The Adrenal Glands

The adrenal glands are two small structures situated one at top each kidney. Both in anatomy and in function, they consist of two distinct regions:

an outer layer, the adrenal cortex, which surrounds
the adrenal medulla.

The Adrenal Cortex

cells of the adrenal cortex secrete a variety of steroid hormones.

glucocorticoids (e.g., cortisol)
mineralocorticoids (e.g., aldosterone)
androgens (e.g., testosterone)

Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.

Glucocorticoids

They Effect by raising the level of blood sugar (glucose). One way they do this is by stimulating gluconeogenesis in the liver: the conversion of fat and protein into intermediate metabolites that are ultimately converted into glucose.

The most abundant glucocorticoid is cortisol (also called hydrocortisone).

Cortisol and the other glucocorticoids also have a potent anti-inflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses.

Mineralocorticoids

The most important of them is the steroid aldosterone. Aldosterone acts on the kidney promoting the reabsorption of sodium ions (Na⁺) into the blood. Water follows the salt and this helps maintain normal blood pressure.

Aldosterone also

acts on sweat glands to reduce the loss of sodium in perspiration;
acts on taste cells to increase the sensitivity of the taste buds to sources of sodium.

The secretion of aldosterone is stimulated by:

a drop in the level of sodium ions in the blood;
a rise in the level of potassium ions in the blood;
angiotensin II
ACTH (as is that of cortisol)

Androgens

The adrenal cortex secretes precursors to androgens such as testosterone.

Excessive production of adrenal androgens can cause premature puberty in young boys.

In females, the adrenal cortex is a major source of androgens. Their hypersecretion may produce a masculine pattern of body hair and cessation of menstruation.

Addison's Disease: Hyposecretion of the adrenal cortices

Addison's disease has many causes, such as

destruction of the adrenal glands by infection;
their destruction by an autoimmune attack;
an inherited mutation in the ACTH receptor on adrenal cells.

Cushing's Syndrome: Excessive levels of glucocorticoids

In Cushing's syndrome, the level of adrenal hormones, especially of the glucocorticoids, is too high.It can be caused by:

excessive production of ACTH by the anterior lobe of the pituitary;
excessive production of adrenal hormones themselves (e.g., because of a tumor), or (quite commonly)
as a result of glucocorticoid therapy for some other disorder such as
- rheumatoid arthritis or
- preventing the rejection of an organ transplant.

The Adrenal Medulla

The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.The adrenal medulla releases:

adrenaline (also called epinephrine) and
noradrenaline (also called norepinephrine)

Both are derived from the amino acid tyrosine.

Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors transmembrane proteins in the plasma membrane of many cell types.

Some of the effects are:

increase in the rate and strength of the heartbeat resulting in increased blood pressure;
blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain;
rise in blood sugar;
increased metabolic rate;
bronchi dilate;
pupils dilate;
hair stands on end (gooseflesh in humans);
clotting time of the blood is reduced;
increased ACTH secretion from the anterior lobe of the pituitary.

All of these effects prepare the body to take immediate and vigorous action.

Clinical Physiology

Heart Failure : Heart failure is inability of the heart to pump the enough amount of blood needed to sustain the needs of organism .
It is usually called congestive heart failure ( CHF) .

To understand the pathophysiology of the heart failure , lets compare it with the physiology of the cardiac output :
Cardiac output =Heart rate X stroke volume

Stroke volume is determined by three determinants : Preload ( venous return ) , contractility , and afterload (peripheral resistance ) . Any disorder of these factors will reduce the ability of the heart to pump blood .

Preload : Any factor that decrease the venous return , either by decreasing the intravenous pressure or increasing the intraatrial pressure will lead to heart failure .

Contractility : Reducing the power of contraction such as in myocarditis , cardiomyopathy , preicardial tamponade ..etc , will lead to heart failure .

Afterload : Any factor that may increase the peripheral resistance such as hypertension , valvular diseases of the heart may cause heart failure.

Pathophysiology : When the heart needs to contract more to meet the increased demand , compensatory mechanisms start to develope to enhance the power of contractility . One of these mechanism is increasing heart rate , which will worsen the situation because this will increase the demands of the myocardial cells themselves . The other one is hypertrophy of the cardiac muscle which may compensate the failure temporarily but then the hypertrophy will be an additional load as the fibers became stiff .

The stroke volume will be reduced , the intraventricular pressure will increase and consequently the intraatrial pressure and then the venous pressure . This will lead to decrease reabsorption of water from the interstitium ( see microcirculation) and then leads to developing of edema ( Pulmonary edema if the failure is left , and systemic edema if the failure is right) .

COPD and Cancer

A. Chronic Obstructive Pulmonary Disease (COPD)

1. Common features of COPD

a.   almost all have smoking history
b.   dyspnea - chronic "gasping" for air
c.   frequent coughing and infections
d.   often leads to respiratory failure

2. obstructive emphysema - usually results from smoking

a.   enlargement & deterioration of alveoli
b.   loss of elasticity of the lungs
c.   "barrel chest" from bronchiole opening during inhalation & constriction during exhalation

3. chronic bronchitis - mucus/inflammation of mucosa

B. Lung Cancer

1.   squamous cell carcinoma (20-40%) - epithelium of the bronchi and bronchioles
2.   adenocarcinoma (25-35%) - cells of bronchiole glands and cells of the alveoli
3.   small cell carcinoma (10-20%) - special lymphocyte-like cells of the bronchi
4.   90% of all lung cancers are in people who smoke or have smoked

Functional Divisions of the Nervous System:

1) The Voluntary Nervous System - (ie. somatic division) control of willful control of effectors (skeletal muscles) and conscious perception. Mediates voluntary reflexes.

2) The Autonomic Nervous System - control of autonomic effectors - smooth muscles, cardiac muscle, glands. Responsible for "visceral" reflexes

Vital Capacity: The vital capacity (VC) is the maximum volume which can be ventilated in a single breath. VC= IRV+TV+ERV. VC varies with gender, age, and body build. Measuring VC gives a device for diagnosis of respiratory disorder, and a benchmark for judging the effectiveness of treatment. (4600 ml)

Vital Capacity is reduced in restrictive disorders, but not in disorders which are purely obstructive.

The FEV₁ is the % of the vital capacity which is expelled in the first second. It should be at least 75%. The FEV₁ is reduced in obstructive disorders.

Both VC and the FEV₁ are reduced in disorders which are both restrictive and obstructive

Oxygen is present at nearly 21% of ambient air. Multiplying .21 times 760 mmHg (standard pressure at sea level) yields a pO₂of about 160. Carbon dioxide is .04% of air and its partial pressure, pCO₂, is .3.

With alveolar air having a pO₂of 104 and a pCO₂of 40. So oxygen diffuses into the alveoli from inspired air and carbon dioxide diffuses from the alveoli into air which will be expired. This causes the levels of oxygen and carbon dioxide to be intermediate in expired air when compared to inspired air and alveolar air. Some oxygen has been lost to the alveolus, lowering its level to 120, carbon dioxide has been gained from the alveolus raising its level to 27.

Likewise a concentration gradient causes oxygen to diffuse into the blood from the alveoli and carbon dioxide to leave the blood. This produces the levels seen in oxygenated blood in the body. When this blood reaches the systemic tissues the reverse process occurs restoring levels seen in deoxygenated blood.

Conductivity :

Means ability of cardiac muscle to propagate electrical impulses through the entire heart ( from one part of the heart to another) by the excitatory -conductive system of the heart.

Excitatory conductive system of the heart involves:

1. Sinoatrial node ( SA node) : Here the initial impulses start and then conducted to the atria through the anterior inter-atrial pathway ( to the left atrium) , to the atrial muscle mass through the gap junction, and to the Atrioventricular node ( AV node ) through anterior, middle , and posterior inter-nodal pathways.
The average conductive velocity in the atria is 1m/s.

2- AV node : The electrical impulses can not be conducted directly from the atria to the ventricles , because of the fibrous skeleton , which is an electrical isolator , located between the atria and ventricles. So the only conductive way is the AV node . But there is a delay in the conduction occurs in the AV node .
This delay is due to:
- the smaller size of the nodal fiber.
- The less negative resting membrane potential
- fewer gap junctions.

There are three sites for delay:
- In the transitional fibers , that connect inter-nodal pathways with the AV node ( 0.03 ) .
- AV node itself ( 0.09 s) .
- In the penetrating portion of Bundle of Hiss ( 0.04 s) .
This delay actually allows atria to empty blood in ventricles during the cardiac cycle before the beginning of ventricular contraction , as it prevents the ventricles from the pathological high atrial rhythm.
The average velocity of conduction in the AV node is 0.02-0.05 m/s

3- Bundle of Hiss : A continuous with the AV node that passes to the ventricles through the inter-ventricular septum. It is subdivided into : Right and left bundle. The left bundle is also subdivided into two branches: anterior and posterior branches .

4- Purkinje`s fibers: large fibers with velocity of conduction 1.5-4 m/s.
the high velocity of these fibers is due to the abundant gap junctions , and to their nature as very large fibers as well.
The conduction from AV node is a one-way conduction . This prevents the re-entry of cardiac impulses from the ventricles to the atria.
Lastly: The conduction through the ventricular fibers has a velocity of 0.3-0.5 m/s.

Factors , affecting conductivity ( dromotropism) :

I. Positive dromotropic factors :

1. Sympathetic stimulation : it accelerates conduction and decrease AV delay .
2. Mild warming
3. mild hyperkalemia
4. mild ischemia
5. alkalosis

II. Negative dromotropic factors :

1. Parasympathetic stimulation
2. severe warming
3. cooling
4. Severe hyperkalemia
5. hypokalemia
6. Severe ischemia
7. acidosis
8. digitalis drugs.

Sensory pathways include only those routes which conduct information to the conscious cortex of the brain. However, we will use the term in its more loosely and commonly applied context to include input from all receptors, whether their signals reach the conscious level or not.

NEET MDS Lessons

Physiology