The equation for all points of a system filled with a constant-density fluid is

  
    
      
        
          
            p
            γ
          
        
        +
        
          
            
              v
              
                2
              
            
            
              2
              g
            
          
        
        +
        z
        =
        
          c
          o
          n
          s
          t
        
        ,
      
    
    {\displaystyle {\frac {p}{\gamma }}+{\frac {v^{2}}{2g}}+z=\mathrm {const} ,}
  where:

p, pressure of the fluid,

  
    
      
        
          γ
        
      
    
    {\displaystyle {\gamma }}
   = ρg, density × acceleration of gravity is the (volume-) specific weight of the fluid,
v, velocity of the fluid,
g, acceleration of gravity,
z, elevation,

  
    
      
        
          
            p
            γ
          
        
      
    
    {\displaystyle {\frac {p}{\gamma }}}
  , pressure head,

  
    
      
        
          
            
              v
              
                2
              
            
            
              2
              g
            
          
        
      
    
    {\displaystyle {\frac {v^{2}}{2g}}}
  , velocity head.
However, the elongated papillae of hairy tongue usually develop discoloration due to growth of pigment producing bacteria and staining from food. Hence the term black hairy tongue, although hairy tongue may also be discolored yellow or brown. Transient, surface discoloration that is not associated with hairy tongue can be brushed off. Drug-induced black hairy tongue specifically refers to BHT that develops because of medication. Treatment
Treatment is by reassurance, as the condition is benign, and then by correction of any predisposing factors. This may be cessation of smoking or cessation/substitution of implicated medications or mouthwashes. Generally direct measures to return the tongue to its normal appearance involve improving oral hygiene, especially scraping or brushing the tongue before sleep. This promotes desquamation of the hyperparakeratotic papillae. Keratolytic agents (chemicals to remove keratin) such as podophyllin are successful, but carry safety concerns. Other reported successful measures include sodium bicarbonate mouthrinses, eating pineapple, sucking on a peach stone and chewing gum. Prognosis
BHT is a benign condition, but people who are affected may be distressed at the appearance and possible halitosis, and therefore treatment is indicated. Epidemiology
Hairy tongue occurs in about 0.5% of adults. However, the prevalence is variable depending on the population studied. References


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