Hormones are carried by the blood throughout the entire body, yet they affect only certain cells.  The specific cells that respond to a given hormone have receptor sites for that hormone.  

This is sort of a lock and key mechanism.  If the key fits the lock, then the door will open.  If a hormone fits the receptor site, then there will be an effect.  If a hormone and a receptor site do not match, then there is no reaction.  All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone.  In some cases, the target tissue is localized in a single gland or organ.  In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.  

Hormones bring about their characteristic effects on target cells by modifying cellular activity.  Cells in a target tissue have receptor sites for specific hormones.  Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.

In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell.  The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm.  Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP.  This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity.  The protein hormone, which reacts at the cell membrane, is called the first messenger.  c Amp that brings about the action attributed to the hormone is called the second messenger.  This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.  

Carbon Dioxide Transport

Carbon dioxide (CO₂) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H⁺) and a bicarbonate ions:

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻

95% of the CO₂ generated in the tissues is carried in the red blood cells:

• It probably enters (and leaves) the cell by diffusing through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is the D antigen that is the most important factor in the Rh system of blood groups.)
• Once inside, about one-half of the CO₂ is directly bound to hemoglobin (at a site different from the one that binds oxygen).
• The rest is converted — following the equation above — by the enzyme carbonic anhydrase into
  • bicarbonate ions that diffuse back out into the plasma and
  • hydrogen ions (H⁺) that bind to the protein portion of the hemoglobin (thus having no effect on pH).

Only about 5% of the CO₂ generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO₂ we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)

When the red cells reach the lungs, these reactions are reversed and CO₂ is released to the air of the alveoli.

 Acute Obstructive Disorders
 1.    Heimlich maneuver
 2.    Bypass, tracheostomy w/catheter to suck up secretion

Each hormone in the body is unique.  Each one is different in it's chemical composition, structure, and action.  With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.

 Amines- these simple hormones are  structural variation of the amino acid tyrosine.  This group includes thyroxine from the thyroid gland and epinephrine and norepinephrine from the adrenal medulla.

Proteins- these hormones are chains of amino acids.  Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins.  Short chains of amino acids are called peptides.  Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.

Steroids- cholesterol is the precursor for the steroid hormones, which include cortisol and aldosterone from the adrenal cortex, estrogen and progesterone from the ovaries, and testosterone from the testes.

Bile - produced in the liver and stored in the gallbladder, released in response to CCK . Bile salts (salts of cholic acid) act to emulsify fats, i.e. to split them so that they can mix with water and be acted on by lipase.

Pancreatic juice: Lipase - splits fats into glycerol and fatty acids. Trypsin, and chymotrypsin - protease enzymes which break polypeptides into dipeptides. Carboxypeptidase - splits dipeptide into amino acids. Bicarbonate - neutralizes acid. Amylase - splits polysaccharides into shorter chains and disaccharides.

Intestinal enzymes (brush border enzymes): Aminopeptidase and carboxypeptidase - split dipeptides into amino acids. Sucrase, lactase, maltase - break disaccharides into monosaccharides. Enterokinase - activates trypsinogen to produce trypsin. Trypsin then activates the precursors of chymotrypsin and carboxypeptidase. Other carbohydrases: dextrinase and glucoamylase. These are of minor importance.

Sensory pathways include only those routes which conduct information to the conscious cortex of the brain. However, we will use the term in its more loosely and commonly applied context to include input from all receptors, whether their signals reach the conscious level or not.