NEET MDS Lessons
Physiology
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.
Membrane Structure & Function
Cell Membranes
- Cell membranes are phospholipid bilayers (2 layers)
- Bilayer forms a barrier to passage of molecules in an out of cell
- Phospholipids = glycerol + 2 fatty acids + polar molecule (i.e., choline) + phosphate
- Cholesterol (another lipid) stabilizes cell membranes
- the hydrophobic tails of the phospholipids (fatty acids) are together in the center of the bilayer. This keeps them out of the water
Membranes Also Contain Proteins
- Proteins that penetrate the membrane have hydrophobic sections ~25 amino acids long
- Hydrophobic = doesn't like water = likes lipids
- Membrane proteins have many functions:
- receptors for hormones
- pumps for transporting materials across the membrane
- ion channels
- adhesion molecules for holding cells to extracellular matrix
cell recognition antigens
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.
Basic Properties of Gases
A. Dalton's Law of Partial Pressures
1. partial pressure - the "part" of the total air pressure caused by one component of a gas
Gas Percent Partial Pressure (P)
ALL AIR 100.0% 760 mm Hg
Nitrogen 78.6% 597 mm Hg (0.79 X 760)
Oxygen 20.9% l59 mm Hg (0.21 X 760)
CO2 0.04% 0.3 mm Hg (0.0004 X 760)
2. altitude - air pressure @ 10,000 ft = 563 mm Hg
3. scuba diving - air pressure @ 100 ft = 3000 mm Hg
B. Henry's Law of Gas Diffusion into Liquid
1. Henry's Law - a certain gas will diffuse INTO or OUT OF a liquid down its concentration gradient in proportion to its partial pressure
2. solubility - the ease with which a certain gas will "dissolve" into a liquid (like blood plasma)
HIGHest solubility in plasma Carbon Dioxide
Oxygen
LOWest solubility in plasma Nitrogen
C. Hyperbaric (Above normal pressure) Conditions
1. Creates HIGH gradient for gas entry into the body
2. therapeutic - oxygen forced into blood during: carbon monoxide poisoning, circulatory shock, asphyxiation, gangrene, tetanus, etc.
3. harmful - SCUBA divers may suffer the "bends" when they rise too quickly and Nitrogen gas "comes out of solution" and forms bubbles in the blood
Tubular secretion:
Involves transfer of substances from peritubular capillaries into the tubular lumen. It involves transepithelial transport in a direction opposite to that of tubular absorption.
Renal tubules can selectively add some substances that have not been filtered to the substances that already have been filtered via tubular secretion.
Tubular secretion mostly function to eliminate foreign organic ions, hydrogen ions ( as a contribution to acid base balance ), potassium ions ( as a contribution to maintaining optimal plasma K+ level to assure normal proceeding of neural and muscular functions), and urea.
Here we will focus on K+ secretion and will later discuss H+ secretion in acid base balance, while urea secretion will be discussed in water balance.
K+ is filtered in glomerular capillaries and then reabsorbed in proximal convoluted tubules as well as in thick ascending limb of loop of Henley ( Na-2Cl-K symporter)
K+ secretion takes place in collecting tubules (distal nephron) . There are two types of cells in distal nephron:
- Principal cells that reabsorb sodium and secrete K+ .
- Intercalated cells that reabsorb K+ in exchange with H+.
Mechanism of secretion of K+ in principal cells : Two steps
- K+ enters tubular cells by Na/K ATPase on the basolateral membrane.
- K+ leaves the tubular cells via K+ channels in apical membrane.
Aldosterone is a necessary regulatory factor.
If there is increased level of K+ in plasma,excessive K+ is secreted , some of which is reabsorbed back to the plasma in exchange with H+ via the intercalated cells.
Respiration involves several components:
Ventilation - the exchange of respiratory gases (O2 and CO2) between the atmosphere and the lungs. This involves gas pressures and muscle contractions.
External respiration - the exchange of gases between the lungs and the blood. This involves partial pressures of gases, diffusion, and the chemical reactions involved in transport of O2and CO2.
Internal respiration - the exchange of gases between the blood and the systemic tissues. This involves the same processes as external respiration.
Cellular respiration - the includes the metabolic pathways which utilize oxygen and produce carbon dioxide, which will not be included in this unit.
Ventilation is composed of two parts: inspiration and expiration. Each of these can be described as being either quiet, the process at rest, or forced, the process when active such as when exercising.
Quiet inspiration:
The diaphragm contracts, this causes an increase in volume of the thorax and the lungs, which causes a decrease in pressure of the thorax and lungs, which causes air to enter the lungs, moving down its pressure gradient. Air moves into the lungs to fill the partial vacuum created by the increase in volume.
Forced inspiration:
Other muscles aid in the increase in thoracic and lung volumes.
The scalenes - pull up on the first and second ribs.
The sternocleidomastoid muscles pull up on the clavicle and sternum.
The pectoralis minor pulls forward on the ribs.
The external intercostals are especially important because they spread the ribs apart, thus increasing thoracic volume. It's these muscles whose contraction produces the "costal breathing" during rapid respirations.
Quiet expiration:
The diaphragm relaxes. The elasticity of the muscle tissue and of the lung stroma causes recoil which returns the lungs to their volume before inspiration. The reduced volume causes the pressure in the lungs to increase thus causing air to leave the lungs due to the pressure gradient.
Forced Expiration:
The following muscles aid in reducing the volume of the thorax and lungs:
The internal intercostals - these compress the ribs together
The abdominus rectus and abdominal obliques: internal obliques, external obliques- these muscles push the diaphragm up by compressing the abdomen.
Respiratory output is determined by the minute volume, calculated by multiplying the respiratory rate time the tidal volume.
Minute Volume = Rate (breaths per minute) X Tidal Volume (ml/breath)
Rate of respiration at rest varies from about 12 to 15 . Tidal volume averages 500 ml Assuming a rate of 12 breaths per minute and a tidal volume of 500, the restful minute volume is 6000 ml. Rates can, with strenuous exercise, increase to 30 to 40 and volumes can increase to around half the vital capacity.
Not all of this air ventilates the alveoli, even under maximal conditions. The conducting zone volume is about 150 ml and of each breath this amount does not extend into the respiratory zone. The Alveolar Ventilation Rate, AVR, is the volume per minute ventilating the alveoli and is calculated by multiplying the rate times the (tidal volume-less the conducting zone volume).
AVR = Rate X (Tidal Volume - 150 ml)
For a calculation using the same restful rate and volume as above this yields 4200 ml.
Since each breath sacrifices 150 ml to the conducting zone, more alveolar ventilation occurs when the volume is increased rather than the rate.
During inspiration the pressure inside the lungs (the intrapulmonary pressure) decreases to -1 to -3 mmHg compared to the atmosphere. The variation is related to the forcefulness and depth of inspiration. During expiration the intrapulmonary pressure increases to +1 to +3 mmHg compared to the atmosphere. The pressure oscillates around zero or atmospheric pressure.
The intrapleural pressure is always negative compared to the atmosphere. This is necessary in order to exert a pulling action on the lungs. The pressure varies from about -4 mmHg at the end of expiration, to -8 mmHg and the end of inspiration.
The tendency of the lungs to expand, called compliance or distensibility, is due to the pulling action exerted by the pleural membranes. Expansion is also facilitated by the action of surfactant in preventing the collapse of the alveoli.
The opposite tendency is called elasticity or recoil, and is the process by which the lungs return to their original or resting volume. Recoil is due to the elastic stroma of the lungs and the series elastic elements of the respiratory muscles, particularly the diaphragm.
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