7.2: Fluid Balance
- Page ID
- 156615
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Maintaining proper fluid balance is essential for physiological function, and water plays a central role in this regulation. Electrolytes are also vital for fluid balance, because where electrolytes go, water will follow. In the extracellular fluid, sodium and chloride predominate, whereas potassium and phosphate predominate in the intracellular fluid (Table 7.1). In the gastrointestinal tract, particularly the colon, fluid absorption is closely linked to sodium transport. For example, sodium is actively transported into colonic epithelial cells, creating a concentration gradient. This increased sodium concentration inside the cells drives osmosis, causing water to move from the intestinal lumen into the epithelial cells. This mechanism is critical for water reabsorption, and without it, the body would lose significant amounts of water in the feces, leading to dehydration.
Fluid balance also plays a major role in the regulation of blood volume and blood pressure. A high intake of sodium chloride (salt) can disrupt this balance. When sodium is not efficiently reabsorbed by the kidneys, it accumulates in the bloodstream. Water then follows sodium into the intravascular space, increasing blood volume. This expanded volume exerts greater pressure on blood vessel walls, leading to elevated blood pressure. Therefore, dietary sodium and the body’s ability to regulate water movement are closely intertwined in maintaining overall fluid homeostasis.
Dehydration
Dehydration occurs when the body loses more fluids than it takes in, resulting in an insufficient amount of water to carry out normal physiological functions. This imbalance can arise from inadequate fluid intake, excessive fluid loss (through sweating, vomiting, diarrhea, or urination), or a combination of both. Symptoms of dehydration range from mild to severe and can include thirst, dry mouth, fatigue, headache, dizziness, decreased urine output, and dark-colored urine. In more severe cases, dehydration can lead to confusion, rapid heart rate, low blood pressure, and even organ failure. The consequences of dehydration are particularly concerning because they can impair cognitive performance, physical endurance, thermoregulation, and kidney function. Certain populations are especially at risk, including infants and young children (due to their higher body water turnover), older adults (who may have a diminished sense of thirst), athletes (because of fluid loss through sweat), and individuals with chronic illnesses or those experiencing fever, diarrhea, or vomiting. Recognizing the signs and risks of dehydration is key to preventing serious health outcomes and maintaining overall hydration and well-being.
The Body’s Response to Dehydration
When the body experiences dehydration, it initiates a complex hormonal response to conserve water and restore fluid balance. Ultimately, the body tries to retain as much fluid as possible.
Body water losses are rapidly reflected in the blood. Changes in blood volume and electrolyte concentrations in response to decreased blood water content (and therefore increased blood osmolality) trigger the hypothalamus in the brain to stimulate the release of antidiuretic hormone (ADH) from the pituitary gland. ADH then signals to the kidneys to conserve water by increasing reabsorption of water into the blood and concentrating the urine (Figure 7.3).
Decreased blood volume due to dehydration also results in a complex series of steps. One of the first steps involves the release of renin from the kidneys, which catalyzes a cascade of reactions. Renin converts angiotensinogen, a protein produced by the liver, into angiotensin I, which is then converted into angiotensin II by an enzyme released from the lungs (angiotensin-converting enzyme, ACE). Angiotensin II plays multiple critical roles: it stimulates the adrenal glands to release aldosterone, promotes the release of ADH, and triggers the sensation of thirst. Aldosterone acts on the kidneys to increase the reabsorption of sodium and chloride, which in turn promotes water retention, as water follows sodium back into the bloodstream via osmosis. Meanwhile, ADH signals the kidneys to conserve water directly by concentrating urine, as previously noted. Together, these actions help increase blood volume and stabilize blood pressure, allowing the body to correct for water loss and prevent further complications from dehydration (Figure 7.4).