A-Z of medical science

A is for Artery

The best definition of an artery is the blood vessel which LEAVES the heart. If you say that you can’t go wrong! MOST of our arteries do carry oxygen–rich blood under high pressure which is delivered to all the organs of the body- but this isn’t the correct definition!  There are apparently 55-60 000 miles of blood vessels in the average body - so approximately half are arteries. The rest are veins! Arteries have to have very strong muscular walls to withstand the pressure of the heart beating and sending the blood round the body and they have to remain intact to prevent loss of blood. 

B is for Bladder

It is that specialised container for urine found in the pelvic cavity and you may be surprised to learn that it doesn’t hang downwards in your body but lies sideways like a rowing boat. It is made of specialised cells (transitional epithelium) to allow it to expand (and not burst!) and can contain up to 300-400 ml of urine (and never ten pints of beer!) It is a myth that bladder incontinence is a natural consequence of ageing but it is certain that Pilates does wonders for the pelvic floor.

C is for Cartilage

It is a tough, fibrous connective tissue also known as ‘gristle’. This is the template material for all bones in the body. Did you know you have 206 bones and many are not ossified (i.e. made bony) until after birth? Bone cells grow on and into the cartilage until they are imprisoned there to give a rigidity and protection to the bony skeleton. Places that don’t ossify are the ‘bones’ of the nose which is just as well when you think how many times you bash your nose throughout life!

D is for Dermis

This is the mysterious layer under the epidermis (epi=on top of- see what happened there?) of the skin which covers your whole body and which contains myriad secret components like hair follicles and sweat glands and pain sensors, along with nerve endings, blood capillaries and fat cells. In fact, did you know that the average adult, as it says in books, apparently has 20 000cmof skin and a piece of skin about the size of a 2p coin has over a million cells, a metre of blood vessels, 80 sweat glands and at least 35 nerve endings in it? The region below the dermis is where all the fat associated with the skin and appearance is laid down.

E is for Eardrum

This is the membrane separating the outer ear from the middle ear upon which the ear bones rattle to help amplify the sounds we hear. You know about your eardrum when you get an infection as the pressure builds up behind it and you get excruciating pain. It may even rupture and scar when it repairs and this can affect your hearing in later life. Otorhinolaryngologists (ENT or ear nose and throat surgeons to you and me!) love to put grommets (which are tiny tubes) in your eardrum to facilitate draining if you have ‘glue ear’ but many fall out before they are any use. Any mechanical damage to the ear drum can be the cause of deafness.

F is for Fimbriae

These are the wavy ends of the 2 fallopian (uterine) tubes attached to the uterus and they are solely responsible for making sure the egg which ‘hatches’ from the ovary actually goes in the right direction ready for fertilisation. Without fimbriae your egg may end up in the body cavity and if the egg is fertilised there you may have an ectopic pregnancy. Sometimes the fimbriae from one fallopian tube bend right round to the opposite ovary to grab the egg there. You can see some lovely time-lapse photography of that occurring.

G is for Gland

By definition a gland is a collection of (epithelial) cells which produce a fluid either into the body - usually into the blood stream - or on to the body surface. So we have endocrine glands producing substances (hormones) into the circulation and exocrine glands producing fluids on to a surface of the body. Sweat glands in the dermis are examples of exocrine glands. The pancreas (see P) is an example of an endocrine gland, which produces the hormones insulin and glucagon into the blood stream to help maintain blood sugar balance. It also functions as an exocrine gland as you will see. There are loads more exocrine glands than endocrine glands in the human body.

H is for Heart

The heart is such an excellent pump that no engineer has managed to make anything like it to beat continuously for maybe 100 years without breaking down! It is so clever that it even manages its own basic rate automatically so you don’t have to set it. Most hearts beat at about 72 times a minute but that rate can be affected by caffeine, alcohol, cannabis and emotional upset and some people need to have an artificial pace maker inserted to make sure it beats regularly and smoothly. It has its own blood supply too, carried in coronary vessels which wrap around the outside of the heart. We know about these when they get clogged up with cholesterol and it becomes painful for the blood to flow through them when we exert ourselves. This is called angina and if these vessels get blocked cardiac surgeons can replace them with veins from the leg or arm! How lucky is that!

I is for Iris

This is the coloured part of the eye. In fact it is a flat, coloured, ring-shaped membrane behind the cornea of the eye, with an adjustable circular opening (pupil) in the centre. Contraction and expansion of the iris controls the size of the pupil which allows light to enter the eye and reach the retina. The colour of the iris is genetically determined by the distribution of the pigment melanin. Our ancestors would all have had a dark brown iris but various mutations have arisen to provide the spectrum of colours through hazel to blue and green. Recent developments in security measures (at airports etc) involve iris scanning because the iris has about 256 unique characteristics, unlike finger tips which have about 40.

J is for Jaw

This is the essential place for making our food small enough to swallow by means of teeth embedded in the bones of the jaws. In the child there are 20 teeth in total in the top and bottom jaws and in adults there are 32. The lower jaw or mandible is the strongest bone in the skull and is the only one that moves by means of muscles attached at the temporomandibular joint or TMJ. That’s the one that hurts when you yawn too hard! The mandible also plays a vital part in speech and facial expression - think of all those emojis! The upper jaw bone is called the maxilla and is a fixed fused bone. The largest jaw bone in the natural world belongs to the Blue Whale, which is quite surprising when you consider that it filter feeds mainly on krill - by the hundreds of gallons!

K is for Kidney

We usually have 2 bean-shaped kidneys and these are the place where all the blood contents are filtered and sorted for being kept or discarded. Discarded blood contents are excreted in urine. Imagine that everything in your blood stream goes through a very fine filter (with a surface area similar to that of a tennis court) which in general doesn’t get blocked! I think of it being a bit like a coffee cafetiere or French press. It is amazing! How well your filter works is associated with your blood pressure and the health of the kidney cells and structures called nephrons. If you have consistently high blood pressure you may have diseased kidneys. When the kidney filtering mechanism fails we become toxic very quickly and may even die.

L is for Lungs

We have 2 lungs positioned either side of the heart. They are protected by our ribs and are like bags filled with air. When you see lungs in the flesh they look like pink blancmange when healthy but can be stained black by the tar in cigarettes. They hold 2 litres of air each when healthy but can be easily damaged by inhaled pollution. They can also be easily punctured if you get stabbed in the neck above the collar bone which, apparently, is a ‘common’ injury at football matches. Vital exchange of oxygen and carbon dioxide occurs on the alveoli in the lungs which can provide an enormous surface area (let’s say equivalent to a tennis court!) for gas exchange when healthy. When active we may need 160 litres of air per hour!

M is for Membrane

Membranes are like barriers or the border control between areas of the body or between cells. They are SO important for making sure that things stay where they should in cells, tissues and organs. They are composed of phospholipids with proteins embedded in the matrix and are kept flexible by the inclusion of cholesterol molecules. Information is passed between cells by complex cell signalling mechanisms, which are thought to originate on the molecules embedded in membranes. Most membranes are double, like a sandwich, and hydrophobic on the outside and hydrophilic on the inside.

N is for Nostrils

The two openings in the nose are called nostrils, or nares. They lead to two nasal cavities that are separated by the septum, a wall of cartilage. This nasal septum can be severely damaged by cocaine taken via the nose. The nostrils are lined with cilia, which function to waft and effectively sieve the air we inhale as a primary defence mechanism to protect our delicate lung tissue. When air is inhaled through the nostrils, it travels through the ciliated nasal passages, where it becomes warmed and moistened, and then travels on through the choanae, the nasopharynx, the oropharynx and the voice box before ending up in the lungs.  All of these areas are also responsible for smelling, tasting, hearing and immune system defence. Every 24 hours 1.5litres of mucus are produced in the goblet cells of the nostrils.

O is for Ossicle

An ossicle is a tiny bone. The three smallest bones in the body are in the ear and connect with the ear drum so that we can hear sounds. The ear ossicles are called the hammer, the anvil and the stirrup because that’s what they resemble. (Everything in anatomy is named by description - the problem is the description is often Latin or Greek which most people don’t understand these days!) The stirrup is the smallest bone in the body at 3 x 2.5mm, which is useful to know for a pub quiz. (The femur (the leg bone) is the largest bone by the way.)

P is for Pancreas

It is such an important organ and only weighs 60g! It has exocrine and endocrine functions essential for life. It nestles between the curve of the duodenum in the small intestine where it can easily release its products for maximum effect. The exocrine function (pouring chemicals on to the internal surface of the alimentary canal) produces a mixture of digestive enzymes and their precursors, ready for the last stage of digestion of all food groups. The endocrine function is vitally important in the regulation of blood sugar levels in the body. The two hormones insulin and glucagon produced in the cells of the pancreas maintain blood sugar levels. Insulin facilitates removal of excess glucose from blood circulation for storage in the liver as glycogen. Glucagon acts on the stored glycogen to convert it back to glucose when levels drop. Clever stuff!

Q is for Quadriceps

Very well-known to athletes as ‘quads’ this group of 4 muscles collectively known as the quadriceps femoris act together to extend the knee joint. Together they form the strongest and leanest muscle in the body at the front of the thigh. The hamstrings are three muscles at the back of the thigh which work together to bend the knee. These two muscle groups work in opposition to each other, though they tend to have a dysfunctional relationship. The quadriceps must shorten for the hamstrings to lengthen, and vice versa. All too often this fails to happen and this failure results in pulled hamstring muscles. Most people stand in such a way that their thighs lean forward, which puts a lot of pressure on the quadriceps and can chronically shorten the hamstrings.

R is for Retina

This is the innermost lining of the eye and covers three quarters of the eyeball. It is an extremely delicate structure composed of light-sensitive cells which are responsible for how we see things. In order to see, light reflected from objects within the visual field is focused on the retina by refraction through the lens. Effective vision is dependent on coordination of the lens, its muscles and ligaments and the cells of the retina. There are four types of light-sensitive receptor cells in the retina - three types of cones which are responsible for colour vision (red, green, blue) and rods which are sensitive to light and dark and give us our visual acuity. Most people have about 120 million rods and about 6 million cones. Someone who is colour-blind may only have one or two types of cones. Colour vision is either absent or limited in cats and dogs - so much for that pink blanket you bought for your pet!

S is for Spleen

This is the largest of our lymph organs and is easily physically damaged. It acts as a reservoir of red blood cells and platelets in case of haemorrhage. It also distinguishes old or worn-out red blood cells, takes them out of circulation and recycles their components. Red blood cells on average only last about 120 days in circulation. The vital component of red blood cells - the haemoglobin - is dismantled in the spleen so that the valuable iron can be recycled and stored in the liver. Other breakdown products of red blood cells are also transported to the liver - such as bilirubin, which becomes a component of bile. It is also an invaluable site for production of T and B lymphocytes which are the white blood cells essential in the immune system. Formation of red blood cells (erythropoiesis) occurs here in the young and may continue into maturity.

T is for Testes

These paired organs are the male reproductive organs and are held in the scrotum outside the body so that they are at the optimum temperature for sperm formation and development. This is three degrees less than normal body temperature. Consequently, men are encouraged to wear boxer shorts, rather than Y fronts. Each testis is made up of 200-300 lobes with loops of seminiferous tubules within them. These are the site of production of millions of sperm throughout the life of a male from puberty onwards. For reproductive success these sperm are nourished by fluids produced en route from the testis to the penis by other organs associated with the male reproductive system. This whole package contributes to the fertility of the male and can be optimised by diet and nutritional supplements. An average male can (apparently!) produce 525 billion sperm over his life time. If a man produces less than 39 million sperm per ejaculation there may be infertility issues.

U is for Uvula

This is a dingly-dangly structure hanging from the back of the throat, between the two visible tonsils, made of muscle and covered in mucus membrane. No one is quite certain what its function is - in fact it features quite often in exams for medical students. Even they call it ‘that dingly-dangly structure of unknown function’ but perhaps it plays a critical role in the gag reflex. Maybe it plays an important role in keeping the mouth moist for speech. There was a time when it was thought to be responsible for snoring and sleep apnoea. The term for the removal or remodelling of the uvula (which could happen in cases of snoring) is longer that it is! (Uvulopalatopharyngoplasty - another useful term for the pub quiz!)

V is for Villus

To my surprise I have noticed that everyone who has done basic biology knows about a villus even if they know nothing else! The task of villi (plural) is to increase the surface area of the small intestine enormously to optimise absorption of food. They are tiny finger-like projections (1-2mm long) of the mucosal layer in the small intestine. Each boundary cell of a villus is covered by a brush border of even smaller projections called microvilli (1 micron long). Their presence increases the surface area of 0.5m square of small intestine to about the size of - guess what - a tennis court, or 250 square metres. The villi protrude into the lumen of the small intestine where food has been broken down into its smallest components ready for absorption. Water soluble substances can pass into the blood capillaries in the villi, whereas fat soluble substances can enter the lymphatic vessels within each villus. All nutrients are then transported via the hepatic portal system to the liver to ensure that nothing toxic enters the circulation.

W is for White Blood Cells

Also known as leucocytes, they have several important roles to play in the protection and defence of the body. They form the basis of the immune system with a complex and wonderful array of unique lymphocytes, as well as having cells which act as gobbler cells (phagocytes), which clean up any cell debris or invading opportunists. Unlike red blood cells, which stay within the circulation, white blood cells travel around using the blood vessels as a motorway system. They can then escape to where they are needed and maybe initiate an inflammatory response to protect the  tissues and organs.  Lymphocytes come in two populations -T and B lymphocytes - with very significant responsibilities in the immune response and auto-immune disorders. A healthy person should have about 4-11 x 10 (to the power of 9) white blood cells per 1ml of blood.

X is for X Chromosome

X-chromosome is one of two kinds of genetic material conferring sexual identity to the individual. There are two X chromosomes in a female human, as a pair, which are distinct in shape (X) from the 22 other pairs of autosomes. The genes on the X chromosomes code for female sexual characteristics, and also the ‘where did I put my car keys?' scenario! A person with Turner syndrome, in which there is only one X chromosome, does develop female genitalia but the ovaries fail to develop in the foetus and the person is infertile.

Y is for Y Chromosome

Y chromosome (because of its shape) confers maleness in the human. One Y chromosome is paired with one X chromosome, which complements the 22 pairs of autosomes. So someone with XY chromosomes is male and someone with XX chromosomes is female. Rather like the car keys scenario for bearers of XX chromosomes, the Y chromosome confers the ‘where is the butter in the fridge?’ scenario in males! If someone has XXY sex chromosomes, due to a genetic mismatching, they are suffering from Klinefelter’s syndrome. They are born with underdeveloped male genitalia but develop female secondary sexual characteristics at puberty, unless they are given testosterone treatment.

Z is for Zygote

Zygote is the name for the production of successful fertilisation of 2 gametes : an ovum from the female with ONE sperm from a male. These gametes were haploid (n) produced by meiosis, and fertilisation means that the zygote is diploid (2n) with combined chromosomes from the male and the female. This is the first stage of producing a new human with mixed characteristics from both parents. Fertilisation usually takes place in the uterine tubes and the zygote is moved along to the uterus by gentle peristalsis where it eventually embeds. Repeated rapid mitotic divisions of the zygote produce the tissues which make up the foetus, its amniotic sac and the placenta, which protect it during the 40 weeks of gestation.

The A-Z of Medical Science. Fun facts about the human body.