Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
I’ve explained already in this series that the human embryo is destined to become female by default unless several chemicals swing into action to make it become a normal male. But that’s only part of the story, because for the first several years of life, humans, whether male or female, cannot reproduce. Puberty involves an as yet unexplained reduction in the feedback inhibition of the hypothalamus and pituitary so they can increase their output of their respective hormones. This leads to the testes being able to produce sperm and more testosterone and the ovaries being able to develop and release an egg and produce more estrogen and also the pregnancy hormone, progesterone.
Once the sexual organs have matured so the male can produce sperm and the female can release an egg into the fallopian tube, all that is needed for new human life to come about is for them to join together to form a zygote. The natural way that human reproduction occurs is by the male and female physically coming together in sexual intercourse. This very intimate union requires the man to deliver semen containing sperm near the cervical opening of the uterus. Over the next several hours, aided by the cervical mucus, the sperm use their flagella to swim through the body of the uterus toward the fallopian tubes. If one of the woman’s ovaries has released an egg around that time, then one of the sperm may be able to penetrate its outer shell to form a zygote in a process called fertilization. Over the next several hours the zygote develops into an embryo, which over the next several days moves into the body of the uterus and implants in its endometrial lining. Once implantation takes place, the embryo continues to develop and grow into the fetus in a process called gestation, exiting the mother’s body about nine months later as a newborn baby.
From the above it is evident that the male’s ability to reproduce is mainly dependent on being able to perform two tasks; produce enough healthy sperm and deposit them deep inside the female.
The first task, of producing enough sperm, relates to his fertility and is accomplished by the testes. This is dependent on having enough of the gonadotropins, Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), producing enough testosterone and having enough properly functioning specific receptors. The sperm produced in the testes is combined with nutrient fluids from the seminal vesicles and the prostate gland to form semen. During sexual intercourse the semen is released from the erect penis into the woman’s vagina. The normal amount of semen usually needed for adequate fertility is at least 2 milliliters and the concentration of sperm per milliliter should generally be greater than 20 million. In addition, the appearance of the sperm (morphology) and their ability to move well (motility) are very important factors that affect fertility as well. If most of the sperm are structurally abnormal and/or are unable to swim vigorously, then reproduction is unlikely to take place. However, clinical experience teaches that just because the male may have the right parts and puberty may have taken place doesn’t automatically mean he will be fertile.
There are several conditions that can cause inadequate sperm counts, or even the total absence of healthy sperm resulting in male infertility. One category, which is not very common, involves inborn errors such as genetic disorders, in which the testes do not develop and mature properly, and metabolic disorders, which affect the production of testosterone. The other more common category involves acquired conditions such as emotional stress, smoking, the use of alcohol and other substances, viral and sexually transmitted infections, vitamin and mineral deficiencies, toxins, radiation, trauma, surgery, medications, diabetes, hypothalamic and pituitary disorders, thyroid disease, aging, obesity and anti-sperm antibodies. In addition, the testes are housed in the scrotum, away from the inner aspect of the body, because adequate sperm production requires the temperature of the testes to be lower than the body’s core temperature. Therefore, overheating of the testes, such as in men whose testes never descended into the scrotum (cryptorchidism), or who frequent saunas or wear tight underwear, can also result in male infertility as well. So, one can see that everything has to be just right so the man can produce enough semen with a high enough concentration of healthy looking active enough sperm for fertility to be accomplished.
The second task, of being able to deposit the sperm inside the female, refers to his potency. This involves having a firm enough normally shaped erection to penetrate deep enough into the vagina and ejaculate semen which is dependent on having a normal penis and normal nerve and vascular function. A penile erection is achieved by hydraulic pressure. Running the length of the penis, surrounding the urethra, above and on each side, are the corpus spongiosa and the corpus cavernosa. These tube-shaped venous chambers are surrounded by strong fibrous tissue. Sex-related thoughts and sensory stimulation of the penis and pelvic region activates the parasympathetic nerves which signals them to send out a neurohormone called acetylcholine. The acetylcholine attaches to specific receptors on the blood vessels in the penis which causes an increase in arterial blood flow into these chambers while at the same time reducing the venous outflow. This combination results in blood collecting within these chambers resulting in penile erection. After erection takes place, with continued stimulation the sympathetic nervous system swings into action and sends out a neurohormone called norepinephrine. The norepinephrine attaches to specific receptors on the associated pelvic muscles causing coordinated contraction and ejaculation of the semen from the penis, which is usually accompanied by the pleasurable sensation of orgasm. As sympathetic nerve stimulation brings on climax and ejaculation to complete the sexual act, the parasympathetic stimulation that caused the erection in the first place turns off and the penis becomes flaccid soon afterwards.
As noted above, adequate erection resulting in ejaculation is a well-coordinated process that involves having adequate pelvic blood flow and nerve function. If there are significant problems, then erective dysfunction, or impotence, is the result.
Fatigue and emotional causes of erectile dysfunction are usually temporary, but there are many conditions that can lead to permanent impotence. Atherosclerosis (hardening of the arteries), usually related to smoking, hypertension, diabetes and hyperlipidemia, can cause peripheral vascular disease and limit blood flow to the penis. Moreover other conditions associated with vascular problems like coronary artery disease, congestive heart failure and kidney disease can all lead to impotence as well. In particular, diabetes not only affects blood flow but also nerve function as well and therefore diabetic men are particularly at risk for impotence. Other conditions that can not uncommonly lead to nerve malfunction and impotence include alcohol and other substance abuse, medications, anemia, testicular dysfunction, spinal cord or pelvic injury and neurodegenerative conditions such as Multiple Sclerosis, Parkinson’s Disease, and stroke.
Although most of the conditions that lead to impotence and male infertility are related vascular and/or nerve dysfunction, there is one that is related to a defect in the fibrous structure of the penis itself. Peyronie’s disease is a condition in which scar tissue, from plaque formation, forms within the walls of the penis resulting in a deformed and often painful erection. This usually makes sexual intercourse not only very uncomfortable but often physically impossible.
In summary, the two main tasks of male fertility (producing enough sperm and depositing them inside the female) require much more than just having male sexual organs. He must be able to produce enough healthy and vigorous sperm which is dependent on having, not only properly working testes, but the right amount of hormones and properly working receptors as well. In addition, it isn’t enough that he has a properly built male internal genital duct system and external genitalia. He must also have proper neurovascular function in the pelvis, which involves the neurohormones acetylcholine and norepinephrine and their specific receptors, to achieve enough of an erection to penetrate deep into the vagina and deposit his semen.
Experience teaches that an electrical appliance must be connected to an energy source to work. At the end of the electric cord of all appliances is a plug. The plug consists of prongs made of metal, which make them solid enough while at the same time being a good conductor of electricity. In fact, the plug is often referred to as the male component of the electrical connection. The plug at the end of the electrical cord has to be firm enough and have the right shape to fit into the electrical socket to connect up with the energy that will allow the appliance to work. Men who cannot produce enough sperm, maintain a firm enough erection, and be able to ejaculate, or have abnormally shaped erections that precludes them from participating in sexual intercourse, are like electrical appliances with defective plugs.
Of course, it goes without saying that all of these parts working together in a coordinated fashion to produce and deliver enough sperm demonstrate not only irreducible complexity but also natural survival capacity. However, as we’ve noted before, the word sex comes from the Latin secare which means to separate or divide. We must still consider what tasks the female needs to accomplish to reproduce. Once more, as well, we will look to evolutionary biologists to give a plausible explanation for their simultaneous development.
Photo credit: Dark Tichondrias via Wikimedia Commons.