Label the Body Regions - Biology

Label the Body Regions - Biology

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Head Region

  • The cranial region or cephalic region is the head and skull
  • The forehead is referred to as the frontal region.
  • The eyes are referred to as the orbital or ocular region.
  • The cheeks are referred to as the buccal region.
  • The ears are referred to as the auricle or otic region.
  • The nose is referred to as the nasal region.
  • The mouth is referred to as the oral region.
  • The chin is referred to as the mental region.
  • The neck is referred to as the cervical region.

The trunk of the body contains, from superior to inferior,

  • the thoracic region encompassing the chest
  • the mammary region encompassing each breast
  • the pectoral region encompassing the muscles of the chest
  • the sternal region encompassing the sternum
  • the abdominal region encompassing the stomach area
  • the umbilicus, or navel,icenter of the abdomen
  • the coxal region encompassing the hip area
  • the pubic region encompassing the area above the genitals.

The pelvis and legs contain, from superior to inferior,

  • the inguinal is the groin region between legs and genitals
  • the pubic region surrounding the genitals,
  • the femoral region encompassing the thighs,
  • the patellar region encompassing the knee,
  • the crural region encompassing the shin area of the leg,
  • the tarsal region encompassing the ankle,
  • the pedal region encompassing the foot
  • the digital/phalangeal region encompassing the toes.
  • The great toe is referred to as the hallux.

The regions of the upper limbs, from superior to inferior, are

  • the axillary region encompassing the armpit,
  • the brachial region encompassing the upper arm,
  • the antecubital region encompassing the front of the elbow,
  • the antebrachial region encompassing the forearm,
  • the carpal region encompassing the wrist,
  • the palmar region encompassing the palm,
  • the digital/phalangeal region encompassing the fingers.
  • The thumb is referred to as the pollex.

The posterior view contains, from superior to inferior,

  • the dorsal region refers to the entire backside
  • the acromial region where the shoulders bones are found
  • the thoracic region is the upper part of the back (also chest)
  • the lumbar region encompassing the lower back.
  • the sacral region occurring at the end of the spine, directly above the buttocks.

The posterior regions of the legs, from superior to inferior, include

  • the gluteal region encompassing the buttocks,
  • the femoral region encompassing the thigh,
  • the popliteal region encompassing the back of the knee,
  • the plantar region encompassing the sole of the foot.

I am looking for:

Students identify the various regions of the human body through drag-and-drop exercises.


By Barbara Liang

In this interactive learning activity, learners review the terms used to describe relative position of body parts in order to have a common set of words to describe their position. Check out our video on relative position:

By Barbara Liang

In this interactive learning activity, learners review the terms used to describe relative position of body parts in order to have a common set of words to describe their position. Check out our video on relative position:

By Dawn Madl

Students make observations and formulate an evaluation that includes diagnosis and microorganism identification and treatment.

By Barbara Liang

In this animated object, learners examine the different types of joints and their movements.

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The learner constructs a nail and identifies the terms relating to the structures of the nail in a matching exercise.

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Using 10 views of the male and female body, learners match Mandarin Chinese words to the body parts they identify. After the part has been correctly named, learners click on the Mandarin Chinese term to hear it pronounced. This activity is particularly helpful for health care professionals.

By Robert Formanek

In this interactive object, learners will match the muscle names to their corresponding locations in the human body.

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Learners identify various therapeutic interaction strategies to be used when working with children and adolescents to enhance self-esteem, self-concept, self-worth, body image, and coping and communication skills.

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Learners review the differences within the right and left parietal lobes. They answer questions based on a case study and describe a client in a second case study.

medical terminology. Guess the word part from the clues given

Spanish body parts vocabulary

Matching electrolyte names with the appropriate description


what is the size of the human brain?

The size of the human brain in length wise is 15 cm and weight wise is around 3 pounds.

Where is the epidermis and dermis located in relationship to each other

Epidermis is the outer most layer of skin and the dermis is the next layer of skin beneath the epidermis.

The Epidermis is the outside layer of skin and the dermis is the layer beneath that.

Using anatomical terminology how would a broken collarbone be described?


Great practice tool, helps to solidify the information learned.

Posted by Christine Klingsten on 1/15/2014 3:42:36 PM Reply

Love these learning objects. I am jsut wondering if the Chin is actually supposed to be labeled with Mental.

Posted by Jack Knaack on 2/28/2006 12:00:00 AM Reply

Posted by lezette arada on 11/16/2006 12:00:00 AM Reply

This is a very thorough review!

On the overall review at the end I would suggest that the human image be larger. I find it difficult to follow the blue lines to the spot on the image. Also, the sequence of the items should be jumbled more, most followed down the line and wasnt enough of a challenge.

Posted by Laura Lien on 2/28/2006 12:00:00 AM Reply

So helpful study tool! Very nice job! Thank you for helping us, great website.

Body Cavities and Organs


The human body has two main body cavities. The first, the ventral cavity, is a large cavity which sits ventrally to the spine and includes all the organs from your pelvis to your throat. This cavity is the true coelom, as it forms during human embryogenesis from the mesoderm. At first it is a single cavity. It then gets subdivided several times, into smaller cavities separated by muscles, bones, and thin tissues. The first subdivision is the diaphragm muscle, which divides the abdominopelvic cavity from the thoracic cavity. This can be seen in the image below.

The abdominopelvic cavity is then further subdivided into the pelvic cavity and the abdominal cavity. The pelvic cavity holds the reproductive organs, bladder, and allows the intestines passage to the anus. The abdominal cavity is where the majority of the body’s organs lie. These are sometimes referred to as the “viscera”, and they include organs like the liver, stomach, spleen, pancreas, kidneys and others involved in digestion, metabolism, and filtering of the blood. A special membrane holds all of these organs in place and is called the peritoneum.

The thoracic cavity above is also subdivided into smaller section. In the human body, each lung is held within its own pleural cavity, which allows it to expand and avoid friction with the ribs and diaphragm as it reaches capacity. Another important feature of the separation of the pleural cavities is that if one lung fails or collapses, the other can go on functioning. The other division of the thoracic cavity is the mediastinum. This cavity surrounds the heart and associated veins and arteries. The heart is further protected by another layer of mesoderm which forms the pericardial cavity. The pericardium is similar to the peritoneum of the ventral body cavity, except it protects the heart. These different layers have fluids between them which act like lubricants, ensuring the heart pumps without friction.

Other Animals

A body cavity in a non-human animal will be easy to recognize. It will be a membrane-bound, fluid-filled space containing organs. Not all animals have body cavities, and if they do, not all are as easy to recognize as the “true coelom” possessed by humans. Arthropods, for example, are considered to have a coelom, but it is hard to recognize as their open circulatory system is much different than our closed circulatory system. This requires a reframing of how a body cavity is viewed, but the same basic principles apply.

It should be noted that by study a body cavity across organisms, scientist have been able to hypothesize about the evolutionary history of life on Earth. For example, hagfish have a connection between the pericardial (heart) cavity and the rest of their coelom. In higher animals, this separation has become very distinct. This may represent the fact that the hagfish is more closely related to animals with a less developed heart, like the lancelet. This compares the fully separated body cavity in the lamprey, an animal very similar to the hagfish. This change represents a derived character in the vertebrate line, and allowed much more complex vertebrate to arise.

1. The liver, spleen, and gall bladder are found in which body cavity?
A. Abdominal cavity
B. Dorsal cavity
C. Pelvic cavity

2. Which of the following is NOT found in the thoracic cavity?
A. Heart
B. Lungs
C. Brain

3. A scientist studying body cavities and sizes of different organisms concludes that large organisms need body cavities to facilitate their movement. Which of the following statements supports that idea?
A. The smallest organisms have body cavities, while some of the largest do not.
B. As organisms increase in size and complexity, they tend to have more body cavities.
C. All organisms have body cavities, and the fastest ones have the fewest.

The dorsal body cavity protects organs of the nervous system and has two subdivisions. The cranial cavity is the area within the skull and encloses the brain. The spinal (vertebral) cavity encases the vertebral column and spinal cord.

Like the dorsal cavity, the ventral cavity has two subdivisions. The superior division is called the thoracic cavity. The thoracic cavity is surrounded by the ribs and muscles in the chest. It’s further sudivided into lateral pleural cavities (each pleural cavity envelopes a lung) and the mediastinum. Within The pericardial cavity lies within the mediastinum. It encloses the heart and remaining thoracic organs (trachea, esophagus, ect.).

The inferior division of the ventral body cavity is called the “abdominopelvic cavity” and is separated from the thoracic cavity by the diaphragm. The abdominopelvic cavity is also separated into two subdivisions, the “abdominal cavity” and “pelvic cavity“. The abdominal cavity contains the stomach, spleen, liver, intestines, and a few other organs. The pelvic cavity (inferior) contains the urinary bladder, rectum, and some reproductive organs.

Membranes in the Ventral body cavity

The walls of the ventral body cavity and outer covering of its organs contain a thin covering called the serosa (also called serous membrane). It is a double-layered membrane made up of two parts called the “parietal serosa” (lines the cavity walls) and “visceral serosa” (covers organs in the cavity). The serous membranes are separated by a thin layer of fluid called “serous fluid“. Serous fluid is secreted by both membranes and acts as a lubricant, allowing organs to slide in the cavity without causing friction.

Typically, the serous membranes are named according to the cavity and organ they associate with. For instance, the parietal pericardium lines the pericardial cavity.

Abdominopelvic regions and quadrants”

Because it’s so large, the abdominopelvic cavity is separated into regions and quadrants. The quadrants are self-explanatory and can be figured out fairly easily by looking at the abdominopelvic cavity. They consist of the:

  • Right upper quadrant (RUQ)
  • Left upper quadrant (LUQ)
  • Right lower quadrant (RLQ)
  • Left lower quadrant (LLQ)

Simply draw a cross over the cavity seperating it into four boxes, then use the directional terms accordingly.

Abdominopelvic Regions: Image by Mary Weis

The 9 regions of the abdominopelvic cavity are listed below (see image above also).

Related Biology Terms

  • Sagittal Plane – The plane dividing an organism along the longitudinal axis, and divides the organism into left and right halves.
  • Coronal Plane – The coronal planes also divides the organism along the longitudinal axis, but separates the body into dorsal and ventral halves.
  • Anterior – The portion of the body towards the head.
  • Posterior – The part of the body towards the anus, or tail.

1. A laboratory manual describing the dissection of an earthworm tell you to cut the worm in half, along the transverse plane. Which of the following describes this action?
A. You cut the worm from top to bottom, creating two full length halves.
B. You cut the worm in half lengthwise, producing two short sections.
C. Worms do not have a transverse plane.

Parts of the heart


Carries deoxygenated blood from the body to the heart

Flaps that prevent backflow of blood

Receives oxygenated blood from the lungs

Region of the heart that pumps oxygenated blood to the body

Carries deoxygenated blood to the lungs

Region of the heart that pumps deoxygenated blood to the lungs

Carries oxygenated blood from the lungs

Segment of the heart that receives deoxygenated blood

The main artery carrying oxygenated blood to all parts of the body

Appears in

Labelling the heart

The heart is a muscular organ that pumps blood through the blood vessels of the circulatory system. Blood transports oxygen and nutrients to the body. It is also involved in the removal of .

One website – many uses

Greta Dromgool of Berkley Normal Middle School uses the interactive Label the heart as an alternative to dissecting a sheep heart. The interactive appeals to students’ natural play tendencies .


In animals, zoologists define metamery as a mesodermal event resulting in serial repetition of unit subdivisions of ectoderm and mesoderm products. [1] Endoderm is not involved in metamery. Segmentation is not the same concept as metamerism: segmentation can be confined only to ectodermally derived tissue, e.g., in the Cestoda tapeworms. Metamerism is far more important biologically since it results in metameres - also called somites - that play a critical role in advanced locomotion.

One can divide metamerism into two main categories:

  • homonomous metamery is a strict serial succession of metameres. It can be grouped into two more classifications known as pseudometamerism and true metamerism. An example of pseudometamerism is in the class Cestoda. The tapeworm is composed of many repeating segments - primarily for reproduction and basic nutrient exchange. Each segment acts independently from the others, which is why it is not considered true metamerism. Another worm, the earthworm in phylum Annelida, can exemplify true metamerism. In each segment of the worm, a repetition of organs and muscle tissue can be found. What differentiates the Annelids from Cestoda is that the segments in the earthworm all work together for the whole organism. It is believed that segmentation evolved for many reasons, including a higher degree of motion. Taking the earthworm, for example: the segmentation of the muscular tissue allows the worm to move in an inching pattern. The circular muscles work to allow the segments to elongate one by one, and the longitudinal muscles then work to shorten the elongated segments. This pattern continues down the entirety of the worm, allowing it to inch along a surface. Each segment is allowed to work independently, but towards the movement of the whole worm. [2]
  • heteronomous metamery is the condition where metameres have grouped together to perform similar tasks. The extreme example of this is the insect head (5 metameres), thorax (3 metameres), and abdomen (11 metameres, not all discernible in all insects). The process that results in the grouping of metameres is called "tagmatization", and each grouping is called a tagma (plural: tagmata). In organisms with highly derived tagmata, such as the insects, much of the metamerism within a tagma may not be trivially distinguishable. It may have to be sought in structures that do not necessarily reflect the grouped metameric function (eg. the ladder nerve system or somites do not reflect the unitary structure of a thorax).

In addition, an animal may be classified as "pseudometameric", meaning that it has clear internal metamerism but no corresponding external metamerism - as is seen, for example, in Monoplacophora.

Humans and other chordates are conspicuous examples of organisms that have metameres intimately grouped into tagmata. In the Chordata the metameres of each tagma are fused to such an extent that few repetitive features are directly visible. Intensive investigation is necessary to discern the metamerism in the tagmata of such organisms. Examples of detectable evidence of vestigially metameric structures include branchial arches and cranial nerves.

Some schemes regard the concept of metamerism as one of the four principles of construction of the human body, common to many animals, along with general bilateral symmetry (or zygomorphism), pachymerism (or tubulation), and stratification. [3] More recent schemes also include three other concepts: segmentation (conceived as different from metamerism), polarity and endocrinosity. [4]

A metamer is one of several segments that share in the construction of a shoot, or into which a shoot may be conceptually (at least) resolved. [5] In the metameristic model, a plant consists of a series of 'phytons' or phytomers, each consisting of an internode and its upper node with the attached leaf. As Asa Gray (1850) wrote: [6]

The branch, or simple stem itself, is manifestly an assemblage of similar parts, placed one above another in a continuous series, developed one from another in successive generations. Each one of these joints of stem, bearing its leaf at the apex, is a plant element or as we term it a phyton,—a potential plant, having all the organs of vegetation, namely, stem, leaf, and in its downward development even a root, or its equivalent. This view of the composition of the plant, though by no means a new one, has not been duly appreciated. I deem it essential to a correct philosophical understanding of the plant.

Some plants, particularly grasses, demonstrate a rather clear metameric construction, but many others either lack discrete modules or their presence is more arguable. [5] Phyton theory has been criticized as an over-ingenious, academic conception which bears little relation to reality. [7] Eames (1961) concluded that "concepts of the shoot as consisting of a series of structural units have been obscured by the dominance of the stem- and leaf-theory. Anatomical units like these do not exist: the shoot is the basic unit." [8] Even so, others still consider comparative study along the length of the metameric organism to be a fundamental aspect of plant morphology. [9]

Metameric conceptions generally segment the vegetative axis into repeating units along its length, but constructs based on other divisions are possible. [5] The pipe model theory conceives of the plant (especially trees) as made up of unit pipes ('metamers'), each supporting a unit amount of photosynthetic tissue. [10] Vertical metamers are also suggested in some desert shrubs in which the stem is modified into isolated strips of xylem, each having continuity from root to shoot. [5] This may enable the plant to abscise a large part of its shoot system in response to drought, without damaging the remaining part.

In vascular plants, the shoot system differs fundamentally from the root system in that the former shows a metameric construction (repeated units of organs stem, leaf, and inflorescence), while the latter does not. The plant embryo represents the first metamer of the shoot in spermatophytes or seed plants.



  • Root of the penis (radix): It is the attached part, consisting of the bulb of penis in the middle and the crus of penis, one on either side of the bulb. It lies within the superficial perineal pouch.
  • Body of the penis (corpus): It has two surfaces: dorsal (posterosuperior in the erect penis), and ventral or urethral (facing downwards and backwards in the flaccid penis). The ventral surface is marked by a groove in a lateral direction. of the penis consists of the shaft skin, the foreskin, and the preputial mucosa on the inside of the foreskin and covering the glans penis. The epithelium is not attached to the underlying shaft so it is free to glide to and fro. [5]


The human penis is made up of three columns of tissue: two corpora cavernosa lie next to each other on the dorsal side and one corpus spongiosum lies between them on the ventral side. [6]

The enlarged and bulbous-shaped end of the corpus spongiosum forms the glans penis with two specific types of sinusoids, which supports the foreskin, or prepuce, a loose fold of skin that in adults can retract to expose the glans. [7] The area on the underside of the penis, where the foreskin is attached, is called the frenum, or frenulum. The rounded base of the glans is called the corona. The perineal raphe is the noticeable line along the underside of the penis.

The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum, and its opening, known as the meatus / m iː ˈ eɪ t ə s / , lies on the tip of the glans penis. It is a passage both for urine and for the ejaculation of semen. Sperm are produced in the testes and stored in the attached epididymis. During ejaculation, sperm are propelled up the vas deferens, two ducts that pass over and behind the bladder. Fluids are added by the seminal vesicles and the vas deferens turns into the ejaculatory ducts, which join the urethra inside the prostate gland. The prostate as well as the bulbourethral glands add further secretions, and the semen is expelled through the penis.

The raphe is the visible ridge between the lateral halves of the penis, found on the ventral or underside of the penis, running from the meatus (opening of the urethra) across the scrotum to the perineum (area between scrotum and anus). [8]

The human penis differs from those of most other mammals, as it has no baculum (or erectile bone) and instead relies entirely on engorgement with blood to reach its erect state. A distal ligament buttresses the glans penis and plays an integral role to the penile fibroskeleton, and the structure is called "os analog," a term coined by Geng Long Hsu in the Encyclopedia of Reproduction. [9] It is a remnant of baculum evolved likely due to change in mating practice. [10]

The human penis cannot be withdrawn into the groin, and it is larger than average in the animal kingdom in proportion to body mass. The human penis is reciprocating from a cotton soft to a bony rigidity resulting from penile arterial flow varied between 2-3 to 60-80 mL/Min implies the most ideal milieu to apply Pascal's law in the entire human body the overall structure is unique. [9]

Penile measurements vary, with studies that rely on self-measurement reporting a significantly higher average size than those which rely on measurements taken by health professionals. As of 2015 [update] , a systematic review of 15,521 men (and the best research to date on the topic, as the subjects were measured by health professionals) concluded that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches). [3] [4]

Among all primates, the human penis is the largest in girth, but is comparable to the chimpanzee penis and the penises of certain other primates in length. [11] Penis size is affected by genetics, but also by environmental factors such as fertility medications [12] and chemical/pollution exposure. [13] [14] [15] The longest officially documented human penis was found by physician Robert Latou Dickinson. It was 34.3 cm (13.5 in) long and 15.9 cm (6.26 in) around. [16]

Normal variations

    are raised bumps of somewhat paler color around the base (sulcus) of the glans which typically develop in men aged 20 to 40. As of 1999, different studies had produced estimates of incidence ranging from 8 to 48 percent of all men. [17] They may be mistaken for warts, but are not harmful or infectious and do not require treatment. [18] are small, raised, yellowish-white spots 1–2 mm in diameter that may appear on the penis, which again are common and not infectious.
  • Sebaceous prominences are raised bumps similar to Fordyce's spots on the shaft of the penis, located at the sebaceous glands and are normal. is an inability to retract the foreskin fully. It is normal and harmless in infancy and pre-pubescence, occurring in about 8% of boys at age 10. According to the British Medical Association, treatment (topical steroid cream and/or manual stretching) does not need to be considered until age 19.
  • Curvature: few penises are completely straight, with curves commonly seen in all directions (up, down, left, right). Sometimes the curve is very prominent but it rarely inhibits sexual intercourse. Curvature as great as 30° is considered normal and medical treatment is rarely considered unless the angle exceeds 45°. Changes to the curvature of a penis may be caused by Peyronie's disease.

Differences between female and male organs

In the developing fetus, the genital tubercle develops into the glans of the penis in males and into the clitoral glans in females they are homologous. The urogenital fold develops into the skin around the shaft of the penis and the urethra in males and into the labia minora in females. [1] The corpora cavernosa are homologous to the body of the clitoris the corpus spongiosum is homologous to the vestibular bulbs beneath the labia minora the scrotum, homologous to the labia majora and the foreskin, homologous to the clitoral hood. [1] [19] The raphe does not exist in females, because there, the two halves are not connected.

Growth in puberty

On entering puberty, the penis, scrotum and testicles will enlarge toward maturity. During the process, pubic hair grows above and around the penis. A large-scale study assessing penis size in thousands of 17- to 19-year-old males found no difference in average penis size between 17-year-olds and 19-year-olds. From this, it can be concluded that penile growth is typically complete not later than age 17, and possibly earlier. [20]


In males the expulsion of urine from the body is done through the penis. The urethra drains the bladder through the prostate gland where it is joined by the ejaculatory duct, and then onward to the penis. At the root of the penis (the proximal end of the corpus spongiosum) lies the external sphincter muscle. This is a small sphincter of striated muscle tissue and is in healthy males under voluntary control. Relaxing the urethra sphincter allows the urine in the upper urethra to enter the penis properly and thus empty the urinary bladder.

Physiologically, urination involves coordination between the central, autonomic, and somatic nervous systems. In infants, some elderly individuals, and those with neurological injury, urination may occur as an involuntary reflex. Brain centers that regulate urination include the pontine micturition center, periaqueductal gray, and the cerebral cortex. [21] During erection, these centers block the relaxation of the sphincter muscles, so as to act as a physiological separation of the excretory and reproductive function of the penis, and preventing urine from entering the upper portion of the urethra during ejaculation. [22]

Voiding position

The distal section of the urethra allows a human male to direct the stream of urine by holding the penis. This flexibility allows the male to choose the posture in which to urinate. In cultures where more than a minimum of clothing is worn, the penis allows the male to urinate while standing without removing much of the clothing. It is customary for some boys and men to urinate in seated or crouched positions. The preferred position may be influenced by cultural or religious beliefs. [23] Research on the medical superiority of either position exists, but the data are heterogenic. A meta-analysis [24] summarizing the evidence found no superior position for young, healthy males. For elderly males with LUTS, however, the sitting position when compared to the standing position is differentiated by the following:

  • the post void residual volume (PVR, ml) was significantly decreased
  • the maximum urinary flow (Qmax, ml/s) was increased
  • the voiding time (VT, s) was decreased

This urodynamic profile is related to a lower risk of urologic complications, such as cystitis and bladder stones.


An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. Spontaneous erections frequently occur during adolescence due to friction with clothing, a full bladder or large intestine, hormone fluctuations, nervousness, and undressing in a nonsexual situation. It is also normal for erections to occur during sleep and upon waking. (See nocturnal penile tumescence.) The primary physiological mechanism that brings about erection is the autonomic dilation of arteries supplying blood to the penis, which allows more blood to fill the three spongy erectile tissue chambers in the penis, causing it to lengthen and stiffen. The now-engorged erectile tissue presses against and constricts the veins that carry blood away from the penis. More blood enters than leaves the penis until an equilibrium is reached where an equal volume of blood flows into the dilated arteries and out of the constricted veins a constant erectile size is achieved at this equilibrium. The scrotum will usually tighten during erection.

Erection facilitates sexual intercourse though it is not essential for various other sexual activities.

Erection angle

Although many erect penises point upwards (see illustration), it is common and normal for the erect penis to point nearly vertically upwards or nearly vertically downwards or even horizontally straight forward, all depending on the tension of the suspensory ligament that holds it in position.

The following table shows how common various erection angles are for a standing male, out of a sample of 1,564 males aged 20 through 69. In the table, zero degrees is pointing straight up against the abdomen, 90 degrees is horizontal and pointing straight forward, while 180 degrees would be pointing straight down to the feet. An upward pointing angle is most common. [25]

Occurrence of erection angles
angle (°)
from vertically upwards
of males
0–30 4.9
30–60 29.6
60–85 30.9
85–95 9.9
95–120 19.8
120–180 4.9


Ejaculation is the ejection of semen from the penis. It is usually accompanied by orgasm. A series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis. Ejaculation usually happens as the result of sexual stimulation, but it can be due to prostatic disease in rare cases. Ejaculation may occur spontaneously during sleep (known as a nocturnal emission or wet dream). Anejaculation is the condition of being unable to ejaculate.

Ejaculation has two phases: emission and ejaculation proper. The emission phase of the ejaculatory reflex is under control of the sympathetic nervous system, while the ejaculatory phase is under control of a spinal reflex at the level of the spinal nerves S2–4 via the pudendal nerve. A refractory period succeeds the ejaculation, and sexual stimulation precedes it. [26]

The human penis has been argued to have several evolutionary adaptations. The purpose of these adaptations is to maximise reproductive success and minimise sperm competition. Sperm competition is where the sperm of two males simultaneously resides within the reproductive tract of a female and they compete to fertilise the egg. [27] If sperm competition results in the rival male's sperm fertilising the egg, cuckoldry could occur. This is the process whereby males unwittingly invest their resources into offspring of another male and, evolutionarily speaking, should be avoided. [28]

The most researched human penis adaptations are testis and penis size, ejaculate adjustment and semen displacement. [29]

Testis and penis size

Evolution has caused sexually selected adaptations to occur in penis and testis size in order to maximise reproductive success and minimise sperm competition. [30] [31]

Sperm competition has caused the human penis to evolve in length and size for sperm retention and displacement. [31] To achieve this, the penis must be of sufficient length to reach any rival sperm and to maximally fill the vagina. [31] In order to ensure that the female retains the male's sperm, the adaptations in length of the human penis have occurred so that the ejaculate is placed close to the female cervix. [32] This is achieved when complete penetration occurs and the penis pushes against the cervix. [33] These adaptations have occurred in order to release and retain sperm to the highest point of the vaginal tract. As a result, this adaptation also leaves the sperm less vulnerable to sperm displacement and semen loss. Another reason for this adaptation is that, due to the nature of the human posture, gravity creates vulnerability for semen loss. Therefore, a long penis, which places the ejaculate deep in the vaginal tract, could reduce the loss of semen. [34]

Another evolutionary theory of penis size is female mate choice and its associations with social judgements in modern-day society. [31] [35] A study which illustrates female mate choice as an influence on penis size presented females with life-size, rotatable, computer generated males. These varied in height, body shape and flaccid penis size, with these aspects being examples of masculinity. [31] Female ratings of attractiveness for each male revealed that larger penises were associated with higher attractiveness ratings. [31] These relations between penis size and attractiveness have therefore led to frequently emphasized associations between masculinity and penis size in popular media. [35] This has led to a social bias existing around penis size with larger penises being preferred and having higher social status. This is reflected in the association between believed sexual prowess and penis size and the social judgement of penis size in relation to 'manhood'. [35]

Like the penis, sperm competition has caused the human testicles to evolve in size through sexual selection. [30] This means that large testicles are an example of a sexually selected adaptation. The human testicles are moderately sized when compared to other animals such as gorillas and chimpanzees, placing somewhere midway. [36] Large testicles are advantageous in sperm competition due to their ability to produce a bigger ejaculation. [37] Research has shown that a positive correlation exists between the number of sperm ejaculated and testis size. [37] Larger testes have also been shown to predict higher sperm quality, including a larger number of motile sperm and higher sperm motility. [30]

Research has also demonstrated that evolutionary adaptations of testis size are dependent on the breeding system in which the species resides. [38] Single-male breeding systems—or monogamous societies—tend to show smaller testis size than do multi-male breeding systems or extra-pair copulation (EPC) societies. Human males live largely in monogamous societies like gorillas, and therefore testis size is smaller in comparison to primates in multi-male breeding systems, such as chimpanzees. The reason for the differentiation in testis size is that in order to succeed reproductively in a multi-male breeding system, males must possess the ability to produce several fully fertilising ejaculations one after another. [30] This, however, is not the case in monogamous societies, where a reduction in fertilising ejaculations has no effect on reproductive success. [30] This is reflected in humans, as the sperm count in ejaculations is decreased if copulation occurs more than three to five times in a week. [39]

Ejaculate adjustment

One of the primary ways in which a male's ejaculate has evolved to overcome sperm competition is through the speed at which it travels. Ejaculates can travel up to 30–60 centimetres at a time which, when combined with its placement at the highest point of the vaginal tract, acts to increase a male's chances that an egg will be fertilised by his sperm (as opposed to a potential rival male's sperm), thus maximising his paternal certainty. [34]

In addition, males can—and do—adjust their ejaculates in response to sperm competition and according to the likely cost-benefits of mating with a particular female. [40] Research has focused primarily on two fundamental ways in which males go about achieving this: adjusting ejaculate size and adjusting ejaculate quality.

The number of sperm in any given ejaculate varies from one ejaculate to another. [41] This variation is hypothesised to be a male's attempt to eliminate, if not reduce, his sperm competition. A male will alter the number of sperm he inseminates into a female according to his perceived level of sperm competition, [29] inseminating a higher number of sperm if he suspects a greater level of competition from other males.

In support of ejaculate adjustment, research has shown that a male typically increases the amount he inseminates sperm into his partner after they have been separated for a period of time. [42] This is largely due to the fact that the less time a couple is able to spend together, the chances the female will be inseminated by another male increases, [43] hence greater sperm competition. Increasing the number of sperm a male inseminates into a female acts to get rid of any rival male's sperm that may be stored within the female, as a result of her potential extra-pair copulations (EPCs) during this separation. Through increasing the amount he inseminates his partner following separation, a male increases his chances of paternal certainty. This increase in the number of sperm a male produces in response to sperm competition is not observed for masturbatory ejaculates. [29]


Males also adjust their ejaculates in response to sperm competition in terms of quality. Research has demonstrated, for example, that simply viewing a sexually explicit image of a female and two males (i.e. high sperm competition) can cause males to produce a greater amount of motile sperm than when viewing a sexually explicit image depicting exclusively three females (i.e. low sperm competition). [44] Much like increasing the number, increasing the quality of sperm that a male inseminates into a female enhances his paternal certainty when the threat of sperm competition is high.

Female phenotypic quality

A female's phenotypic quality is a key determinant of a male's ejaculate investment. [45] Research has shown that males produce larger ejaculates containing better, more motile sperm when mating with a higher quality female. [40] This is largely to reduce a male's sperm competition, since more attractive females are likely to be approached and subsequently inseminated by more males than are less attractive females. Increasing investment in females with high quality phenotypic traits therefore acts to offset the ejaculate investment of others. [45] In addition, female attractiveness has been shown to be an indicator of reproductive quality, with greater value in higher quality females. [46] It is therefore beneficial for males to increase their ejaculate size and quality when mating with more attractive females, since this is likely to maximise their reproductive success also. Through assessing a female's phenotypic quality, males can judge whether or not to invest (or invest more) in a particular female, which will influence their subsequent ejaculate adjustment.

Semen displacement

The shape of the human penis is thought to have evolved as a result of sperm competition. [47] Semen displacement is an adaptation of the shape of the penis to draw foreign semen away from the cervix. This means that in the event of a rival male's sperm residing within the reproductive tract of a female, the human penis is able to displace the rival sperm, replacing it with his own. [48]

Semen displacement has two main benefits for a male. Firstly, by displacing a rival male's sperm, the risk of the rival sperm fertilising the egg is reduced, thus minimising the risk of sperm competition. [49] Secondly, the male replaces the rival's sperm with his own, therefore increasing his own chance of fertilising the egg and successfully reproducing with the female. However, males have to ensure they do not displace their own sperm. It is thought that the relatively quick loss of erection after ejaculation, penile hypersensitivity following ejaculation, and the shallower, slower thrusting of the male after ejaculation, prevents this from occurring. [48]

The coronal ridge is the part of the human penis thought to have evolved to allow for semen displacement. Research has studied how much semen is displaced by differently shaped artificial genitals. [49] This research showed that, when combined with thrusting, the coronal ridge of the penis is able to remove the seminal fluid of a rival male from within the female reproductive tract. It does this by forcing the semen under the frenulum of the coronal ridge, causing it to collect behind the coronal ridge shaft. [49] When model penises without a coronal ridge were used, less than half the artificial sperm was displaced, compared to penises with a coronal ridge. [49]

The presence of a coronal ridge alone, however, is not sufficient for effective semen displacement. It must be combined with adequate thrusting to be successful. It has been shown that the deeper the thrusting, the larger the semen displacement. No semen displacement occurs with shallow thrusting. [49] Some have therefore termed thrusting as a semen displacement behaviour. [50]

The behaviours associated with semen displacement, namely thrusting (number of thrusts and depth of thrusts), and duration of sexual intercourse, [50] have been shown to vary according to whether a male perceives the risk of partner infidelity to be high or not. Males and females report greater semen displacement behaviours following allegations of infidelity. In particular, following allegations of infidelity, males and females report deeper and quicker thrusting during sexual intercourse. [49]

Circumcision has been suggested to affect semen displacement. Circumcision causes the coronal ridge to be more pronounced, and it has been hypothesised that this could enhance semen displacement. [34] This is supported by females' reports of sexual intercourse with circumcised males. Females report that their vaginal secretions diminish as intercourse with a circumcised male progresses, and that circumcised males thrust more deeply. [51] It has therefore been suggested that the more pronounced coronal ridge, combined with the deeper thrusting, causes the vaginal secretions of the female to be displaced in the same way as rival sperm can be. [34]

Organ Systems of the Human Body Worksheets

The human body is a wonderful creation, and organs are the most precious things we own. The harmonious working of these organ systems similar to the machines, along with the intricate network of wires and tubes results in a healthy life. Grade 3 through grade 6 kids get an insight into the major organ systems like digestive, respiratory, circulatory, nervous, excretory, skeletal, reproductive system, the major organs involved in each, and the functions carried out by them as they walk through these printable human body systems worksheets and charts. Our free pdfs are sure to leave you craving for more!

This show-and-tell internal organs of the human body printable chart displays the major organs like heart, lungs, liver, etc., and their location in the human body. Get the basics of human anatomy right with this chart.

Once armed with the knowledge of the vital organs and their position in the human body, check if 3rd grade and 4th grade kids can identify and label the internal human organs like the lungs, liver, and more.

What are the organ systems in our body? Which are the major organs involved? What are the functions performed by each? This collection of 11 human body system flashcards proves handy and provides answers to all such queries.

Get straight to the crux of the matter with this worksheet comprising diagrams of the six organ systems. Examining, identifying, and naming each organ system is all you have to do. Now, this is what we call easy-peasy stuff!

Is the heart a part of the circulatory system or nervous system? Do the kidneys or lungs make up the respiratory system? Identify the organ system associated with each organ in this printable handout.

Bones are to the skeletal system as the intestine is to the digestive system. Correlate the organs and the systems with each other in this organs and organ systems cut and glue activity.

Tabulate the names of the organ systems, the major organs that make up each system and the function each performs in this printable template on human body systems. Serves best in testing comprehension!

Perfect for your 5th grade and 6th grade children, this fun activity helps enhance vocabulary and spelling. Not only do kids identify the organ systems, but also look for the words in the grid and enhance spelling skills.

Trump your peers using this pdf on matching the organ system with its function. Give kids a chance to demonstrate comprehension of the work performed by each human body system.

Regular check-ins like this fill in the blanks pdf worksheet on organ systems are essential to assess knowledge. Grade 6 kids read the definition, fact, or function, and complete it with word(s) from the box.

Break the monotony, and boost the energy level with this internal organs riddle worksheet. "Who am I?" riddles that have a body part reference can paint an interesting picture in the mind and are a witty way to test young learners.

Body Regions And Body Cavities

Anatomically, the human body is divided into eight regions:

1. The cephalic region comprises the head and associated organs which are located at the top of the body.

2. The cervical region comprises the neck and the thorax, and associated organs.

3. The dorsal region comprises the back, from below the neck to the area below the waist, not including the shoulders.

4. The thorax comprises areas below the neck and ends just below the ribcage.

5. The abdomen covers the area covering the bottom of the ribcage up to the hips.

6. The pelvis begins at where the abdomen ends and covers the area between the hip bones.

7. The upper extremities include organs such as the shoulders and hands.

8. The lower extremities include the hips, buttocks, and legs.

Fluid filled spaces (other than blood and lymph vessels) are called cavities. Cavities in the human body are broadly divided into dorsal and ventral cavities. These cavities house internal organs, and allow size and shape changes that occur during normal functioning of the body.

Dorsal cavity comprises cranial cavity and vertebral canal. The cranial cavity houses the brain, while the vertebral or spinal cavity houses the spinal cord.

Ventral cavity comprises thoracic cavity and abdominopelvic cavity. The thoracic cavity houses the heart and lungs. The abdominopelvic cavity is further divided into abdominal cavity and pelvic cavity. The abdominal cavity houses the digestive organs, kidneys, and spleen, and the pelvic cavity houses the reproductive organs and urinary bladder.

Watch the video: Anatomical Regions (August 2022).