question: Why is it we can sometimes feel or hear our heart beating through various body parts, and what makes it happen as opposed to the majority of time where we can't?
comment: There are a lot of factors that can affect this, but the most likely suspect would probably be either increased blood flow or an extreme change in heart rate. Increased blood flow usually happens in your body from infection or wounds since your body wants to get more nutrients and white blood cells there to fight pathogens. Increased blood vessel permeability results in increased interstitial fluid in the affected area. This process causes the swelling/heat/redness, which can 'pulse' because this new mass may place pressure on surrounding large arteries. Heart rate changes occur from stresses. This is usually from stuff like vigorous exercise or getting scared, with a dramatically increased heart rate of 150+, which is definitely more audible. Lastly, but not often, if you are relaxed enough and it is quiet enough, it is perfectly possible to hear your heart beating and feel your blood flowing through your body. There are super quiet rooms intended to kill as much sound as possible that let you hear your own blood moving around clear as day. EDIT: Added more accurate information from /u/TheLiquidSilver.
passages: Section::::Release.
   Section::::Critical reception.
   BULLET::::- Facial pulse: located on the mandible (lower jawbone) on a line with the corners of the mouth (facial artery).
  
  BULLET::::- Temporal pulse: located on the temple directly in front of the ear (superficial temporal artery).
  
  Although the pulse can be felt in multiple places in the head, people should not normally hear their heartbeats within the head. This is called pulsatile tinnitus, and it can indicate several medical disorders.
  
  Section::::Common palpable sites.:Torso.
   Tachycardia that is not sinus tachycardia usually results from the addition of abnormal impulses to the normal cardiac cycle. Abnormal impulses can begin by one of three mechanisms: automaticity, re-entry or triggered activity. A specialised form of re-entry which is both common and problematic is termed fibrillation.
  
  Although the term "tachycardia" has been known for over 160 years, bases for the classification of arrhythmias are still being discussed.
  
  Section::::Differential diagnosis.:Heart defects.
   Section::::Clinical significance.:Diseases.:Cardiac arrhythmias.
  
  While in the healthy heart, waves of electrical impulses originate in the sinus node before spreading to the rest of the atria, the atrioventricular node, and finally the ventricles (referred to as a normal sinus rhythm), this normal rhythm can be disrupted. Abnormal heart rhythms or arrhythmias may be asymptomatic or may cause palpitations, blackouts, or breathlessness. Some types of arrhythmia such as atrial fibrillation increase the long term risk of stroke.
   This phenomenon can be a sign of hypertrophic obstructive cardiomyopathy or of many other types of heart disease (see list below). Other causes include digitalis toxicity, induction of anesthesia, placement of surgical instrumentation into the thorax or as a benign, temporary phenomenon.
   There are multiple theories about the cause of atrial fibrillation. An important theory is that, in atrial fibrillation, the regular impulses produced by the sinus node for a normal heartbeat are overwhelmed by rapid electrical discharges produced in the atria and adjacent parts of the pulmonary veins. Sources of these disturbances are either automatic foci, often localized at one of the pulmonary veins, or a small number of localized sources in the form of either a re-entrant leading circle, or electrical spiral waves (rotors); these localized sources may be found in the left atrium near the pulmonary veins or in a variety of other locations through both the left or right atrium. There are three fundamental components that favor the establishment of a leading circle or a rotor: slow conduction velocity of cardiac action potential, short refractory period, and small wavelength. Meanwhile, wavelength is the product of velocity and refractory period. If the action potential has fast conduction, with a long refractory period and/or conduction pathway shorter than the wavelength, an AF focus would not be established. In multiple wavelet theory, a wavefront will break into smaller daughter wavelets when encountering an obstacle, through a process called vortex shedding; but under proper conditions, such wavelets can reform and spin around a centre, forming an AF focus.
   Section::::Background and recording.
   In AF, the normal regular electrical impulses generated by the sinoatrial node in the right atrium of the heart are overwhelmed by disorganized electrical impulses usually originating in the roots of the pulmonary veins. This leads to irregular conduction of ventricular impulses that generate the heartbeat.
  
  Section::::Pathophysiology.:Pathology.
   Section::::Music and lyrics.
   Atrial fibrillation is one of the most common cardiac arrhythmias. In general, it is an irregular, narrow complex rhythm. However, it may show wide QRS complexes on the ECG if a bundle branch block is present. At high rates, the QRS complex may also become wide due to the Ashman phenomenon. It may be difficult to determine the rhythm's regularity when the rate exceeds 150 beats per minute. Depending on the patient's health and other variables such as medications taken for rate control, atrial fibrillation may cause heart rates that span from 50 to 250 beats per minute (or even higher if an accessory pathway is present). However, new onset atrial fibrillation tends to present with rates between 100 and 150 beats per minute. 
   Any part of the heart that initiates an impulse without waiting for the sinoatrial node is called an ectopic focus and is, by definition, a pathological phenomenon. This may cause a single premature beat now and then, or, if the ectopic focus fires more often than the sinoatrial node, it can produce a sustained abnormal rhythm. Rhythms produced by an ectopic focus in the atria, or by the atrioventricular node, are the least dangerous dysrhythmias; but they can still produce a decrease in the heart's pumping efficiency, because the signal reaches the various parts of the heart muscle with different timing than usual and can be responsible for poorly coordinated contraction.
   The term sinus arrhythmia refers to a normal phenomenon of alternating mild acceleration and slowing of the heart rate that occurs with breathing in and out respectively. It is usually quite pronounced in children and steadily decreases with age. This can also be present during meditation breathing exercises that involve deep inhaling and breath holding patterns.
  
  Section::::Differential diagnosis.:Bradycardias.
   'Multiform' simply describes the variable P wave shapes and is an observation, 'multifocal' is an inference about the underlying cause. Although these are interchangeable terms, some purists prefer the former nomenclature since it does not presume any underlying mechanism.
  
  Section::::Signs and symptoms.
  
  Multifocal atrial tachycardia is characterized by an electrocardiogram (ECG) strip with 3 or more P-waves of variable morphology and varying P–R intervals, plus tachycardia, which is a heart rate exceeding 100 beats per minute. Narrow QRS complexes are visible as well. how?
  
  Section::::Causes.
   Cardiac arrhythmia are often first detected by simple but nonspecific means: auscultation of the heartbeat with a stethoscope, or feeling for peripheral pulses. These cannot usually diagnose specific arrhythmia but can give a general indication of the heart rate and whether it is regular or irregular. Not all the electrical impulses of the heart produce audible or palpable beats; in many cardiac arrhythmias, the premature or abnormal beats do not produce an effective pumping action and are experienced as "skipped" beats.
   BULLET::::- Pulsus paradoxus: a condition in which some heartbeats cannot be detected at the radial artery during the inspiration phase of respiration. It is caused by an exaggerated decrease in blood pressure during this phase, and is diagnostic of a variety of cardiac and respiratory conditions of varying urgency, such as cardiac tamponade.
  
  BULLET::::- Tachycardia: an elevated resting heart rate. In general an electrocardiogram (ECG) is required to identify the type of tachycardia.
   First, second and third degree block also can occur at the level of the sinoatrial junction. This is referred to as sinoatrial block typically manifesting with various degrees and patterns of sinus bradycardia.
  
  Section::::Classification.:Sudden arrhythmic death syndrome.
   If someone with short QT syndrome is examined while their heart is beating in an abnormal rhythm such as atrial fibrillation, this can be detected by feeling their pulse. No abnormal signs will usually be found when examining someone with short QT syndrome while their heart is beating in its normal or sinus rhythm.
  
  Section::::Cause.
   Rhythm disturbances or arrhythmias:
  
  BULLET::::- Atrial fibrillation and atrial flutter without rapid ventricular response
  
  BULLET::::- Premature atrial contraction (PACs) and premature ventricular contraction (PVCs)
  
  BULLET::::- Sinus arrhythmia
  
  BULLET::::- Sinus bradycardia and sinus tachycardia
  
  BULLET::::- Sinus pause and sinoatrial arrest
  
  BULLET::::- Sick sinus syndrome: bradycardia-tachycardia syndrome
  
  BULLET::::- Supraventricular tachycardia
  
  BULLET::::- Atrial fibrillation with rapid ventricular response
  
  BULLET::::- Atrial flutter with rapid ventricular response
  
  BULLET::::- AV nodal reentrant tachycardia
  
  BULLET::::- Atrioventricular reentrant tachycardia
  
  BULLET::::- Junctional ectopic tachycardia
  
  BULLET::::- Atrial tachycardia
  
  BULLET::::- Ectopic atrial tachycardia (unicentric)
  
  BULLET::::- Multifocal atrial tachycardia
  
  BULLET::::- Paroxysmal atrial tachycardia
  
  BULLET::::- Sinoatrial nodal reentrant tachycardia
   Section::::Diagnosis.
  
  On an ECG, the QRS complex will be abnormally shaped when looking at ventricular ectopic activity, often it occurs earlier with an absent P wave. It can be perceived as a skipped beat on both the ECG and through normal pulse-taking. 
  
  During atrial ectopic activity where the P wave is normally rounded can be inverted or peaked. However the QRS complex and T waves appear relatively normal.
  
  Conversely, during junctional ectopic activity the P wave is frequently absent or can be hidden in the QRS complex.
  
  Section::::See also.
  
  BULLET::::- Cardiac ectopy
  
  BULLET::::- Clinical cardiac electrophysiology
   POTS can sometimes be a paraneoplastic syndrome associated with cancer. Autoantibodies have been found in some cases which occur after a viral infection raising the possibility of some cases being autoimmune in nature.
  
  Section::::Diagnosis.
  
  Section::::Diagnosis.:Diagnostic criteria.
  
  A POTS diagnosis requires the following characteristics:
  
  BULLET::::- For patients age 20 or older, increase in heart rate ≥30 bpm within 10 minutes of upright posture (tilt test or standing) from a supine position
  
  BULLET::::- For patients age 12-19, heart rate increase must be 40 bpm
  
  BULLET::::- Associated with related symptoms that are worse with upright posture and that improve with recumbence
   The first mechanism, referred to as the dispersion of repolarisation, occurs because the action potential shortening seen in this condition occurs to a greater extent in some layers of the heart wall than in others. This means that at certain points in the cardiac cycle, some layers of the heart wall will have fully repolarised, and are therefore ready to contract again, while other regions are only partially repolarised and therefore are still within their refractory period and not yet able to be re-excited. If a triggering impulse arrives at this critical point in the cardiac cycle, the wavefront of electrical activation will conduct in some regions but block in others, potentially leading to wavebreak and re-entrant arrhythmias.
   Section::::Diagnosis.:Classification.:Common AVNRT.
  
  In common AVNRT, the anterograde conduction is via the slow pathway and the retrograde conduction is via the fast pathway ("slow-fast" AVNRT).
   BULLET::::- Atrial fibrillation is an irregular and uncoordinated contraction of the cardiac muscle of atria. It can be a chronic condition, usually treated with anticoagulation and sometimes with conversion to normal sinus rhythm. In this condition the normal electrical pulses coming from the sinoatrial node are overwhelmed by disorganized electrical impulses usually originating in the roots of the pulmonary veins, leading to irregular conduction of impulses to the ventricles which generate the heartbeat.
   Since the 1940s, the concept of diastolic depolarization, or "pacemaker potential", has become established; this mechanism is a characteristic distinctive of cardiac tissue. When the threshold is reached and the resulting action potential fires, a heartbeat results from the interactions; however, when this heartbeat occurs at an irregular time, a potentially serious condition known as arrythmia may result.
  
  Section::::Clinical significance.:Arrythmia.:Use of medications.
  