Fluid, Electrolyte, and Acid-Base Balance in Blood Anatomy Ch. 15 Part 2

Introduction The kidneys have 4 major roles to play: Excretion of nitrogenous wastes Water balance Electrolyte balance Ensuring proper blood pH

Maintaining water balance of blood In a healthy young adult, water accounts for half or more of body weight. Water is the universal body solvent within which all solutes (including electrolytes) are dissolved. Water occupies 3 main locations in the body referred to as fluid compartments: Intercellular fluids (ICF) – found in living cells Extracellular fluids (ECF) – all body fluids outside the cells Blood plasma and interstitial (tissue) fluid

Regulation of Water Intake and Output If the body is to remain properly hydrated, we cannot lose more water than we take in. The thirst mechanism is the driving force for water intake. An increase in plasma solute content of only 2 to 3 percent excites cells in the hypothalamus called osmoreceptors which activate the thirst center. A dry mouth also occurs because the salivary glands obtain water from blood. When there is less fluid available in the blood stream, less saliva is produced.

Most water enters the body through beverages Most water leaves the body through urine, vaporization from lungs and skin, and sweat. Antidiuretic hormone (ADH): is a hormone that prevents excess water loss in urine. It travels in the blood to its main target, the kidney’s collecting ducts, where it causes the duct cells the reabsorb more water.

Maintaining electrolyte balance in blood Electrolytes are charged particles (ions) that conduct an electrical current in an aqueous solution. Most electrolytes enter the body in food and mineral rich water. The major organ that regulates the electrolyte composition of body fluids is the kidneys. Reabsorption of electrolytes by the kidneys is regulated by hormones. Very small changes in electrolyte concentrations cause water to move from one area to another.

Sodium is the electrolyte most responsible for osmotic water flow. Aldosterone is another hormone that helps regulate blood composition and blood volume by acting on the kidneys. Aldosterone regulates sodium content of blood plasma and tissue fluid and in the process helps regulate the concentration of other ions such as chlorine, potassium, and magnesium. Sodium is the electrolyte most responsible for osmotic water flow. When too little sodium is in the blood, the blood becomes too dilute. This causes water to leave the blood stream and flow out into tissue space.

Most sodium ions are reabsorbed into the blood from the PCT. If aldosterone is present, most remaining sodium is reabsorbed in the DCT. Generally speaking, for each sodium ion reabsorbed a chloride ion follows and a potassium ion is secreted into the filtrate in the tubule. Thus as sodium content of blood increases, potassium concentration decreases bringing these 2 ions back to their normal balance in the blood.

As a general rule: water follows salt Basically Another effect of aldosterone is to increase water reabsorption into the blood because as sodium is reclaimed water follows it back into the blood. As a general rule: water follows salt Basically If sodium ion levels increase in blood then then water will be reabsorbed into the blood. If sodium ion levels decrease in blood then water will be released into surrounding tissue and transported out of the body.

The most important trigger for aldosterone release is the renin-angiotensin mechanism. This mechanism is regulated by the renal tubules. This apparatus is made up of cells in the afferent arteriole (beginning ) and modified cells in the DCT (end). When cells in this apparatus are stimulated by low blood pressure in the afferent arteriole or changes in solute concentration of filtrate in the DCT, they respond by releasing the enzyme renin into the blood.

Renin starts a series of reactions that produces a substance called angiotensin. Angiotensin acts on the blood vessels to cause vasoconstriction and causes aldosterone to be released. As a result of vasoconstriction and aldosterone release blood volume and blood pressure increases. This reaction is very important in regulating blood pressure.

Maintaining Acid-Base Balance of Blood For the cells of the body to function properly, blood pH must be between 7.35 and 7.45 Alkalosis: pH above 7.45 Acidosis: pH below 7.35 Most hydrogen ions in the body come from cellular metabolism which adds substances to the blood. Many different acids and bases are produced by the body Chemical buffers in the blood and the respiratory system are 2 pH controlling systems The kidneys assume most of the load for acid-base balance.

Blood Buffers Blood buffers act to prevent dramatic changes in hydrogen ion concentration when acids or bases are added. Blood buffers bind to H+ when pH drops (becomes too acidic) and release H+ when pH rises (becomes too basic). Buffers act very quickly and are the first line of defense in resisting pH changes.

Definitions of strong and weak acids and bases Acids: proton (H+) donors Acidity of a solution reflects only the free hydrogen ions Strong acids: dissociate completely and release all H+ ions in water and can cause large changes in pH Weak acids: dissociate only partially and have less of an effect on pH but are very effective in preventing pH changes Bases: proton (H+) acceptors Strong bases: dissociated easily in water and quickly tie up H+ Weak bases: slower to accept H+ ions but as pH drops they become stronger Both weak acids and bases are valuable members of the chemical buffer system

The 3 major chemical buffer systems Bicarbonate Phosphate Protein All 3 of these systems work together and anything that causes a shift in one area, causes a shift in all areas. All 3 systems operate in a similar way The only system covered in this text is the bicarbonate system which is important in preventing changes in blood pH

Bicarbonate buffer system A mixture of carbonic acid (weak acid) and its salt, sodium bicarbonate (weak base) Because carbonic acid is weak it does not dissociate much in neutral or acidic solutions When a strong acid is added, most of the carbonic acid remains intact but the bicarbonate of the salt acts as a base to tie up the H+ ions released by the stronger acid. Because the strong acid is changed into a weak one, it lowers the pH. Strong acid + weak base weak acid + salt

If a strong base is added to a solution containing the bicarbonate buffer system the bicarbonate will not dissociate. The carbonic acid will dissociate in the presence of the strong base which releases more H+. The strong base will be replaced by a weak one. Strong base + weak acid weak base + water

Respiratory system controls When CO2 enters the blood from tissue cells, most of it enters the red blood cells where it converted to bicarbonate for transport as plasma. An increase in CO2 or H+ produces more carbonic acid. In healthy people, CO2 is expelled from the lungs at the same rate as it is formed in the tissue. H+ ion concentration does not increase during the transport of CO2 because it is tied up in the water portion of blood plasma. H+ ions produced by CO2 transport have no effect on blood pH.

When CO2 does accumulate in blood or more H+ is released into the blood, the respiratory centers in the brain are activated. When this occurs, breathing rate and depth increases and the excess H+ are removed as more CO2 is removed. If blood pH begins to rise (become more basic), the respiratory centers are depressed. When this occurs, rate and depth decrease allowing CO2 and H+ to accumulate in blood. As a general rule, H+ follows CO2

Renal Mechanisms Although the kidneys act slowly and require a long time to make changes in blood pH, they are the most important mechanism for regulating blood pH. Mechanisms to maintain acid-base balance: Excreting bicarbonate ions Losing a bicarbonate (base) has the same effect as gaining an H+ (acid) As blood pH rises 0r becomes more basic, bicarbonate ions are excreted which will lower the pH Conserving or generating new bicarbonate ions Reabsorbing a bicarbonate (base) is the same as losing an H+ (acid) As blood pH falls or becomes more acidic, bicarbonate is reabsorbed which will raise pH