Review of All the Human Body Organs
An organ is defined as a specialised structure which is composed of different tissues that join together as a unit to perform a specific function. A variety of organs are found in the body; they will be explored systematically through their anatomy and function(s).
Central Nervous System
The meninges are the three layers that surround all the components of the central nervous system. The pia mater is the layer closest to the nervous system; it is closely attached to the structures. This layer also descends into the sulci on the surface of the brain.
The arachnoid mater is a loose layer, named after its spider like appearance. The layer is also closely related to the underlying structures, but not as tight as the pia mater. Between the arachnoid and pia mater is where we find the blood vessels that supply the brain, termed the subarachnoid space.
The dura mater is the outermost meningeal layer. It is the toughest and closely adheres to the internal surface of the skull.
The cerebrum is the large area of the brain which we classically think of as the brain. It consists of four paired lobes and a large amount of complex subcortical anatomy.
The frontal lobe can be described as the region that inhibits our behaviours. The orbitofrontal region is the specific structure that prevents us from acting impulsively.
There is also a prefrontal cortex, which has a dorsolateral component. This is the region that helps us to plan and scheme. The superior and lateral surfaces of the frontal lobe are divided into superior, middle, and inferior gyri. The inferior gyrus is where we find Broca’s area (the region that enables us to form words). The ,most posterior gyrus of the frontal lobe (just anterior to the central sulcus) is known as the precentral gyrus (primary motor cortex).
This lobe lies posterior to the frontal lobe, and its most anterior gyrus (just posterior to the central sulcus) is known as the postcentral gyrus, or primary somatosensory cortex. The primary motor cortex (anterior to the postcentral gyrus) is arranged into a homunculus, or map of the body, where certain areas correspond to certain body areas. The face, tongue, and hands form the lateral surface of the brain, with the legs and genitalia forming the medial surface.
Association visual cortex - The posterior parietal cortex also assists the occipital lobe in enabling us to see. The basic concept of vision is something that the occipital lobe provides (see below), but the interpretation of movement, colour etc. is something for which the parietal lobe is required. The same is the case for hearing. The basic act of hearing is found in the primary auditory cortex in the temporal lobe. Contrarily, hearing and understanding voices (Wernicke’s area), as well as making sense of what we hear, is where the parietal lobe is important. The parietal lobe also contains Baum’s loop, which receives the inferior visual field from the superior retina and transmits it to the primary visual cortex of the occipital lobe.
The temporal lobe is found on the inferolateral side of the cerebrum. The Sylvian fissure is the large sulcus that separates the frontal and parietal lobes from the temporal lobe. The region is deep, and the insula is found within its recess. The insula can be visualized as the shield to the basal ganglia.
The primary auditory cortex is found in the subcortical layers of the temporal lobe and is the region of the brain that first allows us to hear. The auditory information generated by the cochlear nerve first travels to the cochlear nuclei, and to the superior olivary complex. The second order neurons travel to the lateral lemniscus, which sends the auditory information to other brainstem nuclei. The majority of brainstem nuclei then send the auditory information to the inferior colliculus, two small projections found on the posterior surface of the brainstem. The auditory information then travels to the medial geniculate nucleus of the thalamus and finally, to the primary auditory cortex. The temporal lobe is also where we find Meyer’s loop, which transmits the superior visual field (inferior retina) to the primary visual cortex.
The occipital lobe is located at the posterior surface of the cortex. The main role of the occipital lobe is that of the primary visual cortex, which is located in the depths of the calcarine sulcus.
The brainstem is the region that connects the cerebrum, cerebellum, and the spinal cord. It has three distinct parts, namely the midbrain, pons, and medulla.
This relatively small section of the brainstem has oculomotor nerves that arise from its anterior surface. The superior and inferior colliculi are found on its posterior surface, with the trochlear nerve emerging from the posterior surface of the midbrain, beneath the inferior colliculi. Two nipple-like projections, mammillary bodies, (hence the name) are round neural structures that project from the midbrain. They play a role in memory formation and are degraded in alcoholics with Wernicke’s encephalopathy.
The pons resides inferior to the midbrain. It has an anterior convexity where the middle, superior, and inferior cerebellar peduncles arise posteriorly. These peduncles are the main channels to the cerebellum and allow the relay of information between the structures. The trigeminal nerve arises from the anterolateral surface of the pons.
The medulla is the region of the brainstem inferior to the pons. It gives rise to numerous cranial nerves. On either side of the anterior midline, lie the pyramids, which allow the corticospinal tract to descend into the spinal cord. Lateral to the pyramids, we find the olives. They contain the superior and inferior olivary nuclei. The superior is involved in the perception of sound whereas the inferior mainly has a role in cerebellar motor learning. The inferior olive is further divided into the primary olivary nucleus, the dorsal accessory olivary nucleus, and the medial accessory olivary nucleus.The medulla also contains the vital respiratory centres.
Other important regions
The thalamus is the gateway to the cerebral cortex; it is the major sensory relay station of the brain. The two thalami are said to kiss across the third ventricle, resulting in the thalamic interconnexus. The lateral geniculate and medial geniculate nuclei are important nuclei that receive visual and auditory information respectively, before it is sent to the primary cortex of their respective senses.
This is the region of the brain that lies anterior and inferior to the thalami. It releases small concentrations of hormones that stimulates the pituitary gland to release hormones. These hormones enter the bloodstream and influence the endocrine organs of the body e.g. thyroid gland, adrenal gland, gonads etc.
This is the master endocrine gland of the human body. It sits in the sella turcica (meaning Turkish saddle, which it resembles). The anterior and posterior clinoid processes of the skull project around it, giving it the appearance of a four post bed. The sella turcica is subdural; found between the arachnoid mater and dura mater. The pituitary gland has an anterior and posterior lobe where each have their distinct functions.
The anterior lobe releases the majority of the hormones, which are:
- growth hormone
- luteinizing hormone
- follicle stimulating hormone
- thyroid stimulating hormone
- adrenocorticotropin releasing hormone
The posterior lobe releases only two hormones; oxytocin and vasopressin.
This structure is named after what is resembles; a seahorse when seen in cross section. It is involved in the formation of episodic memories, emotions (as part of the limbic system), and spatial navigation. Since it lies adjacent to the parahippocampal gyrus in the medial temporal lobe, it is also important in declarative memory formation. Medial to the hippocampus proper, we have the dentate gyrus (named after its resemblance to a tooth). The hippocampus proper continues under the dentate gyrus, and here is called the subiculum. Finally, the entorhinal cortex is continuous with the subiculum and resides in the medial temporal lobe. These structures play a role in the hippocampal formation.
This is the ‘danger detector’ and lies anterior to the hippocampus in the medial temporal lobe. It receives the olfactory input and also receives descending cortical input.
Papez circuit and the Limbic system
This is said to be the main pathway for the control of emotional expression. The loop involves the parahippocampal gyrus, which sends information to the entorhinal cortex in the medial temporal lobe. This then sends information to the subiculum via the hippocampus. The subiculum then connects with the mammillary bodies via the fornix, and then sends the information onto the anterior thalamic nucleus via the mammillothalamic tract. This then send the information to the cingulate gyrus (the anterior part of the cingulum), via the internal capsule. The cingulum is a ribbon-like strip of neural tissue that wraps around the corpus callosum. It is then able to send the information back to the parahippocampal gyrus to form the circuit.
These are nuclei that lie in the deep in the cerebral cortex, and have a role in cognition, movement, and emotion. The caudate nuclei (involved in the motor loop) lie in the wall of the third ventricle; along their course they form a large c-shape in profile. It lies superolateral to the thalamus in cross section.
The motor component is known as the corpus striatum, and consists of the putamen, globus pallidus interna and externa. In cross section, the putamen appears like to ice-cream in a cone, with the globus pallidus interna and externa forming the cone. The internal capsule separates the thalami from the motor nuclei. If you can see the thalamus, it is the posterior limb of the internal capsule. If you can’t see the thalamus, it is the anterior limb of the internal capsule. The subthalamic nuclei and substantia nigra (the pleasure centre) lie inferior to the thalamus, in the brainstem.
This comes from Latin meaning ‘little brain.’ It is important in coordinating the smoothness of our movements and our motor control. It receives its input from three cerebellar peduncles, which arise from the pons of the brainstem. They are the superior, middle and inferior, and are named by their location relative to the cerebellar vermis. The superior cerebellar peduncle is the main output of the cerebellum and sends information to the upper motor neurons of the cerebral cortex. The middle cerebellar peduncle receives input from the pontine nuclei that receives proprioceptive information from the lower limbs. The inferior cerebellar peduncle sends information to the vestibular nuclei and reticular formation. It receives input from the inferior olivary nucleus.
The cerebellum itself is subdivided into an anterior and posterior lobe by the primary fissure. The flocculonodular lobe lies posterior to the posterior fissure. However, the functional subdivisions of the cerebellum are best visualized medial to lateral. The middle of the cerebellum is the spinocerebellum; this region is involved in fine tuning movements of the limbs and body. The region receives input from the trigeminal nerve, visual and auditory systems, and proprioceptive input from the dorsal columns of the spinal cord.
The lateral parts of the cerebellum are termed the cerebrocerebellum. This region receives only cortical input and sends information to the ventrolateral thalamus. It is thought to be involved in movement planning and has a role in some cognitive functions. The vermis is the worm like narrow strip of tissue that protrudes along the midline.
The flocculonodular lobe is its own distinct entity and is also called the vestibulocerebellum. It is also called the archicerebellum as it is the oldest part of the three distinct functional areas.
The spinal cord is a continuation of the brainstem and resides in the vertebral canal. It is composed of numerous ascending and descending tracts, and has cervical and lumbar enlargements. These enlargements occur as this is where the plexi for the upper and lower limbs emerge from.
The spinal cord gives off 31 pairs of segmental mixed spinal nerves at each of the vertebral levels. These nerves divide into a ventral ramus and a dorsal ramus. The ventral rami join together to form plexi, which supply the limbs, and body wall. The dorsal rami provide sensation to the back. Each spinal nerve provides sensation to a dermatome (a specific area of skin), and also coalesce to form plexi.
In adults, the spinal cord terminates at vertebral level L1/L2. It typically terminates lower in children. The cone-like termination of the spinal cord is called the conus medullaris. This terminal region gives rise to a strand of non neural ligamentous tissue, which anchors the spinal cord to the sacrum, and is called the filum terminale.
The cauda equina (Latin for ‘horse’s tail’) is the final spinal nerves that emerge after the spinal cord has terminated. They reside in the vertebral canal, but are loosely floating in the cerebrospinal fluid.
Peripheral nervous system
Components & divisions
The peripheral nervous system consists of the all the neural tissues located outside the central nervous system:
- spinal nerves
- cranial nerves
The peripheral nervous system relays sensory and motor information. The motor division consists of the somatic nervous system and the autonomic nervous system. The latter is further divided into sympathetic and parasympathetic divisions.
These are the mixed nerves of the peripheral nervous system that emerge from the spinal cord and leave the vertebral canal via the intervertebral foramina. They are formed by the dorsal root ganglion (sensory input) and the ventral root ganglion (motor output). These two combine to form the mixed spinal nerve. Once the spinal nerve leaves via the intervertebral foramen, it divides to form a ventral and dorsal ramus. The ventral rami are the ones that join to form plexi. The main plexus that supplies the upper limb is the brachial plexus (ventral rami of C5-T1). The lumbar plexus (L1-L4) and the sacral plexus (L4-S4) supply the lower limb.
As their name suggests, these nerves emerge from the cranial region. These nerves are paired and numbered from 1 to 12. They perform various functions; both sensory and motor around the head and neck region, as well as the larynx and oesophagus. Some cranial nerves or their branches also serve a parasympathetic function. The mandibular nerve and glossopharyngeal nerve at the otic ganglion is an example.
The eye is technically a part of the brain, from which it develops. The outer surface of the front of the eye is called the cornea, which itself is covered with conjunctiva (a thin layer of cells). Deep to the cornea, we have the lens which adapts in shape for the focus required. This is possible because the papillary muscles of the eye attach to the lens via suspensory ligaments, which change the shape of the lens. The iris sits superficial to the lens. The cornea and lens are the refractive tools of the eye.
The eye has an anterior chamber (filled with aqueous humour) and a posterior chamber (contains the vitreous humour, a jelly-like substance). The three layers that surround the eye are the retina, sclera, and choroid. The inner layer of the eye is the retina. This layer consists of the rod and cone cells; they receive black and white, and colour information respectively. The macula is the region of the eye with the highest concentration of cone cells where it has the highest acuity. The fovea centralis is the central region of the macula, and provides us with our central vision, which we use to read and see fine detail. The choroid is the intermediate layer, which is rich in blood vessels. The sclera is the outer layer, and gives the eyeball its white appearance. The blind spot is the small region over the head of the optic nerve, where we are unable to see.
The pinna (auricle) forms the external ear. The helix is the outer region, and the antihelix is the indentation inside the helix. The tragus is the small cartilaginous prominence where the antitragus lies posterior to the tragus. The ear canal extends medially, and is part of the outer ear. The tympanic membrane forms the medial boundary of the outer ear.
The tympanic membrane is where the malleus ossicle articulates in the middle ear. The incus articulates with the malleus. Then, the stapes articulates with the round window, and the vibration of the eardrum causes the kinetic energy to translate with the movement of the ossicles. As a result of the movement of the round window, the movement propagates in the endolymph of the inner ear. This movement is translated into an electrical impulse via movement of potassium ions found in the organ of Corti in the cochlea.
The nose is a bony and cartilaginous structure. The proximal part of the nose (bridge) is formed by the nasal bones. The proximal part of the nasal septum is formed by the perpendicular plate of the ethmoid bone superiorly and the vomer posteroinferiorly. The septal cartilage attaches to these bones and forms the flexible part of the nose.
The nares (nostrils) are formed by the alar cartilage (both major and minor). The nasal septum gives rise to the superior, middle, and inferior conchae, with the meatuses (passages) lying in between them. The sphenoethmoidal recess lies above the superior concha which is closely associated with the olfactory bulb. The choanae (large openings) are found in the posterior opening of the nose, either side of the septum.
The nasal epithelium is specialised epithelium; the olfactory cells are bipolar cells that form the olfactory nerve which ascends through the cribriform plate (part of the ethmoid bone).
Salivary & lacrimal glands
The parotid gland is the largest salivary gland that sits on the lateral surface of the face beneath the skin. It overlies the masseter muscle where the branches of the facial nerve permeate the structure but do not innervate it. The parotid gland produces saliva that flows into Stensen’s duct, which opens up into the mouth opposite to the second upper molar.
The sublingual gland lies beneath the tongue and also opens up underneath the tongue in a row of openings on either side of the frenulum.
The lacrimal gland does not produce saliva, but produces tears. The gland lies on the superolateral border of the eye, and produces tears that flow to cover the eye, and prevent dryness. Tears also prevent infection and lubricate the eye.
All of the salivary glands are supplied by the facial nerve via the submandibular ganglion, with the exception of the parotid gland, which is innervated by the glossopharyngeal nerve.
Tongue and Mouth
The tongue is a muscular structure that lies in the floor of the mouth. It is covered in taste buds, and the posterior aspect is covered in circumvallate papillae.
The oesophagus is a muscular tube that connects the pharynx to the stomach. It enables food and fluids to enter the stomach through peristalsis. The lower oesophageal sphincter is formed by circular muscles in the wall of the oesophagus and the crural fibers of the diaphragm.
It is innervated by the oesophageal branches of the vagus nerve, where the sympathetic supply comes from the cervical and thoracic sympathetic trunk. The blood supply to the oesophagus arises from the:
- inferior thyroid arteries (in the superior section)
- bronchial arteries in the thoracic section
- left gastric and inferior phrenic artery branches of the thoracic aorta.
The mediastinum (anatomical location) is the space that lies in between the two lungs. The heart is found in the middle mediastinum. The heart is composed of endocardium, myocardium, and pericardium. It has four chambers; two atria and two ventricles. The left atrium receives oxygenated pulmonary blood via the pulmonary veins, whereas the right atrium receives the deoxygenated venous blood from the superior and inferior vena cava.
The left ventricle is thicker and more muscular than the right, as it must pump blood into the higher pressure of the systemic circulation (rest of the body). The output of the left ventricle is the aorta. The right ventricle simply pumps into the low pressure pulmonary vasculature via the pulmonary trunk. The beating of the heart is controlled by an electric system of nodes (sinoatrial node and atrioventricular node) and fibers (bundle of his, Purkinje fibers).
The coronary arteries supply the heart tissue, and arise from the cusps of the aortic valve. They fill in diastole, unlike other arteries. They drain into the right atrium via the coronary sinus.
The lungs are supplied by the pulmonary arteries which branch from the pulmonary trunk. The oxygenated blood flows from the lungs to the left atrium via the pulmonary veins. The right lung has three lobes; a superior, middle, and inferior lobe. The superior lobe is separated from the middle lobe by the transverse fissure. The oblique fissure separates the middle and inferior lobes. The left lung only has two lobes; a superior and inferior lobe. The left lung also creates space for the heart; it has a cardiac notch and lingula (anterior fold over the apex of the heart). Both lungs have a hilum at their medial border to accommodate the structures going to and from the lungs.
The thyroid gland lies in the cervical region at the anterior region of the neck. It lies below the thyroid cartilage and has two lobes either side of the midline that are connected by a midline isthmus. It produces thyroid hormones (T4 and T3) that influence metabolic rate, growth, and various other functions. The thyroid is largely composed of follicles, which absorbs iodide from the blood. The follicular cells produce the hormones whereas the parafollicular cells produce calcitonin.
This gland lies in the superior thorax and is the location of T lymphocyte maturation. It enlarges in childhood and atrophies during puberty.
The oesophagus flows into the stomach. The stomach lies inferior to the diaphragm and has a lesser and greater curvature. The cardia is the section of stomach that lies near the the oesophagus’ passageway through the diaphragm. The fundus is the superior curvature, and the body forms the majority of the size of the stomach. The antrum is the inferior section of the stomach, before the pylorus. The pylorus churns the food before it enters the small intestine. The stomach contains gastric acid and enzymes which degrade food and eliminate the majority of pathogens.
The initial section of the small intestine is called the duodenum. This is a c-shaped loop of intestine that houses the major duodenal papilla which receives the biliary system secretions.
The duodenum connects to the jejunum. This forms two thirds of the small intestine and has a large lumen. It has numerous folds on its inner surface and is the location of the majority of absorption of nutrients and minerals.
The last section of the small intestine is the ileum. This is a narrower longer section of the small intestine (it forms the remaining two thirds). The ileum is where the majority of fat absorption takes place. The ileum terminates at the ileo-caecal junction, which acts as a valve and prevents backflow from the caecum. The terminal ileum is where vitamin B12 and bile salts are absorbed.
The caecum is the first part of the large intestine and gives rise to the appendix. The caecum then joins the ascending colon (runs superiorly on the lateral right side). The hepatic flexure is where the large bowel turns and runs horizontally (towards the left side), and is termed the transverse colon. This part of the colon then turns again at the splenic flexure, and descends (inferiorly on the lateral left side). It is renamed the descending colon here. Then, it forms the sigmoid (s-shaped) colon which becomes the rectum.
The rectum is the last section of bowel before it becomes the anus. It receives its blood supply from the superior and middle rectal arteries (branches of the inferior mesenteric artery and internal iliac artery). The superior artery supplies the first two thirds of the rectum, and the middle artery supplies the last third. The venous drainage is unique, in that above the pectineal line, it drains into the portal venous system via the inferior mesenteric vein, and below the pectineal line, drainage occurs into the internal iliac vein.
The anus is the last part of the digestive system that joins the rectum into an external opening. The anus serves to control the expulsion of feces.
The liver has a right lobe and a left lobe. It also has a caudate lobe (superior lobe) and a quadrate lobe (inferior lobe). The liver is the largest organ in the abdomen. It receives blood from the hepatic artery (25%) and portal vein (75%). It also gives rise to the biliary system. These three structures form the hepatic triad, and branches of each supply/drain each hepatic lobule.
The pancreas lies in the epigastrium. It has a head (which sits within the c-shaped curvature of the duodenum) a body, and a tail (which reaches the hilum of the spleen). The pancreas has both endocrine and exocrine function. These include the release of insulin and pancreatic enzymes, respectively. The pancreatic duct joins the common bile duct to form the ampulla of Vater, which empties at the major duodenal papilla.
The spleen lies in the left hypochondrium. It is a rounded spongy organ that acts as a blood filter. It has a rich blood supply from the tortuous splenic artery and acts as a reservoir for monocytes (the non circulating form of macrophages).
The kidneys are paired organs that lie in the paracolic gutters. The outer portion is termed the cortex, and the inner is termed the medulla. The kidneys have a rich blood supply as well and filter the waste products out of the blood through ultrafiltration. They also reabsorb important nutrients such as glucose and ions (sodium and potassium). In addition, they balance acid and bases, synthesize 25-cholecalciferol, and stimulate red blood cell synthesis (via erythropoietin).
Pelvis and external genitalia
The bladder is essentially a muscular balloon which stores and expels urine. The detrusor forms the muscular layer, and the bladder neck (internal sphincter) is under non-voluntary control. The external sphincter is under voluntary control by the pudendal nerve. The internal surface of the bladder is where the trigone is found. The ureteric orifices are found at the two superior corners of the trigone. This has pressure detectors. The internal bladder surface is rough as it is covered in folds known as rugae.
The urethra is a tube-like structure that connects the bladder to the external urethral orifice in order to expel urine.
Female reproductive system
This is also known as the womb. It is lined by the endometrium, which is the lining in which the embryo will implant in normal pregnancy. The myometrium forms the muscular wall of the uterus, with the perimetrium surrounding the organ.
These tubes connect the uterus to the ovaries. They have fimbriae (finger like projections) at their lateral ends, and an infundibulum just proximal to the fimbriae. The fimbriae assist in collecting the ovum (egg) from the ovary. The ampulla is the section just proximal to the infundibulum, which narrows to the isthmus at the medial end.
These are the female gonads and they produce oestrogen. They also contain the eggs, which are released monthly (ovulation) in women.
The cervix is the inferior segment of the uterus. It has numerous crypts, as well as an an internal os (the internal opening) and an external os (the external opening into the vagina).
This is a muscular tube that connects the cervix to the external opening.
Male reproductive system
This is a walnut sized organ found in men inferior to the bladder neck. It produces fluid which contributes to the semen. The central part is known as the transition zone, and the outer part is known as the peripheral zone. The point at which the seminal vesicles and vas deferens input into the prostate is called the verumontanum.
This is part of the external male genitalia. It has a root, shaft, and glans (distally). The foreskin overlaps the glans in uncircumcised men.
The testes hang in the scrotum. This to keep them at a cooler temperature than the body, which is required for optimal sperm production. The posterior aspect of the testis is where the epididymis is found. This is a tightly coiled tubular structure that is renamed the vas deferens once you move superiorly. The vas deferens then connects to the prostatic urethra, and opens at the verumontanum.
This is the most superficial layer of the skin and is subdivided into 5 layers. These are (from superficial to deep) the:
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum basale
The stratum lucidum is only found on the palms and soles (glabrous skin). The corneum has the cornified layer on its surface, which protects the skin from minor abrasion. The basement membrane then anchors the epidermis to the dermis.
This middle layer is subdivided into the papillary and reticular dermis. The papillary dermis is the more superficial layer and is closely associated with the basement membrane. The reticular dermis is looser and forms a larger volume of the dermis. There are numerous appendages such as sweat glands and hair follicles anchored in the dermis.
This layer, also called the hypodermis, is deep to the dermis. It is the layer in which we find the blood vessels and relatively large amounts of fat.
The skin acts as a barrier to infection, temperature regulating organ as well as a site of vitamin D synthesis.