Regulation of HCl Secretion in Sympathetic Nervous System

Respiratory System

• The respiratory system consists of four separate processes: pulmonary ventilation, external respiration, transport, and internal respiration.

Organs Involved

• Nose, nasal cavity, paranasal sinuses, pharynx, larynx, trachea, bronchi, bronchioles, lungs, and alveoli
• Muscles involved: diaphragm, external and internal intercostal muscles, and smooth muscle

Conducting Zone

• Nose to terminal bronchioles
• 23 series of branching in bronchioles

Respiratory Zone

• Surface area in lungs where gas exchange happens (respiratory bronchioles to alveoli)

Nose

• Functions:
  • Deflects particulate to mucus surfaces
  • Houses nasal conchae (superior, middle, and inferior)
  • Houses nasal meatuses (grooves under each concha) to increase turbulence and surface area
• Nasopharynx:
  • Pseudostratified ciliated columnar epithelium (respiratory mucosa)
  • Used to move mucus
  • Has some goblet cells
  • Cilia used to filter air and move contaminants to the back to be swallowed

Pharynx

• Includes:
  • Nasopharynx (nose area)
  • Oropharynx (mouth area)
  • Laryngopharynx (lower throat area)
• Mucosal lining changes along the length of the pharynx
• Oropharynx: stratified squamous epithelium
• Palatine and lingual tonsils
• Laryngopharynx: stratified squamous epithelium
• Passageway for food and air

Larynx

• Produces vocal sounds via vocal folds/cords
• The connection between the pharynx and the trachea
• Upper half: stratified squamous epithelium
• Lower half: pseudostratified ciliated columnar epithelium
• Three big cartilages: thyroid cartilage (Adam's apple), epiglottis, and cricoid

Trachea

• Descends into the mediastinum
• Pseudostratified ciliated columnar epithelium
• Carina: marks the division of the trachea into two primary bronchi

Bronchial Tree

• Primary, secondary, tertiary, and bronchioles
• Bronchi: simple columnar epithelium
• Bronchioles and terminal bronchioles: simple cuboidal epithelium
• Smooth muscle in bronchioles allows for widening when sympathetically stimulated and constricting under parasympathetic stimulation

Lungs

• Anatomy: left has 2 lobes, right has 3 lobes
• Cardiac notch: heart
• Pleurae: parietal pleura and visceral pleura
• Respiratory bronchioles: between terminal bronchioles and alveolar ducts
• Alveolar sacs: clusters of alveoli
• Alveoli: respiratory membrane made up of alveolar and capillary walls + their basal lamina (alveolar epithelium -> fused basement membrane -> capillary endothelium)
• Type 1 cells: simple squamous
• Type 2 cells: simple cuboidal
• Secrete a surfactant to prevent attraction of liquid molecules/reduce surface tension
• Surrounded by elastic fibers
• Have alveolar pores

Pulmonary Ventilation

• Physical air in and out of the lungs
• Lung volume changes during inspiration and exhalation
• Causes pressure change, which allows for gases to flow across the membrane to equalize pressure

External Respiration

• Gas exchange between the lungs and the environment
• Based on partial pressure gradients and gas solubility
• Alveolar ventilation and pulmonary perfusion
• Respiratory membrane structure

Dalton's Law

• Total pressure exerted by a mixture of gases is the sum of the pressure of the individual gases
• Partial pressure: the pressure of an individual gas from a mixture of gases

Henry's Law

• The more pressure you exert on a gas over a solution/liquid, the more the gas will dissolve proportionately into the liquid to its own partial pressure
• The amount also depends on how soluble the gas is

Fick's Law

• Law of diffusion
• Factors that affect the rate of diffusion:
  • Tissue surface area
  • Tissue thickness
  • Diffusion coefficient of the gas
  • ΔP (difference in partial pressure)

Internal Respiration

• Gas exchange between the blood and body tissues

Transport of Oxygen

• Hemoglobin: 98% saturated (never higher) if healthy
• 2% dissolved in plasma
• Males: 201 ml O2/Liter
• Females: 174 ml O2/Liter
• A saturated hemoglobin: all four heme groups have an O2 molecule
• Partially saturated: 1-3 heme groups have an O2 molecule
• Rate of hemoglobin binding and releasing O2 affected by:
  • pO2 (partial pressure of O2)
  • pCO2 (partial pressure of CO2)
  • Temperature
  • Blood pH (H+)
  • DPG (2,3-bisphosphoglycerate)
• Hemoglobin saturation curve: normal curve, pO2 40mmHg = 75% saturation, pO2 100mmHg = 98% saturation

Transport of Carbon Dioxide

• Transported in the blood via:
  • Dissolved in plasma (7-10%)
  • Bound to hemoglobin (20%)
  • Bicarbonate in plasma (most - 70%)
• CO2 diffuses into RBCs and combines with water to form carbonic acid, which dissociates to hydrogen and bicarbonate ions
• Chloride shift: bicarbonate quickly diffuses out of the RBC and into the plasma, and chloride ions move into the RBC for balance

Control of Respiration

• Medullary Respiratory DRG (Dorsal Respiratory Group)
  • Sets basic breathing rate
  • Controls muscles of inspiration and stops neural impulses during exhalation
• VRG (Ventral Respiratory Group)
  • Involved in forced breathing (inspiration and expiration)
• DRG and VRG use phrenic and intercostal nerves to communicate actions (efferent pathway)
• Pontine Respiratory Group: smooths breathing

Central Chemoreceptors

• Chemoreceptors in the medulla monitor levels of pCO2 in the CNS

• Signal to make sure enough pH isn't too much

• Rise of pCO2: the original stimulus, but the rate of breathing is actually dependent on the H+ ion concentration in the cerebrospinal fluid

• H+ and CO2 bind to the central chemoreceptor on the medulla, which then sends a signal to the respiratory control center

• Drop in pH results in pulmonary respiration### Intestinal Phase

• Partially digested food enters the duodenum, releasing hormones that control the rate of gastric emptying

• The presence of partially digested food, distension of the duodenum, low pH, and fatty, acidic, or hypertonic chyme inhibit gastric emptying

• Irritants in the duodenum also inhibit gastric emptying

Regulation of HCl Secretion

• HCl is stimulated by ACh, histamine, and gastrin through a G-protein coupled receptor (GPCR) 2nd-messenger system
• HCl is low if only one ligand binds to parietal cells
• HCl is high if all three ligands bind to parietal cells

Regulation of Gastric Emptying

• Gastric emptying is controlled by the enterogastric reflex and enterogastrone (hormonal mechanism)
• The enterogastric reflex inhibits gastric emptying in response to distension of the duodenum
• Hormonal mechanisms (enterogastrone) involve the secretion of hormones into the bloodstream to stop gastric emptying
• Carbohydrate-rich foods move quickly through the duodenum, while fat-rich foods are digested more slowly in the stomach

Gastrointestinal Hormones

• Gastrin is stimulated by protein and functions to increase HCl and pepsinogen secretion, enhance gastric motility, and help maintain the digestive tract lining
• Gastrin is inhibited by the accumulation of acid
• Secretin is stimulated by the presence of acid in the duodenum and functions to stimulate pancreatic duct cells to produce NaHCO3, stimulate the liver to produce NaCO3-rich bile, inhibit gastric emptying, and inhibit gastric secretion
• Secretin is also trophic to the exocrine pancreatic glands
• CCK (cholecystokinin) functions to inhibit gastric motility and secretion, stimulate pancreatic acinar cells to secrete pancreatic enzymes, and contract the gallbladder
• GIP (glucose-dependent insulinotropic peptide) functions to stimulate insulin release, inhibit gastric activity, and inhibit gastric secretion

Digestion and Absorption in the Stomach

• Preliminary digestion of proteins and carbohydrates occurs in the stomach
• Very little nutrient absorption occurs in the stomach, except for lipid-soluble drugs like alcohol

Small Intestine

• The glands of the duodenum function to moisten chyme, buffer acids, and maintain digestive material in solution
• Hormones secreted in the small intestine include secretin, CCK, and GIP

Liver

• The liver is composed of hepatocytes that produce bile, process blood-borne nutrients, store fat-soluble vitamins, and detoxify
• Bile is a yellow-green alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
• Bile salts are cholesterol that emulsify fat and facilitate fat and cholesterol absorption

Regulation of Bile Release

• Fatty and acidic chyme signals the duodenum to secrete CCK and secretin
• Secretin and bile salts stimulate the liver to produce more bile
• CCK stimulates the vagus nerve, causing gallbladder contraction
• The hepatopancreatic sphincter relaxes, allowing bile to be released into the duodenum

Gallbladder

• The gallbladder stores and concentrates bile by absorbing its water and ions
• The gallbladder releases bile via the cystic duct and bile duct

Pancreas

• The pancreas has endocrine functions, including maintaining blood glucose levels through the release of insulin and glucagon
• The pancreas has exocrine functions, including digestion and secretion of pancreatic juice
• Acini (secretory acinar cells) contain zymogen granules with digestive enzymes
• Pancreatic juice enters the duodenum

Regulation of Pancreatic Secretion

• Secretin and CCK are released when fatty and acidic chyme enters the duodenum
• CCK helps the pancreas secrete enzyme-rich pancreatic juice
• Secretin helps the pancreas secrete bicarbonate-rich pancreatic juice
• Vagal stimulation helps release pancreatic juice

Digestion in the Small Intestine

• When chyme enters the duodenum, proteins and carbohydrates are partially digested, and fat digestion has not started yet
• Mixing starts to occur with the combination of juices
• All nutrient absorption takes place in the small intestine

Control of Motility

• Enteric neurons of the GI tract coordinate intestinal movements
• Cholinergic neurons (from the parasympathetic nervous system) cause contractions and distension
• The gastroileal reflex and gastrin relax the ileocecal sphincter, allowing chyme to move to the large intestine

Large Intestine

• Barely any nutrients are left at this stage, as the rest are digested by bacteria
• Main functions of the large intestine include absorbing water and electrolytes, compacting remaining material to feces, absorbing vitamins produced by bacteria, and storing feces
• Defecation is regulated by distension in the rectal walls, which stimulates contraction of the rectal walls

Regulation of Digestion

• Intrinsic control involves autonomous smooth muscle pacesetter cells, intrinsic nerve plexuses, and sensory receptors
• Mechanoreceptors and chemoreceptors detect stimulus, such as stretch, osmolarity, pH, and presence of substrate or end products
• Intrinsic control initiates reflexes that activate or inhibit digestive glands and mix lumen contents

Extrinsic Control

• Extrinsic control arises from outside the GI tract, including the CNS, autonomic nervous system, and GI hormones
• Stimulus from outside the GI tract, such as seeing or smelling food, triggers a response in the CNS, which then sends signals to the local nerve plexus
• The local nerve plexus then creates a response, which is contraction or secretion of smooth muscle or glands