Fluid balance

What is a sensible water loss

Urine, faeces, blood loss

What is an insensible fluid loss

Skin

Lungs

How can water exit the body and roughly how much is lost from each source?

Skin—450 mL daily in temperate climates 

Kidneys—1 to 2 L of urine daily.

Respiratory mucous membranes—breath (250–350 mL daily).

Digestive system, including faeces (200 mL daily).

What causes increased water loss from the skin

Higher temperature

What causes increased water loss from the kidneys

Water loss via urine increases during hyperglycemia (diabetes mellitus) as well as in disorders involving antidiuretic hormone (vasopressin).

What causes increased water loss from respiratory mucous membranes?

Respiratory infections trigger increased mucous secretions, resulting in increased water loss

What causes increased water loss from the digestive system?

Vomiting and diarrhoea

What are the uses of water in the body

• Regulating Body Temperature

• Moistening Tissues

• Protecting Organs and Tissues

• Delivering Nutrients and Oxygen

• Lubricating Joints

• Supporting Digestion

• Flushing Out Waste

Define fluid balance

Fluid balance is the regulation of the body’s fluid compartments to maintain a stable internal environment.

What are the complications of inadequate fluid intake?

Dehydration can leave you feeling tired, nauseous, and often results in frequent headaches, constipation, and, in some cases, urinary tract infections.

What are the main fluid compartments

• Intracellular

• Extracellular

  • Plasma (inside blood vessels)

  • Interstitial (between cells)

  • Transcellular (pericardial, CSF, synovial fluid—i.e., within an epithelial-lined space)

Describe the percentage of body composition that is water and how is this then divided between intracellular and extracellular fluid

Total body water is about 60% of their body weight

Roughly 2/3 of that is intracellular fluid, and the other 1/3 is extracellular fluid.

What ions does intracellular fluid dissolve?

The major intracellular cations are potassium (K+) and magnesium (Mg2+), whereas the major anions are proteins and organic phosphates like ATP.

What ions does extracellular fluids dissolve?

The major cation in extracellular fluid, both in the interstitial fluid and in the plasma, is sodium (Na+), and the major anions are chloride (Cl-) and bicarbonate (HCO3-).

How does plasma and interstitial fluid differ in composition

So compared to the plasma, interstitial fluid tends to have higher concentrations of small anions like Cl- and lower concentrations of small cations like Na+.

Describe plasma

Plasma is the only major fluid compartment that exists as a real fluid collection all in one location.

How does plasma differ from interstitial fluid

It differs from ISF in its much higher protein content and its high bulk flow (transport function)

What is the function of plasma

• Transports nutrients, hormones and proteins

• Removes waste

What is the function of the interstitial fluid

Provide the cells of the body with nutrients and a means of waste removal.

The interstitial ECM contains a great deal of connective tissue and proteins (such as collagen) that are involved in blood clotting and wound healing.

What is Transcellular fluid

Transcellular fluid is the portion of total body water contained within the epithelial-lined spaces

What is the function of transcellular fluid

 lubrication of epithealial lined cavities, and sometimes electrolyte transport.

Describe the composition of ICF

ICF is the fluid within cells. It contains high levels of potassium (K⁺) and negatively charged proteins.

What is the function of ICF (Intracellular fluid)

Supports cell metabolism, nutrient transport, and waste removal.

What separates intracellular and intrastitial fluid

Plasma membranes separate IF from ICF

What factors affect fluid balance

Osmosis:

Hydrostatic Pressure: .

Solute Concentration:

Protein Channels:

Describe osmosis

Water moves across semi-permeable membranes based on solute concentration gradients.

Describe hydrostatic pressure

Differences in pressure drive fluid movement.

Describe solute concentration as a part of fluid balance

Maintaining proper solute balance ensures normal cell function.

Describe Protein Channels role in fluid balance

These regulate solute movement

What is oral rehydration therapy

Oral rehydration therapy (ORT) is a simple, cost-effective treatment for dehydration, especially due to diarrhoea.

What are the key mechanisms involved in oral rehydration therapy?

Sodium-Glucose Co-Transport

Water Absorption

Describe Sodium-Glucose Co-Transport

In the small intestine, sodium (Na⁺) and glucose are absorbed together through a co-transport mechanism.

This process is facilitated by a specific protein called the sodium-glucose linked transporter 1 (SGLT1).

When glucose is absorbed, it pulls sodium along with it into the intestinal cells.

Describe water absorption as a part of ORT

The absorption of sodium and glucose creates an osmotic gradient.

Water follows this gradient and moves from the intestinal lumen into the cells and then into the bloodstream.

This helps to rehydrate the body by replenishing lost fluids.

Describe the practical applications of oral rehydration therapy

ORT solutions typically contain a precise balance of salts (sodium chloride), glucose, and water.

The presence of glucose in the solution enhances the absorption of sodium and, consequently, water.

This method is effective even in severe cases of dehydration and can be administered easily without the need for intravenous fluids.

What are the benefits of ORT

Accessibility: ORT can be administered at home, making it accessible in low-resource settings.

Effectiveness: It quickly restores hydration and electrolyte balance.

Safety: It has a low risk of complications compared to intravenous rehydration.

Draw the kidney structure

Draw the structure of the nephron

Describe the roles of the kidney

• Filtration

• Reabsorption

• Excretion

• Regulation of electrolytes

• Acid-Base balance

• Hormone production

Describe filtration in the kidney

• The kidneys filter blood to remove waste products and excess substances, including water.

• Each kidney contains about a million nephrons, which are the functional units responsible for this filtration process.

Describe reabsorption in the kidney

• After filtration, the kidneys reabsorb necessary substances like water, glucose, and electrolytes (sodium, potassium, calcium) back into the bloodstream.

• This process ensures that essential nutrients and fluids are retained while waste products are excreted.

Describe Excretion in the kidney

• The kidneys excrete waste products and excess substances in the form of urine.

• This helps to regulate the volume and composition of body fluids.

Describe regulation of electrolytes in the kidney

• The kidneys maintain the balance of electrolytes in the body by adjusting the reabsorption and excretion of ions like sodium, potassium, and calcium.

• This balance is vital for proper nerve and muscle function.

Describe acid-base balance in the kidney

• The kidneys help maintain the body’s pH balance by excreting hydrogen ions and reabsorbing bicarbonate from urine.

• This process is essential for preventing acidosis or alkalosis, which can disrupt normal bodily functions.

Describe hormone production in the kidney

• The kidneys produce hormones like renin, which regulates blood pressure, and erythropoietin, which stimulates red blood cell production.

• They also convert vitamin D into its active form, which is important for calcium absorption and bone health.