes: These lack granules in their cytoplasm.
     • Lymphocytes (20-25%): There are two major types, 'B' and 'T' lymphocytes, which are responsible for the body's immune responses.
     • Monocytes (6-8%): Like neutrophils, they are phagocytic cells that destroy foreign invaders.

Platelets (Thrombocytes)

• Origin: Platelets are not true cells but are cell fragments produced from special bone marrow cells called megakaryocytes.
• Number: Blood normally contains 150,000-350,000 platelets per $\text{mm}^{-3}$.
• Function: They are crucial for the coagulation or clotting of blood. A low platelet count can lead to clotting disorders and excessive bleeding.

Blood Groups

Human blood appears similar, but it differs in certain aspects. Two important blood grouping systems are the ABO and Rh systems.

ABO Grouping

This system is based on the presence or absence of two specific chemicals, called antigens, on the surface of RBCs: Antigen A and Antigen B. The plasma, in turn, contains proteins called antibodies that react against foreign antigens.

The compatibility for blood transfusion is shown below:

Blood Group	Antigens on RBCs	Antibodies in Plasma	Can Receive Blood From
A	A	anti-B	A, O
B	B	anti-A	B, O
AB	A, B	nil	AB, A, B, O
O	nil	anti-A, anti-B	O

• Universal Donors: Individuals with blood group O can donate blood to people with any other blood group because their RBCs have no antigens to trigger an immune reaction.
• Universal Recipients: Individuals with blood group AB can receive blood from any group because their plasma has no antibodies to attack the donor's RBCs.

Rh Grouping

Another important antigen found on the surface of RBCs is the Rh antigen, first discovered in Rhesus monkeys.

• Rh positive (Rh+ve): Individuals who have the Rh antigen (about 80% of humans).
• Rh negative (Rh-ve): Individuals who do not have the Rh antigen.

It is crucial to match Rh groups before blood transfusion. If an Rh-ve person receives Rh+ve blood, they will start producing antibodies against the Rh antigen.

Erythroblastosis Foetalis This is a special case of Rh incompatibility between an Rh-ve mother and an Rh+ve foetus.

1. First Pregnancy: The mother's and foetus's blood are separated by the placenta, so there is no mixing. However, during delivery, some of the foetus's Rh+ve blood may enter the mother's bloodstream.
2. Immune Response: The mother's body recognizes the Rh antigen as foreign and starts producing anti-Rh antibodies.
3. Subsequent Pregnancies: If the next foetus is also Rh+ve, the mother's anti-Rh antibodies can cross the placenta and attack the foetus's RBCs.
4. Consequences: This can be fatal to the foetus or cause severe anaemia and jaundice. The condition is called erythroblastosis foetalis.

This can be prevented by administering anti-Rh antibodies to the mother immediately after the first delivery.

Coagulation of Blood

When you get a cut, the bleeding usually stops after a short time. This is due to blood coagulation, or clotting, a mechanism to prevent excessive blood loss.

The clot is a dark reddish-brown scum formed from a network of threads called fibrins, which trap dead and damaged blood cells. The process is a cascade of enzyme reactions:

1. An injury stimulates platelets and damaged tissues to release factors that activate the coagulation mechanism.
2. These factors lead to the formation of an enzyme complex called thrombokinase.
3. Thrombokinase converts an inactive plasma substance, prothrombin, into its active form, thrombin.
4. Thrombin then acts as an enzyme to convert the soluble plasma protein fibrinogen into insoluble fibrin threads.
5. These fibrin threads form a meshwork at the site of the injury, trapping formed elements and forming a clot.

Lymph (Tissue Fluid)

As blood flows through capillaries in the tissues, water and small soluble substances filter out into the spaces between the cells. This fluid is called interstitial fluid or tissue fluid.

• Composition: It has the same mineral distribution as plasma but contains fewer large proteins and formed elements.
• Function: It facilitates the exchange of nutrients, gases, and waste products between the blood and the body cells.
• Lymphatic System: An elaborate network of vessels called the lymphatic system collects this fluid and drains it back into the major veins. The fluid within this system is called lymph.
• Lymph: It is a colourless fluid containing specialized lymphocytes responsible for immune responses. It also transports nutrients, hormones, and absorbed fats (in vessels called lacteals in the intestine).

Circulatory Pathways

There are two main types of circulatory systems in animals:

1. Open Circulatory System: Found in arthropods and molluscs. The heart pumps blood through large vessels into open spaces or body cavities called sinuses.
2. Closed Circulatory System: Found in annelids and chordates (including humans). Blood is always circulated through a closed network of blood vessels (arteries, veins, capillaries). This system is more efficient as the flow of blood can be precisely regulated.

Evolution of the Heart in Vertebrates

• Fishes: Have a 2-chambered heart (one atrium, one ventricle). They exhibit single circulation, where the heart pumps deoxygenated blood to the gills for oxygenation, and this oxygenated blood is then supplied to the rest of the body.
• Amphibians and Reptiles (except crocodiles): Have a 3-chambered heart (two atria, one ventricle). They have incomplete double circulation, where oxygenated and deoxygenated blood mix in the single ventricle before being pumped out.
• Crocodiles, Birds, and Mammals: Have a 4-chambered heart (two atria, two ventricles). They have double circulation, with two separate pathways for oxygenated and deoxygenated blood, preventing any mixing.

Human Circulatory System

The human circulatory system consists of a four-chambered heart, a network of blood vessels, and blood.

Heart Structure

• Location: The heart is a muscular organ, about the size of a clenched fist, located in the thoracic cavity between the lungs, slightly tilted to the left.
• Pericardium: It is protected by a double-walled membranous sac called the pericardium, which contains pericardial fluid to reduce friction.
• Chambers: The heart has four chambers:
  • Two upper, smaller chambers called atria (singular: atrium).
  • Two lower, larger chambers called ventricles.
• Septa: Muscular walls called septa separate the chambers:
  • Inter-atrial septum: Separates the right and left atria.
  • Inter-ventricular septum: A thick wall that separates the right and left ventricles.
• Valves: Valves ensure that blood flows in only one direction and prevent backflow.
  • Tricuspid Valve: Guards the opening between the right atrium and the right ventricle. It has three muscular flaps (cusps).
  • Bicuspid (or Mitral) Valve: Guards the opening between the left atrium and the left ventricle. It has two cusps.
  • Semilunar Valves: Located at the exit of each ventricle, guarding the openings into the pulmonary artery (right ventricle) and the aorta (left ventricle).

Conduction System of the Heart

The heart is made of specialized cardiac muscles called nodal tissue, which can generate electrical impulses on its own (autoexcitable).

• Sino-atrial Node (SAN): A patch of nodal tissue in the upper right corner of the right atrium. It generates the maximum number of action potentials ($70-75 \text{ per minute}$) and initiates the heartbeat. Therefore, it is called the pacemaker.
• Atrio-ventricular Node (AVN): A mass of tissue in the lower-left corner of the right atrium. It receives the impulse from the SAN.
• Atrio-ventricular (AV) Bundle (Bundle of His): Nodal fibres that continue from the AVN and divide into right and left branches.
• Purkinje Fibres: Minute fibres that spread from the AV bundle branches throughout the ventricular walls, causing them to contract.

Cardiac Cycle

The cardiac cycle is the sequence of events that occurs in the heart during one heartbeat. It consists of the contraction (systole) and relaxation (diastole) of the atria and ventricles. With a heart rate of 72 beats per minute, one cardiac cycle lasts about 0.8 seconds.

Steps of the Cardiac Cycle:

1. Joint Diastole: All four chambers are relaxed. The tricuspid and bicuspid valves are open, and blood flows from the veins into the atria and then into the ventricles. The semilunar valves are closed.
2. Atrial Systole: The SAN generates an action potential, causing both atria to contract simultaneously. This pushes about 30% more blood into the ventricles.
3. Ventricular Systole: The impulse travels to the ventricles, causing them to contract.
   • The rising pressure in the ventricles forces the tricuspid and bicuspid valves to shut, producing the first heart sound, "lub".
   • As ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves are forced open, and blood is ejected from the heart.
4. Ventricular Diastole: The ventricles relax, and the pressure inside them falls.
   • To prevent backflow from the aorta and pulmonary artery, the semilunar valves snap shut, producing the second heart sound, "dub".
   • As ventricular pressure drops below atrial pressure, the tricuspid and bicuspid valves open again, and the cycle repeats.

Cardiac Output

• Stroke Volume: The volume of blood pumped out by each ventricle during one cardiac cycle. It is approximately 70 mL.
• Cardiac Output: The total volume of blood pumped by each ventricle per minute. It is calculated by multiplying the stroke volume by the heart rate.
  • Cardiac Output = Stroke Volume × Heart Rate
  • Example: $70 \text{ mL/beat} \times 72 \text{ beats/min} \approx 5040 \text{ mL/min}$ or about 5 litres per minute.

Electrocardiogram (ECG)

An electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during a cardiac cycle. It is recorded by a machine called an electrocardiograph.

A standard ECG shows several peaks, or waves, labeled P to T:

• P-wave: Represents the electrical excitation (depolarisation) of the atria, which leads to atrial contraction.
• QRS complex: Represents the depolarisation of the ventricles, which initiates ventricular contraction (systole).
• T-wave: Represents the return of the ventricles to their normal, relaxed state (repolarisation). The end of the T-wave marks the end of systole.

By counting the number of QRS complexes in a given time, one can determine the heart rate. Any deviation from the standard ECG shape can indicate a possible heart abnormality, making it a valuable diagnostic tool.

Double Circulation

In humans, blood flows through two distinct circuits, a system known as double circulation. This ensures that oxygenated and deoxygenated blood remain separate. The two circuits are:

1. Pulmonary Circulation: This circuit carries deoxygenated blood from the heart to the lungs and brings oxygenated blood back to the heart.

   • Path: Right Ventricle → Pulmonary Artery → Lungs (blood gets oxygenated) → Pulmonary Veins → Left Atrium.

2. Systemic Circulation: This circuit carries oxygenated blood from the heart to all body tissues and returns deoxygenated blood back to the heart.

   • Path: Left Ventricle → Aorta → Arteries → Capillaries (in tissues) → Veins → Vena Cava → Right Atrium.

• The hepatic portal system is a unique vascular connection where the hepatic portal vein carries blood from the intestine to the liver before it enters the systemic circulation.
• The coronary system is a special set of blood vessels that supplies blood exclusively to the heart muscle itself.

Regulation of Cardiac Activity

The heart's normal activity is intrinsically regulated by its nodal tissue, which is why it is called myogenic. However, its function can be moderated by external factors:

• Neural Control: A special centre in the medulla oblongata can control cardiac function through the autonomic nervous system (ANS).
  • Sympathetic nerves: Increase the heart rate and the force of ventricular contraction, thus increasing cardiac output.
  • Parasympathetic nerves: Decrease the heart rate and speed of impulse conduction, thus decreasing cardiac output.
• Hormonal Control: Adrenal medullary hormones (like adrenaline) can also increase cardiac output.

Disorders of Circulatory System

• High Blood Pressure (Hypertension): Blood pressure that is consistently higher than the normal 120/80 mm Hg. A reading of 140/90 or higher indicates hypertension. It can lead to heart disease and damage vital organs like the brain and kidneys.
• Coronary Artery Disease (CAD): Also known as atherosclerosis, this condition affects the arteries that supply blood to the heart muscle. It is caused by the deposition of calcium, fat, cholesterol, and fibrous tissues, which narrows the arteries.
• Angina (Angina Pectoris): A symptom of acute chest pain that occurs when the heart muscle does not receive enough oxygen. It is caused by conditions that affect blood flow to the heart.
• Heart Failure: A condition where the heart is not pumping blood effectively enough to meet the body's needs. It is sometimes called congestive heart failure because congestion (fluid buildup) in the lungs is a common symptom. Heart failure is different from cardiac arrest (heart stops beating) or a heart attack (heart muscle is damaged).