Respiration occurs in three steps :
1- Mechanical ventilation : inhaling and exhaling of air between lungs and atmosphere.
2- Gas exchange : between pulmonary alveoli and pulmonary capillaries.
3- Transport of gases from the lung to the peripheral tissues , and from the peripheral tissues back to blood .
These steps are well regulated by neural and chemical regulation.

Respiratory tract is subdivided into upper and lower respiratory tract. The upper respiratory tract involves , nose , oropharynx and nasopharynx , while the lower respiratory tract involves larynx , trachea , bronchi ,and lungs .

Nose fulfills three important functions which are :

1. warming of inhaled air .

b. filtration of air .

c. humidification of air .

Pharynx is a muscular tube , which forms a passageway for air and food .During swallowing the epiglottis closes the larynx and the bolus of food falls in the esophagus .

Larynx is a respiratory organ that connects pharynx with trachea . It is composed of many cartilages and muscles and

vocal cords . Its role in respiration is limited to being a conductive passageway for air .

Trachea is a tube composed of C shaped cartilage rings from anterior side, and of muscle (trachealis muscle ) from its posterior side.The rings prevent trachea from collapsing during the inspiration. 

From  the trachea the bronchi are branched into right and left bronchus ( primary bronchi) , which enter the lung .Then they repeatedly branch into secondary and tertiary bronchi and then into terminal and respiratory broncholes.There are about 23 branching levels from the right and left bronchi to the respiratory bronchioles  , the first upper  17 branching are considered as a part of the conductive zones , while the lower 6 are considered to be respiratory zone. 

The cartilaginous component decreases gradually from the trachea to the bronchioles  . Bronchioles are totally composed of smooth muscles ( no cartilage) . With each branching the diameter of bronchi get smaller , the smallest diameter of respiratory passageways is that of respiratory bronchiole. 

Lungs are evolved by pleura . Pleura is composed of two layers : visceral and parietal .
Between the two layers of pleura , there is a pleural cavity , filled with a fluid that decrease the friction between the visceral and parietal pleura.
 

Respiratory muscles : There are two group of respiratory muscles:

1. Inspiratory muscles : diaphragm and external intercostal muscle ( contract during quiet breathing ) , and accessory inspiratory muscles : scaleni , sternocleidomastoid , internal pectoral muscle , and others( contract during forceful inspiration).
 

2. Expiratory muscles : internal intercostal muscles , and abdominal muscles ( contract during forceful expiration)

The Cardiac Cycle: the sequence of events in one heartbeat.

systole - the contraction phase; unless otherwise specified refers to left ventricle, but each chamber has its own systole.

diastole - the relaxation phase; unless otherwise specified refers to left ventricle, but each chamber has its own diastole.

1) quiescent period - period when all chambers are at rest and filling. 70% of ventricular filling occurs during this period. The AV valves are open, the semilunar valves are closed.

2) atrial systole - pushes the last 30% of blood into the ventricle.

3) atrial diastole - atria begin filling.

4) ventricular systole - First the AV valves close causing the first heart sound, then after the isovolumetric contraction phase the semilunar valves open permitting ventricular ejection of blood into the arteries.

5) ventricular diastole - As the ventricles relax the semilunar valves close first producing the second heart sound, then after the isovolumetric relaxation phase the AV valves open allowing ventricular filling.

The Parathyroid Glands

The parathyroid glands are 4 tiny structures embedded in the rear surface of the thyroid gland. They secrete parathyroid hormone (PTH) a polypeptide of 84 amino acids. PTH increases the concentration of Ca²⁺ in the blood in three ways. PTH promotes

• release of Ca²⁺ from the huge reservoir in the bones. (99% of the calcium in the body is incorporated in our bones.)
• reabsorption of Ca²⁺ from the fluid in the tubules in the kidneys
• absorption of Ca²⁺ from the contents of the intestine (this action is mediated by calcitriol, the active form of vitamin D.)

PTH also regulates the level of phosphate in the blood. Secretion of PTH reduces the efficiency with which phosphate is reclaimed in the proximal tubules of the kidney causing a drop in the phosphate concentration of the blood.

Hyperparathyroidism

Elevate the level of PTH causing a rise in the level of blood Ca²⁺ .Calcium may be withdrawn from the bones that they become brittle and break.

 Patients with this disorder have high levels of Ca²⁺ in their blood and excrete small amounts of Ca²⁺ in their urine. This causes hyperparathyroidism.

Hypoparathyroidism

This disorder have low levels of Ca²⁺ in their blood and excrete large amounts of Ca²⁺ in their urine.

Platelets

Platelets are cell fragments produced from megakaryocytes.

Blood normally contains 150,000 to 350,000 per microliter (µl). If this value should drop much below 50,000/µl, there is a danger of uncontrolled bleeding. This is because of the essential role that platelets have in blood clotting.

When blood vessels are damaged, fibrils of collagen are exposed.

• von Willebrand factor links the collagen to platelets forming a plug of platelets there.
• The bound platelets release ADP and thromboxane A2 which recruit and activate still more platelets circulating in the blood.
• (This role of thromboxane accounts for the beneficial effect of low doses of aspirin a cyclooxygenase inhibitor in avoiding heart attacks.)

ReoPro is a monoclonal antibody directed against platelet receptors. It inhibits platelet aggregation and appears to reduce the risk that "reamed out" coronary arteries (after coronary angioplasty) will plug up again.

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

1.Rhythmicity ( Chronotropism ) :  means the ability of heart to beat regularly ( due to repetitive and stable depolarization and repolarization )  . Rhythmicity of heart is a myogenic in origin , because cardiac muscles are automatically excited muscles and does not depend on the nervous stimulus to initiate excitation and then contraction . The role of nerves is limited to the regulation of the heart rate and not to initiate the beat.

There are many evidences that approve the myogenic and not neurogenic origin of the rhythmicity of cardiac muscle . For example :
-  transplanted heart continues to beat regularly without any nerve supply.
-  Embryologically the heart starts to beat before reaching any nerves to them.
-  Some drugs that paralyze the nerves ( such as cocaine ) do not stop the heart in given doses.

Spontaneous rhythmicity of the cardiac muscle due to the existence of excitatory - conductive system , which is composed of self- exciting non-contractile cardiac muscle cells . The SA node of the mentioned system excites in a rate , that is the most rapid among the other components of the system ( 110 beats /minute ) , which makes it the controller or ( the pacemaker ) of the cardiac rhythm of the entire heart.

Mechanism , responsible for self- excitation in the SA node and the excitatory conductive system  is due to the following properties of the cell membrane of theses cells :
1- Non-gated sodium channels
2- Decreased permeability to potassium
3- existence of slow and fast calcium channels.

These properties enable the cations ( sodium through the none-gated sodium voltage channels , calcium through calcium slow channels) to enter the cell and depolarize the cell membrane without need for external stimulus.

The resting membrane potential of non-contractile cardiac cell is -55 - -60 millivolts ( less than that of excitable nerve cells (-70) ) . 

The threshold is also less negative than that of nerve cells ( -40 millivolts ).

The decreased permeability to potassium from its side decrease the eflux  of potassium during the repolarization phase of the pacemaker potential . All of these factors give the pacemaker potential its characteristic shape

Repeating of the pacemaker potential between the action potentials of contractile muscle cells is the cause of spontaneous rhythmicity of cardiac muscle cells.

Factors , affecting the rhythmicity of the cardiac muscle :

I. Factors that increase the rate ( positive chronotropic factors) :
1. sympathetic stimulation : as its neurotransmitter norepinephrine increases the membrane permeability to sodium and calcium.
2. moderate warming : moderate warming increases temperature by 10 beats for each 1 Fahrenheit degree increase in body temperature, this due to decrease in permeability to potassium ions in pacemaker membrane by moderate increase in temperature.
3. Catecholaminic drugs have positive chronotropic effect.
4. Thyroid hormones : have positive chronotropic effect , due to the fact that these drugs increase the sensitivity of adrenergic receptors to adrenaline and noreadrenaline .
5. mild hypoxia.
6. mild alkalemia : mild alkalemia decreases the negativity of the resting potential.
7. hypocalcemia.
8. mild hypokalemia

II. Factors that decrease rhythmicity ( negative chronotropic):

1.Vagal stimulation : the basal level of vagal stimulation inhibits the sinus rhythm and decrease it from 110-75 beats/ minute. This effect due to increasing the permeability of the cardiac muscle cell to potassium , which causes rapid potassium eflux , which increases the negativity inside the cardiac cells (hyperpolarization ).
2. moderate cooling
3. severe warming : due to cardiac damage , as a result of intercellular protein denaturation. Excessive cooling on the other hand decrease metabolism and stops rhythmicity.
4. Cholenergic drugs ( such as methacholine , pilocarpine..etc) have negative chronotropic effect.
5. Digitalis : these drugs causes hyperpolarization . This effect is similar to that of vagal stimulation.
6. Hypercapnia ( excessive CO2 production )
7. Acidemia.
8. hyper- and hyponatremia .
9. hyperkalemia
10. hypercalcemia
11. Typhoid or diphteria toxins.