The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.

The Thyroid Gland

The thyroid gland synthesizes and secretes:

• thyroxine (T₄) and
• calcitonin

T₄ and T₃

Thyroxine (T₄ ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T₄ converting it into triiodothyronine (T₃). T₃ is the active hormone. It has many effects on the body. Among the most prominent of these are:

• an increase in metabolic rate
• an increase in the rate and strength of the heart beat.

The thyroid cells responsible for the synthesis of T₄ take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.

Diseases of the thyroid

1. hypothyroid diseases; caused by inadequate production of T₃

• cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
• myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
• goiter: enlargement of the thyroid gland. Can be caused by:
  • inadequate iodine in the diet with resulting low levels of T₄ and T₃;
  • an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).

2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones

Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.

Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.

Calcitonin

Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca²⁺) circulating in the blood. These cells monitor the level of circulating Ca²⁺. A rise in its level stimulates the cells to release calcitonin.

• bone cells respond by removing Ca²⁺ from the blood and storing it in the bone
• kidney cells respond by increasing the excretion of Ca²⁺

Both types of cells have surface receptors for calcitonin.

Because it promotes the transfer of Ca²⁺ to bones, calcitonin has been examined as a possible treatment for osteoporosis

A rise in blood pressure stretches the atria of the heart. This triggers the release of atrial natriuretic peptide (ANP). ANP is a peptide of 28 amino acids. ANP lowers blood pressure by:

• relaxing arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions in the collecting ducts of the kidneys.

The effects on the kidney reduce the reabsorption of water by them thus increasing the flow of urine and the amount of sodium excreted in it (These actions give ANP its name: natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood volume in the system.

The Heartbeat

During rest, the heart beats about 70 times a minute in the adult male, while pumping about 5 liters of blood.

The stimulus that maintains this rhythm is self-contained. Embedded in the wall of the right atrium is a mass of specialized heart tissue called the sino-atrial (S-A) node. The S-A node is also called the pacemaker because it establishes the basic frequency at which the heart beats.

The interior of the fibers of heart muscle, like all cells, is negatively charged with respect to the exterior. In the cells of the pacemaker, this charge breaks down spontaneously about 70 times each minute. This, in turn, initiates a similar discharge of the nearby muscle fibers of the atrium. A tiny wave of current sweeps over the atria, causing them to contract.

When this current reaches the region of insulating connective tissue between the atria and the ventricles, it is picked up by the A-V node (atrio-ventricular node). This leads to a system of branching fibers that carries the current to all parts of the ventricles.

The contraction of the heart in response to this electrical activity creates systole.

A period of recovery follows called diastole.

• The heart muscle and S-A node become recharged.
• The heart muscle relaxes.
• The atria refill. 

The Electrocardiogram

The electrical activity of the heart can be detected by electrodes placed at the surface of the body. Analysis of an electrocardiogram (ECG or EKG) aids in determining, for example, the extent of damage following a heart attack. This is because death of a portion of the heart muscle blocks electrical transmission through that area and alters the appearance of the ECG

Control of the Heart

Although the A-V node sets the basic rhythm of the heart, the rate and strength of its beating can be modified by two auxiliary control centers located in the medulla oblongata of the brain.

• One sends nerve impulses down accelerator nerves.
• The other sends nerve impulses down a pair of vagus nerves

Accelerator Nerves

The accelerator nerves are part of the sympathetic branch of the autonomic nervous system, and  like all post-ganglionic sympathetic neurons  release noradrenaline at their endings on the heart.

They increase the rate and strength of the heartbeat and thus increase the flow of blood. Their activation usually arises from some stress such as fear or violent exertion. The heartbeat may increase to 180 beats per minute. The strength of contraction increases as well so the amount of blood pumped may increase to as much as 25-30 liters/minute.

Vigorous exercise accelerates heartbeat in two ways;

• As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay  by the accelerator nerves  to the heart.
• As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart.

Distention of the wall of the right atrium also triggers the release of atrial natriuretic peptide (ANP) which initiates a set of responses leading to a lowering of blood pressure

The Vagus Nerves

The vagus nerves are part of the parasympathetic branch of the autonomic nervous system. They, too, run from the medulla oblongata to the heart. Their activity slows the heartbeat.

Pressure receptors in the aorta and carotid arteries send impulses to the medulla which relays these  by way of the vagus nerves  to the heart. Heartbeat and blood pressure diminish.

Structure of a nerve:

A peripheral nerve is arranged much like a muscle in terms of its connective tissue. It has an outer covering which forms a sheath around the nerve, called the epineurium. Often a nerve will run together with an artery and vein and their connective coverings will merge. Nerve fibers, which are axons, organize into bundles known as fascicles with each fascicle surrounded by the perineurium. Between individual nerve fibers is an inner layer of endoneurium.

 The myelin sheath in peripheral nerves consists of Schwann cells wrapped in many layers around the axon fibers. Not all fibers in a nerve will be myelinated, but most of the voluntary fibers are. The Schwann cells are portrayed as arranged along the axon like sausages on a string. Gaps between the Schwann cells are called nodes of Ranvier. These nodes permit an impulse to travel faster because it doesn't need to depolarize each area of a membrane, just the nodes. This type of conduction is called saltatory conduction and means that impulses will travel faster in myelinated fibers than in unmyelinated ones.

The myelin sheath does several things:

1) It provides insulation to help prevent short circuiting between fibers.

2) The myelin sheath provides for faster conduction.

3) The myelin sheath provides for the possibility of repair of peripheral nerve fibers. Schwann cells help to maintain the micro-environments of the axons and their tunnel (the neurilemma tunnel) permits re-connection with an effector or receptor  CNS fibers, not having the same type of myelination accumulate scar tissue after damage, which prevents regeneration.

Phases of cardiac cycle :

1. Early diastole ( also called the atrial diastole , or complete heart diastole) : During this phase :

- Atria are  relaxed
- Ventricles are relaxed
- Semilunar valves are closed
- Atrioventricular valves are open
During this phase the blood moves passively from the venous system into the ventricles ( about 80 % of blood fills the ventricles during this phase.

2. Atrial systole : During this phase :

- Atria are contracting
- Ventricles are relaxed
- AV valves are open
- Semilunar valves are closed
- Atrial pressure increases.the a wave of atrial pressure appears here.
- P wave of ECG starts here
- intraventricular pressure increases due to the rush of blood then decrease due to continuous relaxation of ventricles.

The remaining 20% of blood is moved to fill the ventricles during this phase , due to atrial contraction.

3. Isovolumetric contraction : During this phase :

- Atria are relaxed
- Ventricles are contracting
- AV valves are closed
- Semilunar valves are closed
- First heart sound
- QRS complex.
The ventricular fibers start to contract during this phase , and the intraventricular pressure increases. This result in closing the AV valves , but the pressure is not yet enough to open the semilunar valves , so the blood volume remain unchanged , and the muscle fibers length also remain unchanged , so we call this phase as isovolumetric contraction ( iso : the same , volu= volume , metric= length).

4. Ejection phase : Blood is ejected from the ventricles into the aorta and pulmonary artery .

During this phase :

- Ventricles are contracting
- Atria are relaxed
- AV valves are closed
- Semilunar valves are open
- First heart sound
- Intraventricular pressure is increased , due to continuous contraction
- increased aortic pressure .
- T wave starts.

5. Isovolumetric relaxation:  This phase due to backflow of blood in aorta and pulmonary system after the ventricular contraction is up and the ventricles relax . This backflow closes the semilunar valves .

During this phase :

- Ventricles are relaxed
- Atrial are relaxed
- Semilunar valves are closed .
- AV valves are closed.
- Ventricular pressure fails rapidly
- Atrial pressure increases due to to continuous venous return. the v wave appears here. 
- Aortic pressure : initial sharp decrease due to sudden closure of the semilunar valve ( diacrotic notch) , followed by secondary rise in pressure , due to elastic recoil of the aorta ( diacrotic wave)  .
- T wave ends in this phase

Exchange of gases takes place in Lungs

• A person with an average ventilation rate of 7.5 L/min will breathe in and out 10,800 liters of gas each day
• From this gas the person will take in about 420 liters of oxygen (19 moles/day) and will give out about 340 liters of carbon dioxide (15 moles/day)
• The ratio of CO₂ expired/O₂ inspired is called the respiratory quotient (RQ)
  • RQ = CO₂ out/O₂ in = 340/420 = 0.81
  • In cellular respiration of glucose CO₂ out = O₂ in; RQ = 1
  • The overall RQ is less than 1 because our diet is a mixture of carbohydrates and fat; the RQ for metabolizing fat is only 0.7
• All of the exchange of gas takes place in the lungs
• The lungs also give off large amounts of heat and water vapor

Biological Functions are Extremely Sensitive to pH

• H+ and OH- ions get special attention because they are very reactive
• Substance which donates H+ ions to solution = acid
• Substance which donates OH- ions to solution = base
• Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
  • pH = - log [H⁺]
  • [H⁺] is the H ion concentration in moles/liter
  • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
  • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
    • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times

• Human blood pH is 7.4
  • Blood pH above 7.4 = alkalosis
  • Blood pH below 7.4 = acidosis
• Body must get rid of ~15 moles of potential acid/day (mostly CO₂)
  • CO₂ reacts with water to form carbonic acid (H₂CO₃)
  • Done mostly by lungs & kidney
• In neutralization H⁺ and OH^- react to form water
• If the pH changes charges on molecules also change, especially charges on proteins
  • This changes the reactivity of proteins such as enzymes
• Large pH changes occur as food passes through the intestines.