Bile contains:

• bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.

• bile pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.

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.

Excitability ( Bathmotropism ) : Excitability means the ability of cardiac muscle to respond to signals. Here we are talking about contractile muscle cells that are excited by the excitatory conductive system and generate an action potential.

Cardiac action potential is similar to action potential in nerve and skeletal muscle tissue , with one difference , which is the presence of plateau phase . Plateau phase is unique for cardiac muscle cells .
The  resting membrane potential for cardiac muscle is about -80 mV.
When the cardiac muscle is stimulated an action potential is generated . The action potential in cardiac muscle is composed of four phases , which are :

1. Depolarization phase (Phase 0 ) :

A result of opening of sodium channels , which increase the permeability to sodium , which will lead to a rapid sodium influx into the cardiac muscle cell.

2. Repolarization : Repolarization in cardiac muscle is slow and triphasic :

a. Phase 1 (early partial repolarization ) : A small fast repolarization , results from potassium eflux and chloride influx.
b. Phase 2 ( Plateau ) : After the early partial depolarization , the membrane remains  depolarized , exhibiting a plateau , which is a unique phase for the cardiac muscle cell. Plateau is due to opening of slow calcium-sodium channels , delay closure of sodium channels , and to decreased potassium eflux.
c. Phase 3  ( Rapid repolarization) :  opening of potassium channels and rapid eflux of potassium.
d. Phase 4 ( Returning to resting level) in other words : The phase of complete repolarization. This due to the work of sodium-potassium pump.

Absolute refractory period:

Coincides wit phase 0,phase1 , and phase 2 . During this period , excitability of the heart is totally abolished . This prevents tetanization of the cardiac muscle and enables the heart to contract and  relax to be filled by blood ..

Relative refractory period : 

Coincides with the rapid repolarization and allows the excitability to be gradually recovered .
Excitation contraction relationship : Contraction of cardiac muscle starts after depolarization and continues about 1.5 time as long as the duration of the action potential and reaches its maximum at the end of the plateau. Relaxation of the muscle starts with the early partial repolarization.

Factors , affecting excitability of cardiac muscle:

I. Positive bathmotropic effect :

1. Sympathetic stimulation : It increase the heart , and thus reduces the duration of the action potentia; . This will shorten the duration of the absolute refractory period , and thus increase the excitability .
2.  Drugs : Catecholamines and  xanthines derivatives .
3. Mild hypoxia and mild ischemia
4. Mild hyperkalemia as it decreases the K+ efflux and opens excess Na+ channels .
5. Hypocalcemia

II. Negative bathmotropic effect :

1. Parasympathetic stimulation: The negative bathmotropic effect is limited to the atrial muscle excitability , because there is no parasympathetic innervation for the ventricles. Parasympathetic stimulation decreases the heart rate , and thus increases the duration of cardiac action potential and thus increases the duration of the absolute refractory period.
2. moderate to severe hypoxia
3. hyponatremia , hypercalcemia , and severe hyperkalemia.

Clinical Physiology : Extrasystole is a pathological situation , due to abnormal impulses , arising from ectopic focus .It is expressed as an abnormal systole that occur during the early diastole .
Extrasystole  is due to a rising of excitability above the normal , which usually occurs after the end of the relative refractory period ( read about staircase phenomenon of Treppe)

Water: comprises 60 - 90% of most living organisms (and cells) important because it serves as an excellent solvent & enters into many metabolic reactions

• Intracellular (inside cells) = ~ 34 liters
• Interstitial (outside cells) = ~ 13 liters
• Blood plasma = ~3 liters

40% of blood is red blood cells (RBCs)

plasma is similar to interstitial fluid, but contains plasma proteins

serum = plasma with clotting proteins removed

intracellular fluid is very different from interstitial fluid (high K concentration instead of high Na concentration, for example)

• Capillary walls (1 cell thick) separate blood from interstitial fluid
• Cell membranes separate intracellular and interstitial fluids

• Loss of about 30% of body water is fatal

Ions = atoms or molecules with unequal numbers of electrons and protons:

• found in both intra- & extracellular fluid
• examples of important ions include sodium, potassium, calcium, and chloride

Ions (Charged Atoms or Molecules) Can Conduct Electricity

• Giving up electron leaves a + charge (cation)
• Taking on electron produces a - charge (anion)
• Ions conduct electricity
• Without ions there can be no nerves or excitability
  • Na⁺ and K⁺ cations  
  • Ca²⁺ and Mg²⁺ cations  control metabolism and trigger muscle contraction and secretion of hormones and transmitters

Na+ & K+ are the Major Cations in Biological Fluids

• High K+ in cells, high Na+ outside
• Ion gradients maintained by Na pump (1/3 of basal metabolism)
• Think of Na+ gradient as a Na+ battery- stored electrical energy
• K+ gradient forms a K+ battery
• Energy stored in Na+ and K+ batteries can be tapped when ions flow

• Na+ and K+ produce action potential of excitable cells

DNA (Deoxyribonucleic acid) - controls cell function via transcription and translation (in other words, by controlling protein synthesis in a cell)

Transcription - DNA is used to produce mRNA

Translation - mRNA then moves from the nucleus into the cytoplasm & is used to produce a protein . requires mRNA, tRNA (transfer RNA), amino acids, & a ribosome

tRNA molecule

• sequence of amino acids in a protein is determined by sequence of codons (mRNA). Codons are 'read' by anticodons of tRNAs & tRNAs then 'deliver' their amino acid.
• Amino acids are linked together by peptide bonds (see diagram to the right)
• As mRNA slides through ribosome, codons are exposed in sequence & appropriate amino acids are delivered by tRNAs. The protein (or polypeptide) thus grows in length as more amino acids are delivered.
• The polypeptide chain then 'folds' in various ways to form a complex three-dimensional protein molecule that will serve either as a structural protein or an enzyme.

Functional Divisions of the Nervous System:

1) The Voluntary Nervous System - (ie. somatic division) control of willful control of effectors (skeletal muscles) and conscious perception. Mediates voluntary reflexes.

2) The Autonomic Nervous System - control of autonomic effectors - smooth muscles, cardiac muscle, glands. Responsible for "visceral" reflexes