1. Homeostasis

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(08 Febr 2010)

What is physiology? The science of normal function of living things (esp. animal). How do living organisms function? (how animal function) Can be investigated at several level:Molecular level Cellular level Tissue level Organ level

Investigation Core of Animal Physiology


HomeostasisA condition in which the internal environment of the body remains relatively constant despite changes in the external environment. Examples would be the maintenance of body temperature and levels of glucose in the blood

Homeostasis refers to. Relative stability of internal milieu.as opposed to external environment

Dynamic process, not a static onePhysiological variable fluctuates Does not remain constant Balance does not mean unchanging

Glucose, water

HOMEOSTASIS Related to relative stability of internal milieu. What is regulated? Water volume, concentration [Na+], [K+], [Cl-], [HCO3-], [Ca++], [H+] T Blood pressure [Glucose] Oxygen Waste products (urea)

Water balance in an organism

Homeostatic control systems Steady state (fluctuates within a predictable range) Set point Input = output

Feedback Negative Positive

Feed forward regulation

Homeostatic reflex arc

A sequence of events that link a stimulus to the response.

A reflex that regulates body T


Temperature homeostasis Humans, other mammals and birds, are homoeothermic warm-blooded - and so maintain their body temperature within narrow limits despite variations in the external temperature. The advantage of the homoeothermic state may be that critical enzymes in the body's metabolism work optimally at around normal average body temperature: 37 degrees centigrade. The maintenance of a small range of temperature is a good example of a homeostatic physiological control system. At its most basic level, a cerebral integrative centre samples the body temperature at several sites, compares this value with a desired set point, and modulates heat production and heat loss accordingly.

Temp Regulation cont. In vertebrates temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin monitor the external temperature. Both pieces of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature:

Homeostatic control

Glucose homeostasis

Regulation of Ca


Circadian rhythm of Ca8



Ca++ Mm4


0 0










Scheme for negative feedback

Regulating enzymatic processes

Feed forward controls Digestion of food Stomach produces HCl in anticipation of food Stimulated by odor

Regulation of body temperature Anticipation of cooling of body in response to dropping external To Thermal receptors in skin Signaling to neurons in brain

Chemicals that regulateHormones, Neurotransmitters

Local chemical messengers

Positive Feedback Mechanisms:Homeostatic systems utilizing positive feedback exhibit two primary characteristics: 1. Time limitation Processes in the body that must be completed within a constrained time frame are usually modified by positive feedback. 2. Intensification of stress During a positive feedback process, the initial imbalance or stress is intensified rather than reduced as it is in negative feedback.

Typical Positive Feedback ProcessStress Sensor Control Center



Homeostatic Regulation of Child Birth through Positive FeedbackPressure of Fetus on the Uterine Wall Nerve endings in the uterine wall carry afferent messages to the Hypothalamus


Increasing strength of uterine contractions

Production and Release of Oxytocin into the Blood

The birth of the child will bring this process to a close. Other examples of positive feedback regulation occur during milk letdown and blood clotting.

Feedback in Coagulation

Positive feedback mini-loops are built into pathway to speed up production of chemicals needed to form the clot. Entire sequence of clotting is a negative feedback pathway:

Harmful Effects of Positive FeedbackPositive feedback can be harmful. Two specific examples of these harmful outcomes would be: 1. Fever can cause a positive feedback within homeostasis that pushes the body temperature continually higher. If the temperature reaches 45 degrees centigrade (113 degrees Fahrenheit) cellular proteins denature bringing metabolism to a stop and death. 2. Chronic hypertension can favor the process of atherosclerosis which causes the openings of blood vessels to narrow. This, in turn, will intensify the hypertension bring on more damage to the walls of blood vessels.

More mechanisms of homeostasis Adaptation Allen's rule: a rule which states that among endotherms, populations of the same species living near the equator tend to have more protruding body parts and longer limbs than do populations farther away from the equator.

Acclimatization Continuous exposure to stressors lead to improved adaptation

Biological rhythms Variations that occur on a daily or annual basis

Allens Rule in one genus of hares

From the arctic to desert jack rabbits.

Adaptation and Acclimatization Adaptation Inherited characteristic that promotes survival in a specific habitat Includes homeostatic control mechanisms

Acclimatization Lengthy exposure to a condition that improves ability to survive No genetic change involved

PO2 at different altitudes

Acclimatization to different altitudes Mountain sickness due to low barometric pressure. Inc in tidal vol, frequency of breathing, pH, loss of CO2 from blood. At high altitude, larger lung capacity, more red cells, myoglobin, capillaries in tissue, change in bicarbonate conc. in blood that permits acidbase balance.


1. What is homeostasis? Why is it called a dynamic equilibrium? 2. Describe these components of a homeostasis loop: stress, receptor, control center, effector, response. Using an example, put them in order. 3. What are the benefits of a negative feedback response? In what direction does a variable change as a result of a negative feedback response to a stress? 4. What are the benefits of a positive feedback response? In what direction does a variable change as a result of a positive feedback response to a stress? 5. What are the risks associated with positive feedback responses? 6. Draw 2 graphs to show negative & positive feedback responses. 7. Explain how positive feedback events can be built into a negative feedback loop. 8. What is homeostasis failure? What has happened with this