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Chapter 27

Nursing HAP201 Chapter Notes - Chapter 27: Hyperkalemia, Resting Potential, Weak Base

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Nursing HAP201
Judith Card

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HAP201 Week 12-13Chapter 27: Fluid, Electrolyte and Acid-Base Homeostasis
Body fluids are present in two main “compartment” – inside and outside cells
2/3 of body fluids is intracellular fluid (ICF) or cytosol, the fluid within cells
1/3 of body fluids is extracellular fluid (ECF), the fluid outside cells and it includes all other body fluids.
o Most of ECF is interstitial fluid (IF), which occupies the microscopic spaces between tissues cells
o Some of it also plasma, the liquid portion of the blood
other interstitial fluids include
Lymph in lymphatic vessels
Cerebrospinal fluid in the NS
Synovial fluid in joints
Aqueous humor and vitreous body in the eyes
Endolymph and perilymph in the ears
Pleural, pericardial and peritoneal fluids between serous membranes
There are two “barriers” that separate intracellular fluid, interstitial fluid and blood plasma
o Plasma membrane: individual cells separates intracellular fluid from the surrounding interstitial fluid. It is
selectively permeable, i.e. it allows some substance to cross but blocks the movement of other substances. Active
transport pumps work to maintain different concentrations of certain ions in the cytosol and interstitial fluid
o Blood vessel walls divide the interstitial fluid from blood plasma.
Filtration, reabsorption, diffusion, and osmosis allow continual exchange of water and solutes among body fluid
Since osmosis is the primary means of water movement between IF, the concentration of solutes in these fluids determines
the direction of water movement
Most solutes in body fluids are electrolytes (inorganic compounds that dissociate into ions), so fluid balance is closely
related to electrolyte balance
Because intake of water and electrolytes rarely occurs in exactly the same proportions as their presence in bodily fluids, the
ability of the kidneys to excrete excess water by producing dilute urine, or to excrete excess electrolytes by producing
concentrated urine, is important in maintaining homeostasis
Sources of Body Water Gain and Loss
The body gains water by ingestion and by metabolic synthesis
The main sources of body water are ingested liquids and moist foods absorbed from the GI tract
The other source of water is metabolic water that is produced in the body mainly when electrons are accepted by oxygen
during aerobic respiration and during dehydration synthesis reaction
Body fluid volume remains constant because water loss equals water gain
Water loss occurs in four ways
o The kidneys excrete water in urine
o The skin evaporates water through insensible perspiration (sweat that evaporates before it is perceived as moisture)
o The lungs exhale water was vapor
o The GI tract eliminates water in feces
o (in women, water is lost in menstrual flow)
Regulation of Body Water Gain
The volume of metabolic water formed in the body depends entirely on the level of aerobic respiration, which reflects the
demand for ATP in body cells
When more ATP is produced, more water is produced
The thirst centre in the hypothalamus regulates volume intake
When water loss is greater than water gain, dehydration (a decrease in volume and an increase in osmolarity of body fluids)
stimulates thirst
A decrease in blood volume causes blood pressure to fall
o Kidneys then releases renin, which promotes angiotensin II formation
o Increased nerve impulses from osmoreceptors in the hypothalamus, triggered by increased blood osmolarity, and
increased angiotensin II in the blood stimulate the thirst centre in the hypothalamus
Neurons in the mouth also detect dryness due to a decreased flow of saliva
Baroreceptors that detect lowered blood pressure in the heart and blood vessels also stimulate thirst
Regulation of Water and Solute Loss
Even though the loss of water and solutes through sweating and exhalation increases during exercise, elimination of excess
body water or solutes occurs mainly by control of their loss in urine.
The extent of urinary salt (NaCl) loss is the main factor that determines body fluid volume
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