Medical Biology 1 Lecture 2: Homeostasis PDF

MEDICAL BIOLOGY 1 Lecture 2 HOMEOSTASIS 1 Dr Céline Caquineau CC: pexels.com Why is learning about homeostasis important? Core concept in physiology, necessary for understanding the many regulatory mechanisms in physiology. Disr...

MEDICAL BIOLOGY 1 Lecture 2 HOMEOSTASIS 1 Dr Céline Caquineau CC: pexels.com Why is learning about homeostasis important? Core concept in physiology, necessary for understanding the many regulatory mechanisms in physiology. Disruption of homeostatic mechanisms will lead to disease or diseases can cause disturbances to homeostasis L2- Learning objectives By the end of this lecture, you should be able to 1. Define homeostasis and outline its importance in human health and disease. 2. Describe all key components of a homeostatic control system, avoiding common misconceptions. 3. Provide examples of physiological variables homeostatically regulated. 4. Describe thermoregulation as an example of a homeostatic process. Historical perspective Health is a harmonious balance Concept of “Fixite du milieu of the “elements” that made up interieur”. Organisms maintain a “Homeostasis” the human body, while disease stable internal environment Similar + Standing still was a state of systematic despite changing external “staying similar” imbalance. conditions Hippocrates Walter Cannon Claude Bernard 1871-1975 Ca460-ca 377 BC 1813- 1878 Sources: Wikipedia and National library of medicine  Defining homeostasis “Staying similar” Maintenance of a relatively constant, stable internal environment. Homeostasis is a self regulatory process by which biological systems maintain stability while adjusting to environmental changes. It encompasses active regulatory mechanisms that aim to minimize disturbances to the internal conditions. It is a dynamic process, not a static one.  What are these “internal conditions”? -1 Physiological variables that are regulated to be maintained relatively constant over time to prevent a potentially lethal change. Examples of physiological variables homeostatically regulated Body temperature Body weight Blood glucose levels Blood gases Levels of physiologically-important ions (e.g., calcium, iron, sodium, potassium) Blood pressure Blood pH  What are these “internal conditions”? -2 Disruption of the homeostasis of these variables can result in diseases or harmful physiological states: Some examples are: Body temperature - hypothermia Body weight – obesity, anorexia Blood glucose levels - diabetes Blood gases - hypoxia Levels of physiologically-important ions (e.g., calcium, iron, sodium, potassium) – anemia, hyponatremia Blood pressure - hypertension Blood pH - acidemia  What are these “environmental changes”? Anything that would lead to a change to the internal conditions. External or Internal disturbances. Some examples are: Extreme temperatures Low oxygen levels Infectious agents Dietary iron deficiency Blood loss What are these “environmental changes”? - 2 In the case of blood glucose homeostasis, the environmental change is eating.  What are these “active regulatory mechanisms”? Many physiological processes act together to keep a physiological variable within its normal limits. They respond to environmental changes Concept of “Control system”  Homeostatic regulatory (control) system Key components: 1. A SENSOR measuring the value of a physiological variable. 2. A “SET POINT”: the normal range of values for the physiological variable. 3. A “CONTROL CENTRE” that compares the measured value of the variable to the set point. 4. The EFFECTOR(S) which can change the variable. 5. The response of the effectors to change the variable.  Homeostatic control systems: A model SET POINT measures the value SENSOR CONTROL EFFECTORS CENTRE acts to change value of variable compares the value to set point, communicates “action needed” to effectors Effectors’ response Physiological variable Restores the variable towards its Value changes normal range Disturbance ! Negative Feedback  THERMOREGULATION RESPONSES Constriction of skin blood vessels 36.1- 37.4°C Blood vessels Skin receptors & Hypothalamus Hypothalamus Muscles Shivering Reduction heat loss Body Temperature & Increase heat production Reduction Room temperature  Common Misconceptions 1 1. The “set point” is a fixed value. Incorrect! The set point is a range of values for the regulated physiological variable. Body temperature has a circadian rhythm. It fluctuates throughout the day. Source: @circadian.org  Common Misconceptions- 2 2. The set point or normal range is fixed. Incorrect! Set points are changeable. They can be reset either physiologically or as a result of a pathological change in the system. (Kempisty, 2006) Body Temperature set point is increased for fighting the infection  Common Misconceptions- 3 3. Homeostatic regulatory mechanisms operate like an ON/ OFF switch. Incorrect! These mechanisms are always active. Homeostasis regulation is a continuous process. The amplitude of these mechanisms vary when an 36.1- 37.4°C “error signal” is detected. Skin receptors & Hypothalamus Hypothalamus Body Temperature  Common Misconceptions- 4 4. The change in the physiological variable is the response of the effector(s) Incorrect! The change in Skin blood EFFECTORS the physiological variable vessels is the consequence of the effectors’ responses. Constriction skin blood RESPONSE vessels Note also the difference Reduction heat loss OUTCOME between the effector and the physiological response. Increase body temperature CONSEQUENCE Lecture 2- Take home messages Homeostasis is essential for health. Disruptions of the homeostasis will cause diseases. Homeostasis refers to a series of mechanisms that maintain a relatively stable internal environment. These processes are dynamic to keep regulated physiological variables within a range compatible with the viability of an organism. A key feature of the homeostatic control system is a feedback loop.