BPK 205: Fluid & Electrolyte Balance Quiz

BPK 205: Intro to Human Physiology

• Course covers fluid & electrolyte balance, and calcium homeostasis
• Lecture 30 focuses on fluid and electrolyte balance
• Lecture 31 focuses on calcium homeostasis

Lecture Objectives – Fluid & Electrolyte Balance

• Describe the body's fluid & electrolyte homeostasis
• Water balance: Vasopressin regulates water reabsorption in the collecting duct
• Aldosterone regulates sodium reabsorption in the distal tubule and collecting duct
• Explain how the Renin-Angiotensin-Aldosterone System (RAAS) maintains blood pressure
• Integrating renal and cardiovascular systems
• Explain how acid-base balance is maintained

Lecture Objectives – Calcium Balance

• Structure and growth of bone
• Roles of osteoblasts and osteoclasts
• Calcium homeostasis
• Hormones, role of bone, kidneys, and GI system

Readings

• Chapter 20: Fluid and Electrolyte Balance
• Chapter 23: Tissue and Bone Growth, and Calcium Balance

Fluid & Electrolyte Homeostasis

• Importance: maintaining ECF volume and osmolarity, blood volume, cardiac and muscle function, excitable cells, exocytosis, muscle contraction, bone formation, and pH balance
• Mass balance: Input + cellular production = cellular consumption + excretion
• Excess substances are excreted by the kidneys
• Integration of respiratory, renal, and cardiovascular systems is needed for proper balance

Maintaining Water Balance

• Water gain and loss: 2.2 L intake (food and drink), 0.3 L metabolic production, ~0.9 insensible loss from skin and lungs, 1.5 L urine, 0.1 L from feces
• Kidneys are vital for conserving or removing body fluids. Water reabsorption is regulated to maintain needed homeostasis

Filtrate Osmolarity Changes

• Urine concentration varies from highly dilute to concentrated
• Osmolarity changes through the nephron; this is dependent on hormonal regulation of water reabsorption in the distal nephron

Vasopressin (ADH)

• Hormone released by posterior pituitary gland
• Stimuli for release: high plasma osmolarity, low blood volume, and low blood pressure
• Controls water pores (aquaporin-2) into collecting duct cells.
• Increase in water reabsorption -> more concentrated urine

Stimuli for Vasopressin Release

• Mechanisms that trigger vasopressin release
• Stimulus relates to decreased blood pressure, decreased atrial stretch, and osmolarity greater than 280 mOsM.

Aldosterone & Sodium Reabsorption

• Steroid hormone synthesized by adrenal cortex
• Increases sodium reabsorption in distal tubule and collecting duct
• Main stimuli: Angiotensin II (low BP), High potassium (hyperkalemia)

Low Blood Pressure & Renin Secretion

• Low blood pressure stimulates renin secretion via 3 routes -Direct effect by Cardiovascular control center -Paracrines from macula densa or granular cells -Sympathetic activity on afferent arterioles

The Renin-Angiotensin-Aldosterone System (RAAS)

• Complex hormonal system that regulates blood pressure and water balance.
• Liver releases angiotensinogen—an inactive precursor for angiotensin I
• The enzyme renin converts it to angiotensin I
• Angiotension I is converted into Angiotensin II by ACE (angiotensin-converting enzyme)
• Angiotensin II is a powerful vasoconstrictor and stimulates aldosterone production
• Various targets: cardiovascular system, arterioles, hypothalamus

Low BP & The Renin-Angiotensin System

• Increased sympathetic activity, decreased blood pressure and decreased GFR
• Blood vessels constrict, resulting in increase blood pressure
• Increase in aldosterone and vasopressin release.

Acid-Base Balance

• Maintenance of proper pH levels through physiological mechanisms
• Buffers: HCO3⁻ in extracellular fluid, proteins, hemoglobin, phosphates in cells, phosphates and ammonia in urine to buffer excess protons.
• Ventilation: regulating blood PCO2, and thus H+, regulates rate and depth of breathing
• Kidneys: regulation of H⁺ and HCO₃⁻ secretion and reabsorption

Maintaining pH Homeostasis

• 3 mechanisms used by body
  1. Buffers (seconds); cellular proteins, hemoglobin, phosphate, and HCO3⁻ are used.
  2. Regulation of Ventilation (minutes); blood PCO2 and thus H⁺ is regulated/ adjusted.
  3. Kidneys (hours to days); regulation of H⁺ and HCO₃⁻ secretion and reabsorption.

Maintaining pH Homeostasis: Respiratory Compensation

• Two stimuli: Increased H⁺ and increased PCO₂ trigger the respiratory system adjustment
• Respiratory compensation for pH changes is immediate (rapid effect) and adjusts the breathing rate and depth to compensate for the changes, maintaining a balance of acids and bases.

Maintaining pH Homeostasis: Renal Compensation

• H+ secreted and HCO3⁻ reabsorbed at the proximal tubule during normal conditions.
• Fine regulation of acid-base balance at collecting duct
• Intercalated (I) cells (interspersed between principal cells): High carbonic anhydrase, type A I cells secrete H⁺ and reabsorb HCO3⁻ to adjust to acidosis conditions -> adjusting plasma to be more acidic.. Type B I cells secrete HCO3⁻ and reabsorb H⁺ to adjust to alkalosis conditions -> adjusting plasma to be more basic

Calcium in the Body

• Functions of calcium in extracellular matrix and fluid, and intracellular calcium
• Extracellular matrix: forms calcified matrix of bone and teeth
• Extracellular fluid: neurotransmitter release at synapse, role in myocardial and smooth muscle contraction, cofactor in coagulation cascade, influences excitability of neurons
• Intracellular: muscle contraction, signal in second messenger pathways

Structure of Bone

• Remodeling throughout life: calcified extracellular matrix called hydroxyapatite: Ca10(PO4)6(OH)2 , osteoblasts synthesize bone, and osteoclasts resorb bone. - Both are vital for calcium homeostasis

Calcium Homeostasis

• 3 main mechanisms:
  1. Parathyroid hormone (PTH): Increases calcium reabsorption from bone; increases calcium absorption from kidney.
  2. Calcitriol (Vit D₃): Increases calcium absorption from the GI.
  3. Calcitonin: Increases calcium deposition in bone

Parathyroid Hormone (PTH)

• Origin, chemical nature, biosynthesis, Transport in Circulation, Half-Life, Factors affecting release, Target cells or tissues, Receptor, Whole-body tissue reaction.

Calcitriol (Vitamin D3)

• Origin, chemical nature, mechanism (synthesis and transport), stimulus for synthesis, targets, receptors.
• Roles in whole-body, tissue and molecular levels, feedback regulation.

Calcitonin

• Origin, chemical nature (32 amino acid), biosynthesis, transport, half-life
• Factors affecting release, target cells or tissues, receptor, whole-body and tissue action