homeostasis and urine

HOMEOSTASIS AND URINARY SYSTEM NOTES

HOMEOSTASIS

  • Homeostasis is defined as the maintenance of a stable internal environment within organisms, allowing them to function optimally despite external changes.

    • Important variables maintained include:

    • Temperature

    • Blood pH

Internal Environment Ranges

  • Constant internal environments are critical; only small fluctuations are tolerated.

  • Temperature:

    • Normal body temperature is approximately 97°F (36.1°C).

    • Fever and hyperthermia can lead to dangerous highs of 109.4°F (43°C) causing coma and potentially death.

    • Hypothermia can lead as low as 82°F (28°C), resulting in unconsciousness or death.

  • Blood Pressure:

    • Normal ranges are 90-120 systolic and 60-80 diastolic.

  • Blood pH:

    • Normal: pH = 7.35 - 7.45

    • Critical ranges involve Acidosis (pH=7.0) and Alkalosis (pH=7.8) leading to coma and death.

Feedback Systems in Homeostasis

Feedback Loop Process

  • Homeostatic responses are managed through a feedback loop process that consists of:

    • Sensors that detect levels of variables (e.g., temperature, glucose, etc.).

    • If response is needed, sensors signal the control center.

    • Control Center utilizes input from sensors to direct a response.

    • Once normal levels are restored, the sensors cease signaling to the control center.

Types of Feedback Loops

  1. Positive Feedback Loops:

    • Less common; involves continued signals from sensors that amplify the control center’s response until an endpoint is achieved.

    • Examples:

      • Childbirth: Oxytocin release increases contractions.

      • Mating: Behavioral changes occur to attract a mate.

  2. Negative Feedback Loops:

    • More common; the control center’s response first diminishes the sensor’s signal and eventually shuts it down as conditions return to normal.

    • Examples:

      • Thermoregulation: Maintaining body temperature.

      • Glucose Homeostasis: Managing blood sugar levels.

      • Water Balance: Regulation of hydration status.

Thermoregulation

  • Human body temperature is monitored, with responses driven by sensors in the skin and hypothalamus acting as both sensor and control center.

    • Response to Cold:

    • Vasoconstriction: Blood vessels constrict near skin surface to conserve heat.

    • Shivering: Involuntary muscle contractions generate heat.

    • Response to Heat:

    • Vasodilation: Blood vessels dilate to release heat.

    • Evaporative Cooling: Sweat evaporates, taking heat away from the body.

Glucose Homeostasis

  • Glucose: primary fuel for cellular respiration producing ATP.

    • Maintained within blood via:

    • Insulin: Promotes glucose uptake by cells.

    • Glucagon: Triggers release of glucose from liver and muscle stores.

  • Blood sugar management:

    • Following a meal, insulin is released by the pancreas when blood sugar is high.

    • When blood sugar levels drop, glucagon is secreted to release stored glucose.

Water Balance

  • Multiple hormones regulate water balance crucial for cellular function, preventing cells from either shriveling or bursting due to osmotic pressure.

  • The hypothalamus serves as both sensor and control center:

    • Low Water Volume: Leads to high solute concentration.

    • High Water Volume: Leads to low solute concentration.

Hormonal Responses to Hydration Levels

  1. Dehydration:

    • Blood becomes more concentrated (high solute concentration).

    • Hypothalamus triggers thirst and releases Antidiuretic Hormone (ADH), which reduces water loss in urine.

  2. Over-Hydration:

    • Leads to decreased solute concentration in blood (dilution).

    • Hypothalamus inhibits thirst and decreases release of ADH.

URINARY SYSTEM

  • Key functions of the urinary system include the excretion of metabolic wastes, especially nitrogenous wastes like urea, which is toxic if accumulated.

Nitrogen Waste Removal Modes

  • Excreting Ammonia:

    • High toxicity; requires large volumes of water; typical in aquatic organisms.

  • Excreting Urea:

    • Moderately toxic; requires a moderate amount of water; typical in terrestrial amphibians and mammals.

  • Excreting Uric Acid:

    • Low toxicity; high energy cost; dry excretion method used by birds and reptiles.

Kidneys and Nephrons

  • Human Kidneys: Millions of nephrons process blood for waste removal.

    • Nephron Structures:

    • Nephron capsule, Proximal convoluted tubule, Loop, Distal convoluted tubule, Collecting duct.

    • Surrounded by blood vessels; glomerulus (a ball of capillaries).

Urine Formation in Mammals

  1. Filtration:

    • Occurs at glomerulus into nephron capsule; substances like glucose, water, electrolytes, and urea are filtered out of blood.

  2. Reabsorption:

    • In proximal convoluted tubule; selective reabsorption of important molecules (glucose, electrolytes, water) back into canal.

  3. Tubular Secretion:

    • Active transport from capillaries into distal convoluted tubule; includes ions (H+), electrolytes, and medications like antibiotics, and hormone action (e.g., ADH promotes water reabsorption).

Water Reabsorption and Electrolyte Balance

  • Mechanisms in the nephron (loop and collecting duct) affect water reabsorption through specialized proteins like Aquaporin 2 that facilitate water movement.

Urine Transport

  • Urine collects in the renal pelvis, flows through the ureter to the bladder, and is expelled through the urethra.