Endocrine and Respiratory Systems: Calcium, Gas Exchange, and Neural Control

Parathyroid Hormone (PTH)

Hormone released in response to low calcium (hypocalcemia) that increases calcium uptake from the bone into the bloodstream.

Actions of PTH on Bone

Increases the uptake (resorption) of calcium from the bone into the bloodstream.

Actions of PTH on Kidney

Decreases the loss of calcium in urine, effectively reabsorbing it back into the blood.

Actions of PTH on Vitamin D

Activates Vitamin D by converting it to its active form.

Result of Active Vitamin D

Increases calcium absorption from the gut and enhances reabsorption from the kidneys.

Calcitonin

Hormone released in response to high calcium (hypercalcemia) that stops the reabsorption of calcium from the bone.

Actions of Calcitonin on Bone

Prevents more calcium from being released if there is already enough calcium.

Actions of Calcitonin on Vitamin D

Stops the conversion to active Vitamin D.

Result of Calcitonin

Decreases calcium intake from the gut and reduces calcium reabsorption from the kidneys.

Primary Function of the Respiratory System

To breathe—taking in gases the body needs (like Oxygen, O₂) and pushing out waste gases produced by metabolism (like Carbon Dioxide, CO₂).

Partial Pressure

The individual pressure exerted by each gas in a mixture.

Fundamental Rule of Partial Pressures

In any given environment or enclosed space, the sum of the partial pressures of all gases must equal the total pressure.

Total Pressure at Sea Level

1 atmosphere (atm).

Partial Pressure of O₂ (PO₂)

21% of 1 atm = 0.21 atm.

Driving Force of Gas Movement

Gases move based on differences in partial pressure, NOT concentration.

Rule of Gas Diffusion

A gas will always diffuse from an area of higher partial pressure to an area of lower partial pressure.

Effect of Altitude on Oxygen

The percentage of oxygen in the air remains ~21%, but the total atmospheric pressure decreases, leading to a drop in the partial pressure of oxygen.

Diving and Pressure

As a diver goes deeper, pressure increases, causing the body to absorb more gases into the blood.

Decompression Sickness

Condition caused by ascending too quickly while diving, leading to dissolved nitrogen forming bubbles in the blood and tissues.

Biological Example: Sea Turtle Eggs

Eggs rely on gas exchange with the air in the sand; if covered with water, the partial pressure of O₂ plummets, leading to suffocation.

Biological Example: The Water Beetle

An air-breathing insect that dives underwater, taking a bubble of air with it, needing a high O₂ concentration to survive.

O₂ Concentration in Water vs. Bubble

Water has a low O₂ concentration (0.3 mmol/L) while the beetle maintains a higher concentration (4.3 mmol/L) in its bubble.

Partial Pressure of O₂ in Water

0.21 atm.

Partial Pressure in Beetle's Bubble

0.10 atm.

Convection (Bulk Flow)

The movement of a gas over long distances within the same phase, such as air moving through the trachea or blood being pumped through arteries.

Diffusion

The passive movement of a gas across short distances and across a phase boundary or cell membrane.

Oxygen Cascade

A model showing the stepwise drop in partial pressure of O₂ from the outside air to the mitochondria inside cells.

Tidal Flow

A gas exchange mechanism where air flows in and out of the same dead-end passages, resulting in lower efficiency due to mixing.

Cocurrent Flow

A gas exchange mechanism where blood and medium flow in the same direction, quickly approaching equilibrium and limiting further exchange.

Countercurrent Flow

A gas exchange mechanism where blood and medium flow in opposite directions, maintaining a favorable partial pressure gradient.

Crosscurrent Flow

A gas exchange mechanism where blood capillaries run across a unidirectional flow of air, allowing blood to sample the air stream at multiple points.

Calcium Homeostasis

The regulation of calcium levels in the body using hormones like PTH (low Ca²⁺) and calcitonin (high Ca²⁺).

Efficiency of Gas Exchange Systems

Different systems show varying efficiencies, with countercurrent flow being the most efficient, allowing blood to leave almost fully saturated.

Ambient Air PO₂

The partial pressure of oxygen in ambient air, approximately 0.2 atm.

Alveolar Gas PO₂

The partial pressure of oxygen in alveolar gas, approximately 0.14 atm.

Arterial Blood PO₂

The partial pressure of oxygen in arterial blood, approximately 0.12 atm.

Systemic Capillaries PO₂

The partial pressure of oxygen in systemic capillaries, leading to tissues and mitochondria, approximately 0.01 atm.

O₂ Saturation in Blood

The percentage of oxygen saturation in blood leaving the lungs, approximately 55% for tidal flow.

O₂ Saturation in Cocurrent Flow

The percentage of oxygen saturation achieved in cocurrent flow systems, approximately 50%.

O₂ Saturation in Countercurrent Flow

The percentage of oxygen saturation achieved in countercurrent flow systems, up to 75% or more.

O₂ Saturation in Crosscurrent Flow

The percentage of oxygen saturation in crosscurrent flow systems, between 65-89%.

Gas Movement Dependence

Gas movement depends entirely on partial pressure gradients rather than concentration.

Convection for Transport

The body uses convection for the transport of gases.

Diffusion for Exchange

The body uses diffusion for the actual exchange of gases.

Active Tissues and O₂

Active tissues, such as exercising muscles, create a very low partial pressure of O₂, pulling O₂ from the blood.

Signaling Methods in Endocrine System

The body uses different signaling methods based on the required speed and specificity, differentiated by the concentration of signaling molecules.

Gap Junctions

Cytoplasmic contact between adjacent cells, allowing for the shortest distance communication.

Neurotransmitters

Chemical signals released from a presynaptic neuron that diffuse across a tiny synaptic cleft, characterized by rapid, short-lived effects.

Paracrine Signals

A cell secretes a signal that affects only nearby neighboring cells (local diffusion).

Autocrine Signals

A cell secretes a signal that acts on itself, often for positive feedback.

Hormones

Secreted into the bloodstream (systemic dispersion) and have slower acting, longer-lasting effects on distant target cells throughout the body.

Neurohormones

Hormones secreted by the nervous system into the bloodstream.

Pheromones

Signals released into the outside environment to communicate with other individuals of the same species.

Steroid Hormones

Hormones derived from cholesterol, essential for producing critical hormones.

Amine Hormones

Hormones derived from amino acids (primarily Tyrosine and Tryptophan).

Peptide/Protein Hormones

Hormones that are chains of amino acids.

Insulin Synthesis

Involves translation of mRNA into preproinsulin, processing in the ER to proinsulin, and final cleavage in the Golgi to produce mature insulin.

Oxytocin

Hormone involved in childbirth (uterine contractions) and lactation (milk ejection).

Vasopressin (ADH)

Hormone that regulates water balance and blood pressure.

TSH (Thyroid-Stimulating Hormone)

Stimulates the thyroid gland to produce T3/T4, affecting metabolism and growth.

ACTH (Adrenocorticotropic Hormone)

Stimulates the adrenal cortex to produce glucocorticoids, involved in stress response.

GH (Growth Hormone)

Stimulates the liver, muscle, and fat to produce IGFs and promote growth.

Prolactin

Stimulates mammary glands to produce milk.

LH & FSH (Gonadotropins)

Stimulate gonads (ovaries/testes) to produce sex hormones and support reproduction.

MSH (Melanocyte-Stimulating Hormone)

Stimulates skin pigmentation.

Adrenal Cortex

Outer region of the adrenal glands that secretes hormones like mineralocorticoids and glucocorticoids.

Zona Glomerulosa

Outermost layer of the adrenal cortex that secretes mineralocorticoids (e.g., Aldosterone) to regulate sodium/potassium balance.

Zona Fasciculata

Middle layer of the adrenal cortex that secretes glucocorticoids (e.g., Cortisol) for stress response and metabolism.

Zona Reticularis

Innermost layer of the adrenal cortex that secretes androgens, precursors to sex hormones.

Adrenal Medulla

Core of the adrenal glands that secretes catecholamines (Epinephrine & Norepinephrine) for the fight-or-flight response.

Hypothalamic-Pituitary-Adrenal (HPA) Axis

A pathway involved in the stress response where the hypothalamus releases CRH and Vasopressin, leading to ACTH release from the anterior pituitary and glucocorticoid release from the adrenal cortex.

Corticotropin-Releasing Hormone (CRH)

Hormone released by the hypothalamus that stimulates the anterior pituitary to release ACTH.

Adrenocorticotropic Hormone (ACTH)

Hormone released by the anterior pituitary that stimulates the adrenal cortex to release glucocorticoids.

Glucocorticoids

Hormones released by the adrenal cortex that mobilize energy by promoting protein catabolism, fat catabolism, and gluconeogenesis in the liver.

Negative Feedback

Mechanism where glucocorticoids inhibit the hypothalamus and anterior pituitary to prevent over-activation.

Immune System Interaction

During an infection, immune cells can secrete ACTH independently of CRH and release cytokines that signal the brain to increase CRH production.

Insulin

The 'Feast' Hormone that promotes storage and lowers blood glucose by driving cells to take up glucose and convert it to glycogen or fat.

Glucagon

The 'Famine' Hormone that raises blood glucose by breaking down glycogen and creating new glucose.

Epinephrine

The 'Emergency' Hormone that ensures maximum glucose is available for the brain and muscles during an emergency.

Hormonal Interactions

The effects of insulin, glucagon, and epinephrine on blood glucose levels, including their combined effects.

Mechanism of Action of Vasopressin

Vasopressin binds to receptors in the kidney's collecting duct, triggering a second messenger system that causes water reabsorption.

Renin-Angiotensin-Aldosterone System (RAAS)

A pathway activated by low blood pressure that involves the secretion of renin and the conversion of angiotensinogen to angiotensin II.

Angiotensin II

A hormone that causes vasoconstriction, stimulates thirst, vasopressin release, and aldosterone release.

Aldosterone

Hormone that increases sodium reabsorption in the kidney, leading to increased fluid retention, blood volume, and blood pressure.

ACE Inhibitors

Common blood pressure medications that block the formation of Angiotensin II.

Gluconeogenesis

The process of creating new glucose from non-carbohydrate sources.

Glycogenolysis

The breakdown of glycogen to release glucose.

Protein Catabolism

The breakdown of proteins into amino acids.

Fat Catabolism

The breakdown of fats into fatty acids and glycerol.

Cytokines

Signaling molecules released by immune cells that can influence the brain's production of CRH.

Collecting Duct

Part of the kidney where vasopressin acts to promote water reabsorption.

Aquaporin-2 (AQP-2)

Water channels that facilitate water reabsorption in the kidney.

Atrial Natriuretic Peptide (ANP)

A hormone secreted in response to high blood pressure/volume that increases glomerular filtration rate and decreases sodium and water reabsorption in the kidneys.

Natriuresis

Increased excretion of sodium in the urine.

Diuresis

Increased excretion of water in the urine.

Osteoclasts

Cells that break down bone tissue, releasing calcium into the bloodstream.

Osteoblasts

Cells that promote the deposition of calcium in bone.

Neuronal Circuit

A network of interconnected neurons that transmit signals in one direction.

Excitatory Neuron

A neuron that increases the likelihood of the downstream neuron firing.

Inhibitory Neuron

A neuron that decreases the likelihood of the downstream neuron firing.