Respiratory system plays important role in maintaining homeostasis . Other than its major function , which is supplying the cells with needed oxygen to produce energy and getting rid of carbon dioxide , it has other functions :

1 Vocalization , or sound production.
2 Participation in acid base balance .
3 Participation in fluid balance by insensible water elimination (vapors ).
4 Facilitating venous return .
5 Participation in blood pressure regulation : Lungs produce Angiotensin converting enzyme ( ACE ) .
6 Immune function : Lungs produce mucous that trap foreign particles , and have ciliae that move foreign particles away from the lung. They also produce alpha 1 antitrepsin that protect the lungs themselves from the effect of elastase and other proteolytic  enzymes

Control of processes in the stomach:

The stomach, like the rest of the GI tract, receives input from the autonomic nervous system. Positive stimuli come from the parasympathetic division through the vagus nerve. This stimulates normal secretion and motility of the stomach. Control occurs in several phases:

Cephalic phase stimulates secretion in anticipation of eating to prepare the stomach for reception of food. The secretions from cephalic stimulation are watery and contain little enzyme or acid.

Gastric phase of control begins with a direct response to the contact of food in the stomach and is due to stimulation of pressoreceptors in the stomach lining which result in ACh and histamine release triggered by the vagus nerve. The secretion and motility which result begin to churn and liquefy the chyme and build up pressure in the stomach. Chyme surges forward as a result of muscle contraction but is blocked from entering the duodenum by the pyloric sphincter. A phenomenon call retropulsion occurs in which the chyme surges backward only to be pushed forward once again into the pylorus. The presence of this acid chyme in the pylorus causes the release of a hormone called gastrin into the bloodstream. Gastrin has a positive feedback effect on the motility and acid secretion of the stomach. This causes more churning, more pressure, and eventually some chyme enters the duodenum.

Intestinal phase of stomach control occurs. At first this involves more gastrin secretion from duodenal cells which acts as a "go" signal to enhance the stomach action already occurring. But as more acid chyme enters the duodenum the decreasing pH inhibits gastrin secretion and causes the release of negative or "stop" signals from the duodenum.

These take the form of chemicals called enterogastrones which include GIP (gastric inhibitory peptide). GIP inhibits stomach secretion and motility and allows time for the digestive process to proceed in the duodenum before it receives more chyme. The enterogastric reflex also reduces motility and forcefully closes the pyloric sphincter. Eventually as the chyme is removed, the pH increases and gastrin and the "go" signal resumes and the process occurs all over again. This series of "go" and "stop" signals continues until stomach emptying is complete.

Heart is a hollow muscular organ , that is located in the middle mediastinum  between the two bony structures of the sternum and the vertebral column ( a very important location for applying Cardiopulmonary Resuscitation - CPR- ) .
It has a shape of clenched fist , which weighs about 300 grams ( with mild variation between male and female ).
  Heart has an apex that is anteriorly , inferiorly , and leftward oriented , and a base , that is posteriorly , superiorly and rightward oriented   .
 In addition to its apex and base the heart has anterior , posterior and left surfaces.
 
 The wall of the heart is composed of three layers :
 
1. Endocardium : The innermost layer , which lines the heart chambers and is in direct contact with the blood . It is composed of endothelial cells that are similar to those , that line the blood vessels , and of connective tissue too. 
 Endocardium has a smooth surface that prevents blood clotting, as it ensures laminar blood flow .

 Clinical Physiology 
 Endocarditis is the inflammation of the endocardium , which is resistant to antibiotic treatment and difficult to cure.Endocarditis usually involves heart valves and chordae tendineae too.

 2. Myocardium  : The middle layer of the cardiac wall . It is the thickest among the three layers , and is composed of two types of cardiac muscles :
a. contractile muscle cells (form about 98-99% of the cardiac muscle ) .
 b- non-contractile muscle cells ( form about 1-2 % of the cardiac muscles and are the cells that form excitatory-conductive system of the heart).
 The cardiac muscle cells are similar to the skeletal muscles in that they are striated , but similar to the smooth muscles in being involuntary and connected to each others via gap junctions , that facilitate conduction of electrical potential from one cell to the others. Desmosomes adhere cardiac muscle cells to each others .

 3- Epicardium :  is the outermost and protective layer of the heart . It is composed of connective tissue , and form the inner layer of the pericardium ( visceral pericardium - see bellow).

 Pericardium: 
The heart is surrounded by a fluid-fill sac , which is known as pericardium . Pericardium is composed of two layers ( doubled layer membrane ) , between which a fluid-fill pericardial cavity exist .

 The outer layer is called fibrous pericardium , while the inner layer is called serous pericardium , which is subdivided into parietal pericardium and visceral pericardium . The visceral pericardium is the previously mentioned outermost layer of heart ( epicardium) .
Pericardial sac plays an important role in protection of heart from external hazards and infections , as it fixes the heart and limits its motion. It also prevents excessive dilation of the heart.

Clinical physiology: 

When there is excessive fluid in the pericardial cavity as a result of pericardial effusion , a cardiac tamponade will develop . cardiac tamponade means compression of the heart within the pericardial sac , which will prevent the relaxation of the heart ( heart will not be able to fully expand ) , and thus the circulating blood volume will be decreased (obstructive shock) . This is a life threatening situation which has to be urgently cured by  pericardiocentesis . 

Chambers of the heart : 

Heart has four chambers : two atria and two ventricles . The two right and left atria are separated from the two ventricles by the fibrous skeleton , which involves the right ( tricuspid ) and left ( bicuspid ) valves. Right and left atria are separated from each other by the interatrial  septum .
The two ventricles are separated by the interventricular septum.Interventricular septum is muscular in its lower thick part and fibrous in its upper thin part.
The two atria holds the blood returning from the veins and empty it only in a given right moment into the ventricles. Ventricles pump the blood into the arteries . 

Heart valves : 

There are four valves in the heart : Two atrioventricular valves and two semi-lunar valves:
1. Atrioventricular ( AV ) valves: These valves are found between the atria and ventricles , depending on the number of  the leaflets , the right atrioventricular valve is also called tricuspid valve (has three leaflets ) , while the left one is called bicuspid valve (has two leaflets ) . The shape of the bicuspid valve is similar to the mitre of bishop , so it is also called the mitral valve.
The leaflets of the valves are attached to fibrous threads (composed of collagen fibers ) , known as chordae tendineae , which from their side are attached to papillary muscles in the ventricles. These valves prevent backward flow of blood from ventricles during the systole. 

2. Semi-lunar valves : 

These valves are located on the base of the arteries ( aorta and pulmonary artery ) . They prevent the backward flow of blood from the arteries into ventricles.
The structure of the semilunar valves is quite different from that of the AV valves , as they have crescent-shaped cusps that do not have chorda tendinea , instead these cusps are like pockets which are filled of blood when it returns to the ventricles from the lumen of arteries during the diastole  , so they get closed and prevent the backward flow of blood.

Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials

Typical Concentration Gradients and Membrane Potentials in Excitable Cells

The Na Pump is Particularly Important in the Kidney and Brain

• All cells have Na pumps in their membranes, but some cells have more than others
• Over-all Na pump activity may account for a third of your resting energy expenditure!
• In the kidney the Na pump activity is very high because it is used to regulate body salt and water concentrations
  • Kidneys use enormous amounts of energy: 0.5% of body weight, but use 7% of the oxygen supply
• Pump activity is also high in the brain because Na and K gradients are essential for nerves
  • The brain is another high energy organ; it is 2% of body weight, but uses 18% of the oxygen supply

In the Resting State Potassium Controls the Membrane Potential of Most Cells

• Resting cells have more open K channels than other types
• More K+ passes through membrane than other ions- therefore K+ controls the potential
• Blood K+ must be closely controlled because small changes will produce large changes in the membrane potentials of cells
  • Raising K will make the membrane potential less negative (depolarization)
• High blood K+ can cause the heart to stop beating (it goes into permanent contraction)

During an Action Potential Na Channels Open, and Na Controls the Membrane Potential

• Whichever ion has the most open channels controls the membrane potential
• Excitable cells have Na channels that open when stimulated
• When large numbers of these channels open Na controls the membrane potential

Cell, or Plasma, membrane

• Structure - 2 primary building blocks include

protein (about 60% of the membrane) and lipid, or

fat (about 40% of the membrane).

The primary lipid is called phospholipids, and molecules of phospholipid form a 'phospholipid bilayer' (two layers of phospholipid molecules). This bilayer forms because the two 'ends' of phospholipid molecules have very different characteristics: one end is polar (or hydrophilic) and one (the hydrocarbon tails below) is non-polar (or hydrophobic):

• Functions include:
  • supporting and retaining the cytoplasm
  • being a selective barrier .
  • transport
  • communication (via receptors)