Study guide: Homeostasis and Kidney Regulation

📋 Course Outline

1. Homeostasis and Osmotic Balance
2. Homeostatic Control Components
3. Nervous and Endocrine Regulation
4. Feedback Mechanisms
5. Kidney Roles and Hormonal Control
6. Water and Sodium Regulation
7. Acid–Base and Calcium Balance
8. Other Kidney Functions

📖 1. Homeostasis and Osmotic Balance

🔑 Key Concepts & Definitions

• Homeostasis: The body's ability to maintain relative stability in the components of its internal environment despite constant changes in the external environment.
• Health: A state in which a person can maintain balance among internal functions regardless of instability in the internal and external environment and satisfy needs while maintaining a stable internal balance.

📝 Essential Points

• A multicellular organism survives only while it consistently maintains the composition of its internal environment, including blood, lymph, and interstitial extracellular fluid; this regulation is called osmoregulation.

📌 A hypertonic solution has more solutes than the cell and causes water to leave the cell, dehydrating it; an isotonic solution has the same solute concentration and is ideal for the cell; a hypotonic solution has fewer solutes and causes water to enter, making the cell swell and possibly burst.

💡 Memory Hook

Hypertonic draws water out; hypotonic draws water in.

📖 2. Homeostatic Control Components

★ Must-know

• The five components are:

  • a stimulus
  • a controlled factor or variable
  • a receptor
  • a control or integration centre
  • an effector with its response

• The listed receptor types are:

  • thermoreceptors
  • baroreceptors
  • chemoreceptors

• 🔄 Homeostatic control proceeds as follows:

  1. The control centre compares received information with a set point.
  2. The deviation determines effector activity and compensation intensity.
  3. The effector acts on the regulated parameter to restore balance.

Further detail

• Homeostatic compensation is never perfect and leaves a small deviation from the set point that is necessary to maintain regulatory compensation.

• Dogs have very few eccrine sweat glands, located mainly in their paw pads, and cool themselves primarily by panting, which evaporates water from the tongue, and by vasodilation.

💡 Memory Hook

Stimulus → variable → receptor → control centre → effector.

📖 3. Nervous and Endocrine Regulation

★ Must-know

• The nervous system communicates with different parts of the body through neurons and synaptic spaces and is the fastest, shortest-acting regulatory system, with impulse propagation delays on the order of a second or less.

• The endocrine system communicates through hormones transported by blood and then interstitial fluid.

📌 Peptide hormones act at the cell membrane without entering the cell and can act within minutes, whereas steroid and thyroid hormones act at the DNA of target cells and may take from a few hours to a few months.

Further detail

• 🔄 During prolonged stress:
  1. Cortisol binds to receptors in liver cells.
  2. Cortisol binds to DNA.
  3. Genes are activated.
  4. More glucose is released.

💡 Memory Hook

Nerves act in seconds; hormones range from minutes to months.

📖 4. Feedback Mechanisms

★ Must-know

• Homeostatic regulatory systems cannot maintain perfect constancy, so negative feedback returns a changing variable toward a new dynamic equilibrium.

• In positive feedback, an initial perturbation triggers a response that amplifies the perturbation, producing a potentially explosive snowball effect until the initial disturbance disappears.

Further detail

• Examples of positive feedback are:

  • drug tolerance
  • severe hypertension
  • childbirth
  • blood clotting

• In predictive thermoregulation, thermal receptors in the skin trigger responses in skeletal muscles and blood vessels before body temperature changes, reducing heat production and heat loss.

💡 Memory Hook

Negative feedback reduces a deviation; positive feedback amplifies it.

📖 5. Kidney Roles and Hormonal Control

🔑 Key Concepts & Definitions

• Electrolytes: Chemical elements that dissociate into ions when dissolved in water; mineral salts are present in the body in this form.

★ Must-know

• The kidneys regulate:

  • water balance
  • pH
  • blood sodium concentration
  • blood potassium concentration
  • blood bicarbonate concentration
  • blood calcium concentration

• Aldosterone, synthesized by the adrenal cortex, and vasopressin or antidiuretic hormone (ADH), produced by the hypothalamus and released into the bloodstream by the pituitary, are the two hormones chiefly involved in regulating urinary excretion according to intake and losses.

Further detail

• Each kidney's cortex and medulla contain about one million nephrons, which are tiny filters.

• The kidneys eliminate metabolic wastes and potentially toxic substances while preventing the loss of substances needed for normal body function.

📖 6. Water and Sodium Regulation

★ Must-know

• Sodium is the most abundant ion in extracellular fluid, including interstitial fluid, lymph, and plasma, and comes from dietary sodium chloride.

• When the kidney reabsorbs sodium from the tubule into the blood, water tends to follow the sodium by osmosis.

• When the body needs water, ADH increases the permeability of the distal tubule and collecting duct, allowing more water to be extracted from the filtrate; alcohol or caffeine inhibits ADH and increases water excretion in urine.

• 🔄 The response to low blood volume and pressure proceeds as follows:

  1. The kidneys release renin when blood volume and pressure are too low for glomerular filtration.
  2. Renin triggers aldosterone secretion by the adrenal cortex.
  3. Aldosterone increases sodium reabsorption in the distal tubules and collecting ducts.
  4. Chloride and water follow passively, increasing blood volume and pressure.

Further detail

• Aldosterone promotes potassium excretion and sodium retention by the kidney.

💡 Memory Hook

Retained sodium draws water with it, increasing blood volume and pressure.

📖 7. Acid–Base and Calcium Balance

★ Must-know

📐 Formula - The reversible buffer reactions that regulate pH are $\mathrm{H^+ + HCO_3^- \leftrightarrow H_2CO_3 \leftrightarrow H_2O + CO_2}$.

• Healthy blood has a pH of about 7.4, while urine can range from pH 4.5 to 8 and averages approximately pH 6.6.

• The kidneys regulate acid–base balance with help from respiratory ventilation; when blood is acidic, breathing increases, shifting the buffer reaction toward carbon dioxide production, which is eliminated during exhalation.

Further detail

• 🔄 The response to low blood calcium proceeds as follows:
  1. The parathyroid glands release PTH when blood calcium falls below normal.
  2. PTH stimulates the kidneys to release calcitriol, the active form of vitamin D.
  3. Calcitriol promotes calcium absorption in the digestive tract.

📖 8. Other Kidney Functions

★ Must-know

• The kidneys secrete erythropoietin (EPO), which stimulates red blood cell production in the bone marrow.

Further detail

• The kidneys activate the production of vitamin D in the skin.