1. Bicarbonate Conservation vs. Respiratory CO2 Control:

Renal Bicarbonate Conservation:

• The kidneys play a crucial role in maintaining blood pH by regulating bicarbonate (HCO3-) levels.
• Most filtered bicarbonate (around 90%) is reabsorbed in the proximal tubules.
• This reabsorption involves exchanging H+ ions secreted by the renal tubules for NaHCO3- in the blood.
• Carbonic anhydrase in the proximal tubule cells converts CO2 and water into carbonic acid (H2CO3), which dissociates back into H+ and HCO3-.
• The NaHCO3- is reabsorbed back into the blood, while the H+ is secreted into the urine.
• This process conserves bicarbonate, preventing its loss in the urine and helping maintain blood pH balance.

Respiratory CO2 Control:

• The lungs regulate blood CO2 levels, which directly affect carbonic acid (H2CO3) concentration.
• Increased CO2 levels in the blood stimulate the respiratory center in the brainstem.
• This triggers an increase in breathing rate and depth, promoting CO2 removal from the body.
• As CO2 is exhaled, blood CO2 levels decrease, leading to a decrease in H2CO3 and a shift in blood pH towards less acidic.

Key Differences:

• Bicarbonate conservation by the kidneys regulates the amount of bicarbonate available for buffering, while respiratory control focuses on CO2 removal to indirectly affect blood pH.
• Renal mechanisms are slower but more precise, while respiratory control is faster but less precise.

2. Stomach Protection from Self-Digestion:

• The stomach is lined with a protective mucous layer produced by epithelial cells.
• This mucus acts as a barrier, preventing digestive enzymes (pepsin) and stomach acid (hydrochloric acid) from damaging the stomach lining.
• Bicarbonate ions secreted by epithelial cells also neutralize stomach acid, further protecting the stomach wall.
• Tight junctions between epithelial cells prevent digestive juices from leaking into the underlying tissue.
• Blood flow to the stomach lining helps deliver nutrients and oxygen for repair and regeneration.

Necessity of Protection:

• Self-digestion would damage the stomach wall, leading to ulcers, bleeding, and potentially life-threatening complications.
• The stomach needs to be able to break down ingested food without being harmed by its own powerful digestive enzymes and acid.

3. Conducting vs. Respiratory Zones:

Conducting Zone:

• Function: Transports inhaled air from the nose/mouth to the respiratory zone (alveoli).
• Structures: Includes the nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles.
• Features: Lined with ciliated epithelium that traps dust particles and mucus to prevent them from reaching the alveoli.
• Warms, humidifies, and filters inhaled air.
• Does not participate in gas exchange.

Respiratory Zone:

• Function: Site of gas exchange between blood and inhaled/exhaled air.
• Structures: Alveoli (tiny air sacs with a large surface area).
• Features: Thin epithelium with a rich network of capillaries for efficient gas exchange.
• Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli for exhalation.

4. Smoker’s Damaged Alveoli and Gas Exchange:

• Damaged alveoli lose their elasticity and surface area, hindering gas exchange.
• Diffusion of oxygen from the alveoli into the blood becomes less efficient.
• This reduces the amount of oxygen delivered to tissues throughout the body.
• Carbon dioxide removal from the blood may also be impaired.
• These effects can lead to shortness of breath, fatigue, and respiratory problems.

5. Enteric Nervous System and Digestion:

• The enteric nervous system (ENS) is an intricate network of nerves embedded within the digestive system walls.
• It controls various digestive functions like:
  • Muscle movement (peristalsis) for propelling food through the digestive tract.
  • Secretion of digestive enzymes and juices.
  • Blood flow regulation in the digestive organs.
  • Absorption of nutrients.
• The ENS functions largely independently but can also communicate with the central nervous system.

Autonomic Nervous System’s Negative Impact:

• The autonomic nervous system (ANS) can influence digestion through sympathetic and parasympathetic pathways.
• Stress, anxiety, or fear can activate the sympathetic nervous system, leading to:
  • Decreased blood flow to the digestive tract, hindering nutrient absorption.
  • Reduced digestive enzyme secretion, slowing down digestion.
  • Relaxation of intestinal muscles, potentially leading to constipation.
• Conversely, the parasympathetic nervous system promotes digestion but can sometimes cause excessive gastric secretions or heartburn.