Homeostasis and Fluid Balance Flashcards

Homeostasis

• Maintaining a steady state of the body's internal environment.

• Maintained by effectors (glands or muscles).

• Regulated by sensory information from the internal environment.

• Reaction can be:

  • Direct and quick: nervous system.

  • Indirect and slow: hormones (endocrine system).

Negative Feedback Loops

• If something is not the way it needs to be, the nervous system will cause the opposite to happen.

• Once it's back to normal, the system turns off until the next adjustment is needed.

• It's an "on, off, on, off" cycle.

• In response to sensory information, effectors act to promote a reverse change in the internal environment.

• Effectors are generally muscles or glands.

Basics of a Negative Feedback Loop

1. Stimulus excites a receptor.

2. Receptor signals the integrating center of a disturbance.

3. The integrating center signals effector to correct the disturbance.

Example: Temperature Regulation

• Too Hot:

  • Detected by the hypothalamus.

  • Actions: Seek shade, sweat, shunt blood to the skin.

  • Result: Body cools.

  • Once cooled, actions stop (negative feedback).

• Too Cold:

  • Detected by the hypothalamus.

  • Actions: Put on more clothes, shiver, shunt blood to vital organs, find hot chocolate.

  • Result: Body warms.

  • Once warm, actions stop (negative feedback).

Blood Glucose Levels

• Another example of a negative feedback loop.

• You eat, blood glucose increases.

• Pancreas releases insulin in reaction to high glucose levels.

• Insulin causes cells and liver to uptake glucose.

• Blood glucose decreases.

• Low glucose levels trigger an increase in blood glucose, starting the cycle again.

Direct vs. Indirect Control

• Direct (nervous system) is faster.

• Indirect (hormones) is slower and maintains balance in the longer term.

  • Temperature control: direct (nervous system).

  • Blood glucose levels: indirect (hormones).

Fluid Balance

• Input should equal output (approximately 2.5 liters or 2500 milliliters per day).

• Input:

  • Metabolic processes: 200 ml of water per day.

  • Food: 700 ml.

  • Drinking: 1.5 liters.

• Output:

  • Feces: 200 ml of water.

  • Air: 300 ml of water (transpiration).

  • Sweating: 100 ml.

  • Urine: 1.5 liters.

Reaction to Dehydration

• Dehydration causes blood volume to drop, leading to decreased blood pressure.

• Hypothalamus detects low blood volume, triggering thirst.

• Hypothalamus triggers the posterior pituitary to release antidiuretic hormone (ADH).

Antidiuretic Hormone (ADH)

• Antidiuretic = makes you not pee (holds water).

• ADH causes the distal convoluted tubule (DCT) and collecting ducts to take up less water.

• Therefore, more water is left in the blood.

• This is negative feedback to dehydration or a drop in blood volume.

Cycle of Dehydration Response

1. Dehydration (due to lack of drinking or exercise).

2. Hypothalamus detects low blood volume or low blood pressure.

3. Thirst urge and release of ADH by the posterior pituitary gland.

4. ADH targets the DCT and collecting tubule to secrete more water back to the blood.

5. Increased blood volume and blood pressure, concentrated urine.

6. Once back to normal, the body stops releasing ADH (negative feedback).

Body's Reaction to Drop in Blood Pressure: Aldosterone

• Lower than normal blood pressure is detected by the medulla oblongata and the kidneys.

• Short Term:

  • Arteries vasoconstrict, causing less cross-sectional area and higher blood pressure (nervous system).

• Long Term:

  • The DCT and collecting duct tubules secrete more sodium to the blood.

  • This causes more water to move into the blood (3-step process).

  • More water increases blood volume, thus increasing blood pressure.

Long Term Solution Explained

• Kidneys detect low blood pressure.

• Kidneys release hormones that cause the release of aldosterone from the adrenal cortex (adrenal gland).

• The kidneys detect the low blood pressure and cause aldosterone to be released.

• Aldosterone targets the DCT to hold more sodium in the blood.

• This causes more water to move into the blood.

• More water = increase in blood volume and increase in blood pressure.

Aldosterone Negative Feedback Loop

• Sensors in the afferent arteriole going into the glomerulus detect high or low blood pressure.

• Low blood pressure causes aldosterone to be released.

• Aldosterone is released, causing an increase in sodium ion absorption in the DCT.

• The blood volume goes up.

• The extra hormones being released cause the arterioles to constrict, increasing the blood pressure.

• Both processes raise blood pressure.

• Antidiuretic hormone raises blood pressure by causing the retention of water, more blood volume, thus higher blood pressure.

• However, its goal was to reduce dehydration, not raise your blood pressure.

• Aldosterone is designed to raise blood pressure by retaining sodium, which causes more blood volume, thus higher blood pressure.

Effects of Caffeine and Alcohol

• Caffeine is a natural diuretic (makes you pee).

• This causes too much water to move into the urine.

• Alcohol interferes with antidiuretic hormone and causes too much water to move into the urine.

• Alcohol dehydrates you.

• Both drugs cause dehydration over time.

• Balance with water if consuming caffeine or alcohol.

Positive Feedback Loop

• The action of effectors amplifies those changes that stimulated the effectors.

Example: Childbirth

• The hormone oxytocin from the posterior pituitary is released when the uterus is too stretched.

• Muscles start causing the release of oxytocin, which causes those muscles to contract.

• Contraction stretches the muscles even more, causing more oxytocin to be released, causing more contractions.

• The contractions will finally stop once the stimulus is removed (the baby is born).

• Once the stimulus is being removed, the contractions will finally stop.