Physiology 1.10 homeostatsis

Homeostasis is the process by which biological systems maintain stability while adjusting to changing external conditions.

Typical features include:

• Regulated variables: parameters that are maintained within a certain range.
• Sensors: detect changes in the regulated variables.
• Control center: processes information from sensors and determines the response.
• Effectors: carry out the response to restore balance.
• Set point: the desired level of the regulated variable.

The components include:

1. Regulated variable: the aspect being controlled.
2. Sensor: detects changes in the regulated variable.
3. Control center: evaluates the information and decides on the action.
4. Set point: the target value for the regulated variable.
5. Effector: implements the response to restore the set point.
6. Nonregulated variable: factors that are not actively controlled.

Shivering, sweating, and distribution of blood through blood vessels and skeletal muscle

85-100 mm Hg

Negative feedback is a mechanism that counteracts a change in a regulated variable to maintain homeostasis, while homeostasis refers to the overall process of maintaining stable internal conditions despite external changes.

The body's strategy is to keep the internal environment constant to prevent cells from being in a 'hostile environment' that is difficult to regulate.

TSH stimulates the thyroid gland to release thyroid hormones, which are crucial for regulating metabolism and energy levels.

The regulatory mechanisms include:

• Negative feedback: reduces the output or activity of a system when a variable deviates from its set point.
• Positive feedback: enhances or increases the output of a system in response to a change.
• Feed-forward loops: anticipate changes and adjust the system proactively.

For core body temperature regulation:

• Sensor: thermoreceptors in the skin and hypothalamus.
• Control center: hypothalamus.
• Effector: sweat glands (for cooling) and muscles (for shivering).
• Effector response: sweating to cool down or shivering to generate heat.

For arterial oxygen regulation:

• Sensor: chemoreceptors in the carotid and aortic bodies.
• Control center: medulla oblongata.
• Effector: respiratory muscles.
• Effector response: increased breathing rate to enhance oxygen intake.

The four types of intercellular communication are contact, paracrine, endocrine, and nervous.

Contact and paracrine communication are used for short-distance signaling.

Endocrine and nervous communication are used for long-distance signaling.

The hypothalamus is located below the thalamus in the brain, and the pituitary gland is situated just below the hypothalamus, connected by the infundibulum.

The hypothalamic-pituitary system involves the hypothalamus releasing hormones that regulate the anterior pituitary, which in turn secretes hormones that affect various target organs. The posterior pituitary releases hormones directly into the bloodstream.

The anterior pituitary secretes hormones that are regulated by hypothalamic releasing and inhibiting hormones, while the posterior pituitary releases hormones (like ADH and oxytocin) that are produced in the hypothalamus and stored in the posterior pituitary.

ADH (antidiuretic hormone) helps regulate fluid balance by promoting water reabsorption in the kidneys, thus reducing urine output and increasing blood volume.

The thyroid hormone signaling pathway involves the anterior pituitary secreting TSH (thyroid-stimulating hormone), which stimulates the thyroid gland to produce thyroid hormones (T3 and T4) that regulate metabolism and energy levels in the body.

Steady-state refers to a condition where variables remain constant over time despite ongoing processes, while equilibrium is a state where opposing forces or influences are balanced, leading to no net change.

The key components of homeostasis include:

1. Sensor: Detects changes in the environment.
2. Integrator: Compares the sensor's input to a set point.
3. Effector: Executes the response to restore balance.

Homeostasis is the overall process of maintaining a stable internal environment, while negative feedback is a specific mechanism that counteracts changes to return to a set point, thus playing a role in achieving homeostasis.

Cellular-level homeostasis focuses on maintaining the internal environment of individual cells, while organism-level homeostasis involves the regulation of the entire organism's internal environment, including multiple systems and interactions.

The main types of intercellular signaling include:

• Paracrine signaling: Signals act on nearby cells.
• Autocrine signaling: Signals act on the same cell that produces them.
• Endocrine signaling: Hormones are released into the bloodstream to affect distant cells.
• Neurological signaling: Involves neurotransmitters acting across synapses between neurons.

Claude Bernard stated that the stability of the internal environment is crucial for life.

Charles Richet refined the concept by stating that a living system must be stable to avoid destruction, but it maintains this stability by being excitable and capable of modifying itself in response to external stimuli.

According to Charles Richet, living organisms are stable because they are modifiable; slight instability is necessary for true stability as it allows the organism to adjust to external stimuli.

Walter Cannon coined the term 'homeostasis' in 1963.

Homeostasis is a self-regulating process by which biological systems maintain stability while adjusting to changing external conditions.

Stability refers to a particular physiologic parameter that is monitored and maintained within a relatively narrow range at all times.

Examples of parameters include blood glucose levels, body temperature, blood pressure, and ECF osmolarity.

Equilibrium occurs when a process proceeds in both forward and backward directions at the same rate, resulting in no net change and no energy expenditure. In contrast, homeostasis involves a dynamic steady state where energy is exerted to maintain a particular state, which is not at equilibrium.

A dynamic steady state is characterized by a system exerting energy to maintain a particular state, such as the Na+/K+ pump moving sodium out of the cell to prevent swelling. However, this state is not monitored with clear feedback loops and sensors, distinguishing it from true homeostasis.

The key components include:

1. External Environmentn2. Internal Environment
2. Sensor - measures the regulated variable.
3. Error Detector - compares the measured value to the set point.
4. Set Point - the desired value of the regulated variable.
5. Controller - processes signals from the error detector.
6. Effectors - carry out the control signals to adjust the regulated variable.
7. Regulated Variable - the variable being controlled, which can be affected by internal and external disturbances.

A regulated variable is a parameter measured in the body with sensors, maintained within a set of limits, typically between a low and a high range. For example, extracellular pH is regulated between 7.35 and 7.45.

Sensors measure the regulated variable and deliver signals about that variable. They may signal only when the variable falls outside the normal range, but usually provide constant signaling that reflects the overall state of the regulated variable. Examples include cells, biochemical reactions, channels, or tissues, such as baroreceptors.

The error detector calculates the difference between the set-point value of the regulated variable and the actual value, sending an error signal to the controller.

The controller sends output signals to effectors that can change the regulated variable based on the data received from the error detector.

The set point refers to the range of values of the regulated variable that the system aims to maintain, although identifying the exact set-point in biological systems can be challenging.

It is difficult to determine the set-point because it is not clear how the error detector in the brainstem knows what the normal values, such as pH, are supposed to be.

Effectors respond to information from the controller and change the value of the regulated variable to bring it closer to the setpoint.

Regulated variables are those that are maintained within a setpoint range, while non-regulated variables are manipulated by effectors to influence the regulated variable. For example, pH is a regulated variable, and respiratory rate is a non-regulated variable that can change to affect pH levels.

No, homeostasis and negative feedback are not the same. While homeostasis often utilizes negative feedback loops to maintain stable internal conditions, not all negative feedback loops are related to homeostatic processes.

Homeostasis is the process by which biological systems maintain stability while adjusting to conditions that are optimal for survival. Negative feedback is a mechanism that helps maintain homeostasis by reducing fluctuations in the output of a system. In a negative feedback loop, the output of a process inhibits that process, leading to a stabilization of the system.

In a biological system, negative feedback functions by having the output of a process inhibit the process itself. For example, in the case of ABCase producing products A, B, and C, the product B inhibits ABCase, which slows down the reaction. This mechanism helps to stabilize the system by preventing excessive fluctuations in product levels.

Negative feedback tends to cause the products of a system to oscillate between low and high points. The extent of these fluctuations depends on how quickly the product inhibits the process, which helps maintain balance within the system.

The key components absent in a system not considered homeostasis include:

• Control centre
• Set point
• Error signal
• Regulated variable

The described system is not considered homeostasis because it lacks essential components such as a control centre, set point, error signal, and regulated variable, which are critical for maintaining homeostasis.

The major baroreceptors are located in the carotid arteries and the arch of the aorta.

When blood pressure drops, a message is sent to the brainstem via nerves, leading to:

1. Activation of the sympathetic nervous system
2. Release of epinephrine and norepinephrine
3. Resulting in an elevation in heart rate, constriction of arterioles, and increased stroke volume.

The sensor detects the current blood pressure and sends this information to the error detector.

The error detector compares the measured value of blood pressure to the set point and identifies any discrepancies.

The controller processes the error signal from the error detector and determines the necessary control signals to correct the blood pressure.

Effectors carry out the control signals from the controller to adjust blood pressure, such as altering heart rate or blood vessel diameter.

Input signals are the measured value of blood pressure from the sensor, while output signals are the control signals sent to the effectors to adjust blood pressure.

Non-regulated variables are factors that can influence blood pressure but are not directly controlled by the feedback loop, such as stress levels or hydration status.

The set-point is typically established in the brain, specifically in areas like the hypothalamus, which determines the desired level of blood pressure.

The muscle spindle acts as a proprioceptor that senses muscle stretch, activating the muscle to contract against the stretch by stimulating the motor neuron in the ventral horn and inhibiting the antagonist muscle.

When a muscle is stretched, it activates the muscle to contract against the stretch and inhibits the antagonist muscle, which helps maintain balance and posture.

Change in heart rate, stroke volume, vascular tone, and fluid/salt retention

1. Stretching stimulates the sensory receptor (muscle spindle).
2. The sensory neuron is excited.
3. Within the integrating center (spinal cord), the sensory neuron activates the motor neuron and an inhibitory interneuron.
4. The motor neuron is excited, sending signals through the spinal nerve.
5. The effector (same muscle) contracts to relieve the stretching, while the antagonistic muscles relax due to inhibition of their motor neurons.

Yes, the stretch reflex is considered a negative feedback system because it responds to the stretching of a muscle by initiating a contraction to counteract the stretch, thereby maintaining muscle length and tension.

The stretch reflex is not a homeostatic system in the traditional sense. It lacks components such as a set point and a regulatory mechanism that maintains a stable internal environment over time.

The stretch reflex is missing the following components of a homeostatic system:

1. Set Point: There is no predetermined optimal length for the muscle.
2. Regulatory Mechanism: It does not involve long-term regulation of internal conditions, but rather a quick response to immediate changes.

No, homeostatic mechanisms are constantly monitoring and delivering information to the control center, responding more intensely when there is a larger error signal, but they are always sending input to the effectors at a basal rate.

No, sodium is not a regulated variable despite staying in a relatively narrow range. In contrast, potassium is regulated.

80 – 100 mm Hg

Chemosensors located in the carotid and aortic bodies

Altered K+ reabsorption and secretion by the kidneys and other tissues

The brain stem acts as the control center for respiratory muscles to alter the respiratory rate based on arterial oxygen and CO2 levels.

4 – 8 mmol

The kidneys, bone, and intestine are responsible for altering calcium reabsorption and secretion.

7.35 – 7.45

The hypothalamus detects changes in osmolarity and signals the kidneys to adjust sodium and water reabsorption.

Systolic blood pressure shows a drop close to the 0 mark, followed by a rise as the individual stands up.

Heart rate experiences a sudden spike followed by a gentle decline after the individual stands up.

The oscillations indicate that the controller, error detector, sensor, and effectors are all active at every moment, which is a typical feature of homeostatic systems.

'Normal recovery' refers to the period after the 60-second mark where blood pressure and heart rate stabilize following the initial changes due to standing.

35.5-37.5 Celsius

Thermosensor located in the hypothalamus and skin

Brain stem

They act as sensors for mean arterial pressure, located in the aortic and carotid arteries.

Heart and kidneys

Homeostatic systems tend to overlap rather than be isolated, meaning that different regulatory systems are interconnected. For example, blood pressure regulation is related to fluid volume regulation and osmoregulation in the extracellular fluid (ECF).

Effectors in homeostatic systems can be 'turned up' or 'turned down' to achieve homeostasis. For instance, in blood pressure regulation, effectors can be adjusted, while in glucose regulation, different metabolic pathways are activated depending on whether there is hyperglycemia or hypoglycemia.

Regulated variables fluctuate constantly, and oscillations in these variables are typical. Disease states can cause larger fluctuations that are unstable, and sometimes these states can be due to altered set points, such as hypertension being a 'set point error' disease.

Redundancy is common in homeostatic systems; the more vital the parameter, the greater the number of systems that regulate it.

Positive feedback occurs when the output of a system is fed back in a manner that increases that system's output, often leading to an exponential increase until a limiting event is reached.

An example of positive feedback is parturition (childbirth), where the process continues to amplify until the limiting event of the baby being expelled through the birth canal occurs, ending the feedback loop.

Positive feedback enhances or intensifies the stimulus, leading to increased contractions of the uterus as the baby's body stretches the cervix, which in turn causes more oxytocin to be released, further intensifying contractions until birth occurs.

The release of oxytocin is triggered by the stretching and thinning of the cervix, which is detected by mechanoceptors and transmitted to the brain.

1. Baby's head presses on and thins the cervix.
2. Cervical thinning and stretch are detected by mechanoceptors.
3. The brain interprets this input and the hypothalamus releases oxytocin.
4. Oxytocin causes uterine contractions, forcing the baby against the cervix.

The positive feedback loop ends with the birth of the baby, which decreases the stretching of the cervix, thus interrupting the cycle.

A feed-forward loop is a system where changes in a regulated variable are anticipated, and the controller proactively activates an effector.

An example is visualizing performance prior to an athletic event, which leads to increases in heart rate, stroke volume, blood pressure, and respiratory rate.

Muscle proprioceptors detect an increase in activity and signal the respiratory center to increase ventilatory rate before any changes in blood gases occur.

The cerebral cortex provides voluntary control of respiratory rate, allowing conscious adjustments to breathing patterns.

The hypothalamus adjusts respiratory rate according to emotional state, pain, and body temperature by signaling the brainstem to change ventilation.

Proprioceptors send signals to the brainstem when muscles and joints move, leading to anticipatory increases in ventilation to meet expected increases in oxygen demand and CO2 production.

Chemoreceptors detect arterial O2, CO2, and pH; they increase ventilation when O2 falls or CO2/pH rise (CO2/pH are the primary drivers of ventilation changes).

Homeostasis at the organism level is more easily described and controlled, while cellular homeostasis involves complex molecular and biochemical networks that are difficult to study and measure.

Challenges include the complexity of molecular and biochemical networks, difficulty in measuring or locating controllers, error detectors, and set points.

1. Contact signaling: Involves membrane receptors contacting the extracellular matrix (ECM) or another cell, leading to an intracellular signal in one or both cells.

2. Paracrine signaling: A cell produces a soluble messenger that diffuses to another cell, binding to its membrane receptor and triggering an intracellular signal in that cell.

In endocrine signaling, cells in endocrine organs release a chemical messenger (hormone) into the bloodstream. This hormone circulates and triggers an intracellular response in any cell that has a receptor for that hormone.

In nervous signaling, a neuron (A) sends an electrical signal along an axon to a synapse with another cell (B). This results in the release of a neurotransmitter, which binds to a receptor on cell B, leading to an intracellular response. Cell B can be another neuron, a muscle cell, or an endocrine cell.

Contact-dependent signaling requires cells to be in direct membrane-to-membrane contact, utilizing a membrane-bound signal molecule to bridge the gap between the signaling cell and the target cell.

Paracrine signaling involves a signaling cell releasing local mediators into the extracellular space, which then act on nearby target cells.

Synaptic signaling is performed by neurons that transmit signals electrically along their axons and release neurotransmitters at synapses, which can be located far from the neuronal cell body.

Contact-dependent signaling is important for embryologic development, immune signaling, and for limiting/organizing growth. It helps in determining cell polarity and facilitates communication between adjacent cells.

Integrins are part of the hemidesmosome complex. When they bind to the extracellular matrix (ECM), they generate intracellular signals that help determine the polarity of the cell, indicating which way is 'up'.

Paracrine signaling is a common mechanism where cells signal locally to each other through soluble mediators, affecting nearby target cells.

Paracrine signaling has a wide range of uses including:

• Immunological responses and defense signals for local damage
• Regulation of growth, cell division, and tissue repair
• Local regulation of blood flow

An everyday example of paracrine signaling is when metabolically active tissue releases metabolites like H+, CO2, and K+, which cause local vascular endothelial cells to relax, leading to vasodilation and improved blood flow.

The mechanism of endocrine signaling involves the following steps:

1. An organ secretes a messenger into the bloodstream.
2. The messenger is widely distributed throughout the body.
3. Cells with receptors for the messenger respond to it.

1. Hypothalamic-pituitary axis: the hypothalamus controls the pituitary, which regulates other endocrine glands.
2. Other endocrine glands that act independently of hypothalamic control (e.g., pancreas, parathyroids, adipose tissue).

'Other' endocrine glands are not under hypothalamic or pituitary control. They directly sense a stimulus, acting as both the sensor and the control center, and secrete hormones in response to that stimulus.

Examples of 'other' endocrine glands include:

• Pancreas and gastrointestinal tract
• Parathyroid glands
• Adipose tissue

The glands under hypothalamic control include the pituitary gland, thyroid gland, adrenal glands, and gonads (ovaries and testes).

Some missing endocrine organs could include the thymus, pancreas, and pineal gland. Additionally, organs like the heart (which produces atrial natriuretic peptide) and kidneys (which produce erythropoietin) also have endocrine functions.

The general model is as follows: Hypothalamic signal stimulates pituitary cells, which then release a hormone. This hormone acts on another gland (usually endocrine), prompting the target gland to secrete larger quantities of a hormone, leading to a general systemic response.

The hypothalamus releases hormones that stimulate the anterior pituitary to release its own hormones, thus regulating various physiological processes.

The hypothalamus communicates with the anterior pituitary through the hypophyseal portal system, which consists of a network of blood vessels that transport hypothalamic hormones directly to the anterior pituitary.

Key components include the hypothalamus, anterior pituitary, posterior pituitary, infundibulum, and the hypophyseal portal veins.

The hypophyseal portal veins are crucial for transporting hormones from the hypothalamus to the anterior pituitary, allowing for rapid and direct hormonal communication.

After the hypothalamus releases hormones, these hormones stimulate the anterior pituitary to release its own hormones, which then act on various target organs and tissues in the body.

The hypothalamus secretes releasing or inhibiting hormones into the first set of capillaries, which then travel to the anterior pituitary to modulate hormone secretion from those cells.

Anterior pituitary hormones control a number of other endocrine glands, including the thyroid, adrenal gland, gonads, and liver.

The major hormones secreted by the posterior pituitary are ADH (antidiuretic hormone) and oxytocin. Both hormones act directly on target tissues rather than on other glands.

Hypothalamic neurons project to the posterior aspect of the pituitary, where their axons release hormones into the capillaries in the posterior pituitary.

ADH (anti-diuretic hormone) controls water balance in the body by regulating the amount of water recovered by the kidneys. When blood osmolarity is high (indicating less water), ADH is secreted to retain more water in the bloodstream. Conversely, when ADH levels decrease, more water is lost in urine.

ADH secretion is influenced by blood osmolarity; when blood is more concentrated (higher osmolarity), ADH is secreted to conserve water. When osmolarity decreases, ADH secretion is reduced, leading to increased water loss in urine.

Blood osmolarity is detected by osmoreceptors located in the hypothalamus, which play a crucial role in regulating the secretion of ADH from the posterior pituitary.

ADH (Antidiuretic Hormone) is involved in regulating the body's fluid balance by influencing the kidneys to reabsorb water, thus affecting the regulated variable (fluid volume). It acts as a signal in the feedback loop, where changes in fluid volume are detected by sensors, compared to a set point, and adjustments are made to maintain homeostasis.

The release of thyroid hormone is regulated by the hypothalamus in response to cardiovascular parameters and metabolic parameters such as body temperature.

The anterior pituitary releases TSH (Thyroid Stimulating Hormone) in response to TRH (Thyrotropin-Releasing Hormone) from the hypothalamus, which stimulates the thyroid gland to produce thyroid hormones.

Thyroid hormone is released in a particular rhythm that helps facilitate the growth of the organism.

The thyroid hormone signaling pathway involves negative feedback, where thyroid hormones inhibit the release of TRH from the hypothalamus and TSH from the anterior pituitary.

TRH (Thyrotropin-Releasing Hormone) stimulates the anterior pituitary gland to release TSH (Thyroid-Stimulating Hormone).

The regulation involves a negative feedback mechanism where increased levels of thyroid hormones inhibit the release of TRH from the hypothalamus and TSH from the anterior pituitary.

A decrease in thyroid hormone levels stimulates the hypothalamus to release TRH, which in turn stimulates the anterior pituitary to release TSH, leading to increased thyroid hormone production.

Thyroid hormone negatively feeds back on both the anterior pituitary and the hypothalamus, inhibiting their activity as part of the feedback loop.

TSH (Thyroid Stimulating Hormone) negatively feeds back on the hypothalamus, reducing its activity in the signaling pathway.

The signaling process is characterized by negative feedback loops, where both thyroid hormones and TSH inhibit the hypothalamus and anterior pituitary.