Chapter 25: Fluid, Electrolyte, and Acid-Base Balance (A & P)

What hormone is released when calcium levels are high?

Calcitonin. Its effects in humans are neglible unless levels are high. Released by thyroid

Sources of acid within the body

1) Fat metabolism: produces fatty acids and ketones 2) Glucose metabolism under anerobic conditions: produces lactic acid 3) Disposal of carbon dioxide as bicarbonate 4) HCl in the stomach 5) Breakdown of phosphorus containing proteins which result in phosphoric acid

What are some conditions that cause respiratory acidosis?

1) Impaired lung function (ex. chronic bronchitis) 2) Impaired ventilatory movement (ex. chest injury) 3) Narcotic or barbiturate overdose or injury to brain stem: barbiturates can cause a depression of respiratory centers which results in an accumulation of CO2

What makes up total body water volume? (from greatest percentage to least)

1) Intracellular fluid volume (25 L, about 40% of body weight) 2) Interstitial fluid (12 liters, about 80% of extracellular fluid) 3) Plasma (3 liters, about 20% of extracellular fluid)

What are some conditions which can cause metabolic acidosis?

1) Severe diarrhea: Bicarbonate-rich intestinal (and pancreatic) secretions rushed through GI tract before their solutes can be reaborbed 2) Renal disease: failure of kidneys to rid body of acids formed by normal metabolic processes 3) Diabetes mellitus: lack of insulin or inability of tissue cells to respond to insulin results in inability to use glucose which causes fats to be used as primary energy fuel 4) Starvation: Body proteins and fat reserves are used for energy which yield acidic metabolites 5) Excess alcohol ingestion

What conditions can cause respiratory alkalosis?

1) Strong emotions such as pain, anxiety, fear, or a panic attack 2) Hypoxemia: asthma, pneumonia, high altitude (efforts to raise oxygen levels at the expense of CO2 levels) 3) Brain tumor or injury which results in abnormal respiratory controls

What are some conditions that can cause metabolic alklaosis?

1) Vomiting or gastric suctioning: loss of stomach HCl requires that H+ be withdrawn from blood so there is an inproportionate amount of H+ in comparison to bicarbonate 2) Selected diurectics: causes loss of K+ which stimulates the tubule cells to secrete H+ and elicits the renin-angiotensin-aldosterone mchanism which stimulates sodium reabsorption and H+ secretion 3) Ingestion of excessive sodium bicarbonate (antacid) 4) Excess aldosterone

What is considered a normal pH?

7.40 (slightly alkaline)

What is the importance of calcium in the body?

99% of body calcium is in the skeleton in the form of calcium phosphate salts, which helps to make the skeleton rigid and strong and provides a dynamic reservoir from which calcium and phosphate can be withdrawn or deposited. Calcium is necessary for blood clotting, cell membrane permeability, and neuromuscular functioning.

Describe the importance of sodium

Accounts for about 90-95% of all solutes in the ECF and contributes most of the solute content to plasma. It is the only solute in plasma exerting significant osmotic pressure. Cellular plasma membranes are relatively impermeable to sodium and must be pumped out across an electrochemical gradient. Both its abundance in ECF and cell membranes impermeability to sodium play a huge role in controlling ECF volume and water distribution in the body. Sodium in the ECF remains relatively stable due to water adjustments (water follows salt).

Describe the influence of aldosterone on potassium

Aldosterone can also be released when potassium levels are slightly high. It works by stimulating the principal cells to reabsorb sodium, which simultaneously enhances potassium secretion. Adrenal cortical cells are directly sensitive to the K+ content of the ECF bathing them, so when it increases even slightly it is stimulated to release aldosterone. Aldosterone is most strongly caused to be released in response to the renin-angiotensin-aldosterone mechanism.

What fluid compartments exist within the body?

Almost two-thirds of fluid volume occupies an intracellular fluid compartment (exists as fluid within cells). About 1/3 of the remaining body fluid exists outside of cells in an extracellular fluid (ECF) compartment. This includes plasma (the fluid portion of blood) and interstitial fluid (the fluid which exists in microscopic spaces between cells).

What happens if ADH secretion is high?

Aquaporins are inserted into cells of the kidney (cells in DCT and collecting ducts) which will cause water to be reabsorbed back into the bloodstream. Urine will be concentrated and have a low volume.

Edema

Atypical accumulation of fluid in the interstitial space which leads to tissue swelling. Causes an increase in volume of only the IF. Can impair tissue function by increasing the distance across which oxygen and nutrients must diffuse between the blood and cells. Can be caused by increased hydrostatic pressure, abnormal lymph drainage, or decreased presence of plasma proteins.

Metabolic alkalosis

Caused by rising blood pH and bicarbonate levels. Much less common. Typically caused by vomiting the acidic contents of the stomach or intake of excess base (such as antacids).

Chemical Buffers

Chemical buffers are the first line of defense and act within a fraction of a second to resist pH changes.

Compare/contrast what electrolytes are found in the different fluid compartments

Chief cation of extracellular fluids is sodium and their major anion is chloride (plasma contains less due to being primarily neutral). Intracellular compartment fluid only contains small amounts of sodium and chloride ions and its most abundant cation is potassium and its major anion is hydrogen phosphate. ICF also contains a large amount of soluble proteins.

Potassium (why is it important?)

Chief intercellular cation which is required for normal neuromuscular functioning as well as for several essential metabolic activities. Even slight changes in potassium concentration in the ECF have life-threatening effects on neurons and muscle fibers because it affects the resting membrane potential of these cells. Potassium excess in the ECF causes depolarization and too little causes hyperpolarization. The heart is particularly sensitive to potassium levels and both too much and too little can disrupt its electrical conduction. It is also part of the body's buffer system. Shifts of H+ into cells causes corresponding shifts of K+ in the opposite direction. ECF potassium levels rise with acidsosis, and fall with alkalosis.

Describe the regulation of chloride

Chloride is major anion in ECF along with sodium that helps maintain the osmotic pressure of blood. When blood pH is within normal limits or slightly alkaline, about 99% of filtered chloride is reabsorbed. In the PCT, it moves passively and follows sodium ions out of filtrate and into the peritbulular capillary blood. When acidosis occurs, less chloride accompanies sodium because bicarbonate reabsoprtion is stepped up to restore blood pH.

Cardiovascular baroreceptors

Concept known as pressure diuresis. As blood volume/blood pressure rises, baroreceptors in the heart and the large vessels of the neck and thorax alert the cardiovascular center in the brain stem. This causes sympathetic nervous system impulses to the kidneys to decline and allows the afferent arterioles to dilate. This causes GFR to increase and sodium output and water output to increase which reduces blood volume and pressure.

Hypotonic hydration

ECF has normal water content but low sodium content. Causes net osmosis into tissue cells which causes them to swell and become abnormally hydrated. This can cause severe metabolic disturbances and is damaging to neurons which can lead to coma or death is left untreated.

Describe the influence of female sex hormones

Estrogens are chemically similar to aldosterone and have similar effects (enhance NaCl reabsorption by the renal tubules). Because water follows salt, many women retain fluid as their estrogen levels rise and can cause edema in pregnant women. Progesterone has a direutic effect which is thought to be due to it blocking aldosterone receptors.

How is electrolyte concentration expressed and what is the equation to find it?

Expressed in milliequivalents per liter (mEq/L) which measures the number of electrical charges in 1 L of solution. Can be used to show how much osmolality an electrolyte can contribute through the equation mEq/L = ion concentration (mg/L)/atomic weight of ion (mg/mmol) X number of electrical charges on one ion.

Describe the influence of glucocorticoids

Includes cortisol and hydrocortisol, enhances tubular reabsorption of sodium. When levels are especially high they can exhibit potent aldosterone like effects and promote edema.

Describe the regulation of water intake

Is poorly understood. Is governed by the hypothalamic thrist center which is activated by tiny changes in osmolarity (about 2-3%). Brain cells lose water to the hypertonic ECF which causes the subject to feel thirst. Thirst mechanism is also thought to be activated by an increase in blood osmotic pressure which causes less salivia to be produced (dry mouth), osmoreceptors which detect tiny increases in osmolarity in ECF, and a decrease in blood volume or pressure.

Metabolic acidosis

Low blood pH levels and low bicarbonate levels. Can be caused by ingesting too much alcohol or excessive loss of bicarbonate which might result from excessive diarrhea.

Describe the bicarbonate buffer system

Mixture of carbonic acid and sodium bicarbonate. When a strong acid such as HCl is added to the buffer system, the existing carbonic acid remains intact but the bicarbonate ions of the salt act as weak bases to tie up the H+ released by the stronger acid, forming more carbonic acid, causing only a slight drop in pH. When a strong base is added, it causes the carbonic acid to dissociate further, donating more H+ to tie up the OH- released by the strong base. Under extreme situations this situation can be tied up and become ineffective.

Where is most of calcium reabsorbed?

Most is reabsorbed in the proximal convulated tubule (PCT). About 75% is reabsorbed in the PCT

What happens if ADH secretion is low?

Most of the water will not be reabsorbed in DCT and collecting ducts due to the lack of aquaporins. Urine excreted will be diluted.

Describe the protein buffer system

Most plentiful, best buffer. Most of it resides in cells, so is intracellular. Some of the linked amino acids have exposed side chains of atoms called carboxyl groups which release H+ when pH begins to rise. Other amino acids have exposed groups that can act as bases and accept H+. Some molecules can function reversibly as either an acid or a base depending on pH (molecules known as amphoteric molecules). Hemoglobin in red blood cells are an example of this protein buffer system.

Where does water input come from?

Most water enters the body through ingested liquids and solids foods. Body water can also be produced through cellular metabolism/Krebs cycle (known as metabolic water or water of oxidation).

Describe the phosphate buffer system

Nearly identical to that of the bicarbonate buffer. Components are the salts of dihydrogen phosphate and monohydrogen phosphate. It is present in low concentrations in the ECF so is unimportant for buffering plasma. It is most effective in acting as a buffer in urine and in ICF, where phosphate concentrations are higher.

If osmalility is low in ECF will ADH be released?

No

What occurs if aldosterone concentrations are low?

No sodium reabsorption will occur beyond the distal convulated tube. More sodium will be excreted with urine

Describe the exchange between plasma and intracellular fluid

Occurs across capillary walls. Hydrostatic pressure of blood forces nearly protein-free plasma out of blood and into the interstitial space. This filtered fluid is then almost completely reabsorbed into the bloodstream in response to the colloid osmotic pressure of plasma proteins. Under normal circumstances, lymphatic vessels pick up the small net leakage that remains behind in the interstitial space and return it to the blood.

Describe the exchanges between intracellular fluid and interstitial fluid

Occurs across plasma membranes. Exchanges depend on its permeability properties. Two way osmotic flow of water is substantial. Ion exchange is limited and in most cases is moved selectively by active transport or through channels. Movement of nutrients, respiratory gases, and wastes are typically unidirectional (glucose and oxygen move into the cells and metabolic wastes move out). Plasma is the link between compartments.

Describe how ADH is regulated

Osmoreceptors of the hypothalamus sense the extracellular compartment solute concentration and trigger release of ADH from the posterior pituitary based on this. An increase in ECF osmolality prompts ADH release by stimulating the hypothalamic osmoreceptors. In contrast, a decrease in ECF osmolality inhibits ADH release and allows more water to be excreted in urine (restoring osmolality). ADH is also regulated by large changes in blood volume or blood pressure (not until it drops 10-15%). A decrease in blood pressure increases ADH release by baroreceptors in the atria and various blood vessels and indirectly via the renin-angiotensin-aldosterone mechanism. ADH is only really controlled by decreases in blood volume during trauma such as severe blood loss, burns, or diarrhea.

What are some examples of extracellular fluid?

Plasma, interstitial fluid, lymph, cerebrospinal fluid, humors of the eye, synovial fluid, serous fluid, and GI secretions.

Describe the regulation of potassium

Proximal tubules reabsorb about 65% of filtered K+ and the thick ascending limb of the loop of henle absorbs another 25% which leaves about 10% of potassium remaining at the end of the loop of Henle. The rest of this is controlled by the collecting ducts. Principal cells in the collecting ducts secrete potassium into the filtrate when ECF levels are high. When ECF potassium concentrations are low, the principal cells conserve K+ by reducing its secretion. Type A intercalated cells in the collecting duct can also reabsorb some of the K+ left in the filtrate (in conjunction with active secretion of H+), helping to reestablish K+ (and pH) balance.

Describe the influence of atrial natriuretic peptide (ANP)

Reduces blood pressure and blood volume. It is released by certain cells of the heart atria in response to stretching. It helps to reduce blood volume by cuasing the kidneys to excrete sodium and water and suppresses the release of ADH, renin, and aldosterone. It also inhibits the reabsorption of sodium by the collecting ducts.

Parathyroid hormone (PTH)

Regulates calcium ion levels in ECF. Is released by the parathyroid glands in response to declining plasma levels of calcium. When released it causes osteoclasts to break down the bone matrix, releasing calcium and hydrogen phosphate to the blood. It also increases calcium reabsorption in the renal tubules while decreasing phosphate ion reabsorption. It also enhances intestinal absorption of calcium indirectly by stimulating the kidneys to transform vitamin D to its active form, which is necessary for the small intestine to absorb calcium.

Respiratory alkalosis

Results when carbon dioxide is eliminated from the body faster than it is produced (hyperventilation). Results in the blood becoming more alkaline. Is often due to stress or pain.

What occurs when aldosterone concentrations are high?

Sodium is actively reabsorbed in the distal convulated tubules and collecting ducts. Since water follows sodium, blood volume is increased as water moves back into the cardiovascular system with sodium. Potassium is secreted back into filtrate as sodium is reabsorbed.

What are the three chemical buffer systems?

Systems of one or more compounds that resist changes in pH when a strong acid or base is added. They bind H+ ions when pH drops and release H+ ions when pH rises. These three systems are: the bicarbonate buffer system (contains carbonic acid and sodium bicarbonate), the phosphate buffer system (works for urine and ICF), and the protein buffer system (most plentiful and best buffer, mostly intracellular).

Dehydration

The loss of fluid (can be either the loss of water or loss of water and solutes together). Water output is greater than water input. Is common following hemorrhaging, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, or diuretic abuse.

What is the most important trigger for the release of aldosterone?

The renin angiotensin-aldosterone mechansim which is mediated by the juxtaglomerular complex of nephrons. Low blood volume and blood pressure trigger the release of renin from granular cells of the JGC through sympathetic stimulation, decreased filtrate NaCl concentration, and decreased stretch of the granular cells of the afferent arterioles.

Respiratory acidiosis

Very common in emergency rooms. Often occurs when a person breathes shallowly or when gas exchange is hampered by diseases such as penumonia, emphysema, or cystic fibrosis. Causes CO2 to accumulate in the blood and lower pH.

What are considered insensible losses of water output?

Water loss from the skin and from water vapor lost during expired air from the lungs

What are considered sensible losses of water output?

Water lost in urine (about 60%), water lost in feces, and excessive sweating. It is any type of measurable water loss.

Describe the composition of body fluids

Water serves as the universal solvent in which a variety of solutes are dissolved. Solutes are classified as either electrolytes (chemical compounds that dissociate into charged ions in water) or nonelectrolytes (have bonds, usually covalent). Electrolytes include inorganic salts, inorganic and organic acids, and some proteins. Most non-electrolytes are glucose lipids, creatinine, and urea. Electrolytes have the greatest osmotic power due to their molecule dissociation.

Where does water output result from?

Water that vaporizes out of the lungs in expired air or diffuses directly through the skin (known as insensible water loss). Measurable water loss is known as sensible water loss which includes water lost as urine (about 60%), sweat, and feces.

Describe the respiratory buffer system

Works in a matter of minutes. If CO2 levels rise/pH becomes lower it activates medullary chemoreceptors which cause respiratory rate and depth to increase in order to cause more CO2 expulsion and reduce the H+ concentration. When blood pH rises, the respiratory centers are depressed and the respiratory rate drops and respiration becomes shallower, allowing CO2 to accumulate and cause H+ concentrations to increase (lowering pH).

Describe how the renal buffer system works

Works over a period of days. Rids the body of phosphoric acid, lactic acid, uric acid, and ketones. Also conserves or generates new bicarbonate and excretes bicarbonate when necessary. Generates new bicarbonate by excreting hydrogen ion or by excreting ammonium ion produced by glutamine metabolism in the PCT.

If osmolality is high in ECF will ADH be released?

Yes

What the pH range that would be considered alkalosis?

pH above 7.45

What is the pH range that would be considered acidosis?

pH below 7.35