APC 12- Homeostasis 2

negative feedback ,positive feedback loops,role of autonomic system , role of endocrine systme , how cardiovascular system is regulated to control blood pressure and cardiac output ,how the kidneys contribute to regulation of blood pressure and how negative feedback loops control the release of thyroid hormone and cortisol

Summarise negative and positive feedback loops

negative - means that the stimulus stops , back to normal

positive - the stimulus is increases ,does not go back to normal ,it amplifies the change

Summary of neural control

• fast - within milliseconds ,short lived effects

• cerebrum

• hypothalamus- controls temp ,water

• , medulla oblongata - in brainstem and controls vital signals - hr,br,bp

• cerebellum

• automatic control of body systems

summary of endocrine system

• slower

• longer tern - min,days ,hours

• may have permanent effect

• automatic control of body systems

What are the 2 main branches the autonomic nervous system consist of ?

sympathetic branch :

• increases heart rate

• relaxes ( opens) airways

• contracts most blood vessels - so increase blood pressure

• also inhibits digestive processes - waste energy digesting food

• happens during emergency/stressful situations

• the fight or flight response

• noradrenaline neurotransmitter

parasympathetic branch :

• slows heart rate

• usually when resting

• contricts ( closes ) airways

• also stimulates digestive processes

• the rest and digest response

• acetylcholine neurotransmitter

signals travel down spinal cords , have sympathetic nerves and parasympathetic = both equally control the body

What does the autonomic regulation of the cardiovascular system involve ?

involves

• heart rate = 60-100

• blood pressure

• have cluster of cells that are myogenic

How does the regulation of the heart rate work ?

• at rest the sinoatrial node sets that pace of the heart

• the cardiac centre in the medulla modulates heart rate

Accelerate HR

• activates the cardiac nerve

• via the sympathetic response releases neurotransmitter noradrenaline

• noradrenaline released acts on b1 adrenergic increasing the electrical impulses

Decrease HR

• activates the vagus nerve

• via the parasympathetic response releases acetylcholine neurotransmitter decreases rate of depolarisation

• acetylcholine acts on M 2 muscarinic receptors

What is blood pressure ?

• Pressure exerted by the blood upon the walls

of the blood vessels’

What factors does arterial blood pressure depend on ?

1. cardiac output (CO) - amount of blood pumped out of the left ventricle in 1 minute

2. Peripheral Vascular Resistance ( PVR) - diameter of blood vessels

Note - if CO or PVR increase ,BP will increase

What factors affect cardiac output ?

CO= Heart rate ( -70bpm) x Stroke Volume ( -70ml )

= 4900ml = 5L

• Heart rate is controlled by the cardiac centre in the medulla

• SV is controlled by hypothalamus and kidney ( adh , raas system )

stroke volume - volume of blood ejected from the left ventricle

What factors affect PVR ?

• the diameter of blood vessels

• sympathetic impulses affect the diameter of the blood vessels

• less impulses - vasodilation

• more impulses - vasoconstriction

• to increase BP ,activate vasoconstriction - decreased diameter ,increased pvr = higher blood pressure

• to decrease Bp , activate vasodilation - diameter increased , decreased PVR = lower blood pressure

What are the 2 ways blood pressure can be controlled ?

1. Arterial baroreceptors - the first way the body controls blood pressure ,short term

2. RAAS system ( renin - angiotensin - aldosterone - system ) - longer term

What is the first way used to control blood pressure ?

Arterial Baroreceptors

• bp changes = stimulus ,

Detected by baroreceptors - neuron clusters in muscle wall:

aorta, carotid sinus ( carotid artery ) - sense the amount of stretch in muscular walls in the aortic arch

if blood pressure too low

• if the stretch is lower than the range

• medulla has to increase using co ,pvr

• impulse sent to the cardiac centre

• the cardiac accelerator nerve activated

• heart rate increases , the cardiac output increase

• PVR increases - vasoconstriction

note- medulla only can constrict

If blood pressure is too high

• if the stretch is higher than the range

• medulla sends the impulses to the cardiac centre

• the vagus nerve activated

• the heart rate decreases , the cardiac output decreases

• PVR Decreases - vasodilation

What can control blood pressure ?

• Increasing blood volume increases blood pressure

2 main ways of fluid balance ,modulate blood volume

• Hypothalamus controls blood volume ( ADH)

• Kidneys control blood volume ( RAAS)

kidneys - dehydration ,more conc ions ,detected by hypothalamus ,adh released ,kidney conserve water aquaporins in the walls open -take in more water

How can blood pressure been controlled by the RAAS System ?

raas- renin- angiotensin - aldosterone system

1. renin ( hormone) released from nephrons in juxtaglomerular cells in the kidney in response to low blood pressure ,low sodium in the kidney or sympathetic activation

2. renin converts angiotensinogen ( inactive hormone) into angiotensin ( active form) , this flows through the blood vessels in the lungs

3. it binds to angiotensin converting enzyme which converts angiotensin 1 into angiotensin 2 carried around the body

4. angiotensin 2 increases blood pressure by :

• binding to receptors in arteries causing systemic vasoconstriction increasing pvr

• binds to receptors on adrenal glands - aldosterone released increased na absorption and water reabsorption in kidney increasing sv

• Directly binds to nephron increasing sodium and water reabsorption in kidney , increasing SV

• Adh release from hypothalamus cells conserving water

overall - RAAS raises blood pressure and restores circulating volume

What are the drugs used that affect the raas system to lower blood pressure ?

Describe the regulation of thyroid hormones - negative feedback

thyroid hormone- controls metabolic rate of every cell in the body

• Early morning - hypothalamus releases TRH (thyrotropin-releasing hormone)
  • TRH stimulates pituitary to release thyroid stimulating hormone (TSH)
  • TSH stimulates thyroid to release TH (T4/T3- different forms)
  • High TH inhibits TRH and TSH
  • TH has stable half life (T3 1 day; T4 7 days) so levels remain relatively stable

Describe the regulation of cortisol - negative feedback

linked to circadian rhythm

• Early morning CRH ( corticotropin - releasing - hormone) released
• Stimulates ACTH ( adrenocorticotropic hormones) from the anterior pituitary
• ACTH stimulates adrenal glands (on top of kidney)
• Adrenal gland releases Corticosteroid hormones (mainly
Cortisol)
• Cortisol causes increased glucose, protein and lipid
availability in stressful situations, providing more nutrients
for energy
• When cortisol levels rise during the morning, cortisol
inhibits CRH and ACTH
• Cortisol levels fall slowly, cycle repeats
• This negative feedback loop is known as the Hypothalamic-
Pituitary-Axis (HPA)