Absorption and excretion of calcium and phosphate

2021-06-07 04:26 PM

Normally, the renal tubules absorb about 99% of calcium, and about 100 mg/day is excreted in the urine. About 90% of the calcium in the first urine is reabsorbed in the proximal tubule, loop of Henle, the first part of the distal tubule.

Intestinal absorption and faecal excretion of calcium and phosphate

The recommended daily intake is about 1000mg/day with both calcium and phosphate, equivalent to about 1L of milk. Usually, divalent ions are difficult to absorb in the intestine. However, as will be discussed later, vitamin D supports the absorption of calcium in the intestine, and about 35% of the calcium we swallow will be absorbed, the rest will be excreted in the feces.

Figure. Overview of calcium exchange between organ systems in a person swallowing 1000 mg/day. Indicated that most calcium is excreted in the faeces, although the kidneys are capable of excreting large amounts by reducing renal absorption.

In addition, approximately 250 mg/day of calcium ions enter the intestine through digestive secretions and the shedding of mucinous cells. Therefore about 90% (900 mg/day) of the daily calcium intake is excreted in the faeces.

Absorption of phosphate in the intestine occurs readily. Except for some of the phosphate excreted in the faeces by association with unabsorbable calcium ions, almost all of the dietary phosphate is absorbed into the bloodstream via the intestines and subsequently excreted by the kidneys.

Renal excretion of calcium and phosphate

Approximately 10% (100 mg/day) of ingested calcium is excreted in the urine. Approximately 41% of calcium is bound to plasma proteins and is therefore not filtered by the glomerular membrane. The remainder conjugated with anions such as phosphate (9%) or ionized (50%) is filtered through the glomerulus into the renal tubules.

Normally, the renal tubules absorb about 99% of calcium, and about 100 mg/day is excreted in the urine. About 90% of the calcium in the first urine is reabsorbed in the proximal tubule, loop of Henle, the first part of the distal tubule.

In the posterior distal tubule and the early collecting duct, absorption is about 10%, the remaining 10% is variable, depending on the calcium concentration in the blood.

When blood calcium levels are low, this reabsorption is excellent because no calcium is lost in the urine. Conversely, even a few minutes after serum calcium levels rise above normal, calcium excretion has increased significantly, as we will see later, the most important factor controlling tubular calcium reabsorption. contribute, as well as regulate calcium excretion, is PTH.

Renal phosphate excretion is controlled by an overflow mechanism. That is, when the plasma phosphate concentration is below the critical value of about 1 mmol/L, all the phosphate in the primary urine is reabsorbed and no phosphate is lost in the urine. Once above dangerous concentrations, nonetheless, the rate of phosphate excretion corresponds to an increase in concentration. The kidneys should regulate the concentration of extracellular fluid phosphate by varying the rate of excretion with the concentration of phosphate in the blood and the rate of renal filtration.

Of course, as discussed later, PTH can significantly increase the amount of phosphate excreted by the kidneys, thus playing an important role in regulating blood calcium and phosphate levels.