1. Tubular Transport Flashcards
What are the types of “downhill” transports?
Simple diffusion and facilitated diffusion
What is simple diffusion
movement down electrochemical gradient via lipid bilayer or aqueous channels
What is facilitated diffusion?
Movement down electrochemical gradient; specific carriers are required!
What are the types of energy-dependent transportations?
Primary active transport
Secondary active transport
Pinocytosis
What type of transport moves a substance against electrochemical gradient and needs ATP hydrolysis for energy?
primary active transport
What is a secondary active transport?
Downhill movement of one substance provides energy for uphill movement of another substance
What is paracellular movement?
Transfer of substances across epithelium and passing through intercellular space between cells
Not active transport
What part of the nephron provides the driving force for reabsorption?
Proximal convoluted tubule
2/3 of water reabsorbed here
What provides the ultimate power behind the net unidirectional transport in the proximal convoluted tubule?
Na/K ATPase in basolateral membrane
specifically the Na
keeps the intracellular concentration of the Na lo
The reabsorption of Cl and urea in early PCT is ultimately dependent on what?
Na and water reabsorption
The basal cell membrane faces what space?
the renal interstitial space
Na/K ATPase can be found here in the PCT
What is the basal space?
The basal cell membrane has extensive invaginations or ‘gutters’ called basal space
The leaky tight junctions are found one the interstitial or lumen side of a typical renal tubular cell?
Lumen side where the brush border is
What are the starling forces that promote fluid movement into peritubular capillaries?
- High plasma colloid osmotic pressure in peritubular capillary blood. blood is more concentrated
- Low hydrostatic pressure in the capillaries
What regulates plasma concentrations of glucose and amino acids?
liver and endocrine system. not the kidney
What is the basic mechanism of tubular reabsorption of glucose and amino acids?
Transcellular only: Secondary active transport
uptake across luminal membrane against gradient with Na
Exits basolateral membrane by facilitated diffusion
Filtered load =
GFR * P
Would the filtered load of glucose change if GFR increased but plasma glucose concentration remained constant?
It would increase
What could change the threshold value of Tm?
Only by increasing the amount of transporters
Urine output increases in diabetes. Why?
If there is more sugar in the tubular fluid bc more is being filtered. Tm is met, the water is going to be pulled from the cell into the tubular lumen. Glucose is creating a higher osmolarity and water will follow
Why is a diabetic thirsty?
because water is being pulled from the cells into the tubular lumen to follow the high concentration of glucose
What happens if an osmotically active agent is filtered?
It prevents water reabsorption, thus diluting the Na that is present in the tubule. Thus Na will move from the peritubular capillary to the tubular lumen
Why does GFR increase if there is an increase of osmotically active agent like glucose in the blood?
Because of the higher osmolarity in the plasma, water will be lost from the cells into the plasma systemically and increasing the fluid that will go to the kidneys thus increasing GFR
How does mannitol help with brain swelling?
decreases intracerebral pressure in head injuries. systemic administration of mannitol creates an osmotic gradient within vascular space. It pulls fluid from cells into the tubular spaces. Reduces swelling in brain.
What are the consequences of osmotic diuretics?
increased water excretion and increased sodium excretion to maintain the osmolar gradient
also when Na reabsorption decreases it causes an increase of Na and HCO3 in urine and acidosis (Na/HCO3 transport in PCT)
What is reabsorbed in increasing order in the PCT?
Urea, Cl, K, Ca, Na, PO, bicarbonate, amino acids, and glucose
What molecules have an iso-osmotic absorption type in PCT?
Na and K
What molecules are not reabsorbed at all in the PCT and have a TF/P of +3 or greater?
Inulin and PAH
and creatinine
What is obligatory water reabsorption?
The amount of water reabsorption that must occur
85%
65% in PCT
and 20% in distal tubules
What is facultative water reabsorption?
The water reabsorption that is optional
~15%
From the collecting tubule through the renal medulla under control of ADH
What are the organic anions that are secreted in the PCT?
PAH, bile salts, uric acid, and creatinine
How are organic anions secreted in the PCT? what else is secreted this way?
Through tertiary active transport
Brings sodium from the blood into the cells to bring in OA with it
Also drugs such as penicillin, salicylates, and some antiviral drugs
When does the amount of PAH excreted become parallel to the amount filtered ?
When PAH secretion is saturated and Tm is met
Refer to graph (fig 6.17)
As PAH and Glucose plasma concentration increases, what does the clearance become closer to?
The GFR
PAH clearance decreases and glucose clearance increase to GFR
What organic cations are secreted in PCT and how?
Catecholamines, ACh, and dopamine etc. tertiary active transport. gradient established by Na/K
Organic anions and cations are ionized forms of what?
weak acids and bases, respectively
What form of organic anions and cations cannot readily diffuse through lipid bilayer?
charged forms bc highly polar compounds
When are weak acids neutral?
When protonated
When are weak bases neutral?
When deprotonated
Acidic solutions generate neutral forms of ________
Weak acids
The tubular handling of organic acids and bases is affected by what?
pH of luminal fluid
H in the tubular lumen favors reabsorption of organic acids, but traps organic bases in the lumen
What is transcellular solute movement?
Substance travels through a cell
Passes through apical membrane and basolateral membrane
How would you excrete the overdose of aspirin of a patient?
Luminal acidification (H) favors reabsorption of organic acids and excretion of bases. If there is an increase of organic acids due to aspirin, then to get rid of the protons you must inject bicarbonate to alkalized the urine to help excrete the over dose of aspirin
