The body contains a large variety of ions, or electrolytes, which perform a variety of functions. Electrolytes in living systems include sodium, potassium, chloride, bicarbonate, calcium, phosphate, magnesium, copper, zinc, iron, manganese, molybdenum, copper, and chromium. These six ions aid in nerve excitability, endocrine secretion, membrane permeability, buffering body fluids, and controlling the movement of fluids between compartments.
Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium.
A relative decrease in blood sodium can occur because of an imbalance of sodium in one of the body’s other fluid compartments, like IF, or from a dilution of sodium due to water retention related to edema or congestive heart failure. Some insulin-dependent diabetic patients experience a relative reduction of potassium in the blood from the redistribution of potassium. Hyperkalemia, an elevated potassium blood level, also can impair the function of skeletal muscles, the nervous system, and the heart.
Hypochloremia, or lower-than-normal blood chloride levels, can occur because of defective renal tubular absorption. Bicarbonate ions result from a chemical reaction that starts with carbon dioxide (CO2) and water, two molecules that are produced at the end of aerobic metabolism.
The bidirectional arrows indicate that the reactions can go in either direction, depending on the concentrations of the reactants and products. About two pounds of calcium in your body are bound up in bone, which provides hardness to the bone and serves as a mineral reserve for calcium and its salts for the rest of the tissues. Hypocalcemia, or abnormally low calcium blood levels, is seen in hypoparathyroidism, which may follow the removal of the thyroid gland, because the four nodules of the parathyroid gland are embedded in it. Sodium is reabsorbed from the renal filtrate, and potassium is excreted into the filtrate in the renal collecting tubule. Recall that aldosterone increases the excretion of potassium and the reabsorption of sodium in the distal tubule. The Aldosterone Feedback Loop Aldosterone, which is released by the adrenal gland, facilitates reabsorption of Na+ and thus the reabsorption of water. In the distal convoluted tubules and collecting ducts of the kidneys, aldosterone stimulates the synthesis and activation of the sodium-potassium pump ([link]).
The Renin-Angiotensin System Angiotensin II stimulates the release of aldosterone from the adrenal cortex. Calcium and phosphate are both regulated through the actions of three hormones: parathyroid hormone (PTH), dihydroxyvitamin D (calcitriol), and calcitonin.
Electrolytes serve various purposes, such as helping to conduct electrical impulses along cell membranes in neurons and muscles, stabilizing enzyme structures, and releasing hormones from endocrine glands.
Drinking seawater dehydrates the body as the body must pass sodium through the kidneys, and water follows. Explain how the CO2 generated by cells and exhaled in the lungs is carried as bicarbonate in the blood. How can one have an imbalance in a substance, but not actually have elevated or deficient levels of that substance in the body? Without having an absolute excess or deficiency of a substance, one can have too much or too little of that substance in a given compartment.
Thu vi?n H?c li?u M? Vi?t Nam (VOER) du?c tai tr? b?i Vietnam Foundation va v?n hanh tren n?n t?ng Hanoi Spring. Large food molecules (for example, proteins, lipids, nucleic acids, and starches) must be broken down into subunits that are small enough to be absorbed by the lining of the alimentary canal. In the small intestine, pancreatic amylase does the ‘heavy lifting’ for starch and carbohydrate digestion ([link]).
The digestion of protein starts in the stomach, where HCl and pepsin break proteins into smaller polypeptides, which then travel to the small intestine ([link]). The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase.
The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. Absorption can occur through five mechanisms: (1) active transport, (2) passive diffusion, (3) facilitated diffusion, (4) co-transport (or secondary active transport), and (5) endocytosis. Because the cell’s plasma membrane is made up of hydrophobic phospholipids, water-soluble nutrients must use transport molecules embedded in the membrane to enter cells. In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane.
Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides.
The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport.
The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. In general, all minerals that enter the intestine are absorbed, whether you need them or not.
Iron—The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport.
Bile salts and lecithin can emulsify large lipid globules because they are amphipathic; they have a nonpolar (hydrophobic) region that attaches to the large fat molecules as well as a polar (hydrophilic) region that interacts with the watery chime in the intestine.
Intrinsic factor secreted in the stomach binds to the large B12 compound, creating a combination that can bind to mucosal receptors in the ileum. Some ions assist in the transmission of electrical impulses along cell membranes in neurons and muscles. In terms of body functioning, six electrolytes are most important: sodium, potassium, chloride, bicarbonate, calcium, and phosphate. In a clinical setting, sodium, potassium, and chloride are typically analyzed in a routine urine sample.
It is responsible for one-half of the osmotic pressure gradient that exists between the interior of cells and their surrounding environment. An absolute loss of sodium may be due to a decreased intake of the ion coupled with its continual excretion in the urine. At the cellular level, hyponatremia results in increased entry of water into cells by osmosis, because the concentration of solutes within the cell exceeds the concentration of solutes in the now-diluted ECF. It can result from water loss from the blood, resulting in the hemoconcentration of all blood constituents.
It helps establish the resting membrane potential in neurons and muscle fibers after membrane depolarization and action potentials. Similar to the situation with hyponatremia, hypokalemia can occur because of either an absolute reduction of potassium in the body or a relative reduction of potassium in the blood due to the redistribution of potassium. When insulin is administered and glucose is taken up by cells, potassium passes through the cell membrane along with glucose, decreasing the amount of potassium in the blood and IF, which can cause hyperpolarization of the cell membranes of neurons, reducing their responses to stimuli. Chloride is a major contributor to the osmotic pressure gradient between the ICF and ECF, and plays an important role in maintaining proper hydration. Its principal function is to maintain your body’s acid-base balance by being part of buffer systems.
A deficiency of vitamin D leads to a decrease in absorbed calcium and, eventually, a depletion of calcium stores from the skeletal system, potentially leading to rickets in children and osteomalacia in adults, contributing to osteoporosis. Hypercalcemia, or abnormally high calcium blood levels, is seen in primary hyperparathyroidism. Bone and teeth bind up 85 percent of the body’s phosphate as part of calcium-phosphate salts.
Aldosterone is released if blood levels of potassium increase, if blood levels of sodium severely decrease, or if blood pressure decreases.

This action increases the glomerular filtration rate, resulting in more material filtered out of the glomerular capillaries and into Bowman’s capsule.
Sodium passes from the filtrate, into and through the cells of the tubules and ducts, into the ECF and then into capillaries. All three are released or synthesized in response to the blood levels of calcium.PTH is released from the parathyroid gland in response to a decrease in the concentration of blood calcium. The ions in plasma also contribute to the osmotic balance that controls the movement of water between cells and their environment. It is transformed into carbonic acid and then into bicarbonate in order to mix in plasma for transportation to the lungs, where it reverts back to its gaseous form. Such a relative increase or decrease is due to a redistribution of water or the ion in the body’s compartments. Cac tai li?u d?u tuan th? gi?y phep Creative Commons Attribution 3.0 tr? khi ghi chu ro ngo?i l?.
Glucose, galactose, and fructose are the three monosaccharides that are commonly consumed and are readily absorbed.
After amylases break down starch into smaller fragments, the brush border enzyme ?-dextrinase starts working on ?-dextrin, breaking off one glucose unit at a time. Chemical digestion in the small intestine is continued by pancreatic enzymes, including chymotrypsin and trypsin, each of which act on specific bonds in amino acid sequences. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains. However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine.
Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease, which digests DNA, and ribonuclease, which digests RNA. As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration (up the concentration gradient). Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions. Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of 120 grams per hour.
Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion.
However, bile salts and lecithin resolve this issue by enclosing them in a micelle, which is a tiny sphere with polar (hydrophilic) ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids. The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. When blood levels of ionic calcium drop, parathyroid hormone (PTH) secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys. Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary lipids in micelles via simple diffusion.
Chemical digestion breaks large food molecules down into their chemical building blocks, which can then be absorbed through the intestinal wall and into the general circulation. More than 90 percent of the calcium and phosphate that enters the body is incorporated into bones and teeth, with bone serving as a mineral reserve for these ions. In contrast, calcium and phosphate analysis requires a collection of urine across a 24-hour period, because the output of these ions can vary considerably over the course of a day. Hormonal imbalances involving ADH and aldosterone may also result in higher-than-normal sodium values.
An absolute loss of potassium can arise from decreased intake, frequently related to starvation. In such a situation, potassium from the blood ends up in the ECF in abnormally high concentrations.
Chloride functions to balance cations in the ECF, maintaining the electrical neutrality of this fluid.
Hyperchloremia, or higher-than-normal blood chloride levels, can occur due to dehydration, excessive intake of dietary salt (NaCl) or swallowing of sea water, aspirin intoxication, congestive heart failure, and the hereditary, chronic lung disease, cystic fibrosis. Carbon dioxide is converted into bicarbonate in the cytoplasm of red blood cells through the action of an enzyme called carbonic anhydrase. A little more than one-half of blood calcium is bound to proteins, leaving the rest in its ionized form. Phosphate is found in phospholipids, such as those that make up the cell membrane, and in ATP, nucleotides, and buffers.Hypophosphatemia, or abnormally low phosphate blood levels, occurs with heavy use of antacids, during alcohol withdrawal, and during malnourishment.
Its net effect is to conserve and increase water levels in the plasma by reducing the excretion of sodium, and thus water, from the kidneys. Angiotensin II also signals an increase in the release of aldosterone from the adrenal cortex.
The hormone activates osteoclasts to break down bone matrix and release inorganic calcium-phosphate salts.
Imbalances of these ions can result in various problems in the body, and their concentrations are tightly regulated. This may be due to the loss of water in the blood, leading to a hemoconcentration or dilution of the ion in tissues due to edema. Chemical digestion, on the other hand, is a complex process that reduces food into its chemical building blocks, which are then absorbed to nourish the cells of the body ([link]).
At the same time, the cells of the brush border secrete enzymes such as aminopeptidase and dipeptidase, which further break down peptide chains. Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride. The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes (nucleosidase and phosphatase) into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine.
In this type of transport, proteins within the cell membrane act as “pumps,” using cellular energy (ATP) to move the substance. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid. The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP. All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells (enterocytes) directly.
During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells. When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off.
PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption.
This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements. Intestinal brush border enzymes and pancreatic enzymes are responsible for the majority of chemical digestion. With the help of bile salts and lecithin, the dietary fats are emulsified to form micelles, which can carry the fat particles to the surface of the enterocytes. In the event that calcium and phosphate are needed for other functions, bone tissue can be broken down to supply the blood and other tissues with these minerals.

This excess sodium appears to be a major factor in hypertension (high blood pressure) in some people. The low levels of potassium in blood and CSF are due to the sodium-potassium pumps in cell membranes, which maintain the normal potassium concentration gradients between the ICF and ECF. This can result in a partial depolarization (excitation) of the plasma membrane of skeletal muscle fibers, neurons, and cardiac cells of the heart, and can also lead to an inability of cells to repolarize.
The paths of secretion and reabsorption of chloride ions in the renal system follow the paths of sodium ions. In people who have cystic fibrosis, chloride levels in sweat are two to five times those of normal levels, and analysis of sweat is often used in the diagnosis of the disease. Calcium ions, Ca2+, are necessary for muscle contraction, enzyme activity, and blood coagulation. In the face of phosphate depletion, the kidneys usually conserve phosphate, but during starvation, this conservation is impaired greatly. In a negative feedback loop, increased osmolality of the ECF (which follows aldosterone-stimulated sodium absorption) inhibits the release of the hormone ([link]). PTH also increases the gastrointestinal absorption of dietary calcium by converting vitamin D into dihydroxyvitamin D (calcitriol), an active form of vitamin D that intestinal epithelial cells require to absorb calcium.PTH raises blood calcium levels by inhibiting the loss of calcium through the kidneys.
Aldosterone and angiotensin II control the exchange of sodium and potassium between the renal filtrate and the renal collecting tubule. In this section, you will look more closely at the processes of chemical digestion and absorption. Your bodies do not produce enzymes that can break down most fibrous polysaccharides, such as cellulose.
The fatty acids include both short-chain (less than 10 to 12 carbons) and long-chain fatty acids. Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine. Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane.
Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein.
The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport (that is, co-transport with sodium ions). Despite being hydrophobic, the small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells. After being processed by the Golgi apparatus, chylomicrons are released from the cell ([link]). To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in. When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream.
Most water-soluble vitamins (including most B vitamins and vitamin C) also are absorbed by simple diffusion.
Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells.
All of the ions in plasma contribute to the osmotic balance that controls the movement of water between cells and their environment. Once in the lungs, the reactions reverse direction, and CO2 is regenerated from bicarbonate to be exhaled as metabolic waste.
In addition, calcium helps to stabilize cell membranes and is essential for the release of neurotransmitters from neurons and of hormones from endocrine glands. Hyperphosphatemia, or abnormally increased levels of phosphates in the blood, occurs if there is decreased renal function or in cases of acute lymphocytic leukemia. PTH also increases the loss of phosphate through the kidneys.Calcitonin is released from the thyroid gland in response to elevated blood levels of calcium. While indigestible polysaccharides do not provide any nutritional value, they do provide dietary fiber, which helps propel food through the alimentary canal. Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum.
Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher.
The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts.
Short chains of two amino acids (dipeptides) or three amino acids (tripeptides) are also transported actively. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals.
Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men. The fats are then reassembled into triglycerides and mixed with other lipids and proteins into chylomicrons that can pass into lacteals. Sodium is freely filtered through the glomerular capillaries of the kidneys, and although much of the filtered sodium is reabsorbed in the proximal convoluted tubule, some remains in the filtrate and urine, and is normally excreted. Potassium is excreted, both actively and passively, through the renal tubules, especially the distal convoluted tubule and collecting ducts. Because of such effects on the nervous system, a person with hyperkalemia may also exhibit mental confusion, numbness, and weakened respiratory muscles. Additionally, because phosphate is a major constituent of the ICF, any significant destruction of cells can result in dumping of phosphate into the ECF. The hormone increases the activity of osteoblasts, which remove calcium from the blood and incorporate calcium into the bony matrix. Finally, endocytosis is a transportation process in which the cell membrane engulfs material. The monosaccharide fructose (which is in fruit) is absorbed and transported by facilitated diffusion alone. However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion. Intrinsic factor secreted in the stomach binds to vitamin B12, preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis.
Other absorbed monomers travel from blood capillaries in the villus to the hepatic portal vein and then to the liver. Potassium participates in the exchange with sodium in the renal tubules under the influence of aldosterone, which also relies on basolateral sodium-potassium pumps. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue (mainly plant fibers like cellulose), some water, and millions of bacteria ([link]).
The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down.
The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system. Adjustments in respiratory and renal functions allow the body to regulate the levels of these ions in the ECF. Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol.
These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.

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