Fluids

Question 1

What percent of the body is made up of water?

Answer 1

60% in men, 50% in women. The difference is that men have more muscle and muscle contains more water.

Question 2

Where is the intracellular fluid? Extracellular fluid?

Answer 2

Intercellular fluids are the fluids inside cells. Extracellular fluids are found in two places: blood plasma and the fluids that bathe all body cells (interstitial fluids).

Question 3

Extracellular fluid is found in two places. What are they?

Answer 3

see above.

Question 4

Define the term electrolyte:

Answer 4

Nonelectrolyte: Electrolytes dissassociate into ions when placed in water. Nonelectrolytes do not dissassociate in water.

Question 5

Define the terms isotonic, hypertonic, hypotonic and explain what way water moves in each environment.

Answer 5

Isotonic means that the ratio of solutes to water on each side of the membrane is the same. Hypertonic means that a solution has more solutes to water as compared to another solution. Hypotonic means that a solution has less solutes to water as compared to another solution. In an isotonic environment water and solutes will move across the membrane, but they will move equally - no net movement in either direction. They system is in equilibrium. If you place a cell in a hypertonic environment (meaning that there are more solutes/water in the solution as compared to inside the cell), water will move out of the cell. If you place a cell in a hypotonic environment, water will enter the cell. A basic rule of thumb is that water will always move to the solution that has a higher amout of solutes/water. The goal of the system is to reach the point at which there is an equal ration of solutes to water, or an isotonic condition. This will only happen when the solutes can NOT cross the membrane. This is talking about situations where osmosis is occuring.

Question 6

Define the term osmolality:

Answer 6

the number of dissolved solutes per unit of water.

Question 7

Why do electrolytes contribute more to osmolality than nonelectrolytes?

Answer 7

One electrolyte, once it enters water, dissassociates into at least 2 solutes, maybe more. A nonelectrolyte doesn’t dissassociate in water, so it only contributes one molecule, even if it’s bigger than an ion, it is still only one molecule, giving is less power over the osmolality of the solution.

Question 8

Compare and contrast extracellular and intracellular fluids. How much Na+ is in each? K+? What makes up the bulk of dissolved substances in the body?

Answer 8

In the extracellular fluid the main cation is Na+ and the main anion is Cl-. Intracellular fluids have the main cation of K+ and the main anion of HPO4–. Electrolytes are highest in number in all fluids.

Question 9

If something needed to move from the blood to a cell, describe the compartments it would travel through and the membranes it would need to cross.

Answer 9

It would first cross the wall of the capillary (simple squamous epithelium), then it would be in the interstitial fluid. Then, it would cross the wall of the cell (phospholipid bi-layer).

Question 10

What are sources of water intake? Output?

Answer 10

Sources of water input are water from food, water from drinking, water from metabolism. Water outputs are water vapor from our breath, sweat, urine and feces.

Question 11

How does ADH (anti-diuretic hormone) help maintain the narrow range of solute concentration in the body fluids?

Answer 11

ADH is made by the hypothalamus and stored in the posterior lobe of the pituitary gland. It is released when we are dehydrated. It acts on the principal cells of the collecting duct, causing them to increase manufacturing of aquaporins, transport proteins for water. More water then moves from the filtrate into the principal cells. Then it moves from the principal cells to the blood. In this way, we retain more water and create a concentrated urine.

Question 12

What three factors govern the thirst mechanism? Briefly explain each.

Answer 12

First there are osmoreceptors. When cells stretch because they take on more water, mechanoreceptors signal the hypothalamus which decreases secretion of ADH. Second, dry mouth occurs when you are dehydrated. There is a lot of water in saliva and decreasing water here to keep blood pressure up is one way your body counters dehydration. You experience a dry mouth. Dry mouth signals the thirst response in the brain. Third, there is a decrease in blood volume/pressure. Baroreceptors pick this up and signal your brain to turn on the thirst response.

Question 13

Explain why ‘water follows salt’. Give examples.

Answer 13

When Na+ (salt) moves from the blood to the interstitial fluid, the solute concentration in the interstitial fluid increases. Your body is always trying to maintain isotonic conditions across all fluid compartments. If the interstitial fluid becomes hypertonic (more salty), then in order to create an equal ratio of solutes (Na+) to water, more water must enter this compartment. So, water follows the salt from the blood into the interstitial fluid. Then, the salt moves from there into cells. The cells become hypertonic, therefore water follows the Na+ to create isotonic conditions in the cell.

Question 14

What are the two salts that contain sodium in the ECF?

Answer 14

NaHCO3 and NaCl.

Question 15

What are the main functions of sodium in the body?

Answer 15

There are two main functions of Na+. First, to control the ECF volume (remmeber that the ECF refers to fluid in two areas - the blood plasma and the interstitial fluids). Second, to control water distribution between fluid compartments (see my answer to #13).

Question 16

What’s the difference between sodium concentration and sodium content?

Answer 16

Sodium content is the number of Na+ ions in the body. You would count them. The concentration is the ratio of sodium ions to water. The more sodium ions per volume of water, the higher the concentration. If there aren’t many Na+ ions in the same amount of water, there is a lower concentration.

Question 17

How does water retention work? Why do we retain water when we eat too much salt?

Answer 17

If we eat too much salt the salt is absorbed from our digestive tract into the blood. The sodium concentration of the blood increases. Water enters the blood from the interstitial fluid to bring the conditions to isotonic. The more salt we eat, the more water we need in order to maintain isotonic conditions. The less salt we eat, the less water we need, and any excess is lost in the urine.

Question 18

Explain how aldosterone works.

Answer 18

When we are dehydrated aldosterone is secreted by the adrenal cortex. Aldosterone works on the principal cells of the collecting duct of the kidney. It affects those cells by increasing the manufacturing of transport proteins for Na+. More Na+ moves from the filtrate inside the tubule to the cells that line the tubule. From there, the Na+ moves into the blood. When we retain Na+, we retain more water, which helps when we are dehydrated.

Question 19

Explain how Atrial Natriuretic Peptide hormone works.

Answer 19

When blood pressure is high baroreceptors in the atria of the heart are stimulated to secrete ANP. ANP works on the principal cells of the collecting duct in the kidney. It’s effect is to decrease production of these Na+ tranpsort proteins, so more Na+ is excreted in the urine. Less water is retained and more is lost in the urine.

Question 20

Why is the homeostasis of K+ levels important?

Answer 20

K+ ions directly affect the ability of nerves and muscles to fire and contract.

Question 21

Explain how the kidneys make sure that there is just the right amount of K+ in the body.

Answer 21

Almost all of the K+ is reabsorbed in the proximal convoluted tubule and loop of henle (nephron loop). The distal convoluted tubule and collecting duct then secrete K+ into the filtrate if necessary. In this way the amount of K+ in the body is very fine tuned.

Question 22

What is optimal pH in the body?

Answer 22

Optimal blood pH is 7.35-7.45.

Question 23

Where does the excess of H+ come from when fluids are too acidic?

Answer 23

The excess H+ can come from foods we eat, the breakdown of metabolic products like lactic acid, and other less important sources.

Question 24

What three ways does the body buffer any excess H+?

Answer 24

Chemical buffers (like bicarbonate and proteins), brain stem respiratory centers (we breath faster when the blood gets acidic), and the kidneys (intersticial cells can secrete H+ into the filtrate or reabsorb H+ if needed).

Question 25

Define the term acid:

Answer 25

1-6.9 on the pH scale. This is a H+ or proton donor. In other words, it will donate a H+ to the solution it is added to. This happens when you put a salt into water and one of the peices that results is H+. HCl is an example. When you put it into water it comes apart into H+ and Cl-.

Question 26

What’s a strong acid versus a weak one?

Answer 26

A strong acid is a molecule, like HCl, that completely dissassociates when you put it into water. So, every single HCl will come apart into H+ and Cl-. A weak acid, like H2CO3 (carbonic acid) will dissassociate, but not completely. In other words, some molecules will come apart into H+ and CO3, but some won’t come apart.

Question 27

Give an example of a molecule that acts as an acid in solution:

Answer 27

HCl

Question 28

Define the term base: Give an example:

Answer 28

The definition of a base is that it is a proton (H+) acceptor. In this case, you put a molecule (salt) into water and it comes apart. One of the peices is OH-, or something else that will bind with H+. An example is NaOH. If you put this in water it comes apart into OH- and Na+. The OH- will bind with H+ to form H2O. This neutralizes the H+, so that it doesn’t have an affect on the pH.

Question 29

What’s a strong base versus a weak base?

Answer 29

A strong base completely dissassociates and a weak base dissassociates, but not completely.

Question 30

Define the term buffer:

Answer 30

A buffer is a molecule that can accept H+ to neutralize their affect on the pH.

Question 31

Describe how the bicarbonate buffer system works.

Answer 31

The bicarbonate buffer system contains a mixture of carbonic acid (H2CO3) which is a weak acid, and sodium bicarbonate (NaHCO3), a weak base. If you add a strong acid ((HCl) to this mixture, the carbonic acid remains intact, but the NaHCO3 dissassociates incompletely. It’s Na+ combines with the Cl and the HCOe3 combines with the H+, taking it out of solution (neutralizing it). Another way of looking at it is this - HCl comes apart into H+ and Cl-. This happens completely. The weak base comes apart into Na+ and HCO3-. The HCO3- combines with the H+ to form H2CO3 and the Na+ and Cl- combine to form NaCl. The pH falls, but not as much as it would if there wasn’t the weak base around to buffer the effect. If you were to add a strong base like NaOH, nothing would happen to the weak base (NaHCO3), but the weak acid dissassociates incompletely. So, there is some H+ and HCO3-. The OH would drive the pH up quite a bit, but instead some of it combines with the H+ that has been freed up from the weak acid. H+ + OH- forms water, neutralizing the OH- affect on the pH.

Question 32

Describe how the protein buffer system works.

Answer 32

Each of the 20 amino acids has a central carbon, an amine group and a carboxyl group. If the pH increases, the carboxyl group can donate an H+, bringing the pH back down. If the pH decreases, H+ can bind to the amine group of the protein, taking the H+ out of solution so it doesn’t affect the pH so much.

Question 33

How is hemoglobin in red blood cells a good example of the protein buffer system?

Answer 33

CO2 is released from tissues. It combines with water to form H2CO3 (carbonic acid). Carbonic acid is a salt that comes apart into H+ and HCO3- (bicarbonate). So, as levels of CO2 increase the blood pH decreases (becomes more acidic). At the same time, hemoglobin in the blood is unloading oxygen into the tissues. This means that it is becoming more negatively charged. H+ binds to the hemoglobin (to the amine group of the globin protein) and this minimizes the affect of H+ on the blood pH.

Question 34

Describe the respiratory regulation of H+.

Answer 34

When there is too much CO2 it causes blood pH to become acidic (see above). CO2 binds to water to form H2CO3, which breaks down into H+ and HCO3- (bicarbonate). This creates acidity. When this happens it stimulates the respiratory center to increase respiratory rate. We breath out faster, releasing CO2. In the lungs the H+ and HCO3 combine to form H2CO3, which splits to form water and CO2 and then the CO2 is released by breathing it out. This brings the concentration of H+ back down and pH rises. If there isn’t enough H+ in the body, one way to compensate is to slow the breathing rate, which would decrease the rate that CO2 is being released from the body. This keeps the pH up.

Question 35

What does the kidney do when the blood is too acidic?

Answer 35

The kidney’s secrete H+ from the cells to the filtrate. The H+ moves from the blood to the tubule cells and from the tubule cells into the filtrate. Also, HCO3- is conserved. It remains in the blood and binds to H+, neutralizing its affect on the blood pH. More specifically, in the tubule cell, CO2 combines with water to form carbonic acid, which splits into H+ and HCO3-. The H+ is excreted into the filtrate. The HCO3- returns to the blood to bind with another H+. The H+ that is excreted to the filtrate binds to a HCO3- there and is then excreted as water.

Question 36

What does the kidney do when the blood is too basic?

Answer 36

When it’s too basic the intercalated cells in the collecting duct of the kidneys excrete HCO3- and reclaim H+.

Question 37

Describe the phosphate buffer system in the kidney that helps decrease acidity.

Answer 37

Another pathway could be this: CO2 combines with water in the tubule cell to make carbonic acid, which splits into H+ and HCO3-. The H+ is excreted into the filtrate. Once there it binds to hydrogen phosphate (HPO4-) to form H2PO4, hydrogen diphosphate. The HCO3- goes back into the blood to bind to an H+. In this way the body actually adds another bicarbonate to the blood. Instead of gaining one in the blood and using one in the filtrate, for an even exchange as described in #35, HPO4- is used to bind H+ in the filtrate, so the bicarbonate that moves back into the blood is an actual gain of one.