Cell, or Plasma, membrane

• Structure - 2 primary building blocks include

protein (about 60% of the membrane) and lipid, or

fat (about 40% of the membrane).

The primary lipid is called phospholipids, and molecules of phospholipid form a 'phospholipid bilayer' (two layers of phospholipid molecules). This bilayer forms because the two 'ends' of phospholipid molecules have very different characteristics: one end is polar (or hydrophilic) and one (the hydrocarbon tails below) is non-polar (or hydrophobic):

• Functions include:
  • supporting and retaining the cytoplasm
  • being a selective barrier .
  • transport
  • communication (via receptors)

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

Urine is a waste byproduct formed from excess water and metabolic waste molecules during the process of renal system filtration. The primary function of the renal system is to regulate blood volume and plasma osmolarity, and waste removal via urine is essentially a convenient way that the body performs many functions using one process. Urine formation occurs during three processes:

Filtration

Reabsorption

Secretion

Filtration

During filtration, blood enters the afferent arteriole and flows into the glomerulus where filterable blood components, such as water and nitrogenous waste, will move towards the inside of the glomerulus, and nonfilterable components, such as cells and serum albumins, will exit via the efferent arteriole. These filterable components accumulate in the glomerulus to form the glomerular filtrate.

Normally, about 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration; this is called the filtration fraction. The remaining 80% of the blood flows through the rest of the body to facilitate tissue perfusion and gas exchange.

Reabsorption

The next step is reabsorption, during which molecules and ions will be reabsorbed into the circulatory system. The fluid passes through the components of the nephron (the proximal/distal convoluted tubules, loop of Henle, the collecting duct) as water and ions are removed as the fluid osmolarity (ion concentration) changes. In the collecting duct, secretion will occur before the fluid leaves the ureter in the form of urine.

Secretion

During secretion some substances±such as hydrogen ions, creatinine, and drugs—will be removed from the blood through the peritubular capillary network into the collecting duct. The end product of all these processes is urine, which is essentially a collection of substances that has not been reabsorbed during glomerular filtration or tubular reabsorbtion.

 Pain, Temperature, and Crude Touch and Pressure

General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X.

The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively.

The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the spinal nucleus of V.

Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus.

Finally, third-order neurons project to the "face" area of the cerebral cortex in areas 3, 1, and 2 .

Discriminating Touch and Pressure

Signals are conducted from general somatic mechanoreceptors over GSA fibers of the trigeminal nerve into the principal sensory nucleus of V, located in the middle pons.

Second-order neurons then conduct the signals to the opposite side of the brainstem, where they ascend in the medial lemniscus to the VPM of the thalamus.

 Thalamic neurons then project to the "face" region of areas 3, I, and 2 of the cerebral cortex.

 Kinesthesia and Subconscious Proprioception

Proprioceptive input from the face is primarily conducted over GSA fibers of the trigeminal nerve.

The peripheral endings of these neurons are the general somatic mechanoreceptors sensitive to both conscious (kinesthetic) and subconscious proprioceptive input.

Their central processes extend from the mesencephalic nucleus to the principal sensory nucleus of V in the pons

The subconscious component is conducted to the cerebellum, while the conscious component travels to the cerebral cortex.

Certain second-order neurons from the principal sensory nucleus relay proprioceptive information concerning subconscious evaluation and integration into the ipsilateral cerebellum.

Other second-order neurons project to the opposite side of the pons and ascend to the VPM of the thalamus as the dorsal trigeminothalamic tract.

Thalamic projections terminate in the face area of the cerebral cortex.

Remember the following principles before proceeding :
- Reabsorption occurs for most of substances that have been previously filterd .
- The direction of reabsorption is from the tubules to the peritubular capillaries
- All of transport mechanism are used here.
- Different morphology of the cells of different parts of the tubules contribute to reabsorption of different substances .
- There are two routes of reabsorption: Paracellular and transcellular : Paracellular reabsorption depends on the tightness of the tight junction which varies from regeon to region in the nephrons .Transcellular depends on presence of transporters ( carriers and channels for example).

1. Reabsorption of glucose , amino acids , and proteins :

Transport of glucose occurs in the proximal tubule . Cells of proximal tubules are similar to those of the intestinal mucosa as the apical membrane has brush border form to increase the surface area for reabsorption , the cells have plenty of mitochondria which inform us that high amount of energy is required for active transport , and the basolateral membrane of the cells contain sodium -potassium pumps , while the apical membrane contains a lot of carrier and channels .

The tight junction between the tubular cells of the proximal tubules are not that (tight) which allow paracellular transport.
Reabsorption of glucose starts by active transport of  Na by the pumps on the basolateral membrane . This will create Na gradient which will cause Na to pass the apical membrane down its concentration gradient . Glucose also passes the membrane up its concentration gradient using sodium -glucose symporter as a secondary active transport.

The concentration of glucose will be increased in the cell and this will enable the glucose to pass down concentration gradient to the interstitium by glucose uniporter . Glucose will then pass to the peritubular capillaries by simple bulk flow.

Remember: Glucose reabsorption occurs via transcellular route .
          Glucose transport has transport maximum . In normal situation there is no glucose in the urine , but in uncontrolled diabetes mellitus patients glucose level exceeds its transport maximum (390 mg/dl) and thus will appear in urine .
                   
                   
                   
2. Reabsorption of Amino acids : Use secondary active transport mechanism like glucose.

3. Reabsorption of proteins : 

Plasma proteins are not filtered in Bowman capsule but some proteins and peptides in blood may pass the filtration membrane and then reabsorbed . Some peptides are reabsorbed paracellulary , while the others bind to the apical membrane and then enter the cells by endocytosis , where they will degraded by peptidase enzymes to amino acids .

4. Reabsorption of sodium , water , and chloride:

65 % of sodium is reabsorbed in the proximal tubules , while 25% are reabsorbed in the thick ascending limb of loob of Henle , 9% in the distal and collecting tubules and collecting ducts .
90% of sodium reabsorption occurs independently from its plasma level (unregulated) , This is true for sodium reabsorbed in proximal tubule and loop of Henle , while the 9% that is reabsorbed in distal ,collecting tubules and collecting ducts is regulated by Aldosterone. 

In proximal tubules : 65% of sodium is reabsorbed . The initial step occurs by creating sodium gradient  by sodium-potassium pump on the basolateral membrane . then the sodium will pass from the lumen into the cells down concentration gradient by sodium -glucose symporter , sodium -phosphate symporter and by sodium- hydrogen antiporter and others                    
                   
After reabsorption of sodium , an electrical gradient will be created , then chloride is reabsorbed following the sodium  . Thus the major cation and anion leave the lumen to the the interstitium and thus the water follows by osmosis . 65% of water is reabsorbed in the proximal tubule.

Discending limb of loop of Henle is impermeable to electrolytes but avidly permeable to water . 10 % of water is reabsorbed in the discending thin limb of loob of Henle .

The thick ascending limb of loop of Henly is permeable to electrolytes , due to the presence of Na2ClK syporter . 25% of sodium is reabsorbed here .

In the distal and collecting tubules and the collecting ducts 9% of sodium is reabsorbed .this occurs under aldosterone control depending on sodium plasma level. 1% of sodium is excreted .

Water is not reabsorbed from distal tubule but 5-25% of water is reabsorbed in collecting tubules .

1. PATHOPHYSIOLOGY OF THE CONDUCTION SYSTEM


2. Cardiac arrhythmias = deviation from normal rate, rhythm

    

   1. Heart block (types) = conduction system damage
      1. Complete Heart Block = 3rd degree block
         1. idioventricular beat (35-45/min)
         2. Atria at normal sinus rhythm
         3. Periods of asystole (dizziness, fainting)
         4. Causes = myocardial infarction of ventricular septum, surgical correction of interseptal defects, drugs
      2. Incomplete Heart Block = 2nd degree block
         1. Not all atrial beats reach ventricle
         2. Ventricular beat every 2nd, 3rd, etc. atrial beat, (2:1 block, 3:1 block)
      3. Incomplete Heart Block = 1st degree block
         1. All atrial beats reach ventricle
         2. PR interval abnormally long = slower conduction
      4. Bundle branch blocks (right or left)
         1. Impulses travel down one side and cross over
         2. Ventricular rate normal, QRS prolonged or abnormal
   2. Fibrillation
      1. Asynchronous contractions = twitching movements
      2. Loss of synchrony = little to No output
      3. Atrial Fibrillation
         1. Irregular ventricular beat & depressed pumping efficiency
         2. Atrial beat = 125 - 150/min, pulse feeble = 60 - 70/min
         3. Treatment = Digitalis - reduces rate of ventricular contraction, reduces pulse deficit
      4. Ventricular Fibrillation
         1. Almost no blood pumped to systemic system
         2. ECG = extremely bizarre
         3. Several minutes = fatal
         4. Treatment = defibrillation, cardiac massage can maintain some cardiac output