OCR Gateway B Module B5: The Living Body

Skeletons and bones:

• An internal skeleton grows with the body, is flexible and provides an internal framework. It is easy to attach muscles
• Some animals have an external skeleton instead, which is made from chitin in insects
• Bones consist of a long shaft with marrow inside and cartilage at each end. Long bones are hollow
• Cartilage and bone are living tissues and can repair themselves
• At birth, only the cartilage is present. With the deposition of calcium and phosphorus, it is replaced by bone. This process is called ossification. Lack of these elements in the elderly can cause osteoporosis, making them prone to fractures
• There are different types of fractures: compound fractures are fractures where some fragments of the bone penetrate soft tissue and can break through the skin from the inside. Simple fractures are when this does not happen and no tissue or skin is affected. Greenstick fractures are where the bone is not completely broken, just bent or twisted out of shape. X-rays are used to detect fractures
• Sharks' skeletons are only made from cartilage
• Some human bones, such as the outer ear, nose and ends of long bones is made from some cartilage
• Despite being very strong, bones can easily be broken by a sharp knock

Joints:

• Joints are when two or more bones meet, joined by ligaments
• Bones are moved by muscles, attached by tendons
• Synovial joints (e.g. hinge, or ball and socket) contain synovial fluid (lubricant), a membrane, ligaments and cartilage
• A hinge joint moves up and down. The forearm is raised and lowered by antagonistic muscles (biceps/triceps). To raise the forearm, biceps (on top) contract and the triceps (below) relax
• The ulna is the bottom section of the arm and the radius is the top part. The humerus is at the top of the bone
• Ligaments hold bones in place. Synovial membranes hold in the synovial fluid

The circulatory system:

• Some animals (like amoeba) don't have a blood circulatory system. Some (like insects) have an open circulatory system (explained below), and some (e.g. humans) have a closed circulatory system
• Open circulatory systems are when blood flows freely through their body cavity. Closed circulatory systems use blood vessels to transport blood around the body
• Mammals have a double circulatory system: collecting oxygen and delivering it. Fish, however, only have one circuit
• A double circulatory system requires 4 chambers in the heart: two atria to receive blood and two ventricles to deliver blood. This pumps blood at a higher pressure so materials can be transported more quickly around the body
• In the second century, Galen knew that the heart was a pump and about pulse, but thought that the liver made blood. In the 17^th century, Harvey corrected this
• The cardiac cycle is the events as blood enters and leaves the heart. In humans, two atria contract and two ventricles relax to receive blood through atrio ventricular valves. Ventricles contract to force blood to the lungs/body; semi-lunar valves prevent backflow
• An increase in heart rate is caused by more muscular activity or by the adrenaline hormone, which prepares for the 'fight or flight' reaction
• Pacemaker cells in the heart control the rate with a current which stimulates muscle contractions; the sino-atrial node (SAN) stimulates atria contraction and the atrio-ventricular node (AVN) stimulates the ventricles to contract
• An electrocardiogram (ECG) shows these electrical impulses
• An echocardiogram displays a video. Both show irregular heart actions
• Pulse can be detected at various places, such as at a wrist, ear or temple

Heart problems:

• The heart cannot work with a hole in the wall, although unborn babies have one as they only have a single circulatory system; their lungs aren't yet functioning
• If there is a hole in the heart, oxygenated and deoxygenated blood is mixed
• A blocked coronary artery reduces the flow to the heart, but can be bypassed through a blood vessel transplant
• Major problems are fixed with heart transplants
• Heart assist devices can provide extra pressure
• Damaged/weak valves reduce blood pressure, but can be replaced with artificial ones
• Surgery is risky and requires donors. Artificial pacemakers/valves are often a better solution

Blood donation:

• Blood must be collected from another person. Anti-coagulants, such as heparin, warfarin and aspirin, prevent clogging
• People with haemophilia have blood which doesn't clot. Clotting is a cascade process (there are many steps involved):
  - Platelets are exposed to air, triggering chemical reactions, which creates a clot, which is red blood cells supported by a mesh of fibrin fibres
• People can only receive blood from a compatible blood group, out of the four possible groups, A, B, AB and O:
  - A can donate to A or AB
  - B can donate to B or AB
  - AB can donate to A or B
  - O can donate to O, or to any of the other groups for a short period of time (a few weeks)
• Agglutination (blood clumping) happens with incompatible blood types, agglutinins in red blood cells react with the plasma
• Antigen A will react with anti-A, likewise, antigen B will react with anti-B. This must be prevented for a safe transfusion
• These four blood groups (A, B, AB, O) are further subdivided into Rhesus positive and negative

Breathing:

• Amphibians use moist, permeable skin to obtain oxygen. Therefore, they need moist habitats
• Fish gills obtain oxygen by water passing over filaments
• The main parts of the human respiratory system are:
  - Trachea (the windpipe): from the nose and mouth to the two bronchi
  - Bronchus: a tube leading from the trachea to a lung
  - Lungs: contain gas exchange surfaces
  - Bronchiole: small tubes inside the lungs, from bronchus to alveoli
  - Alveoili: small air sacs at the end of bronchioles, the location of gas exchange (ventilation)
  - Pleural membranes: allow the lungs to move easily during gas exchange
  - Ribs: enclose and protect the lungs, moveable
  - Intercostal muscles allow the ribcage to change in volume for breathing
  - Diaphragm: works with the intercostal muscles, more on this is explained below
• When we inhale (inspiration):
  - Intercostal muscles contract. The diaphragm is moved downwards, increasing chest volume, which reduces pressure, causing air from outside (high pressure) to enter the lungs
• When we exhale (expiration):
  - Intercostal muscles relax. The diaphragm is moved upwards, decreasing chest volume, increasing pressure, forcing air out
• There is diffusion between the alveoli (bulges of air sacs) and air in the air sacs because air oxygen concentration is higher than in deoxygenated capillaries in the alveoli
• Exchange surfaces have a large surface area and a good blood supply. They are permeable, moist and only 1 cell thick

Measures of lung capacity:

• Tidal air is the volume of air breathed at rest
• Vital capacity is the maximum volume of air that can be exchanged
• Residual air is the air that remains in (cannot be forced out of) the lungs
• Spirometer readings can help diagnose lung diseases, measure lung capacities and rate of air flow

Respiratory diseases:

• Cystic fibrosis (genetic) causes too much mucus to be produced in the bronchioles
• Breathing in lots of asbestos fibres, scarring and causing inflammation to lung tissue, can cause asbestosis (an industrial cause)
• Wheezing, tight chest and breathing difficulties are symptoms of asthma. During an asthma attack, the lining of airways becomes inflamed, mucus and fluid build up in the airways and the muscles around bronchioles contract, narrowing airways. Inhalers are used to relieve these symptoms
• In humans, air must go in and out, so chemical particles such as tar can get trapped in air sacs
• Smoking and other lifestyle factors increase the risk of lung cancer (caused by lifestyle)

Parts of the digestive system:

• The pancreas (between the stomach and intestines) and salivary glands (back of the mouth) produce digestive juices
• The stomach digests food
• The liver, to the right of the stomach, produces bile
• The gall bladder, near the liver, stores this bile
• The small intestine digests and absorbs soluble food
• The large intestine absorbs water from undigested food,

Physical digestion:

• Physical digestion, such as chewing, allows for easy travel in the digestive system and creates a larger surface area so the food can be digested quicker

Chemical digestion:

• In the mouth, carbohydrase breaks down starch into sugar in two processes; starch (many linked units) is broken down into maltose (2 linked units), which is broken down into glucose (1 unit)
• In the stomach, protease breaks down protein into amino acids. The protease pepsin, found in the stomach, works best in strong acids, such as stomach acid. Some proteases work best with a higher pH, however
• In the small intestine, lipase breaks down fat into fatty acids and glycerol
• The gall bladder stores bile, which is released into the small intestine to emulsify fats, increasing the surface area

Food absorption:

• Food molecules need to pass through the small intestine walls, with diffusion, into blood plasma/lymph
• Digested carbohydrates and protein molecules diffuse through walls
• Digested fat molecules are not soluble in water/plasma because they are too large, they diffuse into the lymph
• The small intestine has a large surface area; it has villi in the walls (which contain lots of microvilli) which can absorb molecules. It also has a lymphatic system of lacteals, containing lymph

Kidney structure/function:

• Excretion is getting rid of chemical waste from the body
• Egestion is the removal of undigested semi-solid waste
• Unwanted amino acids are broken down in the liver to form urea, which is taken to the kidneys in the blood, through the renal artery. Blood without waste leaves through the renal vein
• Each kidney has an outer cortex and inner medulla. Waste leaves through the ureters as urine
• Blood flows through the kidneys at high pressure, so filtration is at high pressure. Water, glucose and salt are reabsorbed into the blood
• Kidneys contain millions of kidney tubules (nephrons) where filtration takes place and urine is formed
• Nephrons have the following features:
  - Capillaries (the glomerulus) surrounded by a capsule, a filtering unit
  - A region where some materials (e.g., glucose) are selectively reabsorbed
  - A region where salt and water levels are regulated, depending on body demand
  
• If someone has a kidney failure, a dialysis machine is used, which removes urea from blood with diffusion and uses different sizes of tube to increase pressure
• Dialysis fluid contains sodium salts to maintain the levels
• The kidneys excrete urea, water and salt in urine

Regulating urine concentration:

• After drinking water, urine concentration decreases
• During exercise and/or hot weather, more sweat is produced, increasing urine concentration
• The pituitary gland in the brain produces the anti-diuretic hormone (ADH) which controls urea concentration by increasing permeability of kidney tubules so more water is reabsorbed, and using a negative feedback mechanism to control ADH production

Carbon dioxide concentration:

• CO₂ in high concentrations is toxic, so receptors in the carotid artery cause breathing to increase, reducing these levels

Menstrual cycle control:

• The menstrual cycle has three stages: menstruation (the uterus lining breaks down), thickening of the uterus lining and ovulation (egg release)
• 4 sex hormones control the menstrual cycle; FSH stimulates an egg to develop in the ovary, LH controls ovulation (egg release), progesterone maintains the uterus wall and oestrogen repairs the uterus wall
• Negative feedback mechanisms control the levels of each hormone. The cycle is triggered by receptors in the hypothalamus gland
• Oestrogen and progesterone are produced in the ovaries and testosterone is produced in the testes. These are all hormones
• If fertilisation doesn't occur, oestrogen and progesterone levels decrease, causing menstruation to occur
• If an egg is fertilised, progesterone levels remain high and no FSH is produced, so more eggs develop and the uterus lining doesn't break down
• Artificial sex hormones make the body think that it's pregnant, so no FSH is released

Infertility treatments:

• Some causes of infertility are: blockage of sperm ducts or fallopian tubes, eggs not developed or released, and insufficient fertile sperm produced
• Artificial insemination, where sperm is injected into the vagina
• Using FSH
• IVF (fertilisation outside the body)
• Egg donation, where an egg is donated from another female
• Surrogacy, where a fertilised egg is placed inside another female
• Ovary transplants
• Not all people agree with these treatments. They are also expensive - IVF costs over £6,000, for example, and only has a 25% success rate. There are also medical issues, like rejection, and moral issues

Foetal screening:

• This is to check for abnormalities, although there are ethical issues with this
• Methods:
  - Amniocentesis: extracting and testing cells in the amniotic fluid
  - Chromosomal analysis: a blood test to check for chromosome abnormalities

Growth rate:

• Growth is increased by regular exercise and a balanced diet containing calcium, phosphorus, vitamin D and proteins
• Height extremes are caused by genes or a hormone imbalance
• Different body parts grow at different rates
• The length, mass and head size of babies are regularly measured and compared against average growth charts
• The human growth hormone is produced by the pituitary gland

Life expectancy:

• Life expectancy is increased by a healthier lifestyle, better housing (reducing risk of diseases like tuberculosis), less deaths from industrial diseases and advances in medicine, such as transplants and antibiotics

Organ donation:

• Supply is limited by a shortage of donors and a necessity of size and age matches
• Mechanical replacements (such as heart and lung machines, kidney dialysis machines and mechanical ventilators) require a power source and the body must be resistant to the materials used
• Living people can donate blood, bone marrow and one kidney
• Transplants can be rejected by the body, so they need lifetime immuno-suppressive drugs, which weaken the immune system
• Some countries have 'opt-out' donor registers, but most have 'opt-in' systems like the UK