Human Physiology

The human body requires nutrients to keep its cells functioning. Those nutrients are collected thanks to the digestive system.

Food and water are ingested through the mouth. They go down the esophagus to reach the stomach then they continue on in the small intestine where absorption occurs. All that remains follows in the large intestine to be eliminated via the anus.
Mechanical digestion consists of breaking big molecules into nutrients using chewing (mouth) or mixing (stomach). Chemical digestion is due to enzymes (lipases, peptidases, amylase…) synthetized by glands (salivary, liver, pancreas).
Absorption can be passive (fatty acids), facilitated (glucose), active (calcium) or using endocytosis (triglycerides and cholesterol).

Oxygen is necessary for cells’ functioning when carbon dioxide is a byproduct. Ventilation insure oxygen intake and carbon dioxide release.

Ventilation renews air contained in the respiratory system (trachea, bronchus, bronchioles and alveoli). Inhaling increase thorax volume through ribcage opening and diaphragm lowering.
This process induces a decrease in pressure inside and air is sucked in. Exhaling reduces thorax volume by pushing up the diaphragm and moving the ribcage in. That squeezes the air in the lungs and forces it out.
Ventilation rate increase during physical activity to meet the cells need.
Gas exchange occurs in alveoli. Those structures have thin, moist and large surfaces, permeable to oxygen and carbon dioxide. They are fragile and can be altered by pollution and smoking this leading to emphysema and sometimes lung cancer.

After intake, nutrients and oxygen reach cells thanks to the blood stream. This blood stream is possible thanks to the cardiac pump.

The heart is hollow muscle with two sides, left and right, each structured with an atrium and a ventricle. This organization allows a double circulation, the left part sending oxygenated blood coming from the lungs to all the organs (high pressure systemic circulation) when the right part sends deoxygenated blood coming from the organs back to the lungs (low pressure pulmonary circulation).
This cardiac cycle usually last for 0.8s more or less with 0.3s of ventricles relaxation and 0.5s of ventricles contraction but can be much faster with physical activity.
The heart is the only autonomous organ that can beat outside of the body due to its nodal cells that contract rhythmically.
Blood vessels (arteries, capillaries and veins) properties optimize blood stream and exchanges between the blood and the organs.

The human body is well protected but some pathogens can enter.
The immune system is in charge of getting rid of those intruders.

The human skin is mostly impenetrable and protects from microbe’s invasion. However, some cuts or stings may affect its integrity and lead to vulnerability. Moreover, mucous membranes made for exchanges are possible entry gates for pathogens.
When microbes manage to enter the body, the immune system tries to track them and kill them. Tracking is possible because microbes have antigens, surface molecules with a 3D shape different from those of the body. White blood cells recognize that difference. It leads to the inflammatory response and phagocytosis.
If not enough, the immune system white blood cells produce antibodies to fight the infection that stick to antigens and induce microbe’s destruction. HIV positive patients are fragile because the virus brings down white blood cells.

The nervous system is composed of neurons.
Those cells can transmit a message thanks to their electrical properties.

Neurons are polarized cells (dendrites, cell body and axon) that can receive messages and transmit it to other cells.
Along neurons the message is electrical. It consists of a repetition of a quick inversion of plasma membrane polarity called “the action potential”. From resting potential, depolarization is due to natrium entry through voltage dependent channels. Then repolarization is due to potassium exit through voltage dependent channels.
Between neurons, the message is chemical and coded in neurotransmitters quantity. The electrical impulse causes exocytosis of those neurotransmitters (acetylcholine, norepinephrine…) between the cells. They bind to specific receptors on the receiving neuron and lead to an electrical response.
This nervous system regulates really fast different systems by its effect on muscle contraction (cardiac, skeletal, smooth).

The endocrine system is composed of glands releasing hormones in the blood stream.
Those hormones are molecules leading target cells to react.

Hormones are either proteins or lipids (cholesterol derived). They are produced by specific glands and release in the blood stream. They affect target organs because their cells express specific receptors that hormones bind to. This interaction leads the cell to react.
For example, pancreatic hormones are synthetized in response to different glycemic conditions: high blood glucose for insulin (beta cells) and low blood glucose for glucagon (alpha cells). Insulin binds to liver, muscle and fat tissue to promote glucose entry from the blood stream and storage. Glucagon bonds to liver only and promote glucose clearance and release in the blood stream.
Different forms of diabetes are due to impairments of this hormonal regulation of glycemia.

Until 8 weeks of pregnancy, genitals are not differentiated.
Sex determination is a process that involve genetics and hormones to build a baby girl or boy.

Karyotype studies reveal that the Y chromosome is crucial for sex-determination. In fact one portion of Y (SRY for sex determining region of Y) induces the differentiation of gonads into testes. If SRY is not present, gonads will evolve into ovaries.
Testes produce hormones:

1. Testosterone maintains Wolff ducts
2. AMH (Anti Müllerian Hormone) damages Müllerian ducts

Combined, those two hormones differentiate the reproductive system into male genitals. Ovaries do not produce those two hormones and the reproductive system will turn into female genitals.

When puberty happens, hormones levels rise and individuals become sexually mature.
Men produce spermatozoa and women ova.

Men spermatozoa are produced in the testis in a constant basis and stored in the epididymis until ejaculation. Prostate gland and seminal vesicles produce fluids that will protect and feed the spermatozoa. Those fluids together with the spermatozoa constitute sperm. Sperm transits through the penis, which has an erectile tissue, before being ejected through the urethra.
Women produce an ovum every menstrual cycle. It is done in the ovaries and expelled into the fallopian tube where it eventually meets a spermatozoon.

• If it does, pregnancy will begin by fertilization of the egg that will grow and stick to the uterine wall. The placenta will develop and insure fetus nutrition until delivery.
• If it doesn’t, a part of the uterine wall is destroyed 14 days later (menstruations) and the next cycle begins.

Spermatozoa and ova production are duly regulated by cerebral hormones (hypothalamic and pituitary). However, sexual hormones produced in the gonads (testosterone, progesterone and estrogens) have a feedback action on brain activity. It is usually negative but when estrogens reach a high level at the end the follicular stage, it switches into a positive feedback and leads to ovulation.

Knowledge in reproduction allows physicians to help couples trying unsuccessfully to have a child and avoid undesired pregnancies.

Contraception includes birth control pills, morning-after pills and abortion pills, which are available thanks to the understanding of hormonal control of reproductive system. Other devices, such as IUD, exist to block fertilization and avoid pregnancy.

Medically assisted procreation = Ovarian stimulations, intrauterine insemination or in vitro fertilization can help couples with fertility issues. The method will depend on the origin of the problem (ovulation, sperm count…).

Sexual transmitted infections = To prevent those infections, the only solution is to use a condom to prevent direct contact between mucous membranes.