2. Homeostasis

Question 1

Define homeostasis?

Answer 1

A tendency towards stability of the internal environment of a healthy organism

Question 2

How is homeostasis achieved?

Answer 2

Control measure through (mainly) negative feedback involving continuous activity of sensory receptors

Question 3

Give an example of how homeostasis is achieved?

Answer 3

High levels of CO2 in extracellular fluid triggers increased pulmonary ventilation, which in turn lowers CO2 levels

Question 4

List some internal variables that require control?

Answer 4

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

Question 5

Describe briefly how homeostasis happens?

Answer 5

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.

Question 6

Where is the integrating centre found?

Answer 6

Part of central nervous system

Question 7

Give some examples of places effectors take place?

Answer 7

Muscle cells

Epithelial cells

Secretory cells

Nerve cells

Question 8

What happens when the external temperature increases?

Answer 8

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.

Question 9

What happens when the external temperature decreases?

Answer 9

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

Question 10

What happens in response to hypotension?

Answer 10

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.

Question 11

What kind of feedback is used in blood clotting?

Answer 11

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

Question 12

What kind of feedback is used in child birth?

Answer 12

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

Question 13

What does negative feedback do?

Answer 13

Response decreased effect of original stimulus

Question 14

What does positive feedback do?

Answer 14

Response increases effect of original stimulus

Question 15

Where is the key integrative centre?

Answer 15

Hypothalamus/Pituitary axis

Question 16

What does the hypothalamus do?

Answer 16

Converts synaptic to humoral (hormonal) signals, responds to feedback from the system it controls

Question 17

What role does the hypothalamus play in homeostasis?

Answer 17

Biological clock and circadian rhythms

Reproduction and behaviour (stress, mood, etc)

Regulation of sleep and arousal

Thermoregulation etc

Question 18

What does the hypothalamus have to do with the pituitary gland?

Answer 18

The hypothalamus controls hormone secretion from the pituitary gland which have actions on many systems

Question 19

Where is the hypothalamus and the pituitary gland?

Answer 19

Question 20

What happens in response to change in the system?

Answer 20

Change is detected -> nervous system -> hypothalamus -> pituitary gland -> hormones -> system restored

Question 21

How is the hypothalamus linked to the posterior lobe of the pituitary gland?

Answer 21

By nerve fibres

Question 22

How is the hypothalamus linked to the anterior lobe of the pituitary gland?

Answer 22

By a complex systems of blood vessels

Question 23

How does the hypothalamus work with the anterior pituitary gland?

Answer 23

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

Question 24

Which hormones does the anterior pituitary gland secrete and where do they act?

Answer 24

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

Question 25

How does the hypothalamus work with the posterior pituitary gland?

Answer 25

Neurosecrectory cells produce ADH and Oxytocin

ADH and Oxytocin move down axons to axon ends

Secreted from axon endings into blood stream

Question 26

Where do the hormones secreted by the posterior piuitary gland act?

Answer 26

Oxytocin acts on mammary glands

Oxytocin acts on smooth muscle in uterus

ADH (anti-diuretic hormone) acts on kidney tubules

Question 27

Where are the anterior and posterior pituitarys?

Answer 27

Question 28

What effect does stress have on the hypothalamus and pituitary?

Answer 28

Step 3 and step 1 are controlled by negative feedback

1. ) Corticotrophin releasing hormone (CRH) released from hypothalamus
2. ) Synthesis and release of adrenocorticotropin hormone (ACTH) by the anterior pituitary
3. ) Increased concentration of cholesterol and steroids within the adrenal cortex, particularly cortisol
4. ) Cortisol acts on heart, muscle, GI tract, blood vessels etc

Question 29

Describe the process of milk production?

Answer 29

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

Question 30

How is milk production controlled?

Answer 30