2. Homeostasis Flashcards
Define homeostasis?
A tendency towards stability of the internal environment of a healthy organism
How is homeostasis achieved?
Control measure through (mainly) negative feedback involving continuous activity of sensory receptors
Give an example of how homeostasis is achieved?
High levels of CO2 in extracellular fluid triggers increased pulmonary ventilation, which in turn lowers CO2 levels
List some internal variables that require control?
Temperature 37 degrees C
Blood Glucose 4.5-5.6mmol/L
Blood volume 60-80ml/Kg
Blood pressure <80mmHg
Ca2+ (serum) 2.2-2.67mmol/L
Na+ (serum) 135-146mmol/L
K+ (serum) 3.5-5.0mmol/L
Bicarbonate (serum) 22-30mmol/L
osmolarity of blood (i.e. water) 290+/-5 mosm/kg
oxygen 75-100mmHg
Carbon dioxide 36-46mmHg
Describe briefly how homeostasis happens?
Receptor/sensor detects change, information goes along afferent pathway to the control centre which determines appropriate response. Control centre sends information along efferent pathway to effector. The results of the response feedback to influence stimulus.
Where is the integrating centre found?
Part of central nervous system
Give some examples of places effectors take place?
Muscle cells
Epithelial cells
Secretory cells
Nerve cells
What happens when the external temperature increases?
Receptors in skin and brain sense the change, this uses the afferent pathway and is conveyed to the hypothalamus which sends a message down the efferent pathway to cause blood vessels to dilate and sweat gland cell to secrete fluid.
What happens when the external temperature decreases?
Receptors in skin and brain sense the change, this uses the afferent pathway and is conveyed to the hypothalamus which sends a message down the efferent pathway to cause blood vessels to contract, skeletal muscles to twitch and smooth muscles around hair follicles contract raising hair
What happens in response to hypotension?
Baroreceptors in the carotid sinus and aortic body sense the change, this uses the glossopharyngeal nerve (an afferent pathway) and is conveyed to the medulla oblongata which sends a message down the autonomic nerves (an efferent pathway) which causes heart rate and stroke volume to increase and blood vessels to constrict.
What kind of feedback is used in blood clotting?
Positive as it’s self-amplifying 1.) Damage to blood vessel 2.) activation of clotting factors 3.) activation of thrombin back to step 2 until formation of blood clot
What kind of feedback is used in child birth?
Positive as it’s self-amplifying 1.) Head of fetus pushes against cervix 2.) Nerve impulses from cervix transmitted to brain 3.) Brain stimulates pituitary gland to secrete oxytocin 4.) Oxytocin carried in bloodstream to uterus 5.) Oxytocin stimulates uterine contractions and pushes fetus toward cervix
What does negative feedback do?
Response decreased effect of original stimulus
What does positive feedback do?
Response increases effect of original stimulus
Where is the key integrative centre?
Hypothalamus/Pituitary axis
What does the hypothalamus do?
Converts synaptic to humoral (hormonal) signals, responds to feedback from the system it controls
What role does the hypothalamus play in homeostasis?
Biological clock and circadian rhythms
Reproduction and behaviour (stress, mood, etc)
Regulation of sleep and arousal
Thermoregulation etc
What does the hypothalamus have to do with the pituitary gland?
The hypothalamus controls hormone secretion from the pituitary gland which have actions on many systems
Where is the hypothalamus and the pituitary gland?
What happens in response to change in the system?
Change is detected -> nervous system -> hypothalamus -> pituitary gland -> hormones -> system restored
How is the hypothalamus linked to the posterior lobe of the pituitary gland?
By nerve fibres
How is the hypothalamus linked to the anterior lobe of the pituitary gland?
By a complex systems of blood vessels
How does the hypothalamus work with the anterior pituitary gland?
Neurosecretory cells produce and release inhibiting hormones
These are secreted into the blood
These hormones inhibit or stimulate other hormones in the anterior pituitary
Anterior pituitary secretes its hormones in the bloodstream
Which hormones does the anterior pituitary gland secrete and where do they act?
Gonadotropins (FSH and LH) which act in ovaries and testes
Growth hormones which act on bones and tissues
Prolactin which acts on the mammary glands
Adrenocorticotrophin (ACTH) which acts on the adrenal cortex
Thyroid stimulating hormone (TSH) which acts on the thyroid
How does the hypothalamus work with the posterior pituitary gland?
Neurosecrectory cells produce ADH and Oxytocin
ADH and Oxytocin move down axons to axon ends
Secreted from axon endings into blood stream
Where do the hormones secreted by the posterior piuitary gland act?
Oxytocin acts on mammary glands
Oxytocin acts on smooth muscle in uterus
ADH (anti-diuretic hormone) acts on kidney tubules
Where are the anterior and posterior pituitarys?
What effect does stress have on the hypothalamus and pituitary?
Step 3 and step 1 are controlled by negative feedback
- ) Corticotrophin releasing hormone (CRH) released from hypothalamus
- ) Synthesis and release of adrenocorticotropin hormone (ACTH) by the anterior pituitary
- ) Increased concentration of cholesterol and steroids within the adrenal cortex, particularly cortisol
- ) Cortisol acts on heart, muscle, GI tract, blood vessels etc
Describe the process of milk production?
Prolactin releasing hormone (PRH) is released by the hypothalamus
This stimulates the anterior pituitary to release prolactin
This stimulates mammary gland development and milk production
How is milk production controlled?
