Topic 6: Human Physiology

Question 1

Digestion in the small intestine

Answer 1

• Waves of muscle contraction called peristalsis pass along the intestine.
• Contraction of circular muscle behind the food constricts the gut to prevent food from being pushed back towards the mouth.
• Contraction of longitudinal muscle where the food is located moves it on along the gut.
• Contraction of both layers of muscle mixes food with enzymes in the small intestine.
• Enzymes digest most macromolecules (proteins, starch, glycogen, lipids and nucleic acids) in food into monomers.
• Cellulose remains undigested.
• Lipids - by lipase - fatty acids and glycerol.
• Polypeptides - by endopeptidase - shorter peptides.
• Starch - by amylase - maltose.

Question 2

Digestion of starch

Answer 2

Two types of molecule in starch:
Amylose- alpha glucose, 1,4 bonds, unbranched.
Amylopectin- alpha glucose, 1,4 and 1,6 bonds, branched.

Amylase breaks down 1,4 bonds in chains of four or more glucose monomers, so it can digest amylose into maltose but not glucose.
Due to specifity of its active site, amylase cannot break the 1,6 bonds in amylopectin.
Fragments of the amylopectin molecule containing a 1,6 bond that amylase cannot digest are called dextrins.
Digestion of starch is completed by enzymes in the membranes of microvilli on villus epithelium cells: maltase and dextrinase digest maltose and dextrins into glucose.
Also, in the membranes of the microvilli, are protein pumps - cause the absorption of the glucose produced by digesting starch.
Blood carrying glucose and other products of digestion flows through villus capillaries to venules in the submucosa of the wall of the small intestine.
The blood in these venules is carried via the hepatic portal vein to the liver, where excess glucose can be absorbed by liver cells and converted to glycogen for storage.

Question 3

Intestinal villi

Answer 3

The process of taking substances into cells and the blood is called absorption.
Nutrients absorbed by the epithelium- the single layer of cells forming the inner lining of the mucosa. The rate of absorption depends on the surface area of this epithelium.
Absorption occurs principally in the small intestine. The small intestine in adults in about 7m long and 25-30mm wide, with folds on the inner surface, giving a large surface area.
The area of epithelium is further increased by the presence of villi, which are small finger-like projections of the mucosa on the inside of the intestine wall. A villus is between 0.5 and 1.5 mm long, as many as 40 of them per square mm.
Increase the surface area by a factor of ten and absorb mineral ions and vitamins and also monomers formed by digestion such as glucose.

Question 4

Methods of absorption

Answer 4

Different methods of membrane transport used by epithelium cells to absorb nutrients.

• Simple diffusion - nutrients pass down the conc gradient between phospholipids in the membrane.
• Facilitated diffusion - nutrients pass down the conc gradient through specific channel proteins in the membrane.
• Active transport - nutrients pumped through the membrane against the conc gradient by pump proteins.
• Endocytosis (pinocytosis) - small droplets of the fluid are passed through the membrane by means of vesicles.

Some more complex methods of transport - glucose in absorbed by sodium co-transporter proteins which move a molecule of glucose together with a sodium ion across the membrane into the epithelium cells.

Question 5

Modelling absorption with dialysis tubing

Answer 5

Dialysis tubing can be used to model absorption by the epithelium of the intestine.
Cola drink contains a mix of substances which can be used to model digested and undigested foods in the intestine.
The water outside the bag is tested at intervals to see if substances in the cola have diffused through the dialysis tubing.
The expected result is that glucose and phosphoric acid diffuse through the tubing but caramel, as larger, does not.

Question 6

Harvey and the circulation of blood

Answer 6

• Until the 17th century - Galen thought that blood is produced by the liver, pumped out by the heart, and consumed by other organs of the body.
• William Harvey discovered the circulation of blood - had to overcome opposition as Galen’s theories were so well established. Demonstrated that blood flow through vessels is unidirectional with valves to prevent backflow and also that the rate of flow through major vessels is far too high for blood to be consumed in the body. He showed that heart pumps out blood in arteries and that it returns in veins. He also predicted the presence of other small vessels, however, were too small to be seen as no microscopes were invented yet. Later established as capillaries.

Question 7

The double circulation

Answer 7

• There are separate circulations for the lungs (pulmonary circulation) and other organs (systematic circulation).
• The heart is a double pump with left and right sides.
• The right side pumps deoxygenated blood to the lungs via the pulmonary artery and oxygenated blood returns to the left side of the heart via pulmonary vein. The left side pumps this blood via the aorta to other organs and deoxygenated blood is carried back the right side of the heart in the vena cava.

Question 8

Structure and function of blood vessels

Answer 8

• Arteries - carry blood pumped out at high pressure to tissues of the body. Tough outer coat and wall, elastic fibres, narrow lumen - help maintain high pressure.
• Capillaries - carry blood through tissues. Have permeable walls that allow exchange of materials between the cells of the tissue and the blood in the capillary. Walls consist of a single layer of cells - short distance for diffusion, pores between cells, very narrow lumen - many small capillaries have a larger s.a..
• Veins - collect blood at low pressure from the tissues of the body and return it to the atria of the heart. Thin layers of tissue and thin wall, few or no elastic fibres, wide lumen - low pressure.

Question 9

Cardiac muscle

Answer 9

The walls of the heart are made of cardiac muscle, which has a special property - it can contract on its own without being stimulated by a nerve (myogenic contraction).

• There are many capillaries in the wall of the heart. The blood running through these capillaries is supplied by the coronary arteries which branch off the aorta, close to the semilunar valve.
• The blood brought by the coronary arteries brings nutrients. It also brings oxygen for aerobic cell respiration, which provides energy for cardiac muscle contraction.
• Valves ensure circulation of blood by preventing backflow, the atria are the collecting chambers, and the ventricles are the pumping chambers.

Question 10

The cardiac cycle

Answer 10

The beating of the heart consists of a cycle of actions:

1. The walls of the atria contract, pushing blood from the atria into the ventricles through atrio-ventricular valves, which open. The semi-lunar valves are closed so the ventricles fill with blood.
2. The walls of the ventricles contract and the blood pressure rises in them - causes the atrio-ventricular valves to close and the semilunar valves to open, allowing the blood to be pumped out into the arteries and the atria start to refill.
3. The ventricles stop contracting so pressure falls inside them. The semilunar valves close, preventing back-flow from the arteries to the ventricles. When the ventricular pressure drops below the atrial pressure, the atrio-ventricular valves open and the blood entering the atrium from the veins then flows on to start filling the ventricles.

The next cardiac cycle begins when the walls of the atria contract again.

Question 11

Control of heart rate

Answer 11

• Sinoatrial (SA) node - a region of specialised cardiac muscle cells in the wall of the right atrium that acts as the pacemaker of the heart by initiating each contraction.
• The SA node sends out an electrical signal that stimulates contraction, first through the walls of the atria and then the walls of the ventricles.
• Messages can be carried to the SA node by nerves and hormones:
  1. Impulses brought from the medulla of the brain by two nerves can cause the SA node to change the heart rate. One nerve speeds up the rate and the other slows it down.
  2. The hormone epinephrine aka adrenaline increases the heart rate to help prepare for flight/fight response.

Question 12

Coronary artery disease

Answer 12

Caused by fatty plaque building up in the inner lining of coronary arteries, which become narrowed.
When severe, blood flow to the cardiac muscle is restricted - chest pain.
Minerals become deposited in the plaque, making it hard and rough.
Factors causing it: high blood cholesterol, smoking, high blood pressure, high blood sugar levels, genetic factors.

Question 13

Barriers to infection

Answer 13

A pathogen - an organism or virus that causes disease.

The skin a mucous membranes - primary defence against pathogens - form a barrier.

• The outer layers of the skin are tough and form a physical barrier. Sebaceous glands in the skin secrete lactic acid and fatty acids, which make the surface of the skin acidic. Prevents the growth of most pathogenic bacteria.
• Mucous membranes are soft areas of skin kept moist with mucous. Found in the nose, trachea, vagina and urethra. Do not form a strong physical barrier, but many bacteria are killed by lysozyme, an enzyme in the mucus. In the trachea, pathogens tend to get caught in the sticky mucus; cilia then push the mucus and bacteria up and out of the trachea.

Despite these barriers, pathogens sometimes enter the body and so other defences are needed - two types of white blood cell fight infections - phagocytes and lymphocytes.

Question 14

Phagocytes

Answer 14

Phagocytes (a type of white blood cells) - ingest pathogens by endocytosis.

• The pathogens are then killed and digested inside the cell by enzymes from lysosomes.
• Phagocytes can ingest pathogens in the blood.
• They can also squeeze out through the walls of blood capillaries and move through tissues to sites of infection.
• They then ingest the pathogens causing the infection.
• Large nr of phagocytes at a site of infection form pus.
• Phagocytes give us non-specific immunity against diseases as they ingest any pathogen if stimulated to do so.

Question 15

Blood clotting

Answer 15

Skin is cut - blood escapes from tiny blood vessels - a semi-solid blood clot is formed from liquid blood to seal up the cut - prevent the entry of pathogens.

• Platelets have an important role in clotting.
• Platelets - small cell fragments that circulate with red and white blood cells in blood plasma. The clotting process begins with the release of clotting factors which release a cascade of reactions (product of one reaction is the catalyst of the next).
• Helps to ensure that clotting only happens when needed, also very rapid.
• In the last reaction - fibrinogen (soluble plasma protein) is altered by the removal of sections of peptide that have many negative charges.
• Allows the remaining polypeptide to bind to others, forming long protein fibres called fibrin.
• Fibrin forms a mesh of fibres across wounds. Blood cells are caught in it and soon form a semi-solid clot. If exposed to air, the clot dries to form a protective scab, until the wound has healed.

Question 16

Blood clots in coronary arteries

Answer 16

• If the deposits of plaque in coronary arteries rupture, blood clots form (coronary thrombosis) - can block the artery - cardiac muscle receives no oxygen and stops beating in the right way (heart attack).
  Uncoordinated contraction of cardiac muscle is fibrillation.
  Sometimes, the heart recovers and starts beating again, but severe heart attacks can be fatal as contractions of the heart stop completely.

Question 17

Production of antibodies

Answer 17

1. Antibodies are made by lymphocytes (a type of white blood cell) when stimulated by antigens (foreign substances).
2. A lymphocyte can only make one type of antibody so a huge number of different lymphocyte is needed. Each lymphocyte puts some of the antibody that it can make into its cell surface membrane with the antigen-combining site projecting outwards.
3. When a pathogen enters the body, its antigens bind to the antibodies in the cell surface membrane of one type of lymphocyte.
4. When antigens bind to the antibodies on the surface of a lymphocyte, this lymphocyte becomes active and divides by mitosis to produce a clone of many identical cells.
5. The cells produced by mitosis are plasma cells. They produce large quantities of the same antibody. The antibody binds to the antigens on the surface of the pathogen and stimulate its destruction. After an infection has been cleared, most of the lymphocytes disappear , but some persist as memory cells. These memory cells can quickly reproduce to from a clone of plasma cells if a pathogen carrying the same antigen is re-encountered.

Question 18

HIV and the immune system

Answer 18

• HIV (human immunodeficiency virus) - infects a type of lymphocyte that plays a vital role in antibody production.
• Over a period of years, these lymphocytes are gradually destroyed.
• Without active lymphocytes, antibodies cannot be produced, which leads to AIDS (acquired immunodeficiency syndrome).
• If AIDS is left untreated, leads to death from infections by a variety of pathogens that would normally be controlled easily.
• HIV does not survive long outside the body and cannot easily pass through skin.
• Transmission- through bodily fluids from infected to uninfected (through small cuts in the vagina, penis, mouth or intestine during sex, in traces of blood on hypodermic needles shared by drug abusers, across the placenta, in transfused blood).

Question 19

Antibiotics

Answer 19

Antibiotics - chemicals produced by microorganisms, to kill or control the growth of other organisms.

• For ex. Penicillium fungus produces penicillin to kill bacteria. Antibiotics work by blocking processes that occur in prokaryotic cells but not in eukaryotic cells.
• Viruses lack a metabolism and rely on a host such as a human cell - it is not possible to block these processes using an antibiotic without also harming the human cells. Thus antibiotics cannot be used to treat viral diseases.
• Most bacterial diseases in humans can be treated using antibiotics, but some strains of bacteria have developed resistance to the antibiotic.

Question 20

Testing penicillin

Answer 20

• Penicillin was developed as an antibiotic by Florey and Chain in 1930s.
• 1st test on eight mice infected with a bacterium that causes fatal pneumonia. All the four mice treated recovered while the other four who did not receive the treatment died.
• Then tested on a man, he started to recover but the antibiotic ran out.
• Five other people were tested, and were cured.
• Florey and Chain’s research would not be considered safe today. Extensive animal testing is required nowadays. 1st on animals, then on healthy informed humans, and finally on patients with the disease.

Question 21

Ventilation and Gas Exchange

Answer 21

• Ventilation - the process of bringing in fresh air to the alveoli and removing stale air to maintain the concentration gradients.
• Gas exchange - exchanging one gas (oxygen and carbon dioxide) for another, happens by diffusion in the alveoli in the lungs.
• Cell respiration - happens in the cytoplasm and mitochondria and releases energy in the form of ATP for use inside the cell.

Question 22

Adaptations of an alveolus for gas exchange

Answer 22

The lungs contain hundreds of millions of alveoli.

Adaptations of alveoli:

• Small - give a large surface area for diffusion of gases.
• Permeable to oxygen and carbon dioxide.
• Thin - distance for diffusion is small.
• Moist - oxygen can dissolve.
• Covered by blood capillaries - with low oxygen and high carbon dioxide concentrations so enhances diffusion.
• Type 1 pneumocytes - extremely thin and permeable alveolar cells adapted to carry out gas exchange.
• Type 2 pneumocytes - cells in the alveolus wall that secrete a fluid to keep the inner surface of the alveolus moist and allow gases to dissolve + reduces surface tension.

Question 23

Lung cancer

Answer 23

• Epidemiology (the study of incidence and causes of disease).
• Five main causes of lung cancer:
  Smoking (mutagens), passive smoking (carcinogens), air pollution, radon gas, asbestos and silica (dust).
• Consequences: difficulties with breathing, persistent coughing, fatigue, coughing up blood, chest pain, loss of appetite, weight loss.
• Usually fatal as discovered late, and secondary tumours have already developed and primary tumour is large.

Question 24

Emphysema

Answer 24

Emphysema - a chronic and progressive disease with serious consequences. The surface area for gas exchange in reduced so the oxygen saturation in the blood falls and exercise is difficult. The lungs lose elasticity, mucus in the lungs causes coughing.

• Main causes: smoking, air pollution.
• Cilia that line the airways expel mucus are damaged so mucus builds up in the lungs causing infections.
• Toxins in smoke cause inflammation and damage to the white blood cells.
• A protease is released from the inflamed cells and white blood cells and it digests the elastic fibres in the lungs and causes a breakdown of alveolus walls.
• Results in larger air sacs as alveoli become less permeable and thicker.

Question 25

Monitoring ventilation in humans

Answer 25

• Ventilation rate - the nr of inhalations or exhalations per minute.
• Tidal volume - the volume of air taken in or out with each inhalation or exhalation.
  By monitoring ventilation rate and tidal volume at rest and then during mild and vigorous exercise, the effect of ventilation can be investigated.

1. Monitoring ventilation rate - done by simple observation or using data-logging. An inflatable chest belt placed around the thorax and air is pumped in with a bladder. A differential pressure sensor is used to measure pressure variations inside the belt, the ventilation rate can be deduced and also the relative size of ventilations.
2. Monitoring tidal volume - using a spirometer. Made using a bell jar for instance. In a pneumatic trough. A tube is used to breathe out into the bell jar so the expired volume can be measured.

Question 26

Ventilation of the lungs

Answer 26

• Muscle contractions cause the pressure changes inside the thorax that force the air in and out of the lungs to ventilate them.
• Different muscles are required for inspiration and expiration - when cause opposite movements - antagonistic.

Inhaling: external intercostal muscles contract, ribcage up and out, diaphragm contracts, becomes flatter and moves down, increases the volume of the thorax and the pressure drops below atmospheric pressure, air flows into the lungs from the outside.

Exhaling: internal intercostal muscles contract, ribcage down and in, abdominal muscles contract moving the diaphragm up, decreases the volume of the thorax, the pressure inside the thorax rises above atmospheric pressure, air flows out from the lungs to outside.

Question 27

Structure and function o neurons

Answer 27

Nervous system is composed of ells called neurons.
Neurons carre messages at high speed in the form of electrical impulses.
Many neurons are very elongated and carry impulses long distanes.
Myelinated nerve fibres have a myelin sheath with small gaps called nodes of Ranvier.
Nodes of ranvier allow nerve impulses to jump across nodes - saltatory conduction.
Speeds up the transmission.

Question 28

Synapses and neurotransmission

Answer 28

Synapse - a junction between two neurons or a junction between neurons and receptor or effector cells.
Fluid filled gap - synaptic cleft.

Neurtoransmission:

1. A nerve impulse reaches the end of the pre-synaptic neuron.
2. Depolarization of the pre-synaptic membrane causes vesicles of the neurotransmitter to move to the pre-synaptic membrane and fuse with it, releasing the neurotransmitter into the synaptic cleft by exocytosis.
3. The neurotransmitter diffuses across the synaptic cleft and binds to the receptors.
4. The receptors (sodium channels) open and sodium ions diffuse into the post-synaptic neuron - causes depolarization of the post-synaptic membrane.
5. The depolarization passes on down the post-synaptic neuron as an action potential.
6. Neurotransmitter in the synaptic cleft is rapidly broken down to prevent continuous synaptic transmission.

Question 29

Cholinergic synapses

Answer 29

Synapses do not all use the same neurotransmitter but many use acetylcholine - known as cholinergic synapses.
The acetylcholine is broken down in the post-synaptic cleft by cholinesterase, producing acetyl groups and choline. Choline is reabsorbed by the pre-synaptic neuron.

Question 30

Neonicotinoid pesticides

Answer 30

• Neonicotinoid pesticides - bind to acetylcholine receptors in the post-synaptic membranes of cholinergic synapses in insects.
• Cholinesterase does not break down these pesticides so remain in the receptors - prevents acetylcholine from binding.
• Blocks synaptic transmission - kills the insect.
• Honeybees are killed along with insects that are targeted.

Question 31

Resting potential

Answer 31

• Resting potential - the voltage across the plasma membrane of a neuron when it is not conducting a nerve impulse.
• Sodium-potassium pumps in the membranes of axons - pump sodium out and potassium in by active transport.
• Concentration gradients of both sodium and potassium are established across the membrane - the inside develops a negative charge and outside positive.
• Resting potential - typically -70mV.

Question 32

Action potentials

Answer 32

• Action potential - depolarization and repolarization of a neuron, due to facilitated diffusion of ions across the membrane through voltage-gated ion channels. If the potential across the membrane rises from -70 to -50mV, voltage gated sodium channels open - sodium ions diffuse in down the concentration gradient.
  Positively charged sodium ions enter - inside of the neuron develops a net positive charge compared to the outside - reverses the potential - depolarisation.
  The reversal of membrane polarity causes potassium channels to open - potassium ions diffuse out - inside becomes negative again - potential is restored - repolarisation.

Question 33

Propagation of nerve impulses

Answer 33

A nerve impulse is an action potential that travels along the axon of a neuron from one end to the other.
There is an action potential whenever a part of the axon reaches the threshold potential of -50mV.
An action potential in one part of the axon triggers another one in the next part - propagation of the nerve impulse.
Due to diffusion of sodium ions between a region with an action potential and the next region still at resting potential.
Diffusion of sodium ions - local currents - changes the voltage across the membrane from resting potential -70mV to threshold potential of -50mV - causes an action potential.

Question 34

Memory and learning

Answer 34

Higher functions of the brain including memory and learning are only partly understood - being researched.
Psychologists, biologists, physicists, computer scientists… collaborate.

Question 35

Blood glucose concentration

Answer 35

Usually kept between 4 and 8 millimoles per dm cubed per blood.
Cells in the pancreas monitor the concentration and secrete the hormones insulin and glucagon when the level is too high or low.

Too high:

Insulin is secreted by beta cells. Stimulates the liver to absorb glucose and convert it to glycogen. Stored in these cells in the liver, muscle, and other cells are stimulated to absorb glucose for cell respiration instead of fat - reduces the level.

Too low:

Glucagon is secreted by alpha cells. Stimulates liver cells to break down glycogen into glucose and release it. Raises the glucose level.

Question 36

Diabetes

Answer 36

In some people, the control of blood glucose does not work properly and the concentration can rise or fall beyond the limits. Diabetes mellitus.

Type 1:
Onset during childhood, immune system destroys beta cells so not enough insulin is secreted, insulin injections are needed.

Type 2:
Onset during adulthood, target cells become insensitive to insulin, low carbohydrate diets are used, various risk factors such as diets rich in fat and low in fibre, lack of exercise and genetic factors.

Question 37

Thyroxin

Answer 37

Thyroxin - hormone secreted by the thyroid gland in the neck.
Unusual chemical structure - four atoms of iodine. Prolonged deficiency of iodine in diet prevents the synthesis of thyroxin.
Targets many cells in the body.
Regulates metabolic rate + controls body temperature,
Cooling triggers increased thyroxin secretion - stimulates heat production.

Question 38

Leptin and obesity

Answer 38

Leptin - a hormone secreted by adipose tissue.