Bio 6.1-6.3

Question 1

Name all parts of digestive system

Answer 1

Mouth
Esophagus
Stomach
Small intestine
Pancreas
Liver
Gall bladder
Large intestine
Anus

Question 2

Function of mouth

Answer 2

Voluntary control of eating and swa lowing. Mechanical digestion of food by chewing and mixing with saliva, which contains lubricants and enzymes that start starch digestion

Question 3

Esophagus function

Answer 3

Movement of food by peristalsis from mouth to the stomach

Question 4

Stomach function

Answer 4

Churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of protein digestion

Question 5

Small intestine function

Answer 5

Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralizing stomach acids, plus absorption of nutrients

Question 6

Pancreas function

Answer 6

Secretion of lipase, amylase and protease

Question 7

Liver function

Answer 7

Secretion of surfactants in bile to break up lipid droplets

Question 8

Gall bladder function

Answer 8

Storage and regulation of the release of bile

Question 9

Large intestine function

Answer 9

Re-absorption of water, further digestion especially of carbohydrates by symbiotic bacteria, plus formation end storage of feces

Question 10

Name all building blocks of the wall of small intestine

Answer 10

Serosa
Muscle layer
Sub - mucosa
Mucosa

Question 11

Serosa what is it?

Answer 11

On outer coat of small intestine wall

Question 12

Muscle layer of small intestine

Answer 12

Longitudinal muscle and inside it circular muscle

Question 13

Sub-mucosa

Answer 13

A tissue layer containing blood and lymph vessels

Question 14

Mucosa

Answer 14

The lining of the small intestine, with the epithelium that absorbs nutrients on its inner surface

Question 15

Structure of small intestine

Answer 15

The wall is made out of layers of living tissues, which are usually quite easy to distinguish in sections of the wall. Folds are visible on the inner surface and on those folds are finger-like projections of the mucosa called villi. A villus is between 0.5 and 1.5 mm long and there can be os many of them as 40 per millimetre square of small intestine wall. On a cross section the mucosa is stained darker than the sub-mucosa

Question 16

Peristalsis

Answer 16

Waves of muscle contractions that pass along the small intestine. Both sets of muscles work, the circular muscles contrite to stop the food from being pushed back towards the mouth. Contraction of the longitudinal muscle where the food is located moves it dang the gut. The contractions are involuntary, not controlled by the brain but the enteric nervous system, which is extensive and complex. One continuous peristaltic wave moves the food from esophogus to stomach.The movement is unidirectional. Thus when vomiting abdominal muscles are used rather than these. The process of movement of food through intestines is slow ( food only moves a few centimetrens at a time) to allow for digestion. The main function of peristalsis is churning of the semi - digested food to mix it with enzymes and thus speed up the process of digestion.

Question 17

Pancreatic juice content

Answer 17

All three main types of macromolecules.
Amylase: to digest starch
Lipase: to digest triglyceride, lipids
Proteases: to digest proteins

Question 18

How does pancreas work?

Answer 18

Pancreas contain two types of gland tissues. Small group of cells secrete the hormones insulin and glucagon into the blood. The reminder of me pancreas synthesizes and secretes digestive enzymes into the gut in response to eating a meal. This is mediated by hormones senthesized and secreted by the stomach and also enteric nervous system. Small groups of gland cells cluster round the ends of tubes called ducts, into which the enzymes are secreted.

Question 19

The production of pancreatic juice

Answer 19

The digestive enzymes are produced in pancreatic gland cells ou the ribosomes on the rough endoplasmic reticulum. Then they are processed in Golgi apparatus and secreted by exocytosis. Ducts within the pancreas merge into larger ducts finally forming one pancreatic duct, through which about a litre of pancreatic juice is secreted a day into the lumen of the small intestine.

Question 20

What happens to starch, triglycerides, phospholipids, and proteins when digested in the small intestine

Answer 20

Through nyobolysis reaction the following products are created:
Starch → maltose by amylase
Triglycerides → fatty acids, glycerol and phosphate by phospholipase
Proteins and polypeptides → shorter peptides by protease
Then further digestion occurs
Maltose → glucose by Maltase
DNA and RNA → nucleotides by nucleases
Lactose → glucose and galactose by lactase
Sucrose → glucose and fructose by sucrase
Peptides → removing chain by chain of the amino acids either from the amino terminal of the chain until only a Dipeptide is left by proteases called Exopeptidase’s
Dipeptides → amino acids by dipeptidases
There are ones that are not digested such as cellulose (main component of dietary fibre) because the human body cannot produce enzymes to break them down.

Question 21

Villi and surface area

Answer 21

Villi are used for absorption. The rate of absoubtion is largely dependent on the surface area of the epithelium that carries out the absorption. The small intestine is large however the folds maximize its surface area and vili by themselves increase it by a factor of about 10.

Question 22

Absorption by villi

Answer 22

The epithelium that covers villi acts as a barrier to prevent harmful substances from entering the blood system but allows useful nutrients to pass through.
Macromolecules absorbed:
• glucose, fructose, galactose and other monosaccharides
•any of me twenty amino acids used to make proteins
• fatty acids, monoglicerides and glycerol
• bases from digestion of nucleotides
Other non-digested substances:
• mineral ions
• vitamins such as vitamin C
Harmful substances ove removed from the blood and detoxified by liver. The non-toxic but useless substances are excreted in urine. Some bacteria pass through the epithelium but one quickly removed from the blood by phagocytic cells in the liver.

Question 23

Methods of absorption

Answer 23

Different methods of membrane transport are required to absorb different nutrients.
- Triglycerides must be digested before they can be absorbed. The products of the digestion, which are fatty acids and monoglicerides can be absorbed into villus epithelium cells by simple diffusion as they can pass between phospholipids in the presume membrane
-fatty acids are also absorbed by facilitated diffusion as there are fatty acid transporters, which are proteins in the membrane of the microvilli
- once inside the epithelium cells, fatty acids are combined with monoglycerides to produce triglycerides, which cannot diffuse back out into the lumen
- triglycerides coalesce with cholesterol to form droplets with a diameter of about 0.2 μm, which become coated in phospholipids and protein
-These lipoprotein particles are released by exocytosis through the plasma membrane on the inner side of the villus epithelium cells. They can either enter the lacteal and are carried away in the lymph or enter blood capillaries in the villi
- glucose connot pass through the plasma membrane by simple diffusion because it is polar and thus hydrophilic
- sodium-potassium pumps in the inward-facing part of the plasma membrane pump sodium ions by active transport from the cytoplasm to the interstitial spaces inside the villus and potassium ions in the opposite direction. This creates a low concentration of sodium ions inside villus epithelium cells.
-Sodium- glucose co-transporter proteins in the microvilli transfer a sodium ion and a glucose molecule together from the intestinal lumen to the cytoplasm of the epithelium cells. This type of facilitated diffusion is passive but it depends on the concentration gradient of sodium ions created by active transport.
- glucose channels allow the glucose to move by facilitated diffusion from the cytoplasm to the interstitial spaces inside the villus and on into blood capillaries in the villus.

Question 24

Starch digestion in the small intestine

Answer 24

What does it show?
- catalysis, enzyme specificity and membrane permeability.
What is starch?
Macromolecule, composed of many alpha glucose monomers linked together in plants by condensation reaction. They cannot pass through the membrane, thus they must be digested in the small intestine to allow absorption. All of the reactions involved in the digestion of starch are exomhermic, but without a catalyst they happen of very slow rates.
There are two types of molecules in starch:
- amylose mes unbranched chains of alpha glucose linked by 1,4 bonds;
- amylopectin nas chains of alpha glucose linked sy 1,4 bonds with some 1,6 bonds that make the molecule branched
The enzyme beginning me digestion of both forms of starch is amylase. Saliva contains amylase but most of the digestion happens in small intestine, catalysed by pancreatic amylase. Any 1,4 bond in starch molecule can be broken by this enzyme, as long as there is a chain of at least four glucose monomers. Amylose is therefore digested into a mixture of two- and three- glucose fragments called maltose and meltotriose.
Because of specificity of me active site amylase cannot break 1,6 bonds in amylopectin. Those fragments one called dextrins. Digestion of starch is completed by three enzymes in the membranes of microvilli on villus epithelium cells. Maltase, glucosidase, and dextrinase digest maltose , maltotriose and dextrins into glucose.
Glucose is then absorbed into villus epithelium cells by co-transport with sodium ions. it then moves by facilitated diffusion into the fluid in interstitial spaces inside me villus. The dense network of capillaries close to the epithelium ensures that glucose only has to travel a short distance to enter the blood system. Capillary walls consist of single layer of thin cells, with pores between adjacent calls, but these capillaries have larger pores then usual aiding the entry of glucose.
Blood carrying glucose and other products of digestion flows through villus capillaries to venules in the sub-mucosa of the wall of the small intestine. The blood in these venules is carried Via hepatic portal vein to the liver, where excess glucose is absorbed by liver cells and converted to glycogen for storage. Glycogen is similar in structure to amylopectin but with more 1,6 bonds this more branching.

Question 25

Dynamic gastric model

Answer 25

Computer controlled model of the human stomach that carries out the mechanical and chemical digestion real food samples. it can be used to investigate the effects of diet, drugs, alcohol and other factors on digestion.

Question 26

Dialysis tubing

Answer 26

Made of cellulose. Pores in the tubing allow water and small molecules on ions to pass through freely, but not large molecules. This properties mimic the wall of the gut, which is also more permeable to small rather than large particles.it can be used to model absorption by passive diffusion and by osmosis, it cannot model active transport and other processes that occur in living cells.

Question 27

Discovery of blood circulation and heart being a pump

Answer 27

William Harvey - demonstrated that blood flow through the larger vessel is unidirectional, with valves to prevent back flow. He also showed that the rate of flow through major vessels was far too high for blood to be consumed in the body after being pumped out by the heart, as earlier theories proposed.it must therefore return to the heart and be recycled. Harvey showed that the heart pumps the blood out in the arteries and it returns in veins. He also predicted the existence of numerous small vessels which were unable to be seen under contentporary equipment, that would link arteries to veins. Those were later discovered and named capillaries.

Question 28

Main pumping chambers of the heart

Answer 28

Ventricles
They have thick strong muscle in their walls that pumps blood into the arteries, reaching a high pressure at the peak of each pumping cycle.

Question 29

Arteries

Answer 29

Vessels that convey blood from the heart to the tissues of the body. The artery walls work with the heart to facilitate and control blood flow. Elastic and muscle tissue in the walls are used to do this.
Elastic tissue contains elastin fibres, which store the energy that stretches thee at the peak of each pumping cycle. Their recoil helps propel the blood on down the artery. Contraction of smooth muscle in the artery wall determines the diameter of the lumen and to some extent the rigidity of the arteries, thus controlling the overall flow through them.
Both the elastic and muscular tissues contribute to the toughness of the walls, which have to be strong to withstand the constantly changing and intermittently high blood pressure without bulging outwards (aneurysm) or bursting. The blood’s progress along major arteries is thus pulsatile, not continuous. The puls reflects each heartbeat and can easily be felt in arteries that pass near the body surface, including those in the wrist aut the neck.
Each organ of the body is supplied with blood by one or more arteries. For example, each kidney is supplied by renal entry and the liver by the hepatic artery. The powerful, continuous active muscles of the heart itself are supplied with blood by coronary arteries.

Question 30

Walls of the artery

Answer 30

-Tunica externa: a tough outer layer of connective tissue
- Tunica media: a thick layer containing smooth muscle and elastic fibres made of the protein elastin
- Tunica intima: a smooth endothelium forming the lining of the artery

Question 31

Aorta

Answer 31

The main artery that supplies oxygenated blood to the tissues of the body. Aorta starts of the heart and continues through the thorax and abdomen, with branches serving the liver, kidneys intestines and other organs.

Question 32

Systolic pressure

Answer 32

Peak pressure reached in an artery, it pushes the wall of the artery outwards, widening the lumen and stretching elastic fibres in the wall, thus storing potential energy.

Question 33

Blood pressure in arteries

Answer 33

At the end of each heartbeat the pressure in the arteries falls sufficiently for the stretched elastic fibres to squeeze the blood in the lumen. This mechanism saves energy and prevents the minimum pressure inside the artery, called the diastolic pressure, from becoming too low. Because it is relatively high, blood flow in the arteries is relatively steady and continuous although driven by a pulsating heart.
The circular muscles in the wall of the artery form a ring so when they contract, in a process called vasoconstriction, the circumference is reduced and me lumen is narrowed. Vasoconstriction increases blood pressure in the arteries. Branches of arteries called arterioles have a particularly high density of muscle cells that respond to various hormone and neural signals to control blood flow to downstream tissues. Vasoconstrictions of the atrioles restricts blood flow to the part of the body that they supply and the opposite process called vasodilation, increases it.

Question 34

Capillaries.

Answer 34

Blood flows through tissues in capillaries with permeable walls that allow exchange of materials between cells in the tissue and the blood in the capillary.
The narrowest blood vessels with diameter of about 10pm. They branch and rejoin repeatedly to form capillary network with huge total length. Capillaries transport blood through almost all tissues in the body. Two exceptions being lens and the cornea in the eye which must be transparent so cannot contain any blood tissue.
Capillary wall consists of one layer of very thin endothelium cells, coated by filter-like protein gels, with pores between the cells. The wall is thus very permeable and allows pert of the plasma to leak out and form tissue fluid.
The permeabilities of capillaries differs among tissues, enabling particular proteins and other large particles to reach certain tissues but not others. Permeabilities can also change over time and capillaries repair and remodel themselves continually in response to the needs of the tissues that they perfuse.

Question 35

Tissue fluid

Answer 35

Plasma in which the blood cells are suspended. Tissue fluid contains oxygen, glucose and all other substances in blood plasma apart from large protein muscles, a nice cannot pass through the capillary wall. The fluid flows between the cells in a tissue, allowing the cells to absorb useful substances and excrete waste products. The tissue fluid then re-enters the capillary network.

Question 36

Veins

Answer 36

Collect blood at low pressure from the tissues of the body and return it to the atria of the heart. Veins therefore do not need to have thick walls and they contain for fewer muscle and elastic fibres. They can therefore die late to become much wider thus hold more blood than arteries. Around 80% of a sedentary person’s blood is in the veins though this proportion falls during vigorous exercise.
Ang form of movement helps with veinous blood flow as contraction makes a muscle shorter and wider so it squeezes on adjacent veins like a pump.

Question 37

Blood from arms is carried in

Answer 37

Subclavian vein

Question 38

Blood from head is carried by

Answer 38

Jugular vein

Question 39

The hepatic portal vein

Answer 39

Does not carry blood to the heart! It carries blood from stomach to the liver.it is regarded as a vein rather than an artery because the blood it carries is at low pressure so it is relatively thin.

Question 40

Valves

Answer 40

Valves in veins and the heart ensure circulation of blood by preventing backflow.
Blood pressure in veins is sometimes so low that there is a danger of backflow towards the capillaries and insufficient return of blood to me heart. To maintain circulation, veins contain pocket valves, consisting of three cup-shaped flaps of tissue.
Valves allow for unidirectional blood flow and make efficient us of the intermittent and often transient pressures provided by muscular and postural changes. They ensure the blood circulates request then just flows in linear way.

Question 41

Valves in event of backflow

Answer 41

Blood gets caught in the flaps of the pocket valve, which fill with blood blocking the lumen of the vein.

Question 42

Valves in event of blood flowing towards the heart

Answer 42

It pushes the flaps to the sides of the vein. The pocket valve therefore opens and blood can flow freely.

Question 43

The double circulation (lungs)

Answer 43

There is a separate circulation for the lungs.
Human lungs supply products of gas exchange are supplied through blood by a separate circulation. Blood capillaries in lungs connot withstand high pressures so blood is pumped to them at relatively low pressure. After passing through the capillaries of the lungs the pressure of the blood is low, so it must return to the heart to be pumped again before it goes to other organs. Humans therefore have two separate circulations:
• the pulmonary circulation, to and from the lungs.
• the systematic circulation, to and from all other organs, including the heart muscles
Blood from both circulations do not mix, therefore the heart is a double pump, delivering blood under different pressures separately to the two circulations.

Question 44

Pulmonary circulation

Answer 44

Receives deoxygenated blood that was returned from the systematic circulation

Question 45

Systematic circulation

Answer 45

Receives blood that has been oxygenated by the pulmonary circulation.

Question 46

Heart structure

Answer 46

• The heart has two sidles, that pump to the pulmonary and systematic circulations
• each side of me heart has two chambers, a ventricle that pumps the blood into the arteries and an atrium that collects blood from the veins and passes it to me ventricle
• each side has two values, an atrioventricular valve between the atrium and the ventricle and semilunar value between the ventricle and the artery
• oxygenated blood flows into the left side of the heart through the pulmonary veins from the lungs and out through the aorta
• deoxygenated blood flows into the left side of the heart through the venue cava and out in the pulmonary arteries

Question 47

Atherosclerosis

Answer 47

The development of fatty tissues called atheroma in the artery wall adjacent to the endothelium. Low density lipoproteins (LDL) containing fats and cholesterol accumulate and phagocytes are then attracted by signals from endothelium cells and smooth muscle. The phagocytes engulf the fats and cholesterol by endocytosis and grow very verge. Smooth muscle cells migrate to form a tough cap over atheroma. The artery wall bulges into the women narrowing it and thus impeding blood flow.
Small traces of atheroma are normally visible in children’s arteries by the age of ten, but do not affect their health. In some older people atherosclerosis becomes much more advanced but often goes unnoticed until a major artery becomes so blocked so blocked that the tissues it supplies become compromised.

Question 48

Causes of atherosclerosis

Answer 48

The causes of atherosclerosis are not yet fully understood. Various factors have been shown to be associated with an increased risk of atheroma but are not the sole causes of the condition:
- high blood concentrations of LDL (low density lipoprotein)
- chronic high blood glucose concentrations, due to overeating, obesity or diabetes
- chronic high blood pressure due to smoking, stress or any other cause
- consumption of trans fats, which damage the endothelium of the artery.
There are also some more recent theories that include microbes:
- infection of the artery wall with Chlamydia pneumoniae
- production of trimethylamine N-oxide (TMAO) by microbes in the intestine.

Question 49

Coronary oclusion

Answer 49

Coronary occlusion is a narrowing of the arteries that supply blood
containing oxygen and nutrients to the heart muscle. Lack of oxygen
(anoxia) causes pain, known as angina, and impairs the muscle’s
ability to contract, so the heart beats faster as it tries to maintain
blood circulation with some of its muscle out of action. The fibrous
cap covering atheromas sometimes ruptures, which stimulates the
formation of blood clots that can block arteries supplying blood to the
heart and cause acute heart problems. This is described in sub-topic 6.3.

Question 50

The sinoatrial node

Answer 50

The heartbeat is initiated by a group of specialized muscle cells ina the right atrium called the sinoatrial node.
The heart is unique in the body as its muscles can contract without
stimulation from motor neurons. The contraction is called myogenic,
meaning that it is generated in the muscle itself. The membrane of a
heart muscle cell depolarizes when the cell contracts and this activates
adjacent cells, so they also contract. A group of cells therefore contracts
almost simultaneously at the rate of the fastest.
The region of the heart with the fastest rate of spontaneous beating
is a small group of special muscle cells in the wall of the right atrium,
called the sinoatrial node. These cells have few of the proteins that
cause contraction in other muscle cells, but they have extensive
membranes. The sinoatrial node therefore initiates each heartbeat,
because the membranes of its cells are the frst to depolarize in each
cardiac cycle.

Question 51

Initiating the heartbeat

Answer 51

Sinoatral node acts as a pacemaker.
Because the sinoatrial node initiates each heartbeat, it sets the pace for
the beating of the heart and is often called the pacemaker. If it becomes
defective, its output may be regulated or even replaced entirely by an
artificial pacemaker. This is an electronic device, placed under the skin
with electrodes implanted in the wall of the heart that initiate each
heartbeat in place of the sinoatrial node.

Question 52

Atrial and ventricular contraction

Answer 52

The sinoatrial node initiates a heartbeat by contracting and simultaneously
sends out an electrical signal that spreads throughout the walls of the atria.
This can happen because there are interconnections between adjacent bres
across which the electrical signal can be propagated. Also the fibres are
branched so each fibre passes the signal on to several others. It takes less
than a tenth of a second for all cells in the atria to receive the signal. This
propagation of the electrical signal causes the whole of both left and right
atria to contract.
After a time delay of about 0.1 seconds, the electrical signal is conveyed
to the ventricles. The time delay allows time for the atria to pump
the blood that they are holding into the ventricles. The signal is then
propagated throughout the walls of the ventricles, stimulating them to
contract and pump blood out into the arteries.

Question 53

Diastole

Answer 53

Chamber that is not contracting

Question 54

Blood when both chambers are at rest

Answer 54

Heart values open and close based on blood pressure differences on either side of any one valve. During me period of diastole for both chambers, the atrial pressure is just slightly higher than ventricular pressure, and this keeps the atrioventricular valve open. Much of the blood that slowly returns to the left atrium via pulmonary veins move passively down the left ventricle through this open valve. The pressure in the aorta is much higher then in the left ventricle. This difference keeps the left semilunar valve closed and prevents backflow into the ventricle.

Question 55

Systole

Answer 55

The chamber is active thus in contraction

Question 56

Blood when the atria are in systole and the ventricles are in diastole

Answer 56

The pressure due to the systole is not very high. The wall of each atrium is relatively thin muscle and is not capable of creating great pressure. There is no need for great pressure because much of the volume of blood has already accumulated passively within the ventricle through the open atrioventricular valve. Any remaining blood in the atrium is moved to the ventricle by systole.

Question 57

Blood when the atria are in diastole and the ventricles are in systole

Answer 57

There one two stages of ventricular systole: the early and late ventricular systole.
As soon as ventricular systole begins, the pressure inside the ventricle increases to be greater than that in the atrium, so the atrioventricular valve closes to prevent backflow to the atrium (this creates the “lub” sound that can be heard by the stethoscope). The pressure in the aorta is still far higher than in the ventricle, so the semilunar valve remains closed. There is a relatively large volume of blood in the ventricle during this time, and the ventricle is highly muscular. This combination of factors permits the ventricular pressure to build up considerably as systole continues. Finally, the pressure in the ventricle becomes greater than that in the aorta, and the semilunar valve opens, allowing the ventricle to pump the blood into the aorta. As the ventricle finishes its contraction, the pressure inside it once again drops below the pressure in the aorta, and the semilunar valve closes ( this causes the ‘dub’ sounds that con be heard by a stathoscope). Both chambers go back into diastole and the cardiac cycle repeats itself continuously.

Question 58

Changing the heart rate

Answer 58

The heart rate can be increased on decreased by impulses brought to the heart through two nerves from the medulla of the brain.
The sinoatrial node that sets the rhythm for the beating of the heart responds to the signal from the outside of the heart. These include signals from branches of two nerves originating in a region in the medulla of the brain called the cardiovascular centre. Signals from one of the nerves cause the pacemaker to increase the frequency of heartbeats. In healthy young people the rate can increase to three times the vesting rate. Signals from the other decreases the rate. These two nerve branches act rather like a throttle and a break of a car.

Question 59

The cardiovascular centre monitors…

Answer 59

Receives inputs from receptors that monitor blood pressure and its ph and oxygen concentration. The ph of the blood reflects its carbon dioxide concentration.
• Low blood pressure, low oxygen concentration and low ph all suggest that the heart rate needs to speed up, to increase the flow rate of blood to the tissues, deliver more oxygen and remove more carbon dioxide.
• High blood pressure, high oxygen and high ph are all indicators that the heart rate may need to slow down.

Question 60

Epinephrine

Answer 60

Epinephrine increases the heart rate to prepare for vigorous physical activity. The Sinoatrial node also responds to epinephrine in the blood, by increasing the heart rate.Epinephrine
Epinephrine increases the heart rate to prepare for
vigorous physical activity.
The sinoatrial node also responds to epinephrine in the blood
by increasing the heart rate. This hormone is also sometimes
called adrenalin and is produced by the adrenal glands. The
secretion of epinephrine is controlled by the brain and rises
when vigorous physical activity may be necessary because of a
Wreat or opportunity. So epinephrine has the nickname “fight or
flight hormone”.
in the past when humans were hunter-satherers rather than farmers
pinephrine would have been secreted when humans were hunting
Tor prey or when threatened by a predator. In the modern world
athletes often use pre-race routines to stimulate adrenalin secretion
so that their heart rate is already increased when vigorous physical
activity begins.

Question 61

Skin as barrier to infection

Answer 61

The skin and mucous membranes from a primary defence against pathogens that cause infectious disease. There are many different microbes in the environment that can grow inside the human body and cause disease. Some microorganisms are opportunistic and although they can invite the body they also commonly live outside it. Others are speciallized and can only live inside a human body. Microbesthat cause disease are called pathogens.
Skins outermost layer is tough and provides physical barrier against the entry of pathogens and protection against chemical and physical damage. Sebaceous glands one associated with hair follicles and they secrete a inimical called sebum, which maintains skin moisture and slightly lowers skin ph. The lower ph inhibits the growth of bacteria and fungi.
Mucous membranes are a thinner and softer type of skin that is found in oveces such as the nasal passage and other airways, the head of the penis and foreskin of the vagina. The mucus that these areas of skin secrete is a sticky solution of glycoproteins. Mucus acts as a physical barrier; pathogens and harmful particles are trapped in it and either swallowed or expelled. It also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozyme.