Questions
1. Describe methods of parenteral nutrition administration, i.e. continuous, cyclic, central, peripheral.2. Discuss labs for monitoring parenteral nutrition.3. Describe the possible complications of parenteral nutrition.4. Identify each part of the nephron and discuss its function.5. Identify and discuss metabolic abnormalities associated with renal failure.6. Discuss goals of nutrition therapy in patients with renal failure.
Parenteral Nutrition InformationROUTES OF INFUSIONParenteral nutrition, total parenteral nutrition (TPN) or peripheral parenteral nutrition (PPN) is the administration of nutrient (i.e. glucose, amino acids, fat, vitamins and minerals) intravenously either by central or peripheral vein. Indications for use are only when oral or enteral feeding is inadequate or contraindicated. If the gut works, use it first. Solutions vary and depend on calorie, protein, mineral, vitamin, and fluid requirements of the individual.The components of a parenteral feeding formulation will determine its osmolarity and infusion route. Parenteral nutrition (PN) may be prepared for peripheral venous infusion or infusion through a central venous access device. PN may also be prepared as a total nutrient admixture (TNA) or a 2-in-1 solution. 2-in-1 solutions contain all necessary IV macronutrients and micronutrients in the same container except intralipids, which may be infused separately. Parenteral formulations are hypertonic to body fluids, and, if administered inappropriately, may result in venous thrombosis, thrombophlebitis, and extravasation. Specifically, the osmolarity of a parenteral feeding formulation is dependent primarily on the dextrose, amino acid, and electrolyte content. The maximum osmolarity of PN through a peripheral line is 900 mOsm/L.Central Parenteral NutritionCentral Parenteral Nutrition (CPN) is often referred to as total parenteral nutrition (TPN) because the entire nutrient needs of the patient may be delivered by this route. A complete, balanced formulation includes dextrose, amino acids, intravenous fat emulsion (IVFE), electrolytes such as potassium, magnesium, and phosphorus, vitamins, and multiple trace elements such as zinc, copper, manganese, chromium, and selenium. CPN is delivered into a large-diameter vein such as the superior vena cava adjacent to the right atrium. The rate of blood flow in these large vessels rapidly dilutes the hypertonic parenteral feeding formulation to that of body fluids. CPN may be concentrated to provide adequate calories and protein for those patients requiring a fluid restriction. CPN is preferred for use in patients who require PN support for longer than 7 to 14 days.Peripheral Parenteral NutritionPeripheral Parenteral Nutrition (PPN) has a similar composition as CPN, but lower concentrations of nutrient components are necessary to allow peripheral venous administration. It has a lower dextrose dose of 150 to 300 g/d (5-10% final concentration) and amino acid (50-100 g/d, or 3% final concentration) content compared to CPN. PPN is usually an undesirable option for patients with a fluid restriction because concentrating the solution frequently results in a hyperosmolar solution that is not suitable for peripheral administration. PPN may be used in patients with mild to moderate malnutrition to provide partial or total nutrition support when they are not able to ingest adequate calories orally or enterally, or when CPN is not feasible. However, this therapy is typically used for short periods (up to 2 weeks). Patients considered for PPN must meet two criteria: (1) have good peripheral venous access, and (2) be able to tolerate large volumes (2.5-3 L) of fluid. They should require at least 5 days but no more than 2 weeks of partial or total PN. The use of PPN is controversial, with many believing that the risk of complications outweighs any potential benefit because candidates for this therapy have only minor, if any, nutritional deficits. IVFE may be used to increase the caloric density of the peripheral parenteral feeding formulation without increasing the osmolarity, and the addition of IVFE has also been reported to improve peripheral vein tolerance of PPN. Contraindications for peripheral parenteral nutrition include:
Significant Malnutrition
Severe Metabolic Stress
Large Nutrient or Electrolyte Needs
Fluid Restriction
Need for Prolonged Parenteral Nutrition
Renal or Liver Compromise
Indications for PN were derived from A.S.P.E.N guidelines and defined as patients with: Peritonitis
Intestinal hemorrhage
Intestinal obstruction
Intractable Vomiting
Paralytic Ileus
Severe pancreatitis
Stool output greater than 1 L/day
High output fistula
Short bowel syndrome
Bone Marrow recipients having nausea, vomiting, and severe mucositis lasting longer than 3 days.Other circumstances where PN has been shown to be of benefit include: perioperative support of patients with moderate to severe malnutrition, acute exacerbation of Crohns disease, and critical care patients who will be NPO for prolonged periods of time who are unable to utilize their gut. PN is costly and may result in serious complications when used inappropriately. The therapy should only be used in those patients who will benefit.
Considerations for PN use
1. Patients who are candidates for PN support cannot, should not, or will not eat adequately to maintain their nutrient stores. These patients are already, or have the potential, of becoming malnourished.
2. PPN may be used in selected patients to provide partial or total nutrition support for up to 2 weeks in patients who cannot ingest or absorb oral or enteral tube-delivered nutrients, or when central-vein PN is not feasible.
3. CPN support is necessary when parenteral feeding is indicated for longer than 2 weeks, peripheral venous access is limited, nutrient needs are large, or fluid restriction is required, and the benefits of PN support outweigh the risks.
Use CPN when:
1. Patient has failed EN trial with appropriate tube placement (postpyloric).
2. EN is contraindicated or the intestinal tract has severely diminished function due to underlying disease or treatment. Possible applicable conditions are as follows:
Paralytic Ileus
Mesenteric ischemia
Small bowel obstruction
GI fistula except when enteral access may be placed posterior to the fistula or volume of output (less than 200 mL/d) supports a trail of EN.
3. As occurs in postoperative nutrition support, the exact duration of starvation that can be tolerated without increased morbidity is unknown. Expert opinion suggests that wound healing would be impaired if PN is not started within 5 to 10 days postoperatively for patients unable to eat or tolerate enteral feeding.
4. The patients clinical condition is considered in the decision to withhold or withdraw therapy. Conditions where nutrition support is poorly tolerated and should be withheld until the condition improves are severe hyperglycemia, azotemia, encephalopathy and hyperosmolality, and severe fluid and electrolyte disturbances.Energy SubstratesCarbohydrate- is used in the form of dextrose, which provides 3.4 kcal/g. Dextrose is commercially available in multiple concentrations ranging from 2.5% to 70%. Higher dextrose concentrations (greater than 10%) are generally reserved for central venous administration because of the propensity to cause thrombophlebitis in peripheral veins. Another carbohydrate energy substrate used less frequently is glycerol, a sugar alcohol which provides 4.3 kcal/g. Glycerol, or glycerin, is contained in certain pre-mixed PN formulations marketed for peripheral administration.IVFE- is used to provide energy as well as essential fatty acids for PN formulations. IVFE components include soybean oil or 50:50 mixes of soybean and safflower oils, egg yolk phospholipid as an emulsifier, glycerin to render the formulation isotonic, vitamin K, and sodium hydrozide to adjust the final pH. IVFE is commercially available in 10% (1.1 kcal/mL), 20% (2 kcal/mL), and 30% (3 kcal/ML) concentrations. The IVFE 30% formulation is only approved for compounding of TNA, not for direct IV administration.Because of enhanced microbial growth potential with infusion of IVFE separate from dextrose and amino acid formulations, the Centers for Disease Control and Prevention (CDC) recommends a 12-hour hang-time limit for IVFE. However, an admixture containing IVFE, dextrose, and amino acids in the same container may be administered over 24 hours. The hang time and infusion of this formulation is extended when compared with infusion of IVFE alone because bacterial growth is inhibited at a reduced pH and there is an increased total osmolarity when the three substrates are combined in one container. Whether infused separately from amino acids and dextrose or as a TNA, the IVFE infusion rate should not exceed 0.11 g/kg/h. Greater infusion rates are associated with an increased risk of side effects such as hypertriglyceridemia and infectious complications.Protein- is used in the form of crystalline amino acids in PN formulations and yield 4 kcal/g. Nitrogen content varies; however, for nitrogen balance calculations amino acid products are generally assumed to be 16% nitrogen (6.25 g of protein = 1 g of nitrogen). Concentrations are available ranging from 3% to 20%.Electrolytes- maintenance and therapeutic amounts, are added to PN formulations depending upon the patients requirements. The six major electrolytes are sodium, potassium, magnesium, calcium, phosphate, and chloride, all of which have an active role in the metabolic processes of the body. Potassium, phosphate, and magnesium are the major intracellular electrolytes, where as sodium and chloride are located predominantly in the extracellular compartment. When refeeding severely malnourished patients or when using high levels of dextrose in the TPN formulation, the requirement for the intracellular electrolytes increases.The electrolyte content of TPN solutions is dependent on individual patient requirements, taking into consideration maintenance requirements; gastrointestinal and urinary losses; renal, cardiac, nutritional and endocrine status; and the nutrient composition of the TPN solution. Appropriate prescription of electrolytes requires regular monitoring of serum values. The composition of the electrolyte profile in patients receiving TPN depends on their acid-base balance.Bicarbonate buffers are not compatible with TPN solutions; however, acetate is used in place of bicarbonate as a buffer precursor. There are a number of commercially available electrolyte solutions that facilitate the preparation of TPN. Most of the premixed electrolyte solutions do not contain calcium and phosphate for compatibility reasons. Calcium and phosphate have a tendency to form insoluble precipitates that can result in catheter occlusion or embolus formation. Electrolytes are also added to some of the commercially prepared amino acid solutions in standard amounts.Vitamins- are commercially available in single vitamin products and multivitamin products that contain both fat-soluble and water-soluble vitamins. Vitamins should be given as a standard daily dose in parenteral nutrition. Delaying IV multivitamin therapy until development of clinical signs of vitamin deficiency is inappropriate. Certain situations may warrant special attention. Patients receiving PN and warfarin therapy need close monitoring of the desired anticoagulation level because of the inclusion of vitamin K in the parenteral multivitamin preparation. PN supplementation with additional thiamin is reasonable in PN patients with a history of alcohol abuse, especially if the patient did not receive thiamin upon hospital admission.Trace Elements- that are commonly used in PN formulations include zinc, copper, chromium, manganese, and selenium. They are commercially available as single-entity products and in various multiple trace element combinations with concentrations for adults, pediatrics, and neonates. Some formulations may also contain electrolytes. Other trace elements that may be supplemented in PN include molybdenum, iodine, and iron.Disease-Specific Formulations and Specific NutrientsRenal failure formulations are composed primarily of essential amino acids based upon the theory that nonessential amino acids can be physiologically recycled from urea, while essential amino acids must be provided from the diet. These formulations are relatively dilute (5.2%-6.5%) and offer no significant advantage over the standard formulations and may result in metabolic complications; therefore, indications for these formulations are limited.Hepatic encephalopathy formulations contain increased amounts of branched chain amino acids (BCAA) and decreased amounts of aromatic amino acids (AAA) compared with standard amino acid formulations. Altered metabolism in patients with hepatic failure can result in a high serum ratio of AAA to BCAA. This imbalance is thought to cause an increased transport of AAA into the brain, where they serve as precursors to neurotransmitters that may be responsible for altered mental status. Indications for these formulations are very limited.Metabolic stress, trauma, thermal injury, and/or hypercatabolic state formulations are based on the theory that higher BCAA amounts are beneficial during severe metabolic stress because of increased skeletal muscle catabolism; therefore, significantly increased amounts of leucine, isoleucine, and valine are provided in these products. While the use of BCAA-enriched formulations slightly improves nitrogen balance in certain population groups, clinical evidence does not support improved outcomes.Parenteral Nutrient Preparation: Admixtures- Dextrose-Amino Acids versus TNAPN admixtures may be prepared for administration in either of two formats- the traditional or dextrose-amino acids (2-in-1) formulation, or the TNA system which is also referred to as a 3-in-1 or all-in-one admixture. The dextrose-amino acids format incorporates the dextrose and amino acid base solutions alone with the prescribed electrolytes, minerals, vitamins, and trace elements in one or multiple containers each day. IVFE is administered separately as a piggyback infusion. In contrast, the TNA system incorporates dextrose, amino acids, and IVFE along with the prescribed micronutrients together in the same container for final administration. There are advantages and disadvantages of each.Advantages and Disadvantages of the Total Nutrient Admixture System
Advantages
All components are aseptically compounded by the pharmacy
Preparation is more efficient for pharmacy personnel
Less manipulation of the system during administration
Less risk of contamination during administration
Inhibited or slower bacterial growth if contamination does not occur compared to separate IVFE
Less nursing time needed
Less supply and equipment expense for only one infusion pump and IV tubing
More convenient storage, fewer supplies, and easier administration in home care settings
Dextrose and venous access tolerance may be better in some situations
Possible applications in fluid restricted patients because IVFE 30% is restricted for use in TNA
May be more cost-effective
Fat clearance may be better when IVFE is administered over more than 12 hours
Disadvantages
Large particle size of admixed IVFE precludes use of 0.22 micron (bacteria-eliminating) filter, and requires larger pore size filter of 1.2 microns.
Admixed IVFE is less stable and more prone to separation of the lipid components
Formulations are more sensitive to destabilization with certain electrolyte concentrations
Formulations are more sensitive to destabilization with low concentrations of dextrose and amino acids.
Lower pH amino acid formulations may destabilize the IVFE-portion of admixture
Formulation may be unstable when the final concentration of IVFE is low
Difficult to visualize precipitate or particulate material in the opaque admixture
Certain medications are incompatible with the IVFE portion of the admixture
Catheter occlusion is more common with daily IVFE
Less stable over time than dextrose-amino acid PN formulations with separate IVFEEither system may be infused via a central venous access device. If PN is to be administered via a true peripheral line, certain criteria are important to decrease risk of thrombophlebitis and damage to peripheral vein(s). Osmolarity should generally be kept below 900 mOsm/L, calcium and potassium concentrations should be kept low, and IVFE is generally given daily to provide adequate kcals and decrease osmolarity. In a dextrose-amino acids system for PPN, the IVFE is usually piggybacked into the same line and infused over 24 hours, replacing the IVFE bag at 12 hours to adhere to the 12-hour hang-time limit. It has been speculated that using daily IVFE along with the dextrose-amino acids formulation may confer a modest protective effect on venous tolerance by dilution of the PPN formulation and/or by some buffering action in the vein. There has been some success in reducing or preventing thrombophlebitis through the addition of heparin and/or small amounts of hydrocortisone or the use of a nitroglycerin patch at the venous insertion site.Central versus Peripheral AccessCentral or peripheral access is not defined by the initial point of entry into the vascular system, but rather by the position of the distal catheter tip. A peripheral catheter is defined as one whose tip position is outside of the central vessels, the inferior and superior vena cava. Peripheral catheters include standard peripheral cannulas, midline catheters, and midclavicular catheters. Midline catheters are catheters with the tip terminating in the proximal portion of the upper extremity.Central Venous AccessCentral venous access is defined as a catheter whose distal tip lies in the distal vena cava or right atrium. The most common sites of venipuncture for central access include the subclavian, jugular, femoral, cephalic, and basilic veins. CVCs provide access for infusion as well as blood aspiration for laboratory monitoring. The primary indications for central venous access are chemotherapy, antibiotics, and PN. Central venous infusions are not limited by drug pH, osmolarity, or volume. CVADs can be grouped into three broad categories: nontunneled, tunneled, and implanted.Nontunneled CVADs are most commonly used in the acute care setting for therapies of short duration. Advantages of nontunneled catheters include decreased placement costs, ease of removal, and the ability to exchange these catheters over a guidewire. Peripherally inserted CVC (PICC) is defined as a catheter inserted via a peripheral vein whose tip lies in the vena cava. These catheters are classified as nontunneled CVCs. PICCs are common means for securing central vascular access in both acute care and home settings.Tunneled catheters decrease the risk of catheter infection by separating the exit and venipuncture sites. Tunneled catheters have been demonstrated to be safe and effective in long-term therapies ranging from months to years. Advantages to these catheters include ease of self-care by the patient, decrease risk of dislodgement, and the ability to repair the external lumen in the event of catheter breakage.Implanted catheters consist of a silicone elastomer catheter attached to a plastic or titanium disk with a self-sealing silicone elastomer septum. The port is placed into a subcutaneous pocket that is most often located in the anterior chest. Ports can be accessed up to 1000 to 2000 times. Advantages to these include minimal alteration in body image and ease of self-care. Implanted systems do not require routine site care when not in use and often are maintained with a monthly routine heparin flush. Ports are ideal for those who require infrequent IV therapies, such as intermittent chemotherapy. Lower rates of infection and thrombosis have been demonstrated.Macronutrient-Related ComplicationsHyperglycemia- is the most common complication associated with PN administration and can be caused by various factors such as stress-associated hyperglycemia and excess carbohydrate administration. PN should be initiated at half of the estimated energy needs or approximately 150 to 200 g for the first 24 hours. Lesser carbohydrate delivery (approximately 100-150 g of dextrose) is warranted in the hyperglycemic patient requiring insulin therapy or a hypoglycemic agent. Carbohydrate administration should not exceed a rate of 4 to 5 mg/kg/min or 20-25 kcal/kg/d. Blood glucose concentrations can be controlled with regular insulin therapy, which may be given subcutaneously or added directly to the PN solution. An insulin drip provides a more consistent and safe glucose control. Uncontrolled hyperglycemia may result in hyperosmolar hyperglycemic nonketotic dehydration, coma, and death secondary to osmotic diuresis.Hypoglycemia- can occur from excess insulin administration via the PN solution, IV drip, or subcutaneous injection. Treatment includes initiation of a 10% dextrose infusion, administration of an ampule of 50% dextrose, and/or stopping any source of insulin administration. Abrupt discontinuation of PN solutions has been associated with rebound hypoglycemia. To reduce the risk in susceptible patients, a 1- to 2-hour taper down of the infusion may be necessary. If PN is discontinued quickly, 10% dextrose should be infused for 1 to 2 hours following PN discontinuation to avoid possible rebound hypoglycemia.Essential Fatty Acid Deficiency- may result from IVFE-free PN, depending on the length of the PN therapy and the patients nutritional status. Intravenous fat emulsion is generally provided as a source of essential fatty acids and as a nonprotein calorie source. Although the incidence is low, there are several complications associated with IVFE use, such as infusion related adverse reactions and allergy to the IVFE components. Two polyunsaturated fatty acids, linoleic and alpha-linolenic, cannot be synthesized by the body and are considered essential. To prevent EFAD, 1% to 2% of daily energy requirements should be derived from linoleic acid and about 0.5% of energy from linolenic acid. This translates into approximately 500 mL of a 10% IVFE or 250 mL of 20% IVFE administered over 8 to 10 hours, twice weekly. Alternatively, 500 mL of a 20% IVFE can be given once a week. A trial of topical skin application or oral ingestion of oils to alleviate biochemical deficiency of EFAD may be given to patients who are intolerant to IVFE.Hypertriglyceridemia- can occur with dextrose overfeeding or with rapid administration of IVFE (greater than 110 mg/kg/h). Hyperlipidemia may impair immune response, alter pulmonary hemodynamics, and increase the risk of pancreatitis. Reducing the dose and/or lengthening the IVFE infusion time will help minimize these effects. IVFE intake should be limited to less than 30% of total calories or 1 g/kg/d and be provided slowly over no less than 8 to 10 hours if administered separately. Serum triglyceride concentrations should be measured before IVFE administration in any patient with a known history of hyperlipidemia. Pancreatitis due to IVFE-induced hyperlipidemia is rare unless serum triglyceride concentrations exceed 1000 mg/dL. IVFE is considered safe for use in patients with pancreatitis without hypertriglyceridemia. However, IVFE should be withheld from the PN regimen if serum triglyceride concentrations exceed 400 mg/dL.Azotemia- excessive protein administration results in an increased metabolic demand on the body for disposing of the byproducts of protein metabolism. Prerenal azotemia can result from dehydration, excess protein, and/or inadequate nonprotein calories. Increased blood urea nitrogen (BUN) may occur as a result of intolerance to the protein load. Patients with hepatic or renal disease are prone to developing azotemia because of the impaired ability to metabolize and eliminate urea. When urea clearance is impaired, dialysis may be required to assist with the elimination of urea and allow for adequate intake of protein.Refeeding Syndrome- refers to the metabolic and physiological shifts of fluid, electrolytes, and minerals (eg, phosphorus, magnesium, and potassium) that occur as a result of aggressive nutrition support. The delivery of calories, particularly in the form of carbohydrate, may induce refeeding syndrome in a malnourished patient. Carbohydrate delivery stimulates insulin secretion, which causes an intracellular shift of these electrolytes and minerals with the potential for severe hypophosphatemia, hypomagnesemia, and hypokalemia. For patients who are at risk for refeeding, calories should be initiated and advanced slowly.Cyclic infusion- refers to the infusion of a PN formulation over less than a 24-hour period, allowing a period of time off PN. Continuous PN infusion can result in hyperinsulinemia and fat deposition in the liver and, thereby, potentially increase risk of liver complications. Cyclic PN infusion has been shown to reduce serum liver enzymes and conjugated bilirubin concentrations when compared to continuous PN infusion.Enteral Nutrition- should be optimized because even small amounts of enteral intake may be beneficial in promoting enterohepatic circulation of bile acids. A trial of jejunal feeding at a slow rate during a portion of the day or night may be beneficial for the patient with chronic intestinal pseudo-obstruction who is receiving gastric decompression. Patients with short bowel syndrome should be encouraged to maximize oral intake because at least some of their intake will be absorbed.Monitoring Parenteral NutritionAs with any invasive therapy TPN must be monitored closely. The dietitian should carefully observe those items that are relevant to nutrition. Baseline data, preferably obtained before TPN is begun, usually includes: Complete Blood Count, Blood Glucose, Serum Creatinine, Blood Urea Nitrogen, Serum Electrolytes, Albumin, Transaminoases (Serum Glutamic Oxaloactic Transaminase (SGOT), Serum Glutamic Pyruvate Transaminase (SGPT)), Calcium, Phosphorus, Magnesium.During TPN, blood glucose, electrolytes, and urea nitrogen should be used to monitor hydration, renal function, and electrolyte balance. Calcium, phosphorus, and magnesium are monitored until serum levels stabilize. Urine should be checked daily for glucose and acetone to indicate carbohydrate tolerance. Hepatic function may be monitored by serum bilirubin and the liver enzymes, SGOT or SGPT, lactic dehydrogenase (LDH), and alkaline phosphatase. Plasma triglycerides are used to assess the capacity to clear plasma lipids. Body weight is obtained daily to check fluid overload.NUTRITIONAL THERAPY IN RENAL DISEASEREVIEW OF ANATOMY & PHYSIOLOGYThe major function of the renal system is to help maintain homeostasis in the body by controlling the composition and volume of blood. This is accomplished by removing and restoring secreted amounts of water and solutes.The two kidneys are located in the back abdominal cavity, one on each side of the spinal cord. They are embedded in fatty tissue for protection and surrounded by a sheath of fibrous tissue which helps to hold them in place. Each kidney is approximately 4.5 in. long, 1 inch thick, 2-3 inches wide and weighs 4-6 ounces. The average urinary output is 1.2 1.5 liters/day; however, the total volume depends on many variables such as water intake, nature of the diet, activity, environmental and body temperature and age.The ureters deliver urine from the kidneys where it is made and to the bladder where urine is stored. The average storage capacity of the bladder is 0.5 liters. The urethra takes the urine from the bladder to the outside of the body.The outer portion of the kidney is called the cortex, the inner portion the medulla. Urine is formed by the kidneys, collected in the renal pelvis, and from here flows through the ureters into the bladder.Blood is delivered to the kidneys from the abdominal aorta via the renal artery. It is sent back to the heart via the renal veins to the vena cava.Nephrons
The functional unit of the kidney is the nephron. There are greater than one million nephrons in each kidney, each capable of forming urine.There are two types of nephrons: cortical and juxtamedullary. Cortical nephrons have glomeruli lying close to the surface of the kidney and a short loop of Henle which penetrates varying distances into the outer medulla. The juxtamedullary nephrons have glomeruli which lie very close to the renal medulla and have a very long loop of Henle which dips deep into the medulla.The efferent arteriole of the cortical nephron goes on to form the peritubular capillaries which surround the tubules in the cortex. These short cortical nephrons have poor concentrating ability and poor sodium-retaining capacity.The efferent arteriole of the juxtamedullary nephrons continue not only as peritubular capillaries but as a series of vascular loops called the vasa recta. The deep medullary loops of Henle form the counter current mechanism, which works to concentrate the urine and conserve sodium. Any renal disease where there is a loss of concentrating ability indicates damage to the juxtamedullary nephrons.
Approximately 1 liter of blood, or 25% of the entire cardiac output at rest, flows through the kidneys each minute. Thus, in 4-5 minutes a volume of blood equal to the bodys total volume passes through the renal circulation.Blood enters the nephron through the afferent arteriole, which divides into a tuft-like network of approximately 50 capillaries. This is called the glomerulus. Pressure from the blood causes fluid and dissolved substances to pass through the capillary walls into the Bowmans capsule. This fluid is called the glomerular filtrate and is approximately the same composition as blood plasma, with the exception of protein and blood cells, which are too large to pass through healthy capillary walls. Approximately 180 liters of filtrate is formed daily, yet only 1-2 liters of this becomes urine. Most of what is filtered is reabsorbed into the blood by a series of steps that conserve the nutritious substances such as glucose and amino acids. It also allows the waste products such as urea and creatinine to leave the body in the filtrate, which is later called urine.Some products enter the glomerular filtrate by a process known as secretion. These products enter the tubular lumen from the peritubular blood by active and passive transport. Examples of these secreted substances are: potassium, hydrogen, ammonia, and certain medications. Some of these secretions are partially formed as a result of three processes: filtration, selective reabsorption, and secretion.Important blood constituents are entirely or almost entirely reabsorbed and are excreted in the urine, only when blood concentrations are above normal. Examples of these highly reabsorbed substances include water, glucose, chlorides of sodium, calcium, and magnesium. Normally 99% of water filtered is reabsorbed.The end-products of metabolism such as urea, uric acid and creatinine are seen in high concentrations in the urine because they are reabsorbed in limited quantities or not at
all.The process of reabsorption takes place in a series of renal tubules which are surrounded by peritubular capillaries. These capillaries take the reabsorbed products back into the general circulation for use by the body.The glomerular filtrate flows through the proximal tubule, the loop of Henle, the distal tubule, and the collecting tubule into the renal pelvis of the kidney.Proximal Tubule
Approximately 60-80% of the glomerular filtrate is reabsorbed by the proximal tubule. The active reabsorption of water and chloride occurs in the peritubular capillaries.
Loop of Henle
In the descending limb of the loop of Henle approximately 75% of the remaining water is reabsorbed. The tubule wall is impermeable to NaCl so the filtrate becomes very concentrated (hypertonic). In the ascending limb, the epithelial tissue is impermeable to water, but freely permeable to NaCl. Thus, the volume of the filtrate does not change, but NaCl is reabsorbed and the filtrate becomes hypotonic.Distal Tubule
The distal nephron consists of the distal convoluted tubule, cortical collecting tubule and the medullary collecting duct. Each of these segments is sensitive to antidiuretic hormone (ADH), a hormone secreted by the pituitary gl