Body Maintenance and Homeostasis Notes

Body Maintenance

  • Organ systems coordinate to maintain body functions.
  • Maintaining a specific range of:
    • Body temperature
    • Blood pressure
    • pH
    • Body fluids
  • Is crucial for proper organ function.
  • The body takes in food and water and excretes waste without significant changes in body fluid volume or composition.
  • Maintaining constant volume and composition of intracellular fluid (ICF) and extracellular fluid (ECF) is complex and involves all organ systems.
  • Epithelial cells in the gastrointestinal tract, kidneys, and lungs control the intake and excretion of substances and water.
  • The cardiovascular system transports nutrients and removes waste.
  • The nervous and endocrine systems regulate and integrate these functions.
  • The kidneys and urinary system are primarily responsible for fluid balance.

Learning Objectives

  • Discuss how the body maintains internal balance.
  • Determine components of a homeostatic control mechanism.
  • Categorize sources of body fluids.
  • Determine the role of the urinary system in maintaining fluid balance.
  • Explain the importance of regulating and maintaining the internal environment.

Lesson 18: Homeostasis

  • Homeostasis is a state of body balance or stable internal environment.

  • The body regulates its internal environment despite external and internal fluctuations.

  • Organs involved:

    • Liver: metabolizes toxic substances, maintains carbohydrate metabolism (with pancreas signaling), regulates lipid metabolism, and is the primary site of cholesterol production.
    • Pancreas: Insulin signaling is important for glucose level regulation
    • Kidneys: regulate blood water levels, reabsorb substances, maintain salt and ion levels, regulate blood pH, and excrete urea and waste products.
    • Brain (hypothalamus, autonomic nervous system, and endocrine system): The hypothalamus regulates body temperature, heart rate, blood pressure, and circadian rhythms (wake/sleep cycles).

  • Homeostasis is influenced by intrinsic (internal) or extrinsic (external) factors.

  • Homeostatic control mechanisms have three interdependent components:

    • Sensor/Receptor: detects changes in the internal or external environment.
      • Example: Peripheral chemoreceptors detect changes in blood pH.
    • Integrating/Control Center: receives information from sensors and initiates a response.
      • Example: The hypothalamus controls body temperature, heart rate, blood pressure, satiety, and circadian rhythms.
    • Effector: organ or tissue that receives information and acts to maintain homeostasis.
      • Example: The kidney retains water if blood pressure is too low.

  • Positive and negative feedback are mechanisms that enable these components to maintain homeostasis.

Negative Feedback Mechanism

  • Negative feedback mechanisms reduce the activity of a process to return an organ or system to its normal range of functioning.
  • Most homeostatic processes use negative feedback regulation to maintain a parameter around a set point.
  • Negative feedback processes are also used for non-homeostatic processes.
  • Example: Temperature control.
    • Nerve cells (sensors) relay information about body temperature to the hypothalamus (integrating center).
    • The hypothalamus signals effectors to return body temperature to 37∘C37^\circ C (set point).
    • Effectors:
      • Sweat glands cool the skin.
      • Blood vessels undergo vasodilation to release heat.
    • Once the core temperature returns to normal, sensors send negative feedback to turn off the process.
  • Both internal and external events can induce negative feedback mechanisms.
    • Internal: Shivering to generate heat when body temperature is low.
    • External: Removing a warm hat or drinking cool water to cool the body.
  • The hypothalamus can change the body’s temperature set point (e.g., raising it during a fever).

Positive Feedback Mechanism

  • Positive feedback enhances or upregulates the process that initiated it to create a stronger response.
  • Designed to accelerate or enhance the output of a stimulus.
  • Not as commonly used in homeostatic responses.
  • A series of events initiates a cascading process to increase the stimulus's effect.
  • Example: Amplification of labor contractions.
    • The cervix contains stretch-sensitive nerve cells (sensors).
    • These nerve cells send messages to the brain, causing the pituitary gland to release oxytocin.
    • Oxytocin causes stronger uterine contractions (effectors), pushing the baby further down the birth canal, causing more stretching of the cervix.
    • The cycle stops when the baby is born and the stretching of the cervix halts, stopping oxytocin release.

Lesson 19: Regulation of Body Fluids

  • The body is at least 60% water.
  • Essential biochemical processes occur in an aqueous solution.
  • Main fluid compartments:
    • Intracellular Fluid (ICF): 40% of total body weight, the cytoplasm of the cell, generally stable.
    • Extracellular Fluid (ECF): 20% of total body weight, subdivided into:
      • Plasma: 5% of body weight.
      • Interstitial Space: 12% of body weight.
  • The chemical composition depends on the body part or organ.
    • Extracellular fluid and interstitial fluid: high concentrations of sodium, chloride, bicarbonate, and proteins; lower in potassium, magnesium, and phosphate.
    • Interstitial fluids: have a low concentration of proteins.
    • Intercellular fluid: high levels of phosphate, magnesium, potassium, and proteins; lower content of sodium, chloride, and bicarbonates.
  • Fluid is regulated through passive diffusion based on concentration gradients and hydrostatic pressures.
  • Osmotic pressure: the minimum pressure applied to prevent solvent flow across a semipermeable membrane.
  • Osmotic pressure differences between ECF and ICF cause movement between compartments.
  • Changes in osmolarity of ICF or ECF result in rapid water movement.
  • Osmolality: the number of particles per liter of fluid.
    • Blood plasma osmolality: approximately 286 mOsmoles/L286 \text{ mOsmoles/L}.
      • Lower: Hypoosmotic.
      • Greater: Hyperosmotic.
  • Cellular osmotic concentration gradients are maintained by active pumping of transmembrane ionic transport proteins.
  • Rapid changes in fluid volume without ionic changes cause dilation or concentration of components.
  • Blood plasma osmotic gradients are maintained through absorption or secretion of solutes in the gastrointestinal tract or urine.
  • Osmolarity is partially composed of proteins like albumin and glucose.
  • Fluid moves towards hyperosmotic compartments and away from hypoosmotic compartments.
  • Body fluids have a net electrical charge close to zero (balance of cations and anions).
  • Ionic components diffuse selectively depending on membrane permeability.
  • Non-permeable membranes create a concentration gradient.
  • Solute gradients are created by membrane pumping proteins using ATP to move components against their diffusion gradient.
  • Hydrostatic pressure: the "push" factor on fluid movement, forcing fluid out of a space.
  • The combined push of hydrostatic forces and the pull of osmotic forces create net fluid movement.
  • Alterations to extracellular fluid osmolality can cause swelling or shrinking, which can lead to cell death.
  • The kidneys regulate extracellular fluid osmolality via:
    • The osmoreceptor-antidiuretic hormone (ADH or vasopressin) mechanism.
    • The thirst mechanism.
  • Antidiuretic hormone (ADH):
    • Secreted from the posterior pituitary gland.
    • Influenced by the osmolality of body fluids, and the volume and pressure of the vascular system.
    • Sensitive to small changes in osmolality (1% change alters ADH release).
  • Increased extracellular fluid osmolality causes shrinkage of osmoreceptor cells in the anterior hypothalamus.
    • Leads to nerve signals being sent to hypothalamic ADH-producing cells.
    • Results in ADH release from axon termini in the posterior pituitary.
  • ADH interacts with V2 receptors on principal cells in the kidney, promoting aquaporin-2 water channel translocation to the apical membrane.
    • Increases water reabsorption and excretion of a small volume of concentrated urine.
    • Water conservation dilutes extracellular solutes, correcting hyperosmotic extracellular fluid.
  • The opposite occurs with hypo-osmotic extracellular fluid.
  • Fluid intake is regulated by the thirst response (conscious desire to drink water).
  • Neural centers in the hypothalamus (thirst center) respond to stimuli.
  • ADH and thirst systems work together to maintain water balance.
  • ADH secretion occurs at a lower threshold than thirst.
  • Increased plasma osmolality evokes thirst and ADH secretion.
  • Decreased plasma osmolality suppresses thirst and ADH release, enhancing renal water excretion.