Remember the following principles before proceeding :
- Reabsorption occurs for most of substances that have been previously filterd .
- The direction of reabsorption is from the tubules to the peritubular capillaries
- All of transport mechanism are used here.
- Different morphology of the cells of different parts of the tubules contribute to reabsorption of different substances .
- There are two routes of reabsorption: Paracellular and transcellular : Paracellular reabsorption depends on the tightness of the tight junction which varies from regeon to region in the nephrons .Transcellular depends on presence of transporters ( carriers and channels for example).

1. Reabsorption of glucose , amino acids , and proteins :

Transport of glucose occurs in the proximal tubule . Cells of proximal tubules are similar to those of the intestinal mucosa as the apical membrane has brush border form to increase the surface area for reabsorption , the cells have plenty of mitochondria which inform us that high amount of energy is required for active transport , and the basolateral membrane of the cells contain sodium -potassium pumps , while the apical membrane contains a lot of carrier and channels .

The tight junction between the tubular cells of the proximal tubules are not that (tight) which allow paracellular transport.
Reabsorption of glucose starts by active transport of  Na by the pumps on the basolateral membrane . This will create Na gradient which will cause Na to pass the apical membrane down its concentration gradient . Glucose also passes the membrane up its concentration gradient using sodium -glucose symporter as a secondary active transport.

The concentration of glucose will be increased in the cell and this will enable the glucose to pass down concentration gradient to the interstitium by glucose uniporter . Glucose will then pass to the peritubular capillaries by simple bulk flow.

Remember: Glucose reabsorption occurs via transcellular route .
          Glucose transport has transport maximum . In normal situation there is no glucose in the urine , but in uncontrolled diabetes mellitus patients glucose level exceeds its transport maximum (390 mg/dl) and thus will appear in urine .
                   
                   
                   
2. Reabsorption of Amino acids : Use secondary active transport mechanism like glucose.

3. Reabsorption of proteins : 

Plasma proteins are not filtered in Bowman capsule but some proteins and peptides in blood may pass the filtration membrane and then reabsorbed . Some peptides are reabsorbed paracellulary , while the others bind to the apical membrane and then enter the cells by endocytosis , where they will degraded by peptidase enzymes to amino acids .

4. Reabsorption of sodium , water , and chloride:

65 % of sodium is reabsorbed in the proximal tubules , while 25% are reabsorbed in the thick ascending limb of loob of Henle , 9% in the distal and collecting tubules and collecting ducts .
90% of sodium reabsorption occurs independently from its plasma level (unregulated) , This is true for sodium reabsorbed in proximal tubule and loop of Henle , while the 9% that is reabsorbed in distal ,collecting tubules and collecting ducts is regulated by Aldosterone. 

In proximal tubules : 65% of sodium is reabsorbed . The initial step occurs by creating sodium gradient  by sodium-potassium pump on the basolateral membrane . then the sodium will pass from the lumen into the cells down concentration gradient by sodium -glucose symporter , sodium -phosphate symporter and by sodium- hydrogen antiporter and others                    
                   
After reabsorption of sodium , an electrical gradient will be created , then chloride is reabsorbed following the sodium  . Thus the major cation and anion leave the lumen to the the interstitium and thus the water follows by osmosis . 65% of water is reabsorbed in the proximal tubule.

Discending limb of loop of Henle is impermeable to electrolytes but avidly permeable to water . 10 % of water is reabsorbed in the discending thin limb of loob of Henle .

The thick ascending limb of loop of Henly is permeable to electrolytes , due to the presence of Na2ClK syporter . 25% of sodium is reabsorbed here .

In the distal and collecting tubules and the collecting ducts 9% of sodium is reabsorbed .this occurs under aldosterone control depending on sodium plasma level. 1% of sodium is excreted .

Water is not reabsorbed from distal tubule but 5-25% of water is reabsorbed in collecting tubules .

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 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.

Hyperventilation

1. Treatments :Rebreath air, hold breath (Increase CO₂)
   Give oxygen for Hypoxemia

Bile contains:

• bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.

• bile pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.

Respiratory system plays important role in maintaining homeostasis . Other than its major function , which is supplying the cells with needed oxygen to produce energy and getting rid of carbon dioxide , it has other functions :

1 Vocalization , or sound production.
2 Participation in acid base balance .
3 Participation in fluid balance by insensible water elimination (vapors ).
4 Facilitating venous return .
5 Participation in blood pressure regulation : Lungs produce Angiotensin converting enzyme ( ACE ) .
6 Immune function : Lungs produce mucous that trap foreign particles , and have ciliae that move foreign particles away from the lung. They also produce alpha 1 antitrepsin that protect the lungs themselves from the effect of elastase and other proteolytic  enzymes