The Digestive System
Digestion is the process of breaking down nutrients throughout the body. Beginning with the mouth and ending with the anus, digestion is a very involved process that requires organs and enzymes to complete.
The picture above shows all the organs involved in digestion. There are also many glands that secrete enzymes to help with the process.
Organs Involved
Mouth: the object of food is placed in the mouth; teeth begin chewing (mastication)
Tongue: the tongue moves the food around in the mouth which helps it break down and forms a bolus
Salivary glands (parotid, sublingual, submandibular): these glands secrete salivary amylase which helps the food break down chemically
Pharynx: when swallowed, the food goes through the pharynx
Esophagus: the esophagus is the pathway that the food takes after it goes through the pharynx where peristalsis (contraction and relaxation of the throat muscles to push the food down) occurs
Stomach: stores food; HCl and pepsin are released which turns the bolus into chyme
Liver: stores fat and makes bile
Gall bladder: bile made in the liver is stored here
Pancreas: secretes pancreatic enzymes through the common bile duct into the small intestine for further digestion
Small intestine: absorbs nutrients and they are carried to cells through the blood system; villi line the walls
Large intestine: made up of the ascending, transverse, and descending colon; absorbs water, salts, vitamins, and stores indigestible material until eliminated
Rectum: also part of the large intestine; receives and holds the stool until it is eliminated from the body
Anus: has two sphincters
Internal: always tight to keep stool in until ready to release
External: holds stool in body until opportunity to use the restroom is available
Digestion involves the breaking down of nutrients such as carbohydrates, proteins, and fats for the body to use. Carbohydrates are broken down into glucose, proteins are broken down into amino acids, and fats are broken down into fatty acids. The body uses all of these breaking down processes to produce energy.
There are two metabolic processes that take place in a cell: anabolic and catabolic. Anabolic is the buildup of larger molecules from smaller ones using energy and catabolic is the breakdown of large molecules to smaller ones. The catabolic process releases energy.
An example of an anabolic process is the buildup of glucose to glycogen.
An example of a catabolic process is the breakdown of glycogen to glucose.
Different nutrients take different pathways throughout the body to be digested.
Carbohydrates are put into the mouth and salivary amylase starts to break down the starch. The carbohydrates becomes a mass of the chewed food and salivary amylase called a bolus. The bolus then proceeds down the esophagus through peristalsis and to the stomach where HCl and pepsin break it down further into glucose and turns the bolus into chyme. From there, it travels through the small intestine and then the large intestine where some nutrients are absorbed in the small intestine. After the intestines, it travels through the colon and is excreted from the body.
Proteins are put into the mouth and mastication occurs. They are then swallowed and peristalsis helps them travel through the esophagus. After the esophagus, the nutrients are broken down in the stomach by HCl and pepsin. This changes the nutrient from protein to peptides. From there, they travel through the small and large intestines where the last of the nutrients are absorbed. It then goes to the colon and waits to be eliminated from the body.
Fats are put into the mouth and mastication occurs which breaks down the fats into smaller molecules easier for digestion. Peristalsis occurs in the esophagus and the fats travel to the stomach. Here they are affected by HCl and pepsin and broken down even smaller into lipids. Next comes the small and large intestines where they travel to colon. After the colon, the broken down materials can leave the body when ready.
All of these pathways of digestion give the body energy in one way or another. Whether it is the process of breaking down or the substances that are created from the breaking down, energy is produced. The body needs this energy to survive and keep other bodily functions occurring. The energy created from the breakdown of the molecules is ATP which stands for adenosine triphosphate. ATP is then used in the Krebs cycle. The Krebs cycle is a series of reactions where molecules are decomposed and carbon dioxide and hydrogen atoms are released. The Krebs cycle also produces more ATP.
Water Intake
Water is a very important part of life. It makes up about 60% of our bodies and we cannot survive without it. Without enough water, a person can become dehydrated, often indicated by thirst. Water also helps the process of digestion, helping the food breakdown. When the body water total decreases by 1% or 2%, one feels thirst. Dehydration is a process that has many steps:
Extracellular fluid is more concentrated
The hypothalamus is stimulated
A decline in the signal's posterior pituitary gland which releases ADH (antidiuretic hormone)
Blood carries the ADH to the kidneys
Causes water reabsorption
Urine output decreases because the body needs to conserve more water
When there is excessive water intake, extracellular fluid becomes less concentrated, the pituitary gland decreases the release of ADH, kidneys decrease their water absorption, and urine output increases. ADH (anti-diuretic hormone) is released from the pituitary gland and kidneys receive a large amount of ADH when there is an excessive amount of water. The kidneys are organs that control water.
The Endocrine System
Hormones are a very important part of the endocrine system. They play a major role in the actions of the system as a whole. To understand more about the endocrine system and hormones, one must know some basic information.
The endocrine system is made up of glands. Glands are able to secrete hormones which enables the endocrine system to function properly. Some glands are in the shape of an acorn, such as the pituitary and pineal glands. Because the endocrine system is made of endocrine glands, they have no ducts. They are always secreting something, whether it is insulin, glucagon, etc. The amount of substance secreted varies depending on how much stimulus there is. More often than not, the endocrine system secretes hormones very slowly, like a dripping faucet.
Endocrine Glands
There are many different primary endocrine glands. In a female there is the pineal, hypothalamus, pituitary, thyroid, parathyroids, thymus, adrenals, pancreas, and ovary. In a male there is the pineal, hypothalamus, pituitary, thyroid, parathyroids, thymus, adrenals, pancreas, and testes.
Pineal: secretes the hormone melatonin that helps individuals sleep at night and wake in the morning
Hypothalamus: main link between endocrine and nervous systems; stimulate secretions of pituitary gland
Pituitary: "master gland;" makes hormones that control other glands
Thyroid: produces thyroxine an triiodothyronine which helps burn food to produce energy
Parathyroids: regulates levels of calcium in blood
Thymus: develops a child's immune system
Adrenals: two parts
Adrenal Cortex: regulates salt and water intake of the body, body's response to stress, metabolism
Adrenal Medulla: produces adrenaline, increases blood pressure
Pancreas: maintains blood sugar by secreting glucagon
Gonads: gives individual sex characteristics
Ovaries: produce eggs and release estrogen
Testes: secrete androgens
Target Organs
Target organs are organs that are affected by a certain hormone. A specific protein receptor must be on an organ's plasma membrane so the hormone can attach. It is similar to a lock and key. If the key does not fit in the lock, it cannot work. There are two kinds of hormones that have mechanisms that trigger changes in the cells. They are steroidal hormones and nonsteroidal hormones.
Steroidal hormones are lipid soluble. Their mechanism process starts with diffusing through a plasma membrane.
After the plasma membrane, the hormone enters the nucleus
It then binds to a specific receptor protein
The receptor binds to a site on the cell's DNA
It then activates certain genes to transcribe mRNA
mRNA translated into cytoplasm which makes new proteins
Nonsteroidal hormones are protein and peptide hormones that bind to receptors on target organs.
After he hormone binds to the receptor, a series of reactions are set of to activate the enzyme
The enzyme carries out a reaction and produces a second messenger molecule
This molecule oversees additional intracellular changes to promote response of target cell
Hormone Release
Negative feedback mechanisms regulate blood levels of hormones. Hormone secretion can be triggered by external or internal stimulus. If there is too much of a hormone in the body, negative feedback stops production; but if there is not enough hormone production, hormone production increases. There are three main categories that the stimuli fall into.
Hormonal stimulus: endocrine organs are stimulated by other hormones (hypothalamic hormones stimulate anterior pituitary to secrete its hormones)
Humoral stimuli: changing blood levels of ions and nutrients stimulate release of hormones (parathyroid hormone is secreted when there is a decline in calcium)
Neural stimuli: nerve fibers stimulate hormone release and target cells respond (nervous system stimulates adrenal medulla to release norepinephrine in response to stress)
gif} Diseases and Conditions There are many diseases and conditions that relate to hormones. Some have to deal with the hyposecretion of hormones and some have to deal with hypersecretion of hormones. Pituitary dwarfism: hyposecretion (deficit) of growth hormone; normal body proportions but a very small individual Gigantism: hypersecretion of growth hormone; extremely tall person with normal body proportions Acromegaly: hypersecretion of growth hormone after bones are done growing; facial bones enlarge and face becomes malformed Diabetes Insipidus: hyposecretion of antidiuretic hormone; excessive urine out put, very thirsty as well Goiters: lack of iodine; enlargement of the thyroid gland; not enough thyroxine produced to keep up with the excessive iodine need Cretinism: hyposecretion of thyroxine; dwarfism where proportions are childish; mentally retarded; scanty hair, dry skin Tetany: not enough calcium; muscle impulses/spasms Addison's Disease: hyposecretion of adrenal cortex hormones; bronze-toned skin; weak muscles Cushing's Syndrome: excessive output of glucocorticoids; buffalo hump on back of excess fat; high blood pressure Masculinization: hypersecretion of sex hormones; both genders; masculine body hair appears Diabetes Mellitus: lack of insulin; glucose spills into urine; dehydration {goiter-4.jpg} Hormones There are many hormones throughout the body that carry out different functions. They are secreted from different glands and are vital for life processes. Prolactin (PRL): after childbirth, it stimulates milk production in the mother's breasts Adrenocorticotropic hormone (ACTH): regulates endocrine activity of adrenal cortex Thyroid-stimulating hormone (TSH): growth and activity of thyroid gland Follicle-stimulating hormone (FSH): In females - follicle development in ovaries, produce estrogen In males - stimulates sperm development Luteinizing hormone (LH): In females: ovulation of an egg and production of progesterone and some estrogen In males: also referred to as interstitial cell-stimulating hormone (ICSH); stimulates testosterone production Oxytocin: released during childbirth to stimulate contractions and aids in production of milk for breast-feeding Antidiuretic hormone (ADH): causes kidneys to absorb more water from urine so urine decreases and blood increases Thyroxine: secreted by thyroid molecules to help thyroid processes; has four iodine atoms Triiodothyronine: formed at target tissues from thyroxine; has three iodine atoms Calcitonin: decreases blood calcium levels Parathyroid hormone (PTH): regulates calcium ion homeostasis of the blood by breaking down bone if levels get too low Mineralocorticoids: i.e. aldosterone; regulate mnineral content of the blood Atrial natriuretic peptide (ANP): prevents aldosterone release; reduces blood volume and pressure Glucocorticoids: i.e. cortisone/cortisol; normal cell metabolism; resist long-term stressors by increasing blood glucose levels Sex hormones: androgens and estrogens to help reproductive functions Epinephrine/adrenaline: helps one determine "fight-or-fight" response and stage to stress; short term stressors Insulin: stimulated by high levels of glucose; speed up glucose oxidation; sweeps out glucose from blood Melatonin: at night there are high levels and cause drowsiness; during the day low levels Thymosin: as a child, incubator for white blood cells to help immunity Estrogen: stimulate development of secondary sex characteristics in females and prepares uterus to receive fertilized egg; menstrual cycle; prepare breasts for lactation Progesterone: menstrual cycle; helps embryo stay in uterus during pregnancy Androgens: i.e. testosterone; male sex characteristics; growth and maturation of reproductive organs; secondary sex characteristics Human chorionic gonadotropin (hCG): stimulates corpus luteum of ovary to continue producing estrogen {PatThomas.jpg} Aging and the Endocrine System When most adults hit middle age and elderly years, the endocrine system changes and alters many aspects of the body. Females, for example, go through menopause when they are middle-aged. Menopause is when the efficiency of the ovaries begins to decline and child bearing years come to an end. Osteoporosis, wrinkling, and the operation of sympathetic nervous system occur because of the decrease in estrogen. Women may get hot flashes, fatigue, nervousness, and mood changes during this time. In both sexes, muscles weaken and they are not able to fend off stress and infection as they used to. They either have overproduction or underproduction of hormones that are able to deal with these stressors. Elderly individuals also have slower metabolisms because they do not have enough thyroid hormone being produced. Thyroid problems in the elderly often lead to type II diabetes. {hormones.jpg} The Skeletal System The Skeleton The skeleton makes up the framework of the body, meaning we would be a pile of skin and organs without it. There are 206 bones in the adult body ranging in sizes. There are short bones, long bones, flat bones, and irregular bones. An example of a short bone would be the patella, an example of a long bone would be the humerus, an example of a flat bone is the sternum, and an example of an irregular bone is a cervical vertebrae. Short bones are usually cube shaped and spongy bone. Long bones are longer than they are wide with heads at both ends and mostly spongy bone. Flat bones are thin and flat with two thin layers of compact bone surrounding a layer of spongy bone. Irregular bones do not fit into any other bone category stated. These categories of bones are further broken down into two classifications of bones: spongy and compact Spongy bone: composed of small needle-like pieces of bone and lots of open space Compact bone: dense, appears smooth and homogeneous {index.php.jpeg} Parts of the Bone There are many parts that make up a bone. These parts are essential for muscles, ligaments, and various other parts of the body to attach to the bone. The bone markings are: Tuberosity: large, rounded projection; sometimes roughened Crest: prominent, narrow ridge of bone Trochanter: large, irregularly shaped process Line: narrow ridge of bone Tubercle: rounded projection/process Epicondyle: raised area on a condyle Spine: pointed projection Process: bony prominence Head: bony expansion on a narrow neck Facet: flat articular surface Condyle: rounded articular projection Ramus: arm-like bar of bone Meatus: canal-like passageway Sinus: air-filled cavity in bone lined with mucous membrane Fossa: shallow depression in bone Groove: furrow Fissure: slit-like opening Foramen: oval opening through a bone {xrayelbow.jpg} Fractures Fractures are breaks or cracks in bone. These can occur from an impact or a twist of a bone even though bones are very strong. There are six types of fractures that can occur in all ages. Comminuted: bone breaks into many fragments and is most common in the elderly because they have brittle bones Compression: bone is crushed and is most common in porous bones because it makes them more brittle Depressed: bone is broken inward and often occurs in the skull Impacted: broken bone ends are forced into each other and often occurs when a person falls with outstretched arms Spiral: ragged break when a twisting force occurs and is most common in sports Greenstick: bone breaks incompletely and is most common in children whose bones are more flexible {cow157lg.jpg} The Skull The skull has an abundance of bones. To make it easier to understand, it is broken up into two main parts - the cranium and facial bones. The cranium part of the skull protects the brain whereas the facial bones enable us to show emotion when attached to the muscles. There are many sutures in the head that keep these bones together. The sagittal suture holds the parietal bones in place. The coronal suture is where the two parietal bones meet the frontal bone. The squamous sutures are where where the temporal bones and parietal bones are attached. The cranium has eight large flat bones and two paired bones. These bones are: Frontal bone: forms the forehead Parietal bones: (paired bones) walls of cranium Temproal bones: (paired bones) are inferior to the parietal bones Occipital bone: floor and back wall of skull Sphenoid bone: width of skull and floor of cranial cavity Ethmoid bone: forms the roof of the nasal cavity The facial bones consist of twelve paired bones and two single bones. These bones are: Maxilla: form the upper jaw Palatine bones: form the posterior hard palate Zygomatic bones: cheekbones Lacrimal bones: are a passageway for tears Nasal bones: forms the bridge of the nose Vomer bone: (single bone) forms most of the nasal septum Inferior nasal conchae: form walls of nasal cavity Mandible: (single bone) forms the chin {skull-anatomy-bones-2.jpg} Hyoid Bone The hyoid bone is the only bone in the body that is unattached. It is suspended in mid-neck above the larynx. It is "attached" to the body by ligaments that attach to the temporal bone. It helps us speak and move our tongue as well. {hyoid_bone.jpg} Other Bones There are many other bones in the body with all sorts of different parts in them. These parts often make up larger parts of the body. Such as the pelvis and spine. Spine: made up of vertebra and forms the backbone of a person Pelvis: forms the hips and is the foundation for the lower extremities Femur: largest bone of the body; helps with mobility and attaches muscles Tibia: helps with mobility in the leg Fibula: supports body and helps with movement; smaller bone in the leg Ulna: extends arm and flexes it back and forth Radius: helps with motion and support of the arm; connects with joints and muscles Humerus: helps with mobility; enables you to reach, pull, lift, push and rotate; many muscles are attached which helps arms move Hand: many bones make up the hand and help it to move (pictured below) {bones_hand.jpg} The Muscular System The Muscles Muscles are a very important part to the body. They help a person move and partake in everyday activities. The muscles in the body are attached at different points to certain bones. There is an insertion point and an origin on a bone. The origin is where the muscle starts and there is no movement. The insertion is where the muscle ends and there is mobility in that area of the muscle. {image002.gif} What Are Muscles Used For? Muscles have many functions in the body. They support the body by opposing the force of gravity and allow a person to stay upright. They also enable the bones to move so everyday actions can be carried out. When ATP breaks down in a muscle contraction, heat is released and distributed throughout the body to maintain a constant body temperature. Muscles are also important in moving cardiovascular and lymphatic vessels to help the body function properly. They protect the internal organs and stabilize the joints to make movements easier.

Origin/Insertion Points
Muscles have an insertion and origin point. The origin point is where the muscle begins and does not move at that point. The insertion is where it ends and it moves freely in that area.
Frontalis: cranial aponeurosis/skin of eyebrows
Orbicularis oculi: frontal bone/tissue around eyes
Orbicularis oris: mandible/skin around mouth

Temporalis: temporal bone/ mandible Zygomaticus: zygomatic bone/corner of lips Masseter: temporal bone/mandible Buccinator: maxilla/orbicularis oris Sternocleidomastoid: sternum and clavicle/temporal bone Platysma: connective tissue/tissue around mouth Pectoralis major: sternum/proximal humerus Rectus abdominis: pubis/sternum External oblique: lower eight ribs/iliac crest Biceps brachii: scapula/proximal radius Brachialis deltoid: distal humerus/proximal ulna Iliopsoas: ilium/femur Adductur muscles: pelvis/proximal femur Sartorius: ilium/proximal tibia Quadriceps: femur/tibial tuberosity Tibialis anterior: proximal tibia/first cuneiform Extensor digitorum: proximal tibia/distal toes 2-5 Fibularis muscles: fibula/metatarsals Trapezius: occipital bone/scapular spine Latissimus dorsi: lower spine/proximal humerus Erector spinae: iliac crests/ribs Deltoid: scapular spine/humerus Triceps brachii: shoulder girdle/olecranon Flexor carpi radialis: distal humerus/second and third metacarpals Flexor carpi ulnaris: distal humerus/carpals of wrist Flexor digitorum superficialis: distal humerus/middle phalanges Extensor carpi radialis: humerus/base of second and third metacarpals Extensor digitorum: distal humerus/distal phalanges Gluteus maximus: sacrum and ilium/proximal femur Gluteus medius: ilium/proximal femur Hamstring muscles: ischial tuberosity/ proximal tibia Gastrocnemius: distal femur/calcaneus Soleus: proximal tibia/calcaneus Muscle TypesThere are three kinds of muscles in the body. They each have different properties that make them unique. They are the cardiac, smooth, and skeletal muscle. Cardiac muscle Forms the heart wall Single nucleus fibers Striated, tubular, and branches which allows the fibers to interlock

Contractions spread quickly throughout the heart
Rhythmic contractions
Spindle muscle fibers
Single nucleus fibers
Cells arranged in parallel lines

No striations Located in the walls of internal organs Slower contractions Does not fatigue easily Skeletal muscle Tubular fibers Multinucleated cells Make up the skeletal muscle attached to the skeletal system Contraction controlled by nervous system {19917.jpg} Muscular Contraction Muscle contractions are an essential part to a person moving. Contractions require many different parts of the muscle. The muscle fiber is a cell that has cellular components. The sarcolemma is the plasma membrane that forms the T tubules. The sarcoplasm is the cytoplasm of a muscle fiber that has organelles. Myoglobin is the red pigment that stores O2 for muscular contraction. Muscles store potential energy in many ways. There are three different storage aspects of muscles. ATP: some ATP molecules are stored within muscle fibers which provide short term energy supply for contraction and relaxation Creatine phosphate: Three to six times as much creatine phosphate is stored within skeletal muscle fibers compared to ATP; creatine phosphate is long term energy storage and can be used by the muscle for energy

Muscle fiber glycogen: produces energy during muscle contractions and uses it during them
Muscle Fibers
There are two main colors of muscle fibers: red/dark fibers and white/light.

Red fibers get their red pigment from high concentrations of myoglobin which is a globe shaped muscle protein. Myoglobin functions as a temporary store for O2 in the actual muscle fiber. It does this because it temporarily attaches and stored one O2 molecule and contracts aerobically. The red fibers do not tire easily because the oxidative catabolism of glucose produces 38 ATPs, giving them lots of energy and glucose. They are considered high oxidative because they contract aerobically.
White fibers lack the red pigment because they do not have a lot of myoglobin. They are considered low oxidative because because they are anaerobic muscle fibers. They produce ATP quickly by glycolysis but they are tire quickly. They tire quickly because they only get 2 ATPs produced from glucose.
Based on the oxidative factor and the speed of twitch, there are three types of skeletal muscle fiber types.
HOST (high oxidative slow twitch) - red and aerobic; twitch slowly; resistant to fatigue; good for endurance
HOFT (high oxidative fast twitch) - red and aerobic; good for endurance; twitch rapidly; help with long distance running
LOFT (low oxidative fast twitch) - white and anaerobic; good for sprinters and lifters
Muscle LeversThere are three types of muscle levers: first class, second class, and third class.

An example of first class is a head nod, an example of second class is a toe raise, and an example of third class is a bicep curl.
There are many movements that muscles help a person carry out. They are:
  1. Extension: moving away from the body
  2. Flexion: moving towards the body
  3. Abduction: away from the body; bringing body parts away
  4. Adduction: to the body; bringing body parts close to the body
  5. Inversion: bringing the body part in
  6. Exversion: bringing the body part out
  7. Suppination; turning palms forward
  8. Pronation: turning palms backward
  9. Semipronate: hands to the side
  10. Dorsaflexion: up toward the body
  11. Plantarflexion: pointing away from the body
  12. Opposition: the thumb and forefinger coming together
  13. Rotation: rotating the shoulder
  14. Circumduction: moving the wrist in a controlled movement
  15. Lateral rotation: rotating out from the body
  16. Medial rotation: rotating in toward the body

In order for the muscles to contract, the sliding filament theory must take place. This happens in various steps.
  1. A neurotransmitter sends the muscle a signal to contract
  2. Calcium is produced and and binds to troponin and changes its shape
  3. It becomes tropomyosin and
  4. Tropomyosin moves deeper into the actin helix and becomes a block
  5. Myosin heads are activated and attracted to the sites on actin
    1. This causes cross bridges to form
  6. As the myosin heads binds, ATP is generated and attaches
  7. The ATP breaks down into phosphate and ADP which each bind to different receptor sites
  8. As the power stroke takes place, the myosin head changes angle and the ADP and phosphate are released
  9. A new ATP molecule attaches to the myosin heads and myosin lets go of the actin and the cross bridge detaches
  10. The myosin cocks its head and hydrolyzes ATP to ADP and Pi which gives the energy needed to return the myosin head to the cocked position
  11. This gives enough energy to carry out the next steps to restart the process

The Reproductive System


The reproductive system is a part of the body that is essential to life. It helps in the reproduction process and keeps humans growing from generation to generation. The male and female reproductive systems are different; they have different parts and carry out different processes though used for the same thing - reproduction.

The male reproductive system consists of:
  1. Penis: the penis is the main male reproductive organ
  2. Testes: produce sperm and the male hormone testosterone
  3. Epididymis: stores sperm and feeds it glycogen
  4. Vas deferens: takes sperm up
  5. Seminal vesicle: gives sperm fructose; gateway to freedom for the sperm
  6. Prostate gland: gives sperm the thin, milky fluid that neutralizes the acidity of the vagina
  7. Cowper's gland: has preseminal fluid and sperm
  8. Urethra: urination and semen exit here


The female reproductive system consists of:
  1. Vagina: main female reproductive part
  2. Ovaries: stores the eggs
  3. Fallopian tubes: where the egg is fertilized by sperm
  4. Uterus: where the baby is housed as it grows
  5. Cervix: opening of the uterus
  6. Mons pubis: padding near the pelvic bone
  7. Labia majora: outer folds of skin
  8. Labia minora: fatty tissues inside
  9. Hymen: membrane that covers vagina opening
  10. Vulva: made up of mons pubis, labia majora, and labia minora


The Menstrual Cycle

The menstrual cycle is a part of the female reproductive system. It is when follicle stimulating hormone from the anterior pituitary gland really comes into play. The follicular cells produce and secrete estrogen which is a female hormone. Estrogen helps in the menstrual cycle. The anterior pituitary gland secretes luteinizing hormone and follicle stimulating hormone to mainly stimulate ovulation. The estrogen and progesterone hormones inhibit the secretion of follicle stimulating hormone and luteinizing hormone from the anterior pituitary gland when it is time for ovulation and the menstrual cycle to come to a stop. Menstrual flow is when the uterine lining disintegrates and estrogen and progesterone levels decline. Once the anterior pituitary gland is no longer inhibited by estrogen and progesterone, follicle stimulating hormone is secreted again. The menstrual cycle lasts for 28 days.



Sperm is part of the male reproductive system. It is what fertilizes the egg in a female to form a zygote. The scrotum is what holds the sperm. Scrotum means pouch. The Dartos muscle is what pulls the scrotum closer to the body when it is cold. Spermatogenesis is the sequence of events int he semiferous tubules of testicles that produce sperm. ST are sperm carrying tubules.



Pregnancy is what keeps life continuing. It requires both the male and female reproductive organs. This occurs when one male sperm penetrates the female egg. It is then fertilized and becomes a zygote. During the first six weeks after the fertilization, the embryo grows rapidly in length and gains weight. At the start of the third week, the embryo is about an inch long, which is 10,000 times the size of the original egg. At the end of the eighth week, the embryo becomes a fetus.

There can be two types of twins. Twins are when two babies grow in the uterus at the same time. Fraternal twins are when two eggs are fertilized by two different sperm. The twins may look alike and they can be either male and/or female twins. Identical twins is when the egg is fertilized and it splits. The twins will look exactly alike and can be either female/female or male/male. Identical twins will share the same DNA.

A miscarriage is a spontaneous abortion where the embryo or fetus is expelled from the female body.

A stillborn at birth is a dead fetus that is delivered.



Heredity helps determine what an individual looks like. It is determined by chromosomes that contain DNA. The genes can either be dominant or recessive. Dominant genes are the ones that actually show in a person (the phenotype shows, the genotype is what causes the phenotype). A male chromosome is XY and a female is XX. The Y in the male makeup is what causes testicle development.