1.2. Basic Renal Process - Reabsorption Flashcards
What occurs in reabsorption?
Substances that the body wants are reabsorbed
Those it does not want will stay in the Tubule
How much plasma per day is filtered through the Glomerular Capillaries into the Bowman’s Capsule?
180 litres per day
How much urine do we excrete (on average) per day?
1-2 litres per day
As 180 litres of plasma is filtered per day, and only 1-2 litres is excreted, what must happen?
Most of the plasma must be reabsorbed back into the peritubular capillaries
What allows for a large amount of reabsorption in the peritubular capillaries?
- Very low hydrostatic pressure
2. An increase in oncotic pressure
What causes the very low hydrostatic pressure, favouring reabsorption?
There is little frictional resistance offered along the entire length of the peritubular capillary
What causes the high oncotic pressure?
The plasma in the peritubular capillaries having a higher concentration of plasma proteins
what are the methods by which substances are reabsrobed by the kidney?
- Carrier mediated transport systems
2. Diffusion into the epithelial cells
What are reabsorbed through carrier mediated transport mechanisms?
- Glucose
- Amino-acids
- Organic-acids
- Sulphate
- Phosphate ions
What do all of the carrier proteins have? And why does this occur?
A maximum transport capacity, due to the saturation of the carriers (Tm)
What is the “Renal threshold”?
The plasma threshold at which saturation occurs
If Tm is exceeded, that what happens to the substrates left in the tubule?
They are excreted in the plasma
What is the renal plasma threshold for glucose?
10mmol/L - at a higher concentration that this it will appear in the urine (on urinalysis)
How is the Tm of amino-acid reabsorption set in the kidneys?
Very high - so that urinary excretion does not occur
What is amino-acid reabsorption regulated by?
Insulin and Counter-Regulatory hormones
- What substances does the kidney regulate by Tm mechanism?
- How does this occur?
- Sulphate and Phosphate ions
- The Tm is set at a level whereby the normal plasma concentration causes saturation, and so any increase will be excreted
What is reabsorbed by non-carrier mediated transport?
- Sodium (Na)
- Anions (e.g. Cl)
- Water (H2O)
- Other Solutes (e.g. K. Ca, Urea)
What is the process of non-carrier mediated transport reabsorption?
- Na pumps actively excrete Na from the Epithelial Cell into the Capillary Lumen
- Na diffuses passively into the Epithelial Cells
- This causes an electrochemical gradient, leading to anions to also be reabsorbed
- Water moves by osmosis, following the solute reabsorption
- The concentration of other solutes increases as fluid volume decreases, so permeable solutes are reabsorbed by diffusion
What is non-actively reabsorbed solutes dependent on?
- The amount of H2O removed (determining the concentration gradient)
- The permeability of the membrane to any particular solute
Why are all substances not reabsorbed then, due to the concentration gradient favouring it?
As not all substances can gain access through the tubule membrane
What does glucose share its transporter with?
Sodium (Na)
What is the process of glucose reabsorption?
- Na moves down the electrochemical gradient (into the epithelial cell) using the SGLT protein, which pulls glucose into the cell (against its concentration gradient)
- Glucose diffuses out of the basolateral side of the cell using the GLUT protein
- Na is pumped out of the basolateral side of the cell by an NaKATPase transporter
- What causes a disruption in renal function?
2. Does this have effect on the Tm transport mechanisms?
- Anything which will decrease active transport, e.g. a decrease in blood flow
- Yes
How does sodium concentration in the tubule effect the reabsorption of glucose?
- High Na concentration facilitates it
2. Low Na concentration inhibits it
Other than glucose, what else is the reabsorption of sodium (Na) related to?
Bicarbonate - for the acid base balance
