Describe in brief on any five. a) Write short note on lymph. [3] b) Write down the fate of mesoderm. [3] c) Discuss CO₂ transport. [3] d) Explain working of heart. [3] e) Differentiate between…

(a) Short Note on Lymph

Lymph is a transparent, pale yellow fluid derived from interstitial fluid that enters lymphatic capillaries. It plays a crucial role in the immune system and circulatory processes.

Composition of Lymph:

• Cells: Lymphocytes (T-cells, B-cells, NK cells), macrophages, and dendritic cells.
• Proteins: Plasma proteins (e.g., immunoglobulins, albumin) and enzymes.
• Other Components: Water, salts, fats (absorbed from the digestive tract via lacteals), and metabolic waste products.

Functions of Lymph:

1. Immune Defense:
   • Lymphocytes and macrophages in lymph nodes filter out pathogens (bacteria, viruses, cancer cells).
   • Antibody production (B-cells) and cell-mediated immunity (T-cells).
2. Transport of Fats:
   • Absorbs dietary fats (chylomicrons) from the small intestine via lacteals and transports them to the bloodstream.
3. Fluid Balance:
   • Returns excess interstitial fluid to the bloodstream, preventing edema.
4. Waste Removal:
   • Transports metabolic waste products to the blood for excretion.

Lymphatic System Pathways:

• Lymphatic capillaries → Lymphatic vessels → Lymph nodes (filtration) → Lymphatic trunks → Thoracic duct (left side) or Right lymphatic duct (right side) → Subclavian veins (returns to blood).

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(b) Fate of Mesoderm

The mesoderm is the middle germ layer in triploblastic embryos and gives rise to diverse tissues and organs. Its differentiation follows three main regions:

1. Paraxial Mesoderm:

• Forms somites (segmented blocks):
  • Sclerotome: Develops into vertebrae, ribs, and intervertebral discs.
  • Myotome: Forms skeletal muscles (e.g., back, limb muscles).
  • Dermatome: Contributes to the dermis of the skin.
• Head mesoderm: Forms muscles of the head, neck, and pharyngeal arches.

2. Intermediate Mesoderm:

• Gives rise to the urogenital system:
  • Kidneys (metanephros), ureters, gonads (testes/ovaries), and associated ducts.

3. Lateral Plate Mesoderm:

• Splits into two layers:
  • Somatic layer: Forms the body wall (muscles, bones of limbs, dermis).
  • Splanchnic layer: Forms the visceral organs (heart, blood vessels, connective tissue of the digestive and respiratory tracts).

4. Extraembryonic Mesoderm:

• Contributes to placental membranes (amnion, chorion, allantois) and supports fetal development.

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(c) CO₂ Transport

Carbon dioxide (CO₂) produced during cellular respiration is transported in the blood in three main forms:

Carbon dioxideCarbon dioxide

1. Dissolved CO₂ (7–10%):

• CO₂ is slightly soluble in plasma (~0.03 mL/dL/mmHg).
• Diffuses from tissues to blood and is carried to the lungs.

2. Bicarbonate Ions (HCO₃⁻) (70–75%):

• Reaction in RBCs (catalyzed by carbonic anhydrase): $${\text{CO}}_2+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2{\text{CO}}_3\rightleftharpoons {\text{H}}^{+}+{\text{HCO}}_3^{-}$$
• Chloride Shift: HCO₃⁻ exits RBCs into plasma; Cl⁻ enters to maintain electroneutrality.
• In Lungs: Reaction reverses to release CO₂ for exhalation.

3. Carbaminohemoglobin (20–23%):

• CO₂ binds to amino groups of hemoglobin (not the heme group): $${\text{Hb-NH}}_2+{\text{CO}}_2\to \text{Hb-NH-COOH}$$
• Bohr Effect: Lower pH (higher CO₂) reduces hemoglobin’s O₂ affinity, facilitating O₂ unloading in tissues.

Factors Affecting CO₂ Transport:

• Partial Pressure (PCO₂): Higher in tissues, lower in lungs (drives diffusion).
• Temperature: Higher temperatures increase CO₂ release (e.g., active tissues).
• pH: Acidic conditions (e.g., exercise) enhance CO₂ transport via bicarbonate.

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(d) Working of the Heart

The heart functions as a double pump with four chambers (two atria, two ventricles) and follows the cardiac cycle (systole and diastole).

Mechanism:

1. Atrial Systole (0.1 s):

   • Atria contract → Blood flows into ventricles (AV valves open).
   • Pressure: Atria > Ventricles.

2. Ventricular Systole (0.3 s):

   • AV valves close (lub sound) → Ventricles contract.
   • Pressure: Ventricles > Aorta/Pulmonary artery → Semilunar valves open → Blood ejected.
   • Systemic circuit: Left ventricle → Aorta → Body.
   • Pulmonary circuit: Right ventricle → Pulmonary artery → Lungs.

3. Diastole (0.4 s):

   • Heart relaxes → Semilunar valves close (dub sound).
   • Blood flows from veins into atria → AV valves open → Ventricles fill passively.

Conduction System:

• SA Node (Pacemaker): Initiates impulse (70–80 bpm).
• AV Node: Delays impulse → Ventricular contraction.
• Bundle of His & Purkinje Fibers: Distributes impulse to ventricles.

Heart Sounds:

• First Heart Sound (Lub): Closure of AV valves (start of systole).
• Second Heart Sound (Dub): Closure of semilunar valves (start of diastole).

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(e) Sympathetic vs. Parasympathetic Nervous System

GlucoseGlucose

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(f) Significance of Surrogacy

Surrogacy is a reproductive technique where a woman (surrogate) carries and delivers a child for another couple or individual. Its significance includes:

1. Medical Benefits:

• For Infertile Couples: Enables parents with uterine issues (e.g., absent/receptive uterus, recurrent miscarriages) to have biological children.
• Genetic Disorders: Allows screening of embryos for hereditary diseases (e.g., via IVF + PGT).
• Same-Sex Couples/LGBTQ+: Provides biological parenthood options where traditional methods fail.

2. Ethical and Social Considerations:

• Altruistic Surrogacy: Surrogates may donate embryos without compensation (ethical debates on exploitation).
• Commercial Surrogacy: Regulated in some countries (e.g., Nepal, India) but banned in others (e.g., UK, France) due to ethical concerns.
• Legal Clarity: Ensures parental rights for intended parents (varies by jurisdiction; e.g., Nepal’s Surrogacy Regulation Act, 2075).

3. Psychological Impact:

• For Intended Parents: Reduces emotional stress of infertility; fulfills desire for genetic lineage.
• For Surrogates: May provide financial stability but risks emotional attachment or health complications.

4. Scientific Advancements:

• IVF + Surrogacy: Combines with techniques like ICSI (intracytoplasmic sperm injection) for male infertility.
• Gestational vs. Traditional Surrogacy:
  • Gestational: Surrogate has no genetic link (embryo from intended parents).
  • Traditional: Surrogate is genetically related (donor sperm + surrogate’s egg).

5. Challenges:

• Legal Loopholes: Disputes over parental rights (e.g., cases where surrogates refuse to relinquish the child).
• Health Risks: Surrogates may face gestational diabetes, preeclampsia, or cesarean complications.
• Exploitation: Vulnerable women in developing countries (e.g., Nepal) may face coercion or inadequate medical care.

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(g) Mode of Transmission of Hepatitis B

Hepatitis B is a viral liver infection caused by the Hepatitis B Virus (HBV). Its transmission occurs via:

1. Bloodborne Transmission (Primary Route):

• Direct Contact with Infected Blood:
  • Sharing needles/syringes (IV drug use, tattoos, piercings).
  • Blood transfusions (rare in screened blood banks).
  • Needlestick injuries (healthcare workers).
• Vertical Transmission (Mother-to-Child):
  • During childbirth (90% risk if mother is HBeAg-positive).
  • Perinatal exposure (placental or breast milk transmission is rare but possible).

2. Sexual Transmission:

• Unprotected sexual contact with an infected partner.
• High-risk behaviors (multiple partners, STIs).

3. Mother-to-Infant Transmission:

• Perinatal: During vaginal delivery (viral load in birth canal).
• Postnatal: Breastfeeding is low-risk unless nipples are cracked/bleeding.

4. Other Routes (Less Common):

• Saliva/Body Fluids: Rare, but possible via open wounds (e.g., sharing razors, toothbrushes).
• Organ Transplants: If donor organs are HBV-positive (screened in modern medicine).

Prevention Strategies:

• Vaccination: Hepatitis B vaccine (3-dose series) is 95% effective (recommended for all infants and high-risk groups).
• Safe Practices:
  • Sterile needles, condoms, and screening of blood products.
• Post-Exposure Prophylaxis (PEP): HBV immunoglobulin (HBIG) + vaccine for exposed individuals (e.g., healthcare workers).

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