Tubular secretion:

Involves transfer of substances from peritubular capillaries into the tubular lumen. It  involves transepithelial transport in a direction opposite to that of tubular absorption.

Renal tubules can selectively add some substances that have not been filtered to the substances that already have been filtered via tubular secretion.

Tubular secretion mostly function to eliminate foreign  organic ions, hydrogen ions ( as a contribution to acid base balance ), potassium ions ( as a contribution to maintaining optimal plasma K+ level to assure normal proceeding of neural and muscular functions), and urea.
Here we will focus on K+ secretion and will later discuss H+ secretion in acid base balance, while urea secretion will be discussed in water balance.

K+ is filtered in glomerular capillaries and then reabsorbed in proximal convoluted tubules as well as in thick ascending limb of loop of Henley ( Na-2Cl-K symporter)

K+ secretion takes place in collecting tubules (distal nephron) . There are two types of cells in distal nephron:

- Principal cells that reabsorb sodium and secrete K+ .
- Intercalated cells that reabsorb K+ in exchange with H+.

Mechanism of secretion of K+ in principal cells : Two steps

- K+ enters tubular cells by Na/K ATPase on the basolateral membrane.
- K+ leaves the tubular cells via K+ channels in apical membrane.

Aldosterone is a necessary regulatory factor.

If there is increased level of K+ in plasma,excessive K+ is secreted , some of which is reabsorbed back to the plasma in exchange with H+ via the intercalated cells.        

Ingestion: Food taken in the mouth is

• ground into finer particles by the teeth,
• moistened and lubricated by saliva (secreted by three pairs of salivary glands)
• small amounts of starch are digested by the amylase present in saliva
• the resulting bolus of food is swallowed into the esophagus and
• carried by peristalsis to the stomach.

The large intestine (colon)

The large intestine receives the liquid residue after digestion and absorption are complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. It nourishes a large population of bacteria (the contents of the small intestine are normally sterile). Most of these bacteria (of which one common species is E. coli) are harmless. And some are actually helpful, for example, by synthesizing vitamin K. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces may consist of bacterial cells. Reabsorption of water is the chief function of the large intestine. The large amounts of water secreted into the stomach and small intestine by the various digestive glands must be reclaimed to avoid dehydration.

 Acute Obstructive Disorders
 1.    Heimlich maneuver
 2.    Bypass, tracheostomy w/catheter to suck up secretion

Concentration versus diluting urine 

Kidney is a major route for eliminating fluid from the body to accomplish water balance. Urine excretion is the last step in urine formation. Everyday both kidneys excrete about 1.5 liters of urine.
Depending on the hydrated status of the body, kidney either excretes concentrated urine ( if the plasma is hypertonic like in dehydrated status ) or diluted urine ( if the plasma is hypotonic) .
This occurs thankful to what is known as countercurrent multiplying system, which functions thankfully to establishing large vertical osmotic gradient .
To understand this system, lets review the following facts:
1. Descending limb of loop of Henle is avidly permeable to water.
2. Ascending limb of loop of Henly is permeable to electrolytes , but impermeable to water. So fluid will not folow electrolytes by osmosis.and thus Ascending limb creates hypertonic interstitium that will attract water from descending limb.
Pumping of electrolytes
3. So: There is a countercurrent flow produced by the close proximity of the two limbs.                   
                                                   
Juxtamedullary nephrons have long loop of Henle that dips deep in the medulla , so the counter-current system is more obvious and the medullary interstitium is always hypertonic . In addition, peritubular capillaries in the medulla are straigh ( vasa recta) in which flow is rapid and rapidly reabsorb water maintaining hypertonic medullary interstitium.

In distal tubules water is diluted. If plasma is hypertonic, this will lead to release of ADH by hypothalamus, which will cause reabsorption of water in collecting tubules and thus excrete concentrated urine.

If plasma is hypotonic ADH will be inhibited and the diluted urine in distal  tubules will be excreted as diluted urine.

Urea  contributes to concentrating and diluting of urine as follows:

Urea is totally filtered and then 50% of filtrated urea will be reabsorbed to the interstitium, this will increase the osmolarity of medullary interstitium ( becomes hypertonic ). Those 50% will be secreted in ascending limb of loop of Henle back to tubular fluid to maintain osmolarity of tubular fluid. 55% of urea in distal nephron will be reabsorbed in collecting ducts back to the interstitium ( under the effect of ADH too) . This urea cycle additionally maintain hypertonic interstitium.

The pancreas

The pancreas consists of clusters if endocrine cells (the islets of Langerhans) and exocrine cells whose secretions drain into the duodenum.

Pancreatic fluid contains:

• sodium bicarbonate (NaHCO₃). This neutralizes the acidity of the fluid arriving from the stomach raising its pH to about 8.
• pancreatic amylase. This enzyme hydrolyzes starch into a mixture of maltose and glucose.
• pancreatic lipase. The enzyme hydrolyzes ingested fats into a mixture of fatty acids and monoglycerides. Its action is enhanced by the detergent effect of bile.
• 4 zymogens— proteins that are precursors to active proteases. These are immediately converted into the active proteolytic enzymes:
  • trypsin. Trypsin cleaves peptide bonds on the C-terminal side of arginines and lysines.
  • chymotrypsin. Chymotrypsin cuts on the C-terminal side of tyrosine, phenylalanine, and tryptophan residues (the same bonds as pepsin, whose action ceases when the NaHCO₃ raises the pH of the intestinal contents).
  • elastase. Elastase cuts peptide bonds next to small, uncharged side chains such as those of alanine and serine.
  • carboxypeptidase. This enzyme removes, one by one, the amino acids at the C-terminal of peptides.
• nucleases. These hydrolyze ingested nucleic acids (RNA and DNA) into their component nucleotides.

The secretion of pancreatic fluid is controlled by two hormones:

• secretin, which mainly affects the release of sodium bicarbonate, and
• cholecystokinin (CCK), which stimulates the release of the digestive enzymes.