Chemical Controls of Respiration

A.    Chemoreceptors (CO2, O2, H+)

1.    central chemoreceptors - located in the medulla
2.    peripheral chemoreceptors - large vessels of neck

B.    Carbon Dioxide Effects

1.    a powerful chemical regulator of breathing by increasing H+ (lowering pH)
    
a. hypercapnia            Carbon Dioxide increases -> 
                        Carbonic Acid increases ->
                        pH of CSF decreases (higher H+)- >
                        
DEPTH & RATE increase (hyperventilation)

b. hypocapnia - abnormally low Carbon Dioxide levels which can be produced by excessive hyperventilation; breathing into paper bag increases blood Carbon Dioxide levels

C.     Oxygen Effects

1.    aortic and carotid bodies - oxygen chemoreceptors

2.    slight Ox decrease - modulate Carb Diox receptors
3.    large Ox decrease - stimulate increase ventilation
4.    hypoxic drive - chronic elevation of Carb Diox (due to disease) causes Oxygen levels to have greater effect on regulation of breathing

D.    pH Effects (H+ ion)

1.    acidosis - acid buildup (H+) in blood, leads to increased RATE and DEPTH (lactic acid)

E.    Overview of Chemical Effects

 Chemical                             Breathing Effect

increased Carbon Dioxide (more H+)     increase
decreased Carbon Dioxide (less H+)     decrease

slight decrease in Oxygen             effect CO2 system
large decrease in Oxygen             increase ventilation

decreased pH (more H+)                 increase
increased pH (less H+)                 decrease

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

A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.

Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.

Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.

Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.

Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects 

Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.

1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.

2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.

3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.

4) Ventricular repolarization occurs.

• The Autonomic Nervous System (ANS) Controls the Body's Internal Environment in a Coordinated Manner

• The ANS helps control the heart rate, blood pressure, digestion, respiration, blood pH and other bodily functions through a series of complex reflex actions
• These controls are done automatically, below the conscious level
• To exert this control the activities of many different organs must be coordinated so they work to accomplish the same goal
• In the ANS there are 2 nerves between the central nervous system (CNS) and the organ. The nerve cell bodies for the second nerve are organized into ganglia:
  • CNS -> Preganglionic nerve -> Ganglion -> Postganglionic nerve -> Organ
• At each junction neurotransmitters are released and carry the signal to the next nerve or organ.

• The ANS has 2 Divisions, Sympathetic and Parasympathetic

   

• Comparison of the 2 systems:

• 	Anatomical Location	Preganglionic Fibers	Postganglionic Fibers	Transmitter (Ganglia)	Transmitter (Organs)
  Sympathetic	Thoracic/ Lumbar	Short	Long	ACh	NE
  Parasympathetic	Cranial/ Sacral	Long	Short	ACh	ACh

   

  The Sympathetic is the "Fight or Flight" Branch of the ANS

• Emergency situations, where the body needs a sudden burst of energy, are handled by the sympathetic system
• The sympathetic system increases cardiac output and pulmonary ventilation, routes blood to the muscles, raises blood glucose and slows down digestion, kidney filtration and other functions not needed during emergencies
• Whole sympathetic system tends to "go off" together
• In a controlled environment the sympathetic system is not required for life, but it is essential for any stressful situation

• The Parasympathetic is the Rest and Digest Branch of the ANS

• The parasympathetic system promotes normal maintenance of the body- acquiring building blocks and energy from food and getting rid of the wastes
• It promotes secretions and mobility of different parts of the digestive tract.
• Also involved in urination, defecation.
• Does not "go off" together; activities initiated when appropriate
• The vagus nerve (cranial number 10) is the chief parasympathetic nerve
• Other cranial parasympathetic nerves are: III (oculomotor), VII (facial) and IX (glossopharyngeal)

• The Hypothalamus Has Central Control of the ANS

• The hypothalamus is involved in the coordination of ANS responses,
• One section of the hypothalamus seems to control many of the "fight or flight" responses; another section favors "rest and digest" activities

• The Adrenal Medulla is an Extension of the Sympathetic Nervous System

• The adrenal medulla behaves like a combined autonomic ganglion and postsynaptic sympathetic nerve (see diagram above)
• Releases both norepinephrine and epinephrine in emergency situations
  • Releases a mixture of epinephrine (E = 80%) and norepinephrine (NE = 20%)
  • Epinephrine = adrenaline
• This action is under control of the hypothalamus

• Sympathetic & Parasympathetic Systems

• Usually (but not always) both sympathetic and parasympathetic nerves go to an organ and have opposite effects
• You can predict about 90% of the sympathetic and parasympathetic responses using the 2 phrases: "Fight or Flight" and "Rest and Digest".
• Special cases:
  • Occasionally the 2 systems work together: in sexual intercourse the parasympathetic promotes erection and the sympathetic produces ejaculation
  • Eye: the sympathetic response is dilation and relaxation of the ciliary muscle for far vision (parasympathetic does the opposite)
  • Urination: the parasympathetic system relaxes the sphincter muscle and promotes contraction of muscles of the bladder wall -> urination (sympathetic blocks urination)
  • Defecation: the parasympathetic system causes relaxation of the anal sphincter and stimulates colon and rectum to contract -> defecation (sympathetic blocks defecation)

• Organ	Parasympathetic Response "Rest and Digest"	Sympathetic Response "Fight or Flight"
  Heart (baroreceptor reflex)	Decreased heart rate Cardiac output decreases	Increased rate and strength of contraction Cardiac output increases
  Lung Bronchioles	Constriction	Dilation
  Liver Glycogen	No effect	Glycogen breakdown Blood glucose increases
  Fat Tissue	No effect	Breakdown of fat Blood fatty acids increase
  Basal Metabolism	No effect	Increases ~ 2X
  Stomach	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
  Intestine	Increased secretion of HCl & digestive enzymes Increased motility	Decreased secretion Decreased motility
  Urinary bladder	Relaxes sphincter Detrusor muscle contracts Urination promoted	Constricts sphincter Relaxes detrusor Urination inhibited
  Rectum	Relaxes sphincter Contracts wall muscles Defecation promoted	Constricts sphincter Relaxes wall muscles Defecation inhibited
  Eye	Iris constricts Adjusts for near vision	Iris dilates Adjusts for far vision
  Male Sex Organs	Promotes erection	Promotes ejaculation

   

Functions

Manufacture - blood proteins - albumen, clotting proteins , urea - nitrogenous waste from amino acid metabolism , bile - excretory for the bile pigments, emulsification of fats by bile salts

Storage - glycogen , iron - as hemosiderin and ferritin , fat soluble vitamins A, D, E, K

Detoxification -alcohol , drugs and medicines , environmental toxins

Protein metabolism -

• transamination - removing the amine from one amino acid and using it to produce a different amino acid. The body can produce all but the essential amino acids; these must be included in the diet.
• deamination - removal of the amine group in order to catabolize the remaining keto acid. The amine group enters the blood as urea which is excreted through the kidneys.

Glycemic Regulation - the management of blood glucose.

• glycogenesis - the conversion of glucose into glycogen.
• glycogenolysis - the breakdown of glycogen into glucose.

gluconeogenesis - the manufacture of glucose from non carbohydrate sources, mostly protein

Events in gastric function:

1) Signals from vagus nerve begin gastric secretion in cephalic phase.

2) Physical contact by food triggers release of pepsinogen and H⁺ in gastric phase.

3) Muscle contraction churns and liquefies chyme and builds pressure toward pyloric sphincter.

4) Gastrin is released into the blood by cells in the pylorus. Gastrin reinforces the other stimuli and acts as a positive feedback mechanism for secretion and motility.

5) The intestinal phase begins when acid chyme enters the duodenum. First more gastrin secretion causes more acid secretion and motility in the stomach.

6) Low pH inhibits gastrin secretion and causes the release of enterogastrones such as GIP into the blood, and causes the enterogastric reflex. These events stop stomach emptying and allow time for digestion in the duodenum before gastrin release again stimulates the stomach.