Obligatory Reabsorption and Secretion in the Proximal Convoluted Tubule Flashcards
What is the hydrostatic pressure of the tubule?
-15 mmHg
What is the Blood Hydrostatic pressure?
+55 mmHg
What is the Blood Oncotic Pressure?
-30 mmHg
What is the net filtration pressure?
+10 mmHg
What are the factors that lead to the blood hydrostatic pressure in the capillaries of renal corpusles not changing much?
Autoregulation of Glomerular Filtration
- Myogenic mechanism
- Tubuloglomerular feedback (nephrogenic)
Between what arterial blood pressures does the renal blood flow not really change?
~ 80 - 150 mmHg
What does constriction of the efferent arteriole cause within the capillary?
Increased Filtration pressure and increased flow rate
What does constriction of the affereant arteriole cause within the capillary?
- Reduced filtration pressure and decreased GFR
What is the main controller of GFR the afferent or efferent arteriole?
The afferent (both usually work together at the same time)
What is the renal blood flow? per min?
1 - 1.2 L/min
RBF essentially constant over wide range of BP
What causes vasoconstriction in response to decreased afferent blood flow?
- Sympathetic nerves releasing NE
- Circulating epinephrine
- Angiotensin II
What causes vasodilation in response to increased afferent blood flow?
- Renal prostaglandins
- Atrial natriuretic peptide
What are the 5 steps in the kidney nephron?
- Filtration by glomerulus
- Obligatory absorption and secretion by proximal tubule
- Generation of osmotic gradient by loop of Henle
- Regulated absorption and secretion by distal tubule
- Regulation of water uptake by collecting ducts
What are the functions of the proximal tubule?
- Most of the recovery of ions, sugars, amino acids, peptides and a considerable amount of the total water is achieved in the first part of the PT
- In addition the PT actively secretes a number of compounds for excretion with urine, and metabolises some of the amino acids
What are the 2 pathways molecules and ions can cross the tubule epithelium?
- Transcellular route (through the cell body)
- Paracellular route (through the leaky ‘tight’ junctions between cell bodies)
What are the forces involved in the obligatory reabsorption from the proximal tubule?
- Ion gradients across the basolateral membrane
- Sets up an electrochemical gradient of about -3mV (tubule lumen negative; causes paracellular efflux of cations)
- Osmotic gradient set up by pumping Na out of the cell into the interstitial space
- Water moving along the paracellular path due to osmotic pressure drags solutes along with it. Known as sovent drag
- Chemical concentration of solutes left behind when water leaves the tubule facilitates a chemical gradient
What is co-transport in the PCT?
Sodium moves substances into the cell
What is counter transport (exchange) in the PCT?
Sodium moves a substance out
What percentage of bicarbonate is reabsorbed?
Proximal Tubule
- Through the action of carbonic anhydrase
Where is the majority of Mg2+ absorbed?
Thick ascending limb
What passes through tight junctions in the proximal tubule?
- H2O
- K+
- Na+
- Ca2+
What is solvent drag (‘bulk transport’)?
Movement of solutes in the ultrafiltrate that are transported back from the renal tubule by the flow of water rather than specifically by ion pumps or other membrane transport proteins by the paracellular route
What is the tubular potential in the proximal tubule?
-4 (causes absorption of anions)
What is transport maximum
- Limit as to how muvch can be moved
- Often discussed in terms of glucose and the PAH
- Measured in mg/min or mmol/min (rate)
- If blood conc. is high, Tmax can be exceeded
How is the amount filtered calculated?
Plasma conc. x GFR
When does a substance appear in urine?
When it is above threshold (when normally 100% reabsorbed)
What is the threshold of a substance?
Point at which the amount filtered is equivalent to Tmax
When is the concentration of glucose in urine proportional to plasma concentration?
Above threshold
How much glucose can the Na-glucose transporter reclaim?
~ 375 mg/min of glucose
What is Para-amino hippurate for?
Measuring secretion through the kidneys can be infused into patient to measuring secretion
Where is PAH secreted?
Distal parts of proximal tubule
How much of a substance appears in urine that is excreted by the kidneys?
Amount filtered plus amount secreted
When does the secretion line level off?
Once maximum secretion level is reached
How much urea is reabsorbed in the proximal tubule?
~ 50% through simple diffusion
What is the obligatory reabsorption of glucose, amino acids and lactate?
100%
What is the obligatory reabsorption of bicarbonate?
90%
What is the obligatory reabsorption of water and sodium?
65%
What is the obligatory reabsorption of potassium?
55%
What is the obligatory reabsorption of chloride?
50%
What is clearence?
- Volume of plasma which is cleared of the substance per unit time (theoretical as in reality no substance is completely cleared)
What are the 3 renal processes that determine and modify composition of urine?
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
What are the requirments of a substance in order to measure GFR?
- Freely filtered at the glomerulus
- Neither secreted or reabsorbed
- Not metabolised
- Not toxic
What can clearence be used to determine?
- Renal transport mechanism
- Net reabsorption
- Net secretion
