1. Negative feedback by stimulated hormone:  
when one hormone stimulates the production of a second, the second suppresses the production of the first.

    Ex.: Follicle stimulating hormone FSH stimulates the release of estrogens from the ovary. A high level of estrogen, in turn, suppresses the further production of FSH.

    1.hormone is synthesized, stored, then secreted when release is triggered 
   2.hormone is transported via circulation to target cells
   3.hormone is recognized by specific cells membrane receptors of target cells
   4. hormone signal is relayed and amplified to the cell interior causing response
   5.reactions of target cells are recognized by the original hormone-producing
       cells leading to a reduction of hormone production in a negative feedback loop.
   6. hormone is degraded

ACE2 is an enzyme found on the surface of many cell types. ACE2 is part of the RAAS system but acts to reduce blood pressure (see pic below).

SARS-CoV-2 virus binds to this enzyme (key/lock)
to gain access and infect the cell.

ACE2 is found on lung, heart, blood vessels, kidneys, liver and gastrointestinal cells...pretty much any epithelial cells. ​ACE2 is highly abundant on lung cells. 

If the virus occupies to ACE2, then ACE2 can't do its regular job in the RAAS pathway.

there are drugs called ACE inhibitors which block the formation of angiotensin II to prevent high blood pressure.

ACE2 naturally converts ANG II to other molecules that counteract the effects of ANG II and lower blood pressure. 

Cortisol is known as the "stress hormone".
it's released by the adrenal glands in response to stress as part of the fight-or-flight response.
Cortisol increases heart rate, blood pressure, and energy availability. 

chronic stress = chronic high cortisol levels.

hormones act as ligands...molecules that bind to specific receptors on target cells triggering a cellular response such as altering membrane permeability, stimulating protein synthesis, or regulating gene expression.
hormone/receptor specificity is similar to a "lock and key" mechanism...
​the hormone only affects the target cells that have the appropriate receptor. 

Type 1 diabetes is an autoimmune disease where the body's immune system attacks and destroys insulin-producing cells in the pancreas. 

a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone," which stimulates the pituitary to secrete a "stimulating hormone" into circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of the target gland hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland.

CRH = 
Corticotropin-releasing hormone stimulates the release of ACTH from the pituitary gland.

RAAS 

                        Sequence of Hormone Action

stress and anxiety...
​nature or nurture and the
​GR = glucocorticoid receptor

    Hormone levels are controlled by one of three homeostatic mechanisms:

TRH
GHRH
GRH
CRH...
are all releasing
hormones

Diabetes is a chronic disease where the body either doesn't produce enough insulin or can't effectively use the insulin it produces. This causes high blood glucose  levels which can lead to serious health problems such as heart disease, stroke, nerve damage, kidney disease, and vision problems if not managed properly. 
Common symptoms include increased thirst and urination, extreme hunger, fatigue, blurred vision, and slow-healing sores. 

diabetes

ANG II increases inflammation in lung cells.
ACE2 reduces this inflammation.
When the SARS-CoV-2 virus binds to ACE2, it prevents ACE2 from performing its normal function to regulate ANG II.
ACE2 action is “inhibited” = more ANG II injures tissues.
Covid-19 patients experience injury to the lungs and heart...

When the amount of ACE2 is reduced because the virus is occupying the receptor, individuals may be more susceptible to severe illness from COVID-19. That is because enough ACE2 is available to facilitate viral entry but the decrease in available ACE2 = more ANG II injury...
inflammation, cell death and organ failure especially in the heart and the lung.

           and related via the hypothalamus

2. Antagonistic pair
        of hormones:

    Ex. Insulin causes the level of                          blood glucose
              to drop when too high.   
          
           Glucagon causes blood                      glucose levels to rise                        when too low.

what type of feedback mechanism is this ->? 

ACE 2 and Covid-19

           the endocrine system

Atrial natriuretic peptide (ANP), produced by the heart, is a hormone that helps regulate blood pressure. it lowers blood pressure by increasing the excretion of sodium and water by the kidneys. this decreases blood volume which decreases blood pressure. ANP is also a vasodilator which also lowers blood pressure.

ANP inhibits the renin-angiotensin-aldosterone system (RAAS), which is involved in blood pressure regulation. ​

1. Low Blood Pressure
    kidneys release renin. 
2. Renin converts                             angiotensinogen into                   angiotensin I.
3. Angiotensin I is converted        to angiotensin II by ACE. 
    (ACE = sueface enzyme)
4. Angiotensin II causes                   vasoconstriction, raising           blood pressure and
     stimulates the release of
     aldosterone by adrenal               glands and ADH
     (hypothalamus/pituitary) 
5. Aldosterone and ADH cause
    the kidneys to retain sodium
     and water increasing blood       volume and blood pressure

just fyi...

thyroid gland

what type of feedback mechanism(s) is this?

STRESS

Thyroid hormones, primarily T3 & T4,
 regulate various bodily functions:
metabolism, heart rate, temperature. 

They influence nearly every organ and play a vital role in energy production, growth, and development. 

 the endocrine and nervous systems are both regulatory 

thyroid gland secretes calcitonin.

parathyroid gland secretes PTH 

the hypothalamus secretes
releasing hormones
​ that target the pituitary

what type of feedback mechanism is this?

other diseases

​As we already learned, insulin is a hormone produced by the pancreas that helps glucose enter cells .

ANP (Atrial natriuretic peptide)
and blood pressure

endocrine disorders

Type 2 diabetes: The body either doesn't produce enough insulin or the cells don't respond to insulin properly = insulin resistance 

the hypothalamus secreting releasing hormones such as TRH 
 triggers the pituitary gland to secrete stimulating hormones such as TSH

ACTH = 
adrenocorticotropic hormone.
produced by the pituitary gland.
stimulates the adrenal glands to release cortisol.

blood calcium regulation via t he thyroid and parathyroid glands.

           3. Negative Feedback by                   controlled molecule:
    Hormone secretion is increased (or decreased) by the concentration of the controlled molecule.

     Ex.: As Ca2+ in the blood rises, it                   suppresses the production of the                     Parathyroid hormone (PTH).               
           
         Low levels of Ca2+ stimulate
​PTH release.