INHALATION AND EXHALATION (BREATH)

 

                                   THE MECHANICS OF 

                                            BREATHING

             PREVIOUSLY THE RESPIRATORY SYSTEM WAS DISCUSSED IN TERMS OF LEVES WHICH ARE NOT READILY VISIBLE TO THE CASUAL OBSERVER: MOLECULAR INNTERACTIONS, MICROSCOPE TRANSPORT, AND INTERNAL ANATOMY. THE FOCUS OF DISCUSSION NOW SHIFTS CLOSER TO THE BODY'S SURFACE IN ORDER TO STUDY THE ORGANIZATION AND ACTION OF THOSE STRUCTURES WHICH CREATE THE DRIVING FORCE FOR MOVING AIR IN AND OUT OF THE BODY.

                     IN LOOKING AT THE BODY, SEVERAL ANATOMICAL DIVISIONS BECOME READILY APPARENT: THE FOUR LIMBS, THE HEAD, AND THE TORSO. EACH COMPRISES A DISTINCT ANATOMICAL UNIT. SINCE IT IS THE TORSO WHICH CONTAINS THE ORGANS RESPONSIBLE FOR MOVEMENT OF AIR INTO THE BODY, A WELL AS THE OTHER MAJOR ORGAN SYSTEMS RESPONSIBLE FOR DISTRIBUTION OF OXYGEN, IT IS HERE THAT A DISCUSSION OF THE MECHANICS OF BREATHING MUST BE CENTERED.

                    ONE CAN SUBDIVIDE THE TORSO INTO THREE REGIONS, THE THORAX OR CHEST, WHICH HOUSES THE HEART AND TWO LUNGS, THE ABDOMEN, WHICH BEGINS IMMEDIATELY BELOW THE DIAPHRAGM, AND WHICH EXTENDS FROM THE ORGANS OF EXCRETION AND REPRODUCTION. FOR THE PURPOSES OF THIS DISCUSSION, THE PELVIS WILL BE CONSIDERED AS ESSENTIALLY CONTINOUS WITH ABDOMEN.

                    LOOKING ONLY AT THE SUPPORTING BONE, MUSCLE, AND SKIN, WITH THE INTERNAL ORGANS REMOVED, THE TORSO CAN BE VIEWED AS FORMING A ROUGH CYLINDER, SLIGHTLY FLATTENED OUT, SO THAT IT IS WIDER THAN IT IS DEEP WHEN SEEN IN CROSS SECTION. THE BONY SPINE, OR VERTEBRAL COLUMN, WHICH RUNS VERTICALLY THROUGH THE BACK PARALLEL TO THE LONG AXIS OF THE TORSO "CYLINDER" PROVIDES STRUCTURAL SUPPORT FOR THE WHOLE TORSO, ACTING AS A FRAMEWORK AROUND WHICH OTHER TISSUES AND ORGANS ARE GROUPED. THE VERTEBRAL COLUMN ITSELF IS COMPOSED OF A NUMBER OF SMALL INDIVIDUAL BONES CALLED VERTEBRATE. THEY ARE STACKED, ONE ON TOP OF THE OTHER AND ARE SEPERATED BY DISCS OF SHOCK-ABSORBING TISSUE. THE FIRST TWELVE OF THESE BONES WITHIN THE TORSO, THE THORACIC VERTEBRAE, EACH ATTACH TO PAIR OF RIBS, ONE ON EITHER SIDE. THE RIBS ALL TRAVEL PARALLEL TO EACH OTHER, CURVING IN FORWARD AND DOWNWARD DIRECTION. THE FIRST TEN OF THESE JOIN IN THE MIDLINE AND FUSE WITH THE STERNUM, OR BREASTBONE, TO FORM THE RIB CAGE. A SERIES OF SMALL JOINTS CONNECT THE RIBS AND VERTEBRAE, THIS ALLOWS THEM TO MOVE SLIGHTLY IN A HINGED FASHION, SOMEWHAT LIKE A CURVED BUCKET HANDLE MOVES. THE RIBS, ALONE WITH THEIR ATTACHMENTS TO THE STERNUM IN FRONT AND VERTEBRAL COLUMN IN BACK, FORM THE WALLS OF THE THORACIC, OR CHEST, PORTION OF THE BODY CYLINDER.

                    BECAUSE THE RIBS GRADUALLY INCREASE IN LENGTH OF CURVATURE FROM THE TOP TO THE BOTTOM OF THE THORAX, THE WIDEST PART OF THE THORAX OCCURS AT ITS LOWER MARGIN. ATTACHED TO THESE LOWER RIBS AND TO THE STERNUM AND VERTEBRAL COLUMN IS A TOUGH, FLAT SHEET OF MUSCLE, THE DIAPHRAGM, WHICH IN EFFECT DIVIDES THE TORSO CYLINDER INTO TWO SMALLER CYLINDERS, ONE STACKED ON TOP OF THE OTHER, THE CHEST CAVITY ABOVE AND THE ABDOMINAL CAVITY BELOW. THE BOUNDARIES OF THE ABDOMINAL CYLINDER INCLUDE THE VERTEBRAL COLUMN AND ITS SUPPORTING MUSCLES IN THE BACK AND THE FLOOR OF THE PELVIS, BOTH OF WHICH ARE RELATIVELY INFLEXIBLE, AND FIXED, AND THE ABDOMINAL CONTENTS WHICH LIE JUST BENEATH THE SURFACE OF THE DIAPHRAGM, EXTENDING PARTLY INTO THE CHEST CAVITY. SEVERAL OVERLAPPING SHEETS OF MUSCLE, EXTENDING FROM THE RIBS ABOVE TO THE PELVIS BELOW, FORM THE FRONT AND SIDES OF THE ABDOMEN. 

                      THE DIAPHRAGM, WHICH SEPARATES THE CHEST AND ABDOMINAL CAVITIES, IS NOT FLAT IN ITS RESTING POSITION, BUT BILLOWS UP INTO THE CHEST CAVITY SOMEWHAT LIKE A PARACHUTE OR DOME. FOR THIS REASON ITS MOVEMENTS ARE NOT DIRECTLY VISIBLE AT THE BODY'S SURFACE, AND ONE HAS TO INFER ITS ACTIVITY AT THE BODY'S SURFACE, AND ONE HAS TO INFER ITS ACTIVITY BASED ON THE EFFECTS ITS MOVEMENTS HAVE ON OTHER BODY TISSUES.

                  IMMEDIATELY ABOVE THE DIAPHRAGM ARE THE RIGHT AND LEFT LUNGS, AND NESTLED BETWEEN THEM, THE HEART. THE LUNGS DO NOT ACTUALLY TOUCH THE DIAPHRAGM DIRECTLY, FOR THEY ARE COMPLETELY COVERED WITH A VERY THIN DOUBLE LAYER OF TISSUE KNOWN AS PLEURA. NORMALLY, THESE TWO LAYERS ARE IN DIRECT CONTACT WITH EACH OTHER, SLIGHTLY MOISTENED BY A SMALL AMOUNT OF PLEURAL FLUID WHICH ACT AS A LUBRICANT, ALLOWING THE PLEURAL LAYERS TO FREELY SLIDE OVER EACH OTHER.

                     THE INNERMOST PLEURAL LAYER COMPLETELY COVERS THE OUTSIDE OF EACH LUNG, WHILE THE OUTER LAYER COVERS THE INNER SURFACE OF THE CHEST WALL AND THE THORACIC SIDE OF THE DIAPHRAGM. SINCE BOTH PLEURAL LAYERS ARE IN SUCH CLOSE CONTACT WITH ONE ANOTHER, A MOVEMENT OF THE CHEST WALL OR DIAPHRAGM WILL BE TRASMITTED TO THE LUNGS, AND VICE VERSA. IF, FOR INSTANCE, THE LUNGS WILL FOLLOW, EXPANDING IN THE PROCESS. THESE ARE, IN FACT, THE TWO MAIN MECHANICS BY WHICH AIR MOVES INTO THE LUNGS.

                  AIRFLOW INTO TEH LUNGS OCCURS WHEN THE STRUCTURES SURROUNDING THEM EXPAND AND PULL THE LUNGS ALONG WITH THEM. THE RESULTING SUCTION PULL AIR INTO THE LUNGS FROM THE UPPER AIRWAYS TO THE TRACHEA AND BRONCHIAL TREE, INTO THE ALVEOLI. THIS IS THE PROCESS OF INHALATION.