Physiology
Renal
What is the approximate renal threshold for glucose:
Answer:
Glucose is normally completely reabsorbed in the proximal tubule via secondary active transport by the Na+/glucose symporter and none excreted in the urine. However, there is a limit to the rate at which any transporter can operate and so for any substance, there is a maximum rate of absorption or secretion, called the tubular transport maximum (Tm). When filtrate glucose concentration rises above the renal threshold concentration, the transporters start to saturate and glucose appears in the urine. Once tubular maximum transport is reached, excretion increases linearly with filtration. The threshold concentration for glucose (~ 11 mmol/L) is somewhat lower than that required to reach tubular transport maximum (~21 mmol/L) because of the variation in transport maxima between nephrons; this is called splay.Renal Tubular Transport
Physiology / Renal / Mechanism of Filtration
Last Updated: 8th June 2023
Modes of Tubular Transport
Reabsorption and secretion involves the transport of substances across the tubular epithelium; this occurs either by diffusion through tight junctions and lateral intercellular spaces (paracellular pathway), driven by concentration, osmotic or electrical gradients, or by transport through the epithelial cells themselves (transcellular pathway).
The latter usually involves an active process on either the apical or basolateral cell membrane, with passive diffusion across the opposite membrane driven by the concentration gradient so created.
The movement of solutes between the peritubular space and capillaries is by bulk flow and diffusion; the movement of water is influenced by Starling's forces.
Active transport involves transporter proteins, and may be primary (which uses ATP directly as an energy source) or secondary (which uses the concentration gradient created by primary active transport as an energy source)
The rate of diffusion across cell membranes is enhanced by ion channels and uniporters which effectively increase membrane permeability to specific substances; this is termed facilitated diffusion, and may be modulated by hormones or drugs.
Maximal Tubular Transport
There is a limit to the rate at which any transporter can operate and so for any substance, there is a maximum rate of absorption or secretion, called the tubular transport maximum (Tm). Once tubular maximum transport is reached, excretion increases linearly with filtration.
Glucose is normally completely reabsorbed in the proximal tubule via secondary active transport by the Na+/glucose symporter and none excreted in the urine. However, when filtrate glucose concentration rises above the renal threshold concentration, the transporters start to saturate and glucose appears in the urine.
The threshold concentration for glucose (~ 11 mmol/L) is somewhat lower than that required to reach tubular transport maximum (~21 mmol/L) because of the variation in transport maxima between nephrons. As the concentration approaches the transport maximum, some of the channels are overwhelmed before others are. The difference between threshold and transport maximum is called "splay".
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
