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The conservation of bicarbonate ions in the renal system

Describe the conservation of bicarbonate ions in the renal system and compare it to the control of blood carbonic acid levels through the respiratory system.
Explain how the stomach is protected from self-digestion and why this is necessary.
Compare and contrast the conducting and respiratory zones.
A smoker develops damage to several alveoli that then can no longer function. How does this affect gas exchange?
Explain how the enteric nervous system supports the digestive system. What might occur that could result in the autonomic nervous system having a negative impact on digestion?

Sample Answer

1. Bicarbonate Conservation and Blood CO2 Control

Renal Bicarbonate Conservation:

The kidneys play a crucial role in maintaining blood pH by conserving bicarbonate ions (HCO3-). Here’s the process:

Glomerular Filtration: Blood plasma is filtered in the glomerulus, allowing most HCO3- to pass into the renal filtrate.
Reabsorption: Most filtered HCO3- is reabsorbed back into the bloodstream in the proximal tubules. This involves converting CO2 and water into carbonic acid (H2CO3) by the enzyme carbonic anhydrase. HCO3- is then reabsorbed along with Na+ ions.
Secretion of Hydrogen Ions (H+): The proximal tubule secretes H+ ions into the filtrate, binding with filtered HCO3- to form H2CO3. This allows for further HCO3- reabsorption.

Full Answer Section

Ammonium Excretion: Excess acid is excreted as ammonium (NH4+), further aiding HCO3- conservation.

Respiratory CO2 Control:

The lungs regulate blood CO2 levels through respiration. Here’s the process:

Cellular Metabolism: Cellular respiration produces CO2 as a waste product.
Diffusion: CO2 diffuses from cells into the bloodstream.
CO2 Transport: CO2 is transported in the blood, some dissolved directly and some bound to hemoglobin as carbaminohemoglobin.
Gas Exchange: CO2 diffuses from the blood into the alveoli and is exhaled.

Comparison:

Focus: Kidneys conserve HCO3-, while lungs control blood CO2 levels.
Mechanism: Kidneys use reabsorption and secretion, while lungs rely on diffusion and gas exchange.
Regulation: Kidneys regulate HCO3- based on blood pH and acid load, while lungs regulate CO2 based on blood CO2 concentration and respiratory rate.

2. Protection of the Stomach

The stomach lining is protected from self-digestion by several mechanisms:

Mucus Layer: A thick layer of mucus produced by epithelial cells acts as a barrier, separating digestive enzymes (pepsin) and stomach acid (hydrochloric acid) from the stomach lining.
Epithelial Cell Renewal: Stomach lining cells constantly regenerate, replacing damaged cells and maintaining the protective barrier.
Bicarbonate Secretion: Stomach epithelial cells secrete bicarbonate (HCO3-) which neutralizes stomach acid in the vicinity of the cells, protecting them from damage.
Tight Junctions: Tight junctions between epithelial cells prevent digestive enzymes and acid from leaking into the underlying tissues.
Blood Flow: Good blood flow to the stomach lining delivers oxygen and nutrients for repair and promotes removal of waste products.

Importance of Protection:

Without these protective mechanisms, stomach acid and enzymes would damage the stomach lining, leading to gastritis, ulcers, and even bleeding.

3. Conducting vs. Respiratory Zones

Feature	Conducting Zone	Respiratory Zone
Function	Transports air to and from the respiratory zone	Gas exchange between air and blood occurs
Structures	Includes nasal cavity, pharynx, trachea, bronchi, and bronchioles	Includes terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli
Cartilage	Supported by cartilage rings or plates to maintain an open airway	Less cartilage for maximum surface area for gas exchange
Epithelium	Pseudostratified ciliated epithelium with goblet cells (secretion of mucus)	Simple squamous epithelium for efficient gas diffusion
Blood Supply	Less blood supply	Dense network of capillaries surrounding alveoli for gas exchange
Muscles	Smooth muscle for bronchoconstriction and bronchodilation	Lack of smooth muscle for passive gas exchange

4. Smoker’s Damaged Alveoli and Gas Exchange

Damage to alveoli in a smoker’s lungs can significantly impact gas exchange:

Reduced Surface Area: Damaged alveoli have a decreased surface area, reducing the space available for gas diffusion.
Scarring: Scar tissue formation further hinders gas exchange.
Reduced Diffusion Capacity: The damaged alveolar walls impede the diffusion of oxygen from the air into the bloodstream and carbon dioxide in the opposite direction.

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