Renal Transport Mechanisms Flashcards
What is tubular reabsorption?
The transfer of materials from kidney tubule lumen to peritubular capillaries
What are the three conclusions to glomerular filtration and tubular reabsorption?
- The filtered loads are enormous, generally greater than the total amounts of substances in the body
- Reabsorption of waste products is relatively incomplete
- Reabsorption of most useful plasma components, is relatively complete
Why would the reabsorption of waste products be relatively incomplete?
So that large fraction of their filtered loads are excreted in the urine, for instance, urea
Why would the reabsorption of useful plasma components be relatively complete?
Amounts excreted in the urine are a very small fraction of their filtered load
What is an example of substance whose filtered load is greater than the total amount of substance in the body?
The body usually contains about 40L of water, but the volume of water filtered each day is about 180L
What are the useful components of the plasma that are relatively completely reabsorbed?
Water
Inorganic ions
Organic nutrients
What is the % of water reabsorbed daily?
99%
What is the % of sodium reabsorbed daily?
99.5%
What is the % of glucose reabsorbed daily?
100%
What is the % of urea reabsorbed daily and why?
44%, urea in the body helps with certain functions
What is an important difference between reabsorptive processes?
Certain can be controlled physiologically, and certain cannot
What is the reabsorption rate of most organic nutrients?
Very high and not physiologically regulated
What happens to the filtered loads of most organic nutrients in healthy kidneys?
Completely reabsorbed with none appearing in the urine
Can the kidneys regulate the plasma concentration of organic nutrients?
No, the kidneys do not regulate the plasma concentration of those substances, they maintain them
What is one of the main functions of the kidneys in regard to reabsorption?
Eliminate soluble waste products
How are soluble waste products eliminated in the kidneys?
The blood is filtered in the glomeruli
What is one of the main consequences when it comes to filtering the blood in the glomerulus?
SUbstances necessary for normal body functions are filtered from the plasma into the tubular fluid
What is the compensatory mechanism of filtration of blood?
Reabsorption of useful substances from the tubular fluid while allowing waste products to be excreted
What is the reabsorptive rate of water and ions like?
Very high and under physiological control
What is the compensatory mechanism of the kidneys when the water intake is decreased?
The kidneys increase reabsorption to minimize water loss, it all depends on the body’s needs
What are the two processes involved with tubular reabsoprtion?
Diffusion
Mediated transport
Where are substances reabsorbed by diffusion?
From the tubular lumen into the peritubular capillaries
When does tubular reabsorption begin?
As soon as the filtrate enters the tubule cells
What is paracellular transport?
It occurs between cells (tight junctions) and is seen mainly with ions
What is transcellular transport?
Moves across the cell itself, no involvement of tight junctions
What can transport be like?
Active (against the concentration gradient) with the use of ATP or passive (with the concentration gradient) with no ATP
What is an example of passive reabsorption by diffusion?
Reabsorption of urea by the PCT
How is urea reabsorbed?
Starts the reabsorption of water, whichoncentration of urea because ure increases the ca moves decreased fluid from a higher concentration gradient to a lower one
What is the result of the fact that the corpuscular membranes are freely filterable to urea?
The urea concentration in the fluid within Bowman’s space is the same as that in the peritubular capillary plasma, and the interstitial fluid surrounding the tubule
What is the effect of water removal on the concentration of urea?
Urea in the tubular fluid is higher than in the interstitial fluid and peritubular capillaries
What is urea reabsorption dependent on?
Reabsorption of water
What is the direction of reabsorption of urea?
From tubular lumen to peritubular capillary
What is the process of transcellular transport?
Substances first cross the apical (luminal)
Then, they diffuse through the cytosol of the cell and across the basolateral membrane
What is the basolateral membrane?
The membrane that covers the sides of the cell, facing towards the peritubular capillaries
Does a substance necessarily mean that it will cross both apical and basolateral membranes actively?
No, in many cases, it will pass one membrane actively and the other one passively
How does Na+ move through the cell?
It moves passively from the tubule into the cell across the apical membrane through specific channels, from a higher concentration to a lower one, because extracellular concentration is always greater than intracellular when it comes to sodium.
Then, it actively gets transported out of the cell across the basolateral membrane via Na/K ATPase
What is the membrane potential?
A potential gradient that forces ions to passively move in one direction, formed
What is the reabsorption of substances often coupled to?
Coupled to the reabsorption of Na+
What is the transportation of the co-transported substance like?
The co-transported substance moves uphill into the cell via secondary active transportation as Na+ moves downhill (passively) into the cell via the same co-transporter
What are examples of co-transported substances?
Glucose, many amino acids, and other organic substances, also several inorganic ions
What is transport maximum (Tm)?
The renal tubule has a limit to the amounts of material they can transport per unit time
What leads to transport maximum?
The saturation of the binding sites on the membrane transport proteins: if no more binding sites are available, then the substance cannot be filtered or transported
What is an example of transport maximum?
Secondary-active transport proteins for glucose, located in the PCT
What happens if the substance reaches transport maximum?
The excess amount of the specific substance is excreted in the urine
Does glucose usually present in the urine?
No, it is not normal for glucose to present in the urine, it is usually 100% reabsorbed
What is the normal plasma concentration of glucose in a healthy person?
150mg/mL
What is the phenomenon of glucose in the urine known as?
Glucosuria
How do you diagnose diabetes or pre-diabetic stage?
Excretion of glucose in the urine, means there is too much glucose in the plasma (and thus needs to escape), so if value of glucose in plasma is high –> detection of diabetes
How do you calculate filtered load of glucose?
GFR x glucose concentration in plasma
How do you calculate excretion rate of glucose?
GFR x glucose concentration in urine
Where does the majority of reabsorption of filtered molecules happen?
PCT
What is tubular secretion?
The transfer of materials from peritubular capillaries to kidney tubule lumen
What are the processes of transportation of tubular secretion?
Diffusion or transcellular-mediated transport
What are the most important substances secreted by the tubules?
H+ & K+
There is a large number of organic anions, such as choline and creatinine that are also secreted
Also foreign chemicals like penicilln
What are the plausible directions of active secretion of a substance undergoing active transport?
Either from the blood side (interstitial fluid) into tubule cell (across the basolateral membrane) (transcellular transport)
OR
Out of the cell into the lumen (across the apical membrane)
What is the purpose of tubular secretion?
Mechanism to increase the ability of the kidneys to dispose of substances at a higher rate than depending only on the filtered load
Where and when is glucose synthesized in the kidneys?
In the cells of the renal tubules synthesize glucose during fasting and add it to the blood (process of gluconeogenesis)
What are the other functions of the renal tubule cells?
Catabolize certain organic substances, such as peptides, taken up from either the tubular lumen or peritubular capillaries
What is the purpose of catabolism of substances?
Eliminates these substances from the body just as if they had been excreted into the urine
What is the control that regulates tubular reabsorption and secretion?
Physiological control; hormones and paracrine or autocrine factors
What % of gluconeogenesis is due to the liver?
80% due to the liver and 20% due to the kidneys
How is control achieved for most of the substances secreted/reabsorbed?
Regulating the activity or concentrations of the membrane channels and transporter proteins involved
Where does the majority of reabsorption occur?
PCT and loop of Henle
What is the purpose of the distal segments of the tubules?
Fine-tuning for most substances, determining the final amounts excreted in the urine by adjusting their rates of reabsorption and secretion
