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During sexual arousal the prostate gland manufactures fluid that accounts for about 2/3 of the volume of ejaculate. The seminal vesicles are paired structures located behind the prostate gland that also manufacture fluid. Sperm from the testicles (which account for only 1-2% of the semen) travel up a series of tubes (epididymis and vas deferens) on each side to join the seminal vesicles forming the paired ejaculatory ducts. These structures empty into the prostatic portion of the urethra.
At the time of ejaculation, fluid is discharged from the prostate gland and ejaculatory ducts into the urethra (urinary canal) forming the semen. The normal semen volume is in the 2-6 cc range. In addition to this, with the onset of sexual excitement, specialized glands in the urethra called Cowper glands, produce a clear, somewhat sticky fluid to lubricate the urethra. this is called this pre-ejaculate (precum).
Normally the semen is somewhat viscous, opalescent and whitish gray in color. However, depending on many factors including frequency of ejaculation, diet, general health, etc., the volume, consistency and color of the semen can vary. A yellow cast to the semen, as you have noted, is very common and a variation of normal.
If you take multivitamins, this may be a cause. Although there is no need to be medically concerned, stopping the vitamin supplement may help. Thickened semen usually occurs if one ejaculates infrequently and is due to concentration of that portion formed in the seminal vesicles. Occasional clear semen is not a specific sign of disease. A brown or red tint to the semen is a sign of bleeding, usually from the prostate gland. Although prostate bleeding is generally of a benign nature, it is best to have this evaluated by a urologist. Hope this helps. Good Luck!!!!