• Paracellular Pathway = Filtration

• Cellular Pathway = Absorption

• Channels

  • Na \ K \ 2 Cl

  • Cl \ HCO3

  • K+ Channels

  • Cl Channels

  • Na Channels

  • Na \ Glucose

Serial Processing

1. Salivary Glands

• Creates hypotonic saliva

  • because we got rid of sodium and chloride

• so we have free , disposable water now

• If some bacteria or other hyperosmotic cell is around , we can cause it to lyse with the free water

2. We have acid being secreted , intrinsic factor , pepsin.

   • all aid in digestion

3. The microbiome helps digest exogenous starches ( cellulose )

4. Pancreas : amylase , peptidase , lipase

   • digests everything

5. Liver : produces bicarbonate, and bile acids

6. Small Intestine : passive absorption in brush border membrane : sugars , amino acids , fats

7. Large Intestine : absorb bulk of fluid

Control

• Hormonal and Neural

1. Cephalic Phase = secreations

2. Gastric Phase :

   • G-cells = gastrin

   • S-cells = secretin

   • i-cells = CCK

   • L-cells = peptide-YY

3. Intestinal Phase = secretin and CCK

Water Absorption Mechanism :

• The sodium-potassium pumps create the gradient for chloride to floow

• All of that osmole movement makes water follow

• Kidney is a multistage filter

• Arteriole End :

  • high pressure

  • filtration

  • pushes water and solutes out of the capillary

• Venous End :

  • low pressure

  • absorption

  • we are left with albumin

  • high albumin = high amount of protein = high "oncotic" pressure = strong water retention in vessel

    • where as low protein concentration = low oncotic pressure = water is released from vessel = edema

• Nephron :

  • glomerulus = filter

  • proximal tubule = absorption of fluid

  • diluting segment = selective salt absorption

  • distal tubule = selective water ( or salt ) absorption

• Glomerulus :

  • podocytes support against pressure

  • has fenestration pores

  • prevents blood cells and plasma proteins from entering nephron

  • afferent arteriole = in flow

  • efferent arteriole = exit flow

• Macula Densa = boundary between proximal and distal sections

• There are ways the body can autonomically control / constrict both the afferent and efferent arterioles

  • myogenic response ( Bayliss reflex )

  • maintains steady hydrostatic pressure of 60 mmHg

  • The glomerulus can also sense if there is high amounts of NaCl in the lumen , or just high osmolarity in general

    • it will decrease GFR

• Fractional Filtration = portion of fluid filtered at the glomerulus

• Filtration = Glomerular Capillaries

• Reabsorption = Peritubular Capillaries

• Absorbed = $\text{Filtered}-\text{Secreted}$$\text{Filtered} - \text{Secreted}$

• Filtered Amount = $P_x\ast GFR$$P_x * GFR$

• Excreted Amount = $U_x\ast UFR$$U_x * UFR$

• FF = ( amount excreted in urine / amount filtered by the glomerulus ) * 100%

Proximal Tubule

• 2/3 of all filtrate is reabsorbed in the proximal tubule

• You also do bulk of nutrient absorption here

  • Na , K , Cl , Ca , Mg , PO4 , Glucose

• Leave undesired stuff in lumen of nephron

• Convolutions = stirring = mixing = increased absorption

• Absorption of all of these osmoles is being driven by :

  • Capillaries = hydrostatic and osmotic

  • Nephron = oncotic / osmotic

• No aquaporins in ascending limb

• At the last possible moment , we dump NaCl into intermedullary space

• Urea enters inner meddula from IMCD

• ADH causes distal nephrons to insert aquaporins

  • allows for water reabsorption when you are dehydrated

• ADH also decreases the NaCl absorption

• Diabetes Insipidus = lack of ADH release = urinate tons of water

• RAAS = $\downarrow$$\downarrow$ Pressure ➡️ $\uparrow$$\uparrow$ renin ➡️ Angiotensin-1 ➡️ Angiotensin-2 ➡️ $\uparrow$$\uparrow$ aldosterone ➡️ $\uparrow$$\uparrow$ Sodium Channels ➡️ $\uparrow$$\uparrow$ Sodium Absorption

• Anion Gap = [ Na ] - ( [ Cl ] + [ HCO3 ] )

  • we can measure sodium and bicarbonate

    • and then indirectly calculate the "gap" of what the concentration of anions MUST be

    • its a neutral solution

• New Bicarbonate = produced by the kidneys and then added to the blood

• Volume Contracted = excrete more bicarb

• Volume Expanded = stop secreting bicarb

• Vomitting = loosing volume and acid

• Diarrhea = loosing volume and base

• A-Type = acid secretion and bicarbonate reabsorption

• B-Type = bicarb secretion

• If you were to exhale a bunch of CO2 , you might deplete the pool of CO2 needed to make bicarbonate ,

  • transport slows down