IB Categories Archives: Topic 6: Human physiology

6.6 – Reproduction

6.6 – Reproduction

6.6.1 – Draw and label diagrams of the adult male and female reproductive systems


6.6.2 – Outline the role of hormones in the menstrual cycle, including FSH (follicle-stimulating hormone), LH (luteinising hormone), oestrogen and progesterone

Follicle Stimulating Hormone (FSH) – Released during days 5-13 from the pituitary gland. This stimulates the maturation of a follicle in the ovary. Also stimulates the secretion of oestrogen.

Luteinising Hormone (LH) – Released on day 14 to bring about ovulation, when the secondary oocyte detaches from the follicle wall, and the Graafian follicle to turn into the corpus luteum, which produces progesterone. Also causes less oestrogen to be secreted and more progesterone.

Oestrogen – Also released during days 5-13, reaching its peak levels around days 13-14, when it stimulates the release of LH. It causes the repair of the uterine tissue.

Progesterone – Released from the corpus luteum and causes the uterine wall to thicken and increases the blood supply, ready for implantation of a fertilised ovum. It inhibits the production of FSH so that no more follicles will mature in the ovary, as well as reducing oestrogen concentration. It will eventually inhibit LH and the activity of the corpus luteum.


6.6.3 – Annotate a graph showing hormone levels in the menstrual cycle, illustrating the relationship between changes in hormone levels and ovulation, menstruation and thickening of the endometrium


6.6.4 – List three roles of testosterone in males

Pre-natal development of male genitalia – During gestation, the testes will secrete testosterone to stimulate the development of the male genitalia in the foetus, including the penis.

Development of secondary sexual characteristics – When the male reaches puberty, the testosterone levels will rise to cause the development of secondary sexual characteristics. These include pubic hair, an enlarged penis and growth of skeletal muscles.

Maintenance of sex drive – When the male reaches adulthood, their sex drive is maintained by testosterone. This instinct is important for encouraging them to have sexual intercourse to pass on their genes to offspring.

6.6.5 – Outline the process of in vitro fertilisation (IVF) 

  • A drug is injected daily for three weeks to stop the normal menstrual cycle
  • The woman is given large daily doses of FSH for 10-12 days to stimulate the release of multiple follicles in the ovary
  • HCG is then injected 36 hours before the ova are collected to mature them and make them loose in the follicle.
  • Semen is collected from the male, which are processed to concentrate the healthiest ones
  • The ova are extracted from the follicle using a device that is inserted through the wall of the vagina
  • The ova are mixed with sperm in individual dishes overnight in an incubator
  • The dishes are checked to see if fertilisation has occurred
  • Two or three embryos are selected and placed in the uterus using a long plastic tube
  • A pregnancy test is performed about 12 days later to see if any embryos implanted
  • Two weeks later, scans are done to ensure that the pregnancy is continuing normally, including a visibly beating heart.

6.6.6 – Discuss the ethical issues associated with IVF 

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6.5 – Nerves, Hormones and Homeostasis

6.5 – Nerves, Hormones and Homeostasis

6.5.1 – State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses

The somatic nervous system includes motor neurons attached to the skeletal muscles, and the sensory neurons attached to the receptor sense organs.

The autonomic nervous system includes the nerves from internal receptors and the nerves attached to smooth muscle.


6.5.2 – Draw and label a diagram of the structure of a motor neuron

6.5.3 – State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motors neurons

The body contains many different receptors which detect specific stimuli and convert them into a nerve impulse. The central nervous system conducts the nerve impulses from sensory nerves along sensory neurons. This impulse is then passed to relay neurons that pass it through the brain and spine, then to a motor neuron and an effector (such as the muscles), where the response is produced.

𝑆𝑡𝑖𝑚𝑢𝑙𝑖 → 𝑅𝑒𝑐𝑒𝑝𝑡𝑜𝑟 → 𝑆𝑒𝑛𝑠𝑜𝑟𝑦 𝑁𝑒𝑟𝑣𝑒 → 𝑅𝑒𝑙𝑎𝑦 𝑁𝑒𝑟𝑣𝑒 → 𝑀𝑜𝑡𝑜𝑟 𝑁𝑒𝑢𝑟𝑜𝑛 → 𝐸𝑓𝑓𝑒𝑐𝑡𝑜𝑟 → 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒

6.5.4 – Define resting potential and action potential (depolarisation and repolarisation) 

Resting Potential – An electrical potential across a cell membrane when not conducting an impulse Action Potential – The localised reversal, or depolarisation, and then restoration, or

Action Potential – The localised reversal, or depolarisation, and then restoration, or repolarisation, of electrical potential between the inside and outside of a neuron as the impulse moves along it

6.5.5 – Explain how a nerve impulse passes along a non-myelinated neuron

Nerve impulses move along the axon using a domino effect, with an action potential causing one in the adjacent part. Sodium and potassium ions move across the plasma membrane via active transport to alter the concentration and cause an action potential and then return to resting potential.

At resting potential:

Na+ ions are concentrated outside the membrane of the axon, while the K+ ions are concentrated inside the membrane. The Na+ and K+ ion channels are closed. For an action potential to be created, it must rise above a certain threshold before the action potential is created.


At action potential:

Na+ rush inside the membrane and K+ ion rush outside to reverse concentrations. The Na+ ion channels open, then close as the K+ ion channels open. The inside now has a net positive charge, while the outside has a net negative charge.

Active Transport

The ion pumps used to create the action potential must use active transport as they are moving the ions against the concentration gradient. This requires ATP for energy. The polarisation of the membrane changes during this process from -70mV at resting potential to +30mV at action potential. It is restored back to -70mV when the K+ ion channels open.

After the neuron is repolarised, there is a brief period in which that section of the axon cannot have an action potential, called the refractory period.

The all-or-nothing law is that a nerve impulse will only be passed on if it reaches the threshold of -55mV. If it does not, then no action potential will be created. If it does, then an action potential will be sent that is the same voltage as any other action potential. On a graph, all action potentials look the same, each rising to +35mV.


6.5.6 – Explain the principles of synaptic transmission

The synapse is the junction between two neurons – the presynaptic neuron passing the signal to the postsynaptic neuron. The synaptic cleft is the fluid-filled space between an axon terminal and the end of a dendrite. They are the location of communication between neurons and glands or muscles. They pass electrical or chemical signals to their target cells. When an action potential reaches the end of the neuron, the Ca+ ion channels open to allow Ca+ to flow in. This cause exocytosis of a neurotransmitter at the synaptic cleft.

The neurotransmitter diffuses across the synaptic cleft and then binds to a post-synaptic receptor. The post-synaptic membrane becomes polarised, causing the Na+ ion channels to open. An action potential is created in the post-synaptic neuron. The post-synaptic membrane then becomes depolarised. The K+ ion channels immediately open to cause hyperpolarisation of the post-synaptic membrane.

An enzyme binds to the neurotransmitter to hydrolyse it and prevent future function. It is recycled in the body.


6.5.7 – State that the endocrine system consists of glands that release hormones that are transported in the blood 

The endocrine system is also called the hormone system. This is because it consists of glands that release hormones to our cells to aid bodily function. They assist in regulating mood, growth and development, tissue function, metabolism, sexual function and the reproductive processes. Whilst the nervous system controls processes that happen quickly such as breathing and movement, the endocrine system controls the slower processes like cell growth.

Hormones, as the body’s chemical messengers, transfer information by circulating through the bloodstream. The glands secrete hormones to be transported to another part of the body. The hormones go straight from the glands to the bloodstream, and may travel to any part of the body. However, they will only transmit messages to the cells that they are intended for.


6.5.8 – State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance

Homeostasis controls the variables in our bodies to maintain health. The result of disrupting this is called stress, which will lead to disease if it is not corrected. These variables include blood glucose concentration, blood pH, body temperature, CO2 concentration and water balance. These levels are maintained at constant levels within narrow limits.


6.5.9 – Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms

For all the variables controlled in homeostasis, there is a set point around which the body fluctuates within a certain range. Negative feedback is used to return the variable to its set point.

The body has many sensors which detect and signal when the variable fluctuates from the set point. This information is passed onto the control centre to direct the action that should be taken to rectify this. The effectors are the mechanism for returning the variable to its set point, and they switch on or off under the direction of the control centre. The response is produced by the effector.

The variables maintained by homeostasis include blood pH, CO2 concentration, blood glucose concentration, body temperature and water balance. Negative feedback relies on the nervous or endocrine systems. It has the opposite effect to try and stabilise the variable back at the set point. However, negative feedback is only triggered when there is a significant deviation from the set point.

6.5.10 – Explain the control of body temperature, including transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering

The hypothalamus, located in the brain, and the cerebral cortex maintain homeostasis. The hypothalamus sends signals to the pituitary gland in the form of hormones. The pituitary gland then secretes stimulating hormone into the bloodstream to the target gland, which in turn secretes its hormones. The hypothalamus and the pituitary gland can regulate the levels of hormones in the blood. The process used for regulating temperature is called negative feedback.

In humans, the set point for body temperature is 37°C. The body detects if the body goes above or below this using sensors such as the hypothalamus, skin warmth receptors and skin cold receptors.

Response Below the Set Point:

If the temperature is lower than 37°C, the sympathetic nervous system has an involuntary response:

  • vasoconstriction to lower blood flow to the skin to decrease heat loss
  • increased metabolism to increase heat production
  • shivering to increase heat production
  • piloerection, or goosebumps, to decrease heat loss

In addition, the cerebral cortex directs the following voluntary responses:

  • rest to decrease heat loss
  • behavioural responses such as warmer clothing, muscular activity, warm drink, curling up and eating

The effectors will increase heat production and decrease heat loss until the temperature is at 37°C.

Response Above the Set Point:

If the temperature is higher than 37°C, the sympathetic nervous system has an involuntary response to try an increase heat loss:

  • decreased metabolism to decrease heat production
  • sweating to increase heat loss
  • lethargy to decrease heat production
  • skin arterioles increase in diameter to increase heat loss
  • relaxed skeletal muscles to lower heat production

Our bodies also have some voluntary responses controlled by the cerebral cortex:

  • rest to decrease heat production
  • behavioural responses such as cool drinks, cooler clothing and fanning

The effectors will try to decrease heat production and increase heat loss until the temperature reaches 37°C

6.5.11 – Explain the control of blood glucose concentration, including the roles of glucagon, insulin and a and b cells in the pancreatic islets

Blood glucose concentration fluctuate throughout the day, usually from about 4 to 8 millimoles dm-3. Using negative feedback, the body can alter the rate at which glucose is taken up into the blood. The set point for blood glucose concentrations is about 90 mg/ 100 mL. The control centre for this is the pancreatic islets.

Response Above Set Point

The beta cells in the pancreatic islets produce insulin, the chemical which stimulates the uptake of glucose from the blood for conversion into glycogen or for respiration. This lowers the blood glucose concentration.

The insulin binds to receptors on the muscle and liver cells to allow glucose to move from the blood into the cells. The glucose is either metabolised or stored. This continues until the concentration is at the set point.

Response Below Set Point

The alpha cells in the pancreatic islets produce the chemical glucagon, stimulating liver cells to convert glycogen into glucose. The blood glucose concentration rises as a result.

The glucagon binds to the receptors of liver cells to activate enzymes that break down glycogen to glucose. The glucose moves in to the blood until the concentration is at the set point.

6.5.12 – Distinguish between type I and type II diabetes

People who have diabetes have blood glucose levels that are too high. The glucose is not able to get form the blood to the cells to provide energy.

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6.4 – Gas Exchange

6.4 – Gas Exchange

6.4.1 – Distinguish between ventilation, gas exchange and cell respiration

Ventilation – The pumping mechanism that moves air in and out of the lungs efficiently, thereby maintaining the concentration gradient for diffusion.

Gas Exchange – The exchange of gases between an organism and its surroundings, including the uptake of oxygen and the release of carbon dioxide in animals and plants.

Cell Respiration – The controlled release of energy in the form of ATP from organic compounds in cells. It is a continuous process in all cells.


6.4.2 – Explain the need for a ventilation system
The ventilation system is needed to maintain a high concentration gradient in the alveoli. Given that humans have such high demand for oxygen, it needs to be able to be delivered to all their cells in order to support respiration.

The lungs are the respiratory surfaces used for gas exchange. The action of ventilation brings air down into the lungs for exchange. Without it, the lungs would be useless, as no air would be able to reach their surface.

The concentration gradient in the alveoli is maintained using air flow and blood flow. Oxygen (O2) enters the lungs, diffuses across, and enters the bloodstream. Carbon dioxide (CO2) leaves the blood. The concentration of oxygen on one side is kept high, while the concentration of carbon dioxide remains low.


6.4.3 – Describe the features of alveoli that adapt themselves to gas exchange

The alveoli have a large total surface area, which increases the amount of gas that can be diffused across at any given time. This is the result of their spherical shape.

They form a thin layer of flattened cells, which allows for close association with the capillaries and a shorter distance for diffusion into the bloodstream. The wall of alveoli is only one cell thick.

The alveoli are surrounded by a dense capillary network. These then carry the oxygen in the blood to the pulmonary vein to be taken to the heart.

They have a film of moisture for the solutions of gases. The oxygen is able to dissolve in the lipoprotein-based lubricating film.

6.4.4 – Draw and label a diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli

6.4.5 – Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostal muscles, the diaphragm and abdominal muscles

𝑨𝒊𝒓 → 𝑷𝒉𝒂𝒓𝒚𝒏𝒙 → 𝑬𝒑𝒊𝒈𝒍𝒐𝒕𝒕𝒊𝒔 → 𝑳𝒂𝒓𝒚𝒏𝒙 → 𝑻𝒓𝒂𝒄𝒉𝒆𝒂 → 𝑳𝒖𝒏𝒈𝒔

When the chest cavity enlarges, the pressure changes, causing air to enter the lungs to equalise it. The air is then pushed back out when the diaphragm relaxes. The diaphragm is attached to the base of the sternum, the lower parts of the rib cage and the spine.

The lungs are surrounded and protected by the rib cage. The intercostal muscles are attached to the rib cage. The area inside is called the thorax, where the lungs are. The inner surface of the thorax holds the pleural membrane, which secrete pleural fluid. This fluid protects the lungs from friction caused by breathing.

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6.3 – Defence Against Infectious Diseases

6.3 – Defence Against Infectious Diseases

6.3.1 – Define pathogen

An organism or virus that causes a disease or sickness. These are usually microorganisms.


6.3.2 – Explain why antibiotics are effective against bacteria but not against viruses

Antibiotics slow or kill off bacterial diseases. They are found in nature, mainly in fungi or bacteria, and slow down the growth of microorganisms. They enter the bacterial cells during their growth and division.

Only some antibiotics are safe to use as drugs. Some are called broad spectrum antibiotics because they are effective on a range of microorganisms.

Antibiotics inhibit growth by disrupting the metabolism of their target pathogens, preventing the synthesis of a new cell wall during mitosis, and causing the cell to rupture. The metabolic pathways that they target are only found in bacteria, and not in eukaryotic cells.



In some cases, the antibiotics may attack the dormant spores of the bacteria instead.

On the other hand, antibiotics have no effect on viruses because they are not living organisms, and therefore have no metabolism. Instead, a virus will replicate using the metabolism of its host cell. Hence, antibiotics should not be prescribed for viral infections.

6.3.3 – Outline the role of skin and mucous membranes in defence against pathogens

Although there are many different types of bacteria that safety live in or on our bodies, others act as pathogens and need to be prevented from entering. Intact skin forms a barrier against the entry of pathogens into the body. Skin is toughened on the surface by the protein keratin. Most microorganisms cannot get past the skin. For further protection, the skin also produces chemical secretions that prevent the growth of fungi and bacteria on the skin. It also has a low pH, which would hinder the colonisation of bacteria.

If a pathogen enters the body, it will because trapped in the sticky mucous and expelled by the cilia. Tears, saliva and mucous all help to wash bacteria away. Many pathogens may try to enter the lungs, but are prevented by the mucous in the trachea, bronchi and bronchioles. Some cells have cilia which move the mucous up to the epiglottis, where the pathogens can then be swallowed and killed in the acidic environment of the stomach.


6.3.4 – Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues

Phagocytes are a form of leukocytes, and are part of the body’s second line of defence against disease. They ingest bacteria, viruses and dust particles and destroy them. They are able to change shape so that they can ingest microbes.

The membrane of the phagocyte changes shape to surround the microbe, then joins together to form a vesicle, or phagozome. The lysosomes in the phagocyte then fuse with it, releasing its enzymes to kill the microbes. This process is called phagoctyosis.


6.3.5 – Distinguish between antigen and antibodies

Antigen – A foreign substance that stimulates the production of antibodies. It is recognised
by the immune system, triggering this immune response.
Antibodies – Proteins, immunoglobin, that recognise and bind to specific antigens. These
have a T or Y shape made from polypeptide chains.


6.3.6 – Explain antibody production

Antibodies are produced by the lymphocytes, which are a form of leukocytes. The lymphocytes are specialised so that each one makes a specific antibody, so there is a huge range of different lymphocytes in the body.

The antibodies form on the surface of the lymphocyte membrane so that the antigen combining site points outwards. These sites will bind with specific antigens on the pathogens.

When the antibody and antigen bind, the lymphocyte becomes activated, and will begin cloning itself through mitotic division. The clone cells will also make antibodies to help fight the pathogen and defend the body.

6.3.7 – Outline the effects of HIV on the immune system

HIV stands for the Human Immunodeficiency Virus. It is a retrovirus, which means it uses RNA to make DNA. The virus contains two strands of RNA inside a protein coat and the enzyme reverse transcriptase. The virus takes advantage of the lysogenic cycle to replicate the virus within the human’s cells. It is capable of resting dormant.

When the virus enters the host cell, the RNA is translated into DNA, and then inserted in the DNA of the cell. From here, the virus in replicated and will remain with the cell even after it divides.

The outside of the virus is covered in binding proteins which attach to the CD4 receptors on helper T-cells. The virus capsule will then fuse with the membrane of the cell and the RNA is able to enter. However, the outside of the virus, which has the antigens, remains on the outside of the cell.

HIV remains dormant for a long period of time. It is not until about 10 years later that the HIV genes may become activated and the cell begins to produce the virus. As it is continuously replicated, the new viruses will infect other T-cells.

The key effects of the HIV virus are a reduction in the number of active lymphocytes and a loss of ability to produce antibodies. Since the lymphocytes are essential for defending the body against pathogens, the immune system is significantly weakened by HIV, allowing diseases to infect the body.

It is difficult to fight the virus because it targets HTC cells, which conduct immune responses. This damages the immune system because it becomes depressed and unable to recognise a pathogen. The B-cells are not properly activated so that they have limited AMI response. Also, fewer B-cells are produced which means that they have less memory cells produced. There is limited activation of KTC so that the person has limited CMI response.

Since the immune system becomes depressed, a small illness could kill the person. Over time, the body has fewer and fewer lymphocytes, resulting in fewer antibodies, and the body is vulnerable to infections.


6.3.8 – Discuss the cause, transmission and social implications of AIDS

AIDS is acquired immune deficiency syndrome. It is the disease that develops when the body becomes infected with the HIV virus. The immune system becomes depressed and the body is vulnerable to infection.

The negative social implications include:

  • Many consider it to be the “Gay Disease,” causing discrimination against those who are homosexual or have AIDS
  • Individuals who have AIDS may be shunned because those who do not understand it may think that they will contract it if they make contact with the person.
  • Those with limited understanding of the disease may have unreasonable fear of it.
  • Families affected by AIDS suffer grief for the victim  Victims may suffer from guilt or loneliness
  • Increased poverty and unemployment, as infected people may find it harder to work and may be refused life insurance. Housing may also refused or difficult to obtain.
  • Sexual activity in the population is reduced.
  • Children may be orphaned, and responsibility on older siblings may be increased so that they cannot continue their education
  • Young children may be infected at birth, leading to early death
  • Treating AIDS patients may lead to other diseases not being treated properly

The positive social implications include:

  • More research is done on disease
  • There is greater public awareness of safe sexual behaviour
  • Sterile needles are given to drug users, making the process safer
  • Blood is screened during blood transfusions to ensure there is no disease present
  • There is treatment available for those with HIV

AIDS is caused by the HIV virus, which can be transmitted though:

  • Vaginal, anal or oral intercourse, especially if there are cuts or tears present
  • Sharing of hypodermic needles between drug abusers.
  • The placenta, from mother to child
  • Blood transfusion
  • Cuts during childbirth or milk from breast feeding
  • Blood factors used to treat haemophilia


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6.2 – The Transport System

6.2 – The Transport System

6.2.1 – Draw and label a diagram of the heart showing the four chambers, associated blood vessels, valves and the route of blood through the heart


6.2.2 – State that the coronary arteries supply the heart muscle with oxygen and nutrients
Coronary arteries are blood vessels that provide oxygen-rich blood and other nutrients to the heart. They attach to and wrap around the heart’s surface. They branch from the aorta to carry blood back to the heart muscle.

The left coronary artery branches off into smaller arteries, including the left anterior descending artery, supplying blood to the front of the heart. The left circumflex artery encircles the heart and supplies blood to the back of the heart. It supplies the left atrium.

The right coronary artery branches to the right marginal branch to supply blood to the lower right side of the heart. This supplies the right atrium.

These vessels carry blood so that the heart muscle cells are supplied with the oxygen and nutrients needed for them to work


6.2.3 – Explain the action of the heart in terms of collecting blood, pumping blood, and the opening and closing of valves

When the heart contracts, the volume of the chambers decreases, increasing the pressure and causing blood to be pushed into a region of lower pressure. The valves in the heart prevent the blood from flowing back again.

The first part to contract is the atria, pushing the blood past the bicuspid valve into the ventricles. The muscles of the atria then relax. The muscles of the ventricles contract, closing the bicuspid valve and forcing blood into the aorta through the semi lunar valves (or pulmonary artery on the right side). Both the atria and ventricle muscles relax, and blood flows into the atria.

Deoxygenated blood enters the right atrium of the heart from the superior and inferior vena cava, whilst oxygenated blood enters the left atrium from the pulmonary veins. As the blood flows into the ventricles, the direction of flow is controlled by the semilunar valves and the atrioventricular valves to prevent any backflow. From the ventricles, the blood is pumped into the arteries.


6.2.4 – Outline the control of the heartbeat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and epinephrine (adrenaline)

The heartbeat is described as myogenic in origin because it originates from the heart itself, and does not require nerve impulses to stimulate it. This is why the heartbeat is not voluntary. The pacemaker is a structure in the wall of the right atrium. Impulses are sent to the muscles of the atria through special muscles muscle fibres to trigger contraction of the atria, followed by the ventricles. There is a rest period while the heart refills with blood, where the heart becomes insensitive to stimulation.

This is an involuntary response, controlled in the medulla to speed up or slow down depending on our body’s needs. Two nerves send these messages, and they are described as antagonistic. The heat rate is changed in response to stress, anticipation, emotion, or to low or high blood pressure. This is the result of increased release of adrenaline.

6.2.5 – Explain the relationship between the structure and function of arteries, capillaries and veins

Arteries carry blood away from the heart. They have thick, strong elastic walls due to the presence of collagen fibres and smooth muscle. Blood in the arteries is under high pressure, and travels in pulses, thus they must have strong wall to be able to cope with this. The pressure is increased by the narrow lumen. The middle layer of muscle is very thick to prevent leaks and bulges.

Veins carry blood back to the heart. They also have strong walls, but these are thinner and have fewer elastic fibres. Blood in the veins is under lower pressure and does not travel in pulses. There is little risk of them bursting so they are not as thick as arteries, with only a thin layer of muscle surrounding them. These nearby muscles contract to help push the blood back to the heart. To prevent blood from flowing backwards, veins have valves. They have a wide lumen.

Capillaries branch out from the arteries in veins to bring blood closer to cells. They consist solely of an endothelium, only one cell thick. This allows for more rapid diffusion. They also have pores in their membranes for the secretion of plasma and phagocytes. Capillaries have a narrow lumen, which allows many capillaries to fit in a small space.

𝒂𝒐𝒓𝒕𝒂 → 𝒂𝒓𝒕𝒆𝒓𝒚 → 𝒂𝒓𝒕𝒆𝒓𝒊𝒐𝒍𝒆 → 𝒄𝒂𝒑𝒊𝒍𝒍𝒂𝒓𝒚 → 𝒗𝒆𝒏𝒖𝒍𝒆 → 𝒗𝒆𝒊𝒏 → 𝒗𝒆𝒏𝒂 𝒄𝒂𝒗𝒂


6.2.6 – State that blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets

Plasma is the liquid medium of blood in which all the other elements are suspended. Through plasma, substances are exchanged between cells and tissues. It transports nutrients, excretory products like urea, hormones, dissolved proteins, antibodies and heat.

Red blood cells are also called erythrocytes, and they transport the respiratory gases to cells. This includes oxygen and carbon dioxide.

White blood cells are also called leucocytes, and combat infection. Lymphocytes form antibodies as part of the immune system, whilst phagocytes ingest bacteria and cell fragments.

Platelets are important for blood clotting.

6.2.7 – State that the following are transported by the blood: nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat

Blood carries oxygen to all tissues for respiration, and then brings the carbon dioxide which is produced back to the lungs. White blood cells form antibodies, which are also suspended in the plasma.

Nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat are all found in the
plasma, which is the transport medium, taking them to their various destinations in the

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6.1 – Digestion

6.1 – Digestion

6.1.1 – Explain why digestion of large food molecules is essential

When we eat food, the carbohydrates and other molecules are very large and insoluble. They would not be able to pass through the cell membranes to get into the bloodstream. Digestion breaks them down into smaller molecules that are soluble and can be taken up into the bloodstream. These smaller molecules can then be used to build up the macromolecules necessary for the function of our bodies. For example, large polypeptides are broken up into their amino acids, which are then used to build up new proteins.

6.1.2 – Explain the need for enzymes in digestion

An enzyme is a biological catalyst which lowers the activation energy in order to speed up a reaction. Each enzyme is specific to a particular substrate molecule, so a number of different digestive enzymes are secreted to be able to break down all the different food types. They are secreted for the hydrolysis of large polymers. These enzymes reach optimum functioning at body temperature. The enzymes lower the activation energy to allow the reaction to proceed more readily, whilst still at body temperature.

Without enzymes, we would not be able to digest our food because the temperatures required to break the bonds between the large molecules would be far too high for living organisms. However, without enzymes, the reactions involved in digestion would take place very slowly at body temperature, and we would not be able to survive. With them, the rate of reaction is significantly increased to allow for respiration at the necessary rate.

6.1.3 – State the source, substrate, products and optimum pH condition for one amylase, on protease and one lipase

Salivary Amylase

This enzyme comes from the salivary glands, found in the mouth, and breaks down polysaccharides like amylose, or starch. This is then broken down into disaccharides like maltose and glucose. The optimum pH of this enzyme is roughly neutral: 6.5-7.5.

Pancreatic Lipase

This is sourced from the pancreas and breaks down fats and oils, or triglycerides. The result is fatty acids and glycerol. The triglycerides form tiny droplets called emulsified lipids. Bile salts must be present for the lipids to be emulsified. This process increases the surface area and exposes the head of the glycerol molecule to allow the enzymes to act on it. The optimum pH of lipase is 7.0.

This is a protease found among the gastric juices in the stomach. It works best at pH 2.0. It breaks down large polypeptide chains into smaller peptides. This is done through the hydrolysis of the peptide bonds in the chain.

6.1.4 – Draw and label a diagram of the digestive system


6.1.5 – Outline the function of the stomach, small intestine and large intestine


The food enters the stomach in the form of a bolus. The opening between the stomach and the oesophagus is called the cardiac sphincter. In the stomach, the food is churned around. Gastric juices are secreted into the stomach containing HCl and protease enzymes. The main enzyme used to break down proteins in the stomach is pepsin. The pH of the stomach is about 1.5-2.0, which is optimum for protein digestion, as well as killing off many harmful microorganisms.

The mucus in the stomach is secreted from the goblet cells, which are located along the lining of the stomach. This protects the lining from being broken down by all the acids, called autolysis. The end result of the churning is that the food becomes semi-liquid chyme.


Small Intestine
The opening between the stomach and the small intestine is called the pyloric sphincter. In the small intestine, all the soluble products of digestion are absorbed into the bloodstream for use around the body. The first section is the duodenum where the chyme is mixed with bile to lower the pH, then with pancreatic juices containing enzymes for the digestion of lipids, carbohydrates and proteins. This further breaks them down to the smaller monosaccharides, amino acids, small peptides, fatty acids and glycerol. Movement through the small intestine is maintained by peristalsis.

The pancreas secretes both enzymes and a buffer solution made up of a bicarbonate. The buffer helps to maintain a higher pH, resisting the addition of the acidic chyme.

An important feature of the small intestine is that its lining is covered with villi, which in turn are covered in microvilli. These increase the surface area to maximise absorption. The villi absorb nutrients via active transport to the blood.

Bile has an additional function, which is to emulsify the lipids in the duodenum. This breaks them apart, speeding up enzyme action later. 

The small intestine is also coated in mucus along the lining to protect the epithelial cells, which provide the ATP for active transport. The nutrients that are absorbed into the blood then travel to

The nutrients that are absorbed into the blood then travel to all the different parts of the body. They are taken up into the cells through assimilation.


Large Intestine

The large intestine has many folds to increase the surface area for absorption. Most of the products that enter the large intestine are indigestible material such as fibre, dead cells, mucus and other things like minerals and water.

The appendix is located near the opening of the large intestine, however in humans it has no function.

In the first section, the colon, water and minerals are absorbed to leave more solid faeces. These are stored in the rectum until they are excreted through the anus, controlled by sphincter muscles. The movement of food is maintained by peristalsis.


6.1.6 – Distinguish between absorption and assimilation


Soluble products of digestion are absorbed into the blood circulation system, or the lymphatic system if they are fats droplets.


Products of digestion are absorbed into the cells from the blood to be stored or used within the tissues.

6.1.7 – Explain how the structure of the villus is related to its role in absorption of transport of the products of digestion

Villi play an important role in absorption of nutrients in the small intestine because their structure increases the surface area to maximise the process. In fact, the surface area is increased tenfold due to their presence. Each villus is covered with microvilli to further increase their

Each villus is covered with microvilli to further increase their surface area. Each villus has capillaries and lacteals inside it, which transport the nutrients to the rest of the body. The epithelial cells provide energy for active transport of nutrients.

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