BL-42 (Po Hu) Po Door

What does BL-42 Po Hu do?

Po Hu BL-42 is the outer Bladder line point at the third thoracic vertebra, level with Fei Shu BL-13, and the tradition treats it as the point of the Lungs' spirit aspect — the po, usually translated as the corporeal soul. Sacred Lotus sources & references record three actions for it, and they are Lung actions: it tonifies and nourishes the Lung, it soothes dyspnoea and alleviates cough, and it activates the channel and alleviates pain. It carries no five-shu, source, connecting, cleft, back-shu or front-mu classification; its standing comes entirely from its position on the outer line beside the Lung Back-Shu point.

• Tonifies and nourishes the Lung — the core action, shared with BL-13 beside it. Applied to the depletion presentations classical theory attributes to weak Lungs: chronic cough, weak voice, shortness of breath on slight exertion, spontaneous sweating and the exhaustion of long respiratory illness. The point's most striking historical indication, pulmonary tuberculosis, belongs here.
• Soothes dyspnoea and alleviates cough — the respiratory action proper: for breathlessness, wheezing, and cough that will not settle.
• Activates the channel and alleviates pain — the local action. BL-42 lies in the thick muscle between the spine and the inner edge of the shoulder blade, and pain, tightness and restriction in exactly that band are among its most common modern uses.
• Addresses the po, the Lungs' spirit aspect — the action the point's name and its position on the outer line give it. In the five-spirits doctrine the po is the corporeal soul: the bodily, instinctive, sensing part of a person, tied to breath and to sensation, and the aspect the classics associate with grief. The outer-line point is the one reached for when a Lung presentation carries that register — unresolved grief, a sense of constriction that is emotional as much as physical, sadness that has settled into the breath — rather than a physical symptom alone.

Source: Sacred Lotus sources & references (channel classification and the recorded actions above — the broken list markup in that record corrected here); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 302; Chinese Acupuncture & Moxibustion, p. 185; Ling Shu (Spiritual Pivot), chapter 8, on the five spirits and the organs that house them; cross-referenced against multiple online sources.

What is BL-42 Po Hu used for?

Po Hu BL-42 is used for chronic Lung complaints and for pain in the upper back and shoulder. The indications recorded in Sacred Lotus sources & references divide into a respiratory group, a musculoskeletal group and, in the classical literature, a constitutional-depletion group. They describe traditional use, not proven treatment, and this point carries the pneumothorax considerations set out under needling.

• Cough, especially chronic cough
• Asthma, wheezing and shortness of breath
• Pulmonary tuberculosis — the historical indication, recorded in our own data and in the standard texts
• Haemoptysis (coughing blood)
• Pain in the shoulder and upper back
• Neck rigidity and difficulty turning the head
• Tightness and restriction between the shoulder blade and the spine
• Night sweats and the wasting presentations classical theory attributes to Lung depletion
• Grief and constriction of the chest accompanying Lung depletion

The last item is the one that distinguishes BL-42 from BL-13 beside it. The physical indications of the two points overlap almost entirely; what the outer line adds is the mental and emotional dimension of the same depletion. It is worth being honest that our own recorded indications for this point are wholly physical — the spirit-aspect use comes from the point's name, its position on the outer line and the doctrine those encode, not from a separate list of symptoms.

Source: Sacred Lotus sources & references (existing indication record, reformatted here as a list); Chinese Acupuncture & Moxibustion, p. 185; Deadman & Al-Khafaji, A Manual of Acupuncture, p. 302.

How is BL-42 Po Hu used in practice?

Po Hu lies in the upper back, three cun out from the midline — a hand's breadth to the side of the spine — level with the lower edge of the third thoracic vertebra, and therefore level with Fei Shu BL-13 an inch and a half in. At that distance it sits on or very near the inner border of the shoulder blade. Those two points, plus the midline of the spine, sit on one horizontal line across the upper back, and knowing that line is how BL-42 is found: locate the T3 spinous process, move out to BL-13, keep going the same distance again.

In practice BL-42 is used in two quite different ways. It is added to BL-13 — needled alongside it, or treated with moxibustion, cupping or herbal point application alongside it — when the practitioner wants the outer-line dimension as well as the organ one. And it is chosen as a local point in its own right for upper-back and shoulder pain that sits out along the edge of the shoulder blade rather than close to the spine, because that is exactly where it lands. The second use is by far the more common in modern practice.

Classic combinations

• With BL-13 Fei Shu — the essential pairing, and the reason the point exists. BL-13 is the Lung Back-Shu point; BL-42 is its outer companion at the same level. Both are held here.
• With Zhong Fu LU-01 — the Front-Mu point of the Lungs, on the front of the chest. Back-Shu with Front-Mu is the oldest structured pairing in the medicine, and pulling BL-42 into it gives front, inner line and outer line at one organ. Held here.
• With Tai Yuan LU-09 — the Yuan-Source point of the Lung channel at the wrist, and the Hui-Meeting point of the vessels; the standard distal partner for supplementing the Lungs. Held here.
• With Gao Huang Shu BL-43 — the outer-line point one level below at T4, which is the classical point of last resort for deep constitutional depletion and consumptive disease, and shares BL-42's tuberculosis history. The two are very commonly moxaed together. Held here.
• With Feng Men BL-12 and Da Zhu BL-11 — the inner-line points above, for cough and wind invasion treated as an exterior problem rather than a deficiency. Both held here.
• With Ding Chuan — the extra point held here beside the seventh cervical vertebra, the modern point directed specifically at wheezing, and a member of the same hazardous band above the lung apex.
• With the other outer-line points — where the presentation is a spirit-aspect one across several organs, the outer line is treated as a set rather than singly. See below.

The outer Bladder line, and why it matters

This is the piece of theory that makes sense of BL-42, and it is genuinely useful. The Bladder channel runs down the back in two parallel lines. The inner line, one and a half cun from the midline, carries the Back-Shu points — the points that connect directly to each organ: BL-13 Fei Shu to the Lungs, BL-15 Xin Shu to the Heart, BL-18 Gan Shu to the Liver, BL-20 Pi Shu to the Spleen, BL-23 Shen Shu to the Kidneys. The outer line, three cun from the midline, runs alongside it, and at the level of each of those five Back-Shu points it carries a point named for the spirit aspect of that same organ. That is the system:

• BL-42 Po Hu, "Po Door" — level with BL-13 Fei Shu. The po, the corporeal soul, is the spirit aspect housed by the Lungs. This point.
• BL-44 Shen Tang, "Spirit Hall" — level with BL-15 Xin Shu. The shen, the spirit proper, is housed by the Heart. Held here.
• BL-47 Hun Men, "Hun Gate" — level with BL-18 Gan Shu. The hun, the ethereal soul, is housed by the Liver. Held here.
• BL-49 Yi She, "Reflection Abode" — level with BL-20 Pi Shu. The yi, thought or intellect, is housed by the Spleen. Held here.
• BL-52 Zhi Shi, "Will Chamber" — level with BL-23 Shen Shu. The zhi, will or determination, is housed by the Kidneys. Held here.

Read the names together and the design is unmistakable: a door, a hall, a gate, an abode and a chamber — five dwellings, one for each of the five spirits, laid out in order down the back beside the five organ points they belong to. The clinical consequence is a real choice at the treatment table. Where a presentation is physical — the cough, the palpitation, the back pain — the inner line is chosen. Where the same organ's depletion shows in the mental and emotional register the classics assign to it — grief that will not lift for the Lungs, agitation for the Heart, frustration and lack of direction for the Liver, worry and rumination for the Spleen, fear and lost resolve for the Kidneys — the outer line is chosen, usually in addition rather than instead. All five outer-line points named above are held in Sacred Lotus sources & references, and cross-referencing is confined to those five; the outer line has other points on it, and this record does not assert anything about ones we do not hold.

BL-42 is the first of the five, the highest on the back and the first a reader meets coming down the outer line. It is also the one whose name most directly states the doctrine, because po is the least familiar of the five spirits and the point is named for nothing else.

Source: Sacred Lotus sources & references (location, and the BL-11, BL-12, BL-13, BL-15, BL-18, BL-20, BL-23, BL-43, BL-44, BL-47, BL-49, BL-52, LU-01, LU-09 and Ding Chuan records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 302; Chinese Acupuncture & Moxibustion, p. 185; Ling Shu (Spiritual Pivot), chapter 8, on the five spirits and the organs that house them; cross-referenced against multiple online sources.

Where is BL-42 located?

• A Manual of Acupuncture (p. 302): 3 cun lateral to the midline, level with the lower border of the spinous process of the third thoracic vertebra (T3) and level with Feishu BL-13.
• Chinese Acupuncture and Moxibustion (p. 185): 3 cun lateral to the Governor Vessel, at the level of the lower border of the spinous process of the third thoracic vertebra, on the spinal border of the scapula.

How is BL-42 Po Hu needled, and what is the pneumothorax risk?

Stated plainly: BL-42 lies over the thorax beside the third thoracic vertebra, with the upper lobe of the lung beneath it, and pneumothorax — a punctured lung — is the documented hazard at this point and at every point in the band it belongs to. The reference texts do not record a perpendicular insertion here. Sacred Lotus sources & references specify oblique insertion of 0.3 to 0.5 inch, with moxibustion applicable, and the angle is the substance of that record rather than a stylistic detail. Everything below is recorded as neutral reference data describing what the textbooks and the published anatomical studies state. It is not instruction, this page does not teach technique, and needling is carried out only by a qualified, licensed practitioner.

The angle and depth the texts record

• Sacred Lotus sources & references record oblique insertion of 0.3 to 0.5 inch, with moxibustion applicable. That is a shallower figure than our records give for Fei Shu BL-13 on the inner line at the same level (0.5–0.7 inch oblique), which is itself bounded.
• Deadman & Al-Khafaji (p. 302) and Chinese Acupuncture & Moxibustion (p. 185) record oblique insertion directed medially, toward the spine, in a comparable range, and both attach an explicit caution against deep or laterally angled insertion at the thoracic points because of the lung beneath. The geometry is the whole point: angling toward the vertebral column keeps the needle in the muscle mass with bone behind it, while angling outward points it at the intercostal space and the pleura behind that.
• Moxibustion, cupping and herbal point application are all recorded at this point and are extremely common at it. None carries the pneumothorax hazard, which is one reason the upper back is so often treated without needles at all.
• An anatomical note that matters here specifically. At three cun lateral, BL-42 lies on or very near the medial border of the scapula in many people, depending on how the arm is positioned. Where the scapula lies beneath the point there is bone between the needle and the chest wall; where the arm is drawn forward and the shoulder blade slides laterally, there is not. That is a real difference between this point and its inner-line neighbour BL-13, and it means the margin at BL-42 is not a fixed quantity even in the same person.

What has actually been measured at this point

• A CT study that measured BL-42 itself. This is the useful finding, and it is genuinely point-specific. A retrospective study measured, directly from chest CT images, the safe needling depth at 23 upper-back acupoints in 319 patients aged 4 to 18 (205 boys, 114 girls), defining safe depth as the distance from the skin surface of the point to the pleura. The 23 points were DU-09 to DU-14, BL-11 to BL-17, BL-41 to BL-46, SI-14, SI-15, GB-21 and SP-21. The measured depth at Po Hu BL-42 was 28.02 ± 8.69 mm in boys and 26.48 ± 7.25 mm in girls — a difference that was not significant at this point (P = 0.110), although it was significant at most others on the list. Across the five age bands the depth at BL-42 rose steadily from 21.97 ± 3.88 mm at ages 4 to 6 to 31.30 ± 9.28 mm at ages 16 to 18. Depths differed significantly by age (P < 0.001), by weight (P ≤ 0.01) and by body mass index (P < 0.05), and multiple regression identified weight as the single most important determinant (Ma YC et al., BMC Complement Altern Med 2016;16:85, PMID 26922245, PMC4769822, DOI 10.1186/s12906-016-1060-x). State the limits precisely: these are children and adolescents from a single Taiwanese hospital population, and the figure is the distance at which the pleura is reached — not a recommended depth. No adult measurement at this point was located and none is quoted.
• Why that figure is worth reading twice. Two to three centimetres, in a child, is the entire margin — and the standard deviation of nearly nine millimetres in boys means individual children differed from one another by more than the whole textbook insertion range. It is also notable that BL-42 was one of only six points on the list at which boys and girls did not differ significantly; the others were BL-44, BL-45, BL-46, GB-21 and SP-21, which is to say the lateral points. Sex differences in this region track the tissue over the shoulder blade, not the depth of the chest.
• The nearest measured adult figure in this region belongs to a different point. Ultrasonography in 101 adults measured the vertical distance from skin to pleural line at Jian Jing GB-21 as a mean of 17.4 mm in men and 14.6 mm in women, increasing with weight, height and body mass index (Lin S-K et al., J Acupunct Meridian Stud 2018;11(6):355–360, PMID 29936338). That is GB-21's figure, not Po Hu's. A second ultrasound study of 52 healthy volunteers imaged GB-21 in real time and found that safe depth changed with sex, with respiratory phase and with body position — a single textbook figure cannot describe every person at every moment (Evid Based Complement Alternat Med 2018;2018:2308102, PMID 29507586). Also GB-21's.
• The cadaveric pleural-dome study did not name BL-42. A dissection study of 46 adult bodies mapped the dome of the pleura against the points around it, naming Da Zhu BL-11 — two levels above BL-42's level, on the inner line — alongside Tian Tu REN-22, Jian Jing GB-21, Qi She ST-11 and Ding Chuan EX-B1. The dome extended beyond the medial third of the clavicle in almost 60 per cent of bodies, and the authors concluded that when a point is located and angled as the standard describes the pleura is not reached, but that exceeding the recorded depth breaches the pleural membrane (Chen Y et al., Zhongguo Zhen Jiu 2006;26(5):346–8, PMID 16739850). BL-42 is not in that list, and the finding is cited for the region rather than as a measurement of this point.
• No single agreed safe depth exists for any point. Two systematic reviews of the safe-depth literature — 33 studies in one, 47 in the other, using MRI, CT, ultrasound and cadaveric dissection — both concluded that measured depths for the same point differ greatly between subject groups and measuring methods, and that "safe depth" has never been standardised (J Altern Complement Med 2011;17(3):199–206, PMID 21417806; Evid Based Complement Alternat Med 2013;2013:740508, PMID 23935678).
• What lies beneath, in plain terms. At the T3 level, three cun from the midline, the needle passes through skin, subcutaneous tissue, trapezius and rhomboid major. Beneath those lies either the shoulder blade — solid bone — or, where the scapula has moved, the third and fourth ribs with the intercostal space between them, the parietal pleura, and the upper lobe of the lung behind it. That is the whole anatomical reason for the recorded medial angle and shallow depth.

What is documented, and in what proportion

Pneumothorax is the second most frequently reported serious adverse event in acupuncture. A systematic review of the Chinese case-report literature from 1956 to 2010 identified 1,038 adverse-event cases across 167 articles, of which 307 were pneumothorax — behind only fainting — with 35 deaths across the whole series; the authors attributed the events chiefly to improper technique and judged most avoidable (He W et al., J Altern Complement Med 2012;18(10):892–901, PMID 22967282). A companion review of 115 Chinese-language articles reporting 479 adverse events including 14 deaths reached the same conclusion (Zhang J et al., Bull World Health Organ 2010;88(12):915–921C, PMID 21124716, PMC2995190). Presentation is not always immediate: an emergency-medicine case report describes a healthy 38-year-old woman who developed pleuritic chest pain the day after dry needling of the back, with bedside ultrasound showing absent lung sliding (Cureus 2021;13(3):e14207, PMID 33948398, PMC8086747). Fatal cases exist: an autopsy report describes bilateral pneumothoraces following electroacupuncture, the authors noting that electrical pulses and the muscle contraction they cause may draw needles deeper than the depth at which they were inserted (J Forensic Sci 2022;67(1):377–383, PMID 34435369).

What we will not claim. None of those reviews identifies the individual points involved, and no published case report naming Po Hu BL-42 specifically as the site of a pneumothorax was located. No such claim is made here. A pneumothorax after needling over the upper back may declare itself hours later with sudden chest pain, breathlessness or a persistent dry cough, and is a medical emergency requiring immediate assessment.

Risk in proportion. A meta-analysis of prospective clinical studies estimated serious adverse events at approximately 1 per 10,000 patients and around 8 per million treatments, with about half of all recorded events being bleeding, pain or flare at the needle site (Bäumler P et al., BMJ Open 2021;11:e045961, PMID 34489268). Both facts belong together: the overall rate is low, and the events that do occur concentrate in the band of points over the lung to which Po Hu belongs. The bounded oblique depth exists to keep it that way.

Source: Sacred Lotus sources & references (existing needling record; the BL-13 record queried directly for comparison); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 302; Chinese Acupuncture & Moxibustion, p. 185; PMID 26922245 (PMC4769822); PMID 29936338; PMID 29507586; PMID 16739850; PMID 21417806; PMID 23935678; PMID 22967282; PMID 21124716 (PMC2995190); PMID 33948398 (PMC8086747); PMID 34435369; PMID 34489268 — each verified resolvable and checked for retraction status before citation.

What does the name Po Hu mean?

Po Hu means "Po Door" — po, the corporeal soul, and hu, a door, specifically a single-leaf door of the kind that opens into a room rather than a gate into a compound. Sacred Lotus sources & references translate it as Po Door. The name is a direct statement of doctrine: in the five-spirits scheme of the Ling Shu, each of the five yin organs houses one aspect of the spirit, and the aspect housed by the Lungs is the po. Naming the point a door says where that faculty is understood to enter and leave.

The po is the hardest of the five to translate and worth a paragraph of its own, because a reader meeting it for the first time will otherwise take it for a vaguer thing than it is. It is not the soul in the sense of a person's essence or continuity. It is the bodily, animal, sensing part: the part that feels sensation, that startles, that breathes without being told to, that is tied to the flesh and, in the classical account, dissipates with the body at death — in contrast to the hun, the ethereal soul of the Liver, which is described as departing. The Lungs govern the breath and the skin, which are precisely the surfaces at which sensation meets the world, and that is why the po is assigned to them. The emotion the classics attach to the Lungs is grief, and the clinical use of BL-42 in the spirit register follows from that.

The choice of hu is worth noticing too. Among the five outer-line names it is the most exterior — a door, the threshold. Read against its four companions, Shen Tang a hall, Hun Men a gate, Yi She an abode and Zhi Shi an inner chamber, the naming forms a graded architecture from the threshold to the innermost room, descending the back. Po Hu is the threshold, and it is the highest of the five.

Why does the outer Bladder line matter?

Because it gives a practitioner a second point at every major organ level, carrying the same organ's pathology in a different register. The inner line at one and a half cun holds the Back-Shu points, which connect directly to the organs; the outer line at three cun holds, at each of five levels, the point named for that organ's spirit aspect. The full theory and the five paired points are set out under practice above. The short version worth memorising: inner line, organ; outer line, spirit of that organ.

Two cautions about that formulation. First, it is a teaching device rather than a rule the classics state in those words — the outer-line points also carry ordinary physical indications, and BL-42's own recorded indications are entirely physical. Second, the outer line runs the whole length of the back and not every one of its points is a spirit point; the five named here are the ones that pair with the five yin-organ Back-Shu points, and they are the five this library holds.

Is there research on BL-42 Po Hu?

There is no point-specific clinical research on BL-42, and that should be said plainly rather than filled in with studies of protocols that happen to contain it. The only published work that measured this point is anatomical.

• The CT safe-depth study measured this point. The imaging work described under needling above (PMID 26922245, PMC4769822) measured Po Hu BL-42 directly among 23 upper-back points in 319 children and adolescents and published a figure for it, with the age breakdown. It is anatomical rather than clinical, and it is the single most useful published finding about this point.
• Nothing else. No randomised trial isolates BL-42; no trial of BL-42 for cough, asthma, upper-back pain or any other recorded indication was located; and no case report or observational study naming the point was located.

A point-code caution, recorded because it was met while assembling this. Searching the literature for this point by name is almost entirely useless and produces exactly the false positives this library exists to avoid. "Pohu" as a search term returns a small set of records in which the string is a surname or an institutional name — among them a hepatocellular carcinoma ferroptosis paper and its correction notice, a pancreatic cancer gene-expression study, two polymer-chemistry papers on the crystallisation of maltodextrins, two French nursing articles on kidney-pancreas transplantation and two 1973 cardiology papers on myocardial infarction. Not one concerns acupuncture. Searching "BL-42" without further qualification returns tens of thousands of records, because the string matches ordinary text throughout the biomedical literature. Every record considered for this page was read rather than counted, and the honest total is one.

What is missing: BL-42's clinical standing rests entirely on the classical record and on its relationship to BL-13. This site does not claim otherwise.

How does Po Hu differ from the points near it?

Fei Shu BL-13 is the point it exists in relation to: the Lung Back-Shu point, one and a half cun from the midline at the same T3 level, carrying the organ connection where BL-42 carries the spirit aspect. In practice they are used together far more often than apart, and BL-13 is by a wide margin the more used of the two. Fu Fen BL-41 lies directly above BL-42 on the outer line at T2 — our records classify it as a Meeting Point of the Bladder and Small Intestine channels — and Gao Huang Shu BL-43 lies directly below at T4, the famous point of last resort for consumptive and deeply depleted conditions, which shares BL-42's tuberculosis history and is far better known. Both are held here and both are easily mistaken for BL-42, since the outer line has no palpable landmark of its own and must be counted from the vertebrae. Shen Tang BL-44 is the next outer-line spirit point down, at T5 beside the Heart Back-Shu point. Da Zhu BL-11 and Feng Men BL-12 on the inner line above are the points for wind invasion and acute cough rather than for depletion. Jian Jing GB-21 lies laterally on the top of the shoulder in the same hazardous band, and the extra point Ding Chuan sits beside the seventh cervical vertebra in the same band again. All of these are held in Sacred Lotus sources & references.

Source: Sacred Lotus sources & references (name, channel, and the BL-11, BL-12, BL-13, BL-41, BL-43, BL-44, BL-47, BL-49, BL-52, GB-21 and Ding Chuan records queried directly); Deadman & Al-Khafaji, A Manual of Acupuncture, p. 302; Chinese Acupuncture & Moxibustion, p. 185; Ling Shu (Spiritual Pivot), chapter 8, on the five spirits; PMID 26922245 (PMC4769822) — verified resolvable and checked for retraction status before citation; cross-referenced against multiple online sources.