Untitled Quiz

Homeostasis: Regulation of Body Water & Electrolyte Balance

• Homeostasis involves regulating body water and electrolyte balance
• Water makes up approximately 60% of the body, ranging from 45-75% depending on factors like age and sex
• Water is crucial for protecting sensitive tissues, removing waste, regulating temperature, lubricating joints, and enabling cell function and chemical reactions in the digestive system

Previously on Homeostasis Pt 1

• Introduction to homeostasis
• Negative feedback mechanisms
• Body temperature regulation
• Blood glucose regulation were discussed

Importance of Body Water

• Functions:
  • Protects spinal cord and other sensitive tissues
  • Removes waste through urination, sweating, and bowel movements
  • Regulates body temperature
  • Lubricates and cushions joints
  • Provides optimal medium for cell function
  • Facilitates chemical reactions
  • Breaks down food particles in the digestive system

Body Fluid Compartments

• Total body fluids make up 55-60% of total body mass
• Intracellular Fluid (ICF) - about 2/3 of fluid
• Extracellular Fluid (ECF) - about 1/3 of fluid includes plasma (~20% of ECF) and interstitial fluid (~80% of ECF).
• Diagram illustrating distribution of water in body compartments
• Diagram illustrating exchange of water between compartments

Fluid Balance

• Two barriers separate intracellular fluid (ICF) and extracellular fluid (ECF):
  • Plasma membrane separates ICF from ECF
  • Capillary membranes separate interstitial fluid (IF) from plasma
• Body is in fluid balance when correct amounts of water and solutes are present and balanced in the body compartments
• Constant exchange of water and solutes among compartments takes place through filtration, reabsorption, diffusion, and osmosis

Daily Water Gain and Loss

• Water gain through metabolic water production, ingested fluids (including food)
• Water losses include those from the GI tract, lungs (respiration), skin (perspiration), and kidneys
• Quantities associated with different loss mechanisms

Regulation of Body Water Gain

• Primarily driven by water intake
• Dehydration occurs when water loss exceeds intake, leading to decreased blood volume, increased blood osmolarity, and reduced blood pressure.
• Reduced blood volume triggers mechanisms to increase thirst, stimulating the release of ADH to retain water, increasing renin release to increase in angiotensin II formation to cause vasoconstriction

Kidneys

• The kidneys act as the primary organs involved in regulating body water and electrolyte balance
• They are responsible for urine formation, and help regulate blood electrolyte levels
• Consist of Cortex, Medulla (pyramids), renal artery, renal vein, ureter, pelvis

Nephron

• Nephron acts as the functional unit of the kidney
• One human kidney contains ~one million nephrons
• Nephron manufactures urine

Nephron: Parts & Function

• Glomerulus filters blood
• Proximal convoluted tubule reabsorbs glucose, salt, and amino acids
• Loop of Henle reabsorbs water and salts from filtrate
• Distal convoluted tubule reabsorbs salt and some water
• Collecting duct reabsorbs water and salt, and makes urine more concentrated

Selective Reabsorption in the Nephron

• Tubules of nephron reabsorb all glucose, amino acids, and most mineral salts
• Majority of water is reabsorbed in loop of henle and parts of collecting ducts.
• Metabolic wastes are directed towards collecting ducts.

Large Volume of Filtrate

• Kidneys produce a large volume of filtrate (~180 liters/day).
• The excess amounts of water and wastes are removed from the blood to maintain homeostasis via urine

Hormone Regulating Water Balance

• Antidiuretic hormone (ADH) is the main hormone regulating water balance
• ADH is produced by the hypothalamus and released from the posterior pituitary
• ADH increases the permeability of the kidney tubules to water, leading to water reabsorption and more concentrated urine
• Increased or decreased ADH released results in changing urine output

Factors Regulating Body Water Balance

• Thirst centre in hypothalamus, stimulus for drinking water
• Angiotensin II triggers aldosterone release in kidneys
• Aldosterone promotes sodium and water reabsorption
• Atrial natriuretic peptide (ANP) promotes natriuresis
• Antidiuretic hormone (ADH) regulates water permeability to maintain water and sodium retention

Plasma Osmolality and Plasma Volume

• Hypothalamic thirst center receives signal based on water and osmolality levels
• This center causes sensation of thirst to prompt drinking water to re-establish homeostasis
• Water is carried to target regions in body, to replenish body and blood hydration levels

Regulation of Water Output

• Obligatory water loss involves insensible loss from lungs and skin, and water loss through feces
• Minimum daily sensible water loss is 500 ml of urine to excrete wastes
• Water and sodium levels are regulated together to maintain cardiovascular function and blood pressure

Influence of ADH on Water Output

• Water reabsorption in collecting ducts is proportional to ADH release
• Low ADH results in diluted urine and low body fluid volume
• Higher ADH results in concentrated urine
• Hypothalamic osmoreceptors trigger and inhibit ADH release
• Other factors like fever, sweating, vomiting, diarrhea, blood loss, or traumatic burns can trigger ADH release

Homeostasis: Water Level in the Blood

• Hypothalamus detects high solute concentration in blood. Posterior pituitary releases more ADH.
• The person drinks more water. Kidneys reabsorb more water.
• Water levels rise above the normal range. Hypothalamus detects low solute concentration. Pituitary releases less ADH. Kidneys reabsorb less water

Disorders of Water Balance: Dehydration

• Dehydration is due to excess fluid loss (ECF)
• Causes include haemorrhage, severe burns, vomiting, diarrhea, diuretic abuse, and excessive perspiration
• Signs and symptoms include thirst, dry skin, oliguria, weight loss, fever, mental confusion, and hypovolemic shock
• Electrolyte loss also occurs

Disorders of Water Balance: Mechanism (Hypotonic Hydration)

• Excessive water intake leads to lower osmotic pressure in ECF, causing water to move to ICF by osmosis
• Cells swell

If Water is Lost but Electrolytes Are Retained

• ECF osmotic concentration (osmolarity) rises
• Water moves from intracellular fluid (ICF) to ECF
• Both ECF and ICF become slightly concentrated relative to the initial state
• Net change in ECF is small
• Homeostatic responses (increasing ADH release) will replenish lost water

If Water is Gained but Electrolytes Are Not

• ECF osmotic concentration (osmolarity) decreases
• Fluid loss (urine) occurs: ICF looses water back to ECF
• Restore both volume and concentration balance

Electrolytes in Body Fluids

• Electrolytes become ions when they dissolve and dissociate in the body fluids
• Electrolytes in body regulate osmosis of water across the body compartments, maintain acid-base balance, carry electrical current, and serve as co-factors

Movement of Water Between Compartments

• Normally, cells neither shrink nor swell because intracellular and interstitial fluids have same osmolarity
• Increasing osmolarity of the interstitial fluid draws water out of the cells and causes them to shrink.
• Decreasing the osmolarity causes cells to swell

ICF Differs Considerably from ECF

• ECF's main cation is sodium (Na+) while ECF's anion is chloride (Cl⁻).
• ICF's main cation is potassium (K+), while the main anion is phosphate (HPO₄⁻²).
• The Na+/K+ ion pump maintains high K+ inside cells and high Na+ outside.

Sodium (Na+)

• Most abundant extracellular cation
• Important for fluid and electrolyte balance
• Contributes to osmolarity of ECF
• Sodium levels are controlled by aldosterone, ADH, and ANP (atrial natriuretic peptide)

Chloride (Cl-)

• Most prevalent anion in ECF
• Helps balance levels of anions in different body fluids
• Regulated by ADH and urine

Potassium (K+)

• Most abundant cation in ICF
• Key role in establishing resting membrane potential of muscle fibers and neurons
• Maintaining normal ICF fluid volume
• Controlled by aldosterone; stimulates principal cells in renal collecting ducts to secrete excess K+

Dangers of Too Much Water

• Drinking too much water can dilute electrolytes, leading to hyponatremia (sodium levels below 135 mmol/L)
• Kidneys have limits on water excretion rate

Series of Events in Water Intoxication

• Excessive water intake, reduced blood osmolarity (lowered Na+). Causes water into cells via osmosis
• Cellular swelling can lead to convulsions, coma, and death

Dialysis

• Technique used to remove wastes and excess fluids from the body when kidneys fail
• Hemodialysis uses a filtration process, while peritoneal dialysis uses the lining of the abdominal cavity

Principles of Homeostasis

• Stimuli cause changes in the internal environment. These changes are detected by receptors.
• The self-corrective mechanism responds and rectifies the changes, often through negative feedback mechanisms.
• Examples of homeostasis regulation include blood glucose, body temperature, and fluid balance.