Body Fluids and Circulation

Given: Formed elements are the cellular components of blood.

Components and their major functions:

1. Red Blood Cells (RBCs / Erythrocytes):
- They contain haemoglobin and are responsible for the transport of respiratory gases (O₂ and CO₂).

2. White Blood Cells (WBCs / Leucocytes): These are of two main types:
- Granulocytes (Neutrophils, Eosinophils, Basophils)
- Agranulocytes (Lymphocytes, Monocytes)
- Major function: Defence against infections and foreign substances (immune response).
- Neutrophils are phagocytic; Eosinophils resist infections and are associated with allergic reactions; Basophils are involved in inflammatory responses; Lymphocytes produce antibodies; Monocytes are phagocytic.

3. Platelets (Thrombocytes):
- Major function: Blood coagulation (clotting) — they release substances necessary for the clotting of blood at the site of injury, thereby preventing excessive blood loss.

Summary Table:

| Formed Element | Major Function |
|---|---|
| RBCs | Transport of O₂ and CO₂ |
| WBCs | Defence / Immunity |
| Platelets | Blood coagulation |

Given: Plasma is the fluid matrix of blood and contains several proteins.

Importance of Plasma Proteins:

1. Fibrinogen: Essential for blood clotting (coagulation). It is converted to fibrin during clot formation.

2. Globulins: Primarily involved in the defence mechanism of the body — they include immunoglobulins (antibodies) that fight pathogens.

3. Albumins: Help in osmotic balance (oncotic pressure) of blood. They maintain the water balance between blood and tissues by retaining water in the blood vessels.

4. Prothrombin: Another plasma protein involved in blood coagulation.

5. Overall, plasma proteins help in transport of substances (e.g., lipids, hormones, ions) and contribute to the viscosity of blood.

Conclusion: Plasma proteins are vital for maintaining osmotic pressure, immunity, blood clotting, and transport functions.

Correct Matching:

| Column I | Column II |
|---|---|
| (a) Eosinophils | (iii) Resist Infections |
| (b) RBC | (v) Gas transport |
| (c) AB Group | (ii) Universal Recipient |
| (d) Platelets | (i) Coagulation |
| (e) Systole | (iv) Contraction of Heart |

Justification:
- Eosinophils are WBCs that resist infections and are involved in allergic reactions.
- RBCs contain haemoglobin and transport O₂ and CO₂.
- AB blood group individuals have both A and B antigens on RBCs and no antibodies in plasma, so they can receive blood from any group — hence called Universal Recipients.
- Platelets release clotting factors and are essential for coagulation.
- Systole refers to the contraction of the heart chambers (atria or ventricles).

Concept: Connective tissues are characterised by having cells scattered in a matrix (intercellular substance).

Blood as a Connective Tissue:

1. Blood is considered a fluid connective tissue because it originates from mesoderm (like all connective tissues).

2. It has a fluid matrix called plasma (analogous to the intercellular matrix of other connective tissues).

3. The formed elements (RBCs, WBCs, platelets) are the cellular components suspended in this matrix — similar to cells embedded in the matrix of other connective tissues.

4. Like other connective tissues, blood connects and integrates different parts of the body by transporting nutrients, gases, hormones, and waste products.

Conclusion: Since blood has a matrix (plasma) with cells (formed elements) suspended in it and is derived from mesoderm, it is classified as a fluid connective tissue.

Differences between Lymph and Blood:

| Feature | Blood | Lymph |
|---|---|---|
| Colour | Red (due to haemoglobin) | Colourless/pale yellow |
| Formed elements | RBCs, WBCs, platelets present | No RBCs; WBCs (mainly lymphocytes) present; no platelets |
| Plasma proteins | Present in high concentration | Present in low concentration |
| Oxygen content | High (oxygenated blood in arteries) | Low |
| Nutrients | Present | Present (absorbed fats from lacteals) |
| Direction of flow | Flows in blood vessels (arteries, veins, capillaries) | Flows in lymph capillaries and lymph vessels towards heart |
| Function | Transport of O₂, CO₂, nutrients, hormones, waste | Returns excess tissue fluid to blood; transport of fats; immune function |

Conclusion: Lymph is essentially a filtrate of blood plasma that lacks RBCs and has fewer proteins, while blood is a complete fluid connective tissue with all formed elements.

Double Circulation:

Double circulation refers to the condition in which blood passes through the heart twice in one complete circuit of the body. It involves two separate pathways:

1. Pulmonary Circulation:
- Deoxygenated blood from the right ventricle is pumped to the lungs via the pulmonary artery.
- In the lungs, blood is oxygenated (CO₂ released, O₂ absorbed).
- Oxygenated blood returns to the left atrium via pulmonary veins.

2. Systemic Circulation:
- Oxygenated blood from the left ventricle is pumped to the aorta and distributed to all body tissues.
- Deoxygenated blood from body tissues is collected by veins and returned to the right atrium via superior and inferior vena cava.

Significance of Double Circulation:

1. It ensures complete separation of oxygenated and deoxygenated blood, so tissues always receive fully oxygenated blood.

2. It maintains high blood pressure in the systemic circulation, ensuring efficient delivery of oxygen and nutrients to all body cells.

3. It allows the body to sustain high metabolic rates, which is essential for warm-blooded (homeothermic) animals like birds and mammals.

4. It increases the efficiency of the circulatory system by preventing mixing of oxygenated and deoxygenated blood.

(a) Differences between Blood and Lymph:

| Feature | Blood | Lymph |
|---|---|---|
| Colour | Red | Colourless |
| RBCs | Present | Absent |
| Platelets | Present | Absent |
| WBCs | All types present | Mainly lymphocytes |
| Plasma proteins | High concentration | Low concentration |
| Flow | Through heart and blood vessels | Through lymph vessels (unidirectional, towards heart) |
| Oxygen | Rich in O₂ (arteries) | Less O₂ |
| Function | Transport of gases, nutrients, hormones, waste | Returns tissue fluid to blood; transports fats; immunity |

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(b) Differences between Open and Closed Circulatory Systems:

| Feature | Open Circulatory System | Closed Circulatory System |
|---|---|---|
| Blood flow | Blood flows into open spaces (sinuses/haemocoel) | Blood flows entirely within blood vessels |
| Blood pressure | Low | High |
| Speed of circulation | Slow | Fast |
| Efficiency | Less efficient | More efficient |
| Examples | Arthropods, molluscs (except cephalopods) | Annelids, vertebrates |
| Capillaries | Absent | Present |

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(c) Differences between Systole and Diastole:

| Feature | Systole | Diastole |
|---|---|---|
| Definition | Contraction of heart chambers | Relaxation of heart chambers |
| Blood flow | Blood is pumped out of the chambers | Blood fills into the chambers |
| Pressure | Blood pressure is higher (systolic pressure ~120 mmHg) | Blood pressure is lower (diastolic pressure ~80 mmHg) |
| Duration | Shorter | Longer |
| Valves | AV valves close; semilunar valves open | Semilunar valves close; AV valves open |

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(d) Differences between P-wave and T-wave (in ECG):

| Feature | P-wave | T-wave |
|---|---|---|
| Represents | Depolarisation of atria (atrial contraction/systole) | Repolarisation of ventricles (ventricular relaxation/diastole) |
| Position in ECG | First wave, before QRS complex | Last wave, after QRS complex |
| Amplitude | Small | Dome-shaped, moderate |
| Associated event | Atrial systole | Ventricular diastole begins |

Evolutionary Changes in the Heart Among Vertebrates:

The heart has undergone progressive evolutionary changes from lower to higher vertebrates, increasing in complexity and efficiency:

1. Fishes (2-chambered heart):
- Heart consists of one atrium and one ventricle.
- Only single circulation — blood flows: Heart → Gills (oxygenation) → Body → Heart.
- Deoxygenated blood is pumped to gills; oxygenated blood goes to body.
- No separation of oxygenated and deoxygenated blood in the heart.

2. Amphibians (3-chambered heart):
- Heart has two atria and one ventricle.
- Left atrium receives oxygenated blood from lungs; right atrium receives deoxygenated blood from body.
- Both types of blood mix in the single ventricle — incomplete double circulation.
- Some mixing of oxygenated and deoxygenated blood occurs.

3. Reptiles (incompletely 4-chambered heart):
- Heart has two atria and two ventricles, but the inter-ventricular septum is incomplete (except in crocodiles).
- Some mixing of blood still occurs between ventricles.
- Crocodiles have a completely 4-chambered heart — most advanced among reptiles.

4. Birds and Mammals (completely 4-chambered heart):
- Heart has two atria and two completely separated ventricles.
- Complete double circulation — pulmonary and systemic circuits are fully separated.
- No mixing of oxygenated and deoxygenated blood.
- This is the most efficient type, supporting high metabolic rates of warm-blooded animals.

Conclusion: The evolutionary trend shows a progression from a 2-chambered heart (fishes) → 3-chambered (amphibians) → incompletely 4-chambered (reptiles) → completely 4-chambered (birds and mammals), ensuring increasingly efficient separation of oxygenated and deoxygenated blood.

Definition: A heart is called myogenic when the heartbeat is initiated by a specialised muscle tissue within the heart itself, without requiring any external nervous stimulation.

Explanation:

- In humans (and all mammals), the heartbeat originates from the Sino-Atrial Node (SAN), which is a specialised patch of modified cardiac muscle tissue located in the right atrium.

- The SAN is capable of self-excitation — it spontaneously generates electrical impulses (action potentials) at a rate of 70–75 per minute without any nerve signal.

- Since the impulse for contraction arises from the myocardium (cardiac muscle) itself, the human heart is called myogenic.

- This is in contrast to neurogenic hearts (found in some invertebrates like crustaceans), where the heartbeat is initiated by nerve impulses from ganglia.

Conclusion: Our heart is called myogenic because its rhythmic contraction is initiated by the cardiac muscle (SAN) itself, not by external nerve stimulation.

Given: The Sino-Atrial Node (SAN) is a specialised mass of modified cardiac muscle tissue located in the upper right corner of the right atrium.

Reason for being called the Pacemaker:

1. The SAN has the ability to spontaneously generate action potentials (electrical impulses) without any external stimulus.

2. It generates action potentials at the highest frequency among all auto-excitable tissues of the heart — approximately 70–75 action potentials per minute.

3. The impulse generated by SAN spreads first to the atria, causing atrial contraction (atrial systole), then reaches the Atrio-Ventricular Node (AVN), and subsequently spreads to the ventricles via the Bundle of His and Purkinje fibres, causing ventricular contraction.

4. Since the SAN sets the rhythm and rate of the heartbeat for the entire heart, it is called the Pacemaker.

5. If the SAN is damaged, other nodes (like AVN) can take over but at a lower rate, confirming that SAN normally dominates and controls the pace.

Conclusion: SAN is called the pacemaker because it generates the maximum number of action potentials per minute and thereby determines the rate and rhythm of the heartbeat.

1. Atrio-Ventricular Node (AVN):

- The AVN is located at the base of the right atrium, near the inter-atrial septum.
- It receives the electrical impulse from the SAN after atrial depolarisation.
- Significance:
- It acts as a relay station — it receives the impulse from the atria and transmits it to the ventricles.
- It introduces a brief delay (approx. 0.1 second) before passing the impulse to the ventricles. This delay ensures that the atria complete their contraction (and blood is pushed into ventricles) before the ventricles contract. This is crucial for efficient filling of ventricles.
- It is the only electrical connection between the atria and ventricles (since the fibrous skeleton of the heart otherwise electrically insulates them).

2. Atrio-Ventricular Bundle (Bundle of His):

- The AV bundle arises from the AVN and passes through the inter-ventricular septum, dividing into right and left bundle branches (Purkinje fibres).
- Significance:
- It conducts the impulse from AVN to the ventricular myocardium rapidly via Purkinje fibres.
- It ensures simultaneous and coordinated contraction of both ventricles from the apex upward, so blood is efficiently pumped into the pulmonary artery and aorta.
- It maintains the proper sequence of contraction: atria first, then ventricles.

Conclusion: The AVN and AV bundle together ensure that the electrical impulse is properly delayed and then rapidly distributed to the ventricles, resulting in coordinated and efficient pumping of blood.

1. Cardiac Cycle:

Definition: The cardiac cycle refers to the sequential series of events that occur in the heart during one complete heartbeat — from the beginning of one heartbeat to the beginning of the next.

- It includes:
- Atrial systole (contraction of atria) — 0.1 sec
- Ventricular systole (contraction of ventricles) — 0.3 sec
- Joint diastole (relaxation of both atria and ventricles) — 0.4 sec
- Total duration of one cardiac cycle = 0.8 seconds
- A healthy person completes approximately 72 cardiac cycles per minute.

2. Cardiac Output:

Definition: Cardiac output is defined as the volume of blood pumped out by each ventricle per minute.

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

Where:
- Stroke Volume = volume of blood pumped by each ventricle per beat ≈ 70 mL
- Heart Rate = number of beats per minute ≈ 72 beats/min

$$\text{Cardiac Output} = 70 \text{ mL} \times 72 = 5040 \text{ mL} \approx 5 \text{ litres/min}$$

Conclusion: A healthy adult has a cardiac output of approximately 5 litres per minute, which means the entire blood volume is circulated once every minute.

Heart Sounds:

During each cardiac cycle, the heart produces two main sounds that can be heard through a stethoscope:

1. First Heart Sound — 'LUBB':
- Produced by the closure of the tricuspid and bicuspid (mitral) valves (Atrio-Ventricular valves / AV valves).
- Occurs at the beginning of ventricular systole (when ventricles start contracting).
- It is louder and longer in duration.
- Indicates the start of ventricular contraction.

2. Second Heart Sound — 'DUPP':
- Produced by the closure of the semilunar valves (pulmonary and aortic valves).
- Occurs at the beginning of ventricular diastole (when ventricles start relaxing).
- It is shorter and sharper in duration.
- Indicates the end of ventricular systole and beginning of diastole.

Summary:

| Sound | Cause | Phase |
|---|---|---|
| 'LUBB' (1st) | Closure of AV valves (tricuspid + bicuspid) | Beginning of ventricular systole |
| 'DUPP' (2nd) | Closure of semilunar valves (aortic + pulmonary) | Beginning of ventricular diastole |

Clinical Significance: Any abnormal heart sounds (murmurs) may indicate defective valves or other cardiac abnormalities.

Conclusion: Heart sounds are produced by the closure of valves and help in assessing the proper functioning of the heart.

Electrocardiogram (ECG):

Definition: An ECG is a graphical recording of the electrical activity of the heart over time, recorded from the body surface using an electrocardiograph.

Standard ECG — Description of Waves:

A normal ECG shows the following distinct waves:

$$\boxed{P \rightarrow Q \rightarrow R \rightarrow S \rightarrow T}$$

*(Note: Draw a standard ECG trace showing P wave, QRS complex, and T wave on the answer sheet. The baseline is the isoelectric line. P is a small upward deflection; QRS is a sharp tall complex; T is a broad upward dome.)*

Explanation of Each Segment/Wave:

1. P-wave:
- Represents depolarisation of the atria (spread of electrical impulse from SAN over both atria).
- Results in atrial contraction (atrial systole).
- Small, rounded, upward deflection.

2. QRS Complex:
- Represents depolarisation of the ventricles (spread of impulse through Bundle of His and Purkinje fibres to ventricular muscle).
- Results in ventricular contraction (ventricular systole).
- Q: small downward deflection; R: tall upward peak; S: small downward deflection.
- The atrial repolarisation is hidden within this complex.

3. T-wave:
- Represents repolarisation of the ventricles (recovery of ventricular muscle).
- Results in ventricular relaxation (ventricular diastole).
- Broad, dome-shaped, upward deflection.

4. P-Q (P-R) Interval:
- Time from start of P-wave to start of QRS complex.
- Represents the time taken for impulse to travel from SAN to ventricles (includes AVN delay).
- Normal duration: 0.12–0.20 seconds.

5. S-T Segment:
- Represents the period when ventricles are in a sustained contracted state (plateau phase).
- Normally isoelectric (flat).
- Elevation or depression of S-T segment indicates myocardial infarction or ischaemia.

6. Q-T Interval:
- Represents the total time of ventricular activity (depolarisation + repolarisation).

Clinical Significance of ECG:
- Helps diagnose arrhythmias, myocardial infarction (heart attack), hypertrophy, electrolyte imbalances, and other cardiac conditions.
- Each cardiac cycle produces one complete PQRST pattern; a healthy person shows 72 such cycles per minute.

Conclusion: ECG is a vital diagnostic tool that records the electrical events of the heart, with each wave corresponding to a specific event in the cardiac cycle.