WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:09.089 Unit S B 7: Animal coordination, control and homeostasis. 00:00:11.767 --> 00:00:15.825 The nervous system sends rapid electrical signals along neurones. 00:00:15.975 --> 00:00:23.226 Hormonal communication uses chemical messengers carried in the blood and is usually slower, with longer-lasting effects. 00:00:25.900 --> 00:00:29.641 Endocrine glands release hormones directly into the bloodstream. 00:00:29.791 --> 00:00:35.570 Blood carries a hormone around the body, but only target cells with suitable receptors respond to it. 00:00:38.233 --> 00:00:42.091 A target organ is an organ affected by a particular hormone. 00:00:42.241 --> 00:00:47.209 A hormone may affect several target tissues; it does not act on every cell it passes. 00:00:49.867 --> 00:00:52.832 The pituitary gland lies at the base of the brain. 00:00:52.982 --> 00:01:00.489 It releases F S H, L H and growth hormone, and hormones such as T S H that influence other endocrine glands. 00:01:03.167 --> 00:01:06.227 The thyroid gland in the neck produces thyroxine. 00:01:06.377 --> 00:01:11.923 The adrenal glands above the kidneys produce adrenalin; adrenaline is an alternative spelling. 00:01:14.600 --> 00:01:17.490 The pancreas produces insulin and glucagon. 00:01:17.640 --> 00:01:22.802 The ovaries produce oestrogen and progesterone; the testes produce testosterone. 00:01:22.952 --> 00:01:27.400 A hormone directory: target cells respond through suitable receptors. 00:01:32.067 --> 00:01:35.477 Growth hormone supports growth of bones and other tissues. 00:01:35.627 --> 00:01:46.389 Puberty involves changes in sex hormones and growth-hormone activity; it is an oversimplification to say that sex hormones alone directly make the pituitary release growth hormone. 00:01:49.067 --> 00:01:55.701 Homeostasis maintains a stable internal environment despite changes inside or outside the body. 00:01:55.851 --> 00:02:00.991 Examples include blood glucose concentration, body temperature and water balance. 00:02:03.667 --> 00:02:09.265 Stable does not mean perfectly unchanging: conditions fluctuate around suitable levels. 00:02:09.415 --> 00:02:13.717 Maintaining these levels helps cells and enzymes function effectively. 00:02:16.367 --> 00:02:19.905 Negative feedback reverses a change from the normal level. 00:02:20.055 --> 00:02:26.059 If a level rises too high, the response brings it down; if it falls too low, the response brings it up. 00:02:26.209 --> 00:02:29.883 The response gets smaller as the level returns towards normal. 00:02:32.533 --> 00:02:39.300 If a regulated level rises too far, feedback can reduce it; if it falls too far, feedback can raise it. 00:02:39.450 --> 00:02:43.134 The response must oppose the change, rather than amplify it. 00:02:45.800 --> 00:02:52.391 Negative feedback is a control mechanism, while homeostasis is the maintenance of the internal environment. 00:02:52.541 --> 00:02:56.163 The two terms are related but are not identical definitions. 00:02:58.833 --> 00:03:04.765 Thyroxine helps control metabolic rate: the rate at which chemical reactions occur in the body. 00:03:04.915 --> 00:03:07.453 It also supports growth and development. 00:03:10.133 --> 00:03:17.727 When blood thyroxine concentration is low, the hypothalamus releases more T R H, thyrotropin-releasing hormone. 00:03:20.400 --> 00:03:26.330 T R H stimulates the pituitary gland to release T S H, thyroid-stimulating hormone. 00:03:29.000 --> 00:03:34.439 T S H stimulates the thyroid gland to produce and release thyroxine into the blood. 00:03:37.100 --> 00:03:44.714 When thyroxine returns to a suitable level, it reduces (inhibits) further T R H release and T S H production. 00:03:44.864 --> 00:03:50.123 The thyroid receives less stimulation, so it does not keep producing too much thyroxine. 00:03:52.800 --> 00:03:58.613 This is negative feedback: the increase in thyroxine reduces the signals that caused its production. 00:03:58.763 --> 00:04:04.506 Do not confuse the thyroid, which makes thyroxine, with the pituitary, which makes T S H. 00:04:04.656 --> 00:04:09.871 Thyroxine inhibits its stimulating signals: an example of negative feedback. 00:04:14.533 --> 00:04:20.057 Adrenalin is released from the adrenal glands during frightening or exciting situations. 00:04:20.207 --> 00:04:23.533 It prepares the body for a rapid fight-or-flight response. 00:04:26.200 --> 00:04:31.415 It increases heart rate and blood pressure, and increases blood flow to skeletal muscles. 00:04:31.565 --> 00:04:35.103 This helps deliver more oxygen and glucose for respiration. 00:04:37.767 --> 00:04:43.674 Adrenalin stimulates the liver to break glycogen down into glucose and release glucose into the blood. 00:04:43.824 --> 00:04:48.551 Glycogen is a stored carbohydrate; glucose is the circulating sugar. 00:04:51.233 --> 00:04:59.244 Adrenalin changes where blood flows: vessels supplying skeletal muscles widen (dilate), while vessels in some other regions narrow. 00:04:59.394 --> 00:05:02.910 More blood reaches the muscles to support rapid activity. 00:05:05.567 --> 00:05:08.296 The nervous system and hormones can work together. 00:05:08.446 --> 00:05:16.061 An adrenalin response is a temporary preparation for activity, rather than the same long-term control pathway as thyroxine. 00:05:18.733 --> 00:05:22.653 The menstrual cycle prepares the uterus for a possible pregnancy. 00:05:22.803 --> 00:05:25.065 An egg matures and is released. 00:05:25.215 --> 00:05:31.507 Cycles start during puberty and stop at menopause; their length and the ages at which they start and stop vary. 00:05:34.167 --> 00:05:36.727 Day 1 is the first day of menstruation. 00:05:36.877 --> 00:05:42.774 The uterus lining is shed when hormone levels fall; menstruation is not simply the loss of an egg. 00:05:45.433 --> 00:05:50.853 After menstruation, the lining is repaired and thickens under the influence of oestrogen. 00:05:51.003 --> 00:05:54.543 It does not wait until a fixed day 11 to begin repairing. 00:05:57.200 --> 00:06:00.165 Ovulation is the release of an egg from an ovary. 00:06:00.315 --> 00:06:06.875 In a simplified 28-day cycle it occurs around day 14, but actual timing varies with the cycle. 00:06:09.533 --> 00:06:14.099 After ovulation, progesterone helps maintain the thickened uterus lining. 00:06:14.249 --> 00:06:20.019 If pregnancy does not occur, progesterone and oestrogen fall and another menstruation begins. 00:06:22.700 --> 00:06:26.166 Sperm can travel through the cervix and uterus to an oviduct. 00:06:26.316 --> 00:06:33.249 Fertilisation is the fusion of sperm and egg nuclei, usually in an oviduct, also called a fallopian tube. 00:06:35.900 --> 00:06:40.637 The fertilised egg divides to form an embryo as it moves towards the uterus. 00:06:40.787 --> 00:06:46.663 Implantation is the embryo attaching to the uterus lining; it is different from fertilisation. 00:06:49.333 --> 00:06:55.871 Fertilisation is associated with the time around ovulation, not a fixed day 17, 21 window. 00:06:56.021 --> 00:07:01.662 A textbook 28-day example cannot reliably predict an individual's fertile days. 00:07:04.333 --> 00:07:08.889 F S H, follicle-stimulating hormone, is released by the pituitary. 00:07:09.039 --> 00:07:15.342 It stimulates an ovarian follicle containing an egg to mature and encourages oestrogen production. 00:07:18.000 --> 00:07:20.731 The developing follicle produces oestrogen. 00:07:20.881 --> 00:07:26.555 Oestrogen repairs and thickens the uterus lining and inhibits F S H for much of the cycle. 00:07:29.233 --> 00:07:34.183 A high oestrogen level just before ovulation stimulates an L H surge. 00:07:34.333 --> 00:07:40.583 This is a change in its feedback effect; do not assume oestrogen always inhibits every pituitary hormone. 00:07:43.233 --> 00:07:46.779 The pituitary releases L H (luteinising hormone). 00:07:46.929 --> 00:07:49.745 A sharp rise in L H triggers ovulation. 00:07:49.895 --> 00:07:53.908 The emptied follicle then forms a structure called the corpus luteum. 00:07:56.567 --> 00:08:00.371 After ovulation, the corpus luteum produces progesterone. 00:08:00.521 --> 00:08:11.517 This maintains the uterus lining and reduces (inhibits) F S H and L H release, helping prevent another follicle maturing and another egg being released in the same cycle. 00:08:14.200 --> 00:08:20.375 If pregnancy does not occur, the corpus luteum breaks down and progesterone and oestrogen decrease. 00:08:20.525 --> 00:08:26.039 The lining is shed and reduced inhibition allows F S H to rise for the next cycle. 00:08:26.189 --> 00:08:31.087 Sequence if pregnancy does not occur; actual cycle lengths and timings vary. 00:08:35.767 --> 00:08:45.120 When interpreting a hormone graph, look for an oestrogen rise before ovulation, a sharp L H surge around ovulation and a progesterone rise afterwards. 00:08:45.270 --> 00:08:49.496 These are schematic patterns, not identical curves in every person. 00:08:52.167 --> 00:08:55.109 Contraception reduces the chance of pregnancy. 00:08:55.259 --> 00:09:02.607 Different methods act in different ways, so preventing ovulation and blocking sperm should not be described as the same mechanism. 00:09:05.267 --> 00:09:11.473 The combined contraceptive pill contains oestrogen and a progestogen, which acts like progesterone. 00:09:11.623 --> 00:09:16.709 These hormones suppress pituitary F S H and L H and prevent ovulation. 00:09:19.367 --> 00:09:24.517 Progestogen can also thicken cervical mucus, making it harder for sperm to pass. 00:09:24.667 --> 00:09:31.320 Hormonal methods can include pills, patches, injections and implants; their mechanisms and suitability vary. 00:09:34.000 --> 00:09:39.257 A condom is a barrier that prevents semen and sperm entering the vagina when used correctly. 00:09:39.407 --> 00:09:47.107 Condoms also reduce transmission of many sexually transmitted infections; hormonal contraception does not provide that protection. 00:09:49.767 --> 00:09:55.687 A diaphragm covers the cervix and is used with spermicide to reduce sperm entry into the uterus. 00:09:55.837 --> 00:09:58.577 It does not stop the ovaries releasing eggs. 00:10:01.233 --> 00:10:09.955 Evaluate a method using effectiveness, correct use, possible side effects, how long it lasts, reversibility and protection against infections. 00:10:10.105 --> 00:10:14.479 No method should be described as guaranteed to prevent every pregnancy. 00:10:17.133 --> 00:10:24.887 A daily pill requires consistent use; a long-acting method reduces that daily requirement but can need a healthcare procedure. 00:10:25.037 --> 00:10:30.199 Barrier methods avoid altering the menstrual hormones but also depend on correct use. 00:10:32.867 --> 00:10:38.242 Assisted reproductive technology, or ART, can help some people with fertility problems. 00:10:38.392 --> 00:10:43.181 The approach depends on the cause of the difficulty and does not guarantee a pregnancy. 00:10:45.833 --> 00:10:51.837 Clomifene is a fertility medicine that can encourage ovulation in people who do not ovulate regularly. 00:10:51.987 --> 00:10:58.311 It promotes pituitary release of F S H and L H by reducing the effect of oestrogen feedback. 00:11:00.967 --> 00:11:08.676 In I V F, in vitro fertilisation, fertility hormones such as F S H stimulate development of several follicles. 00:11:08.826 --> 00:11:14.467 A hormone trigger, often acting like L H, helps eggs complete maturation before collection. 00:11:17.133 --> 00:11:20.599 Eggs are collected and fertilised with sperm in a laboratory. 00:11:20.749 --> 00:11:25.837 Embryos are allowed to develop before a selected embryo is transferred into the uterus. 00:11:28.500 --> 00:11:33.693 Progesterone can be given to help support the uterus lining after embryo transfer. 00:11:33.843 --> 00:11:39.517 Laboratory fertilisation and transfer do not guarantee implantation or a live birth. 00:11:39.667 --> 00:11:44.808 I V F involves laboratory fertilisation followed by embryo transfer. 00:11:49.467 --> 00:11:54.425 Benefits include the possibility of pregnancy despite some fertility problems. 00:11:54.575 --> 00:12:04.206 Limitations include cost, physical and emotional demands, variable success and risks such as excessive ovarian stimulation or multiple pregnancy. 00:12:06.867 --> 00:12:12.989 Evaluate success using comparable data and the stated outcome, such as pregnancy or live birth. 00:12:13.139 --> 00:12:19.687 Age and the cause of infertility can affect success, so results from different groups are not automatically comparable. 00:12:22.367 --> 00:12:29.490 Blood glucose concentration rises after absorption of carbohydrate from food and can fall as cells use glucose. 00:12:29.640 --> 00:12:32.880 The pancreas monitors and responds to these changes. 00:12:35.533 --> 00:12:39.599 When blood glucose is high, the pancreas releases more insulin. 00:12:39.749 --> 00:12:46.957 Insulin promotes glucose uptake by body cells and promotes conversion of glucose into glycogen in the liver and muscles. 00:12:49.633 --> 00:12:54.679 As blood glucose returns towards a suitable level, insulin secretion decreases. 00:12:54.829 --> 00:12:58.474 This is a negative-feedback response to an increase in glucose. 00:13:01.133 --> 00:13:05.125 When blood glucose is low, the pancreas releases more glucagon. 00:13:05.275 --> 00:13:11.033 Glucagon stimulates the liver to break glycogen down into glucose and release it into the bloodstream. 00:13:13.700 --> 00:13:18.893 As blood glucose rises towards a suitable level, glucagon secretion decreases. 00:13:19.043 --> 00:13:23.416 Insulin and glucagon therefore have opposing effects on blood glucose. 00:13:23.566 --> 00:13:29.805 Insulin and glucagon have opposing effects; secretion decreases as glucose returns towards normal. 00:13:34.467 --> 00:13:44.726 Glucose and glycogen are not interchangeable names: glucose is a small sugar that circulates in blood, while glycogen is a storage carbohydrate made from glucose units. 00:13:47.400 --> 00:13:54.205 Glucagon and glycogen are also different: glucagon is a hormone, while glycogen is a stored carbohydrate. 00:13:54.355 --> 00:13:57.350 The liver can supply glucose to the blood between meals. 00:14:00.000 --> 00:14:08.553 In type 1 diabetes, the body's immune system destroys insulin-producing pancreatic cells, so little or no insulin is produced. 00:14:08.703 --> 00:14:10.900 Blood glucose can become too high. 00:14:13.567 --> 00:14:23.624 Type 1 diabetes is controlled with insulin delivered by injections or a pump, together with blood-glucose monitoring and appropriate management of food and activity. 00:14:23.774 --> 00:14:26.761 Diet alone cannot replace the missing insulin. 00:14:29.433 --> 00:14:37.433 In type 2 diabetes, cells respond less effectively to insulin, and the pancreas may also produce insufficient insulin. 00:14:37.583 --> 00:14:40.568 This is often described as insulin resistance. 00:14:43.233 --> 00:14:50.987 Type 2 diabetes may be managed with dietary changes, physical activity and medicines; some people also need insulin. 00:14:51.137 --> 00:14:55.492 It should not be described as always controlled by diet and exercise alone. 00:14:58.167 --> 00:15:07.669 A higher body mass and greater abdominal fat are associated with increased type 2 diabetes risk, but genetics, age and other factors also matter. 00:15:07.819 --> 00:15:10.977 Type 2 diabetes can occur without obesity. 00:15:13.633 --> 00:15:17.555 Glucose may appear in urine if blood glucose becomes high enough. 00:15:17.705 --> 00:15:23.313 This can be a sign of diabetes, but a urine result alone cannot confirm a diagnosis. 00:15:25.967 --> 00:15:36.469 Body mass index, B M I, is calculated as mass in kilograms divided by height in metres squared: B M I equals mass (kg) divided by [height (m)] squared. 00:15:39.133 --> 00:15:49.905 For a mass of eighty one kilograms and height of one point eight zero metres, body mass index equals eighty one divided by one point eight zero squared, which equals twenty five. 00:15:50.055 --> 00:15:54.399 Square the height before dividing, and convert centimetres to metres first. 00:15:57.067 --> 00:16:09.084 Common adult categories are below 18.5: underweight; 18.5 to below 25: healthy-weight range; 25 to below 30: overweight; 30 or above: obesity. 00:16:09.234 --> 00:16:15.527 These adult boundaries are not used directly to assess children and teenagers, whose age and sex are considered. 00:16:18.200 --> 00:16:22.973 Waist:hip ratio equals waist circumference divided by hip circumference. 00:16:23.123 --> 00:16:27.136 Use the same units for both measurements; the ratio has no unit. 00:16:29.800 --> 00:16:35.837 For a waist of 80 centimetres and hips of 100 centimetres, the ratio is 0.80. 00:16:35.987 --> 00:16:41.831 A greater proportion of abdominal fat is associated with increased type 2 diabetes risk. 00:16:44.500 --> 00:16:50.750 A positive correlation means that higher values of one variable tend to occur with higher values of another. 00:16:50.900 --> 00:16:58.739 A trend between B M I and diabetes risk is not proof that every person with a high B M I will develop diabetes. 00:17:01.400 --> 00:17:05.778 B M I does not distinguish muscle from fat or show where fat is stored. 00:17:05.928 --> 00:17:13.949 Waist:hip ratio provides different information, but neither measure alone describes an individual's complete health or proves causation. 00:17:16.600 --> 00:17:21.815 Compare diabetes risk using proportions or percentages when group sizes differ. 00:17:21.965 --> 00:17:29.537 Ten cases out of 100 people and ten cases out of 1,000 people are different rates despite the same case count. 00:17:32.200 --> 00:17:37.159 Percentage equals number in a group divided by total number times 100. 00:17:37.309 --> 00:17:44.229 For example, 12 cases in 200 people is 6 percent; state which population the percentage describes. 00:17:46.900 --> 00:17:52.871 Check whether differences in age, activity or other factors could affect a reported correlation. 00:17:53.021 --> 00:17:57.345 A large sample and comparable groups support a more reliable comparison. 00:18:00.000 --> 00:18:08.915 Benedict's test can investigate reducing sugars in simulated urine: add Benedict's reagent and warm in a hot-water bath under school instructions. 00:18:09.065 --> 00:18:15.019 A colour change from blue towards green, yellow, orange or brick-red indicates reducing sugar. 00:18:17.700 --> 00:18:21.745 Benedict's reagent detects reducing sugars, not only glucose. 00:18:21.895 --> 00:18:29.340 Use simulated samples, appropriate eye protection and controlled heating; compare with a known positive and a negative control. 00:18:32.000 --> 00:18:36.939 A colour test on a simulated sample illustrates the chemistry of sugar detection. 00:18:37.089 --> 00:18:41.517 It does not diagnose diabetes or replace blood-glucose monitoring. 00:18:44.167 --> 00:18:52.453 A stable core temperature supports enzyme activity: low temperatures slow reactions, while high temperatures can denature proteins. 00:18:52.603 --> 00:18:59.727 The hypothalamus coordinates responses using information about blood temperature and signals from skin receptors. 00:19:02.400 --> 00:19:05.131 The epidermis is the outer protective layer. 00:19:05.281 --> 00:19:12.084 The dermis contains blood vessels, receptors, sweat glands and hair follicles involved in temperature responses. 00:19:14.767 --> 00:19:20.323 When the body is too warm, small arteries (arterioles) supplying skin capillaries widen. 00:19:20.473 --> 00:19:26.445 This vasodilation increases blood flow near the surface, so more energy transfers to the surroundings. 00:19:26.595 --> 00:19:30.471 Sweat cools the skin when it evaporates, taking energy with it. 00:19:30.621 --> 00:19:34.179 Changes in blood flow alter heat transfer through the skin. 00:19:38.833 --> 00:19:42.156 When the body is too cold, skin arterioles narrow. 00:19:42.306 --> 00:19:47.121 This vasoconstriction reduces blood flow near the surface and reduces heat loss. 00:19:47.271 --> 00:19:53.563 Shivering is repeated muscle contraction, which increases respiration and releases more thermal energy. 00:19:56.233 --> 00:20:04.745 Blood vessels do not move closer to or farther from the skin surface: vasodilation and vasoconstriction change vessel diameter and blood flow. 00:20:04.895 --> 00:20:08.657 These negative-feedback responses oppose the temperature change. 00:20:11.333 --> 00:20:20.931 Animal cells need controlled water balance: in very dilute surroundings they gain water and may burst; in concentrated surroundings they lose water and shrink. 00:20:21.081 --> 00:20:25.013 Osmoregulation keeps blood water content within suitable limits. 00:20:27.667 --> 00:20:34.652 The urinary system contains two kidneys, ureters carrying urine to the bladder, and a urethra carrying urine out. 00:20:34.802 --> 00:20:40.635 Renal arteries supply blood and renal veins return it; urine and blood have separate pathways. 00:20:43.300 --> 00:20:49.581 Excess amino acids are broken down in the liver, producing urea from their nitrogen-containing parts. 00:20:49.731 --> 00:20:55.875 Urea travels in blood to the kidneys and is removed in urine; kidneys do not manufacture the urea. 00:20:58.533 --> 00:21:01.156 A nephron is a filtering unit in a kidney. 00:21:01.306 --> 00:21:05.777 It starts with a knot of capillaries (the glomerulus) inside Bowman’s capsule. 00:21:05.927 --> 00:21:11.813 High pressure forces small molecules, including water, urea, glucose and ions, out of the blood. 00:21:11.963 --> 00:21:15.077 Blood cells and most large proteins stay in the blood. 00:21:15.227 --> 00:21:20.399 Filtration is followed by selective reabsorption; A D H acts on water balance. 00:21:25.067 --> 00:21:30.305 The kidney takes useful substances back into the blood from the filtered liquid (the filtrate). 00:21:30.455 --> 00:21:32.567 This is selective reabsorption. 00:21:32.717 --> 00:21:40.556 Normally all filtered glucose is reabsorbed, partly by active transport, along with much of the water and the ions the body needs. 00:21:43.233 --> 00:21:48.607 (Higher tier) When blood water content is too low, the pituitary releases more A D H. 00:21:48.757 --> 00:21:53.824 This makes the collecting ducts more permeable to water, so more water returns to the blood. 00:21:53.974 --> 00:21:57.247 A smaller volume of concentrated urine is produced. 00:21:57.397 --> 00:22:03.081 When blood water content is higher, less A D H is released and more dilute urine is produced. 00:22:05.733 --> 00:22:13.027 Kidney failure can be treated by dialysis: blood flows alongside dialysis fluid across a partially permeable membrane, 00:22:13.177 --> 00:22:17.138 allowing urea to diffuse out while cells and proteins stay in blood. 00:22:17.288 --> 00:22:20.602 Fluid composition limits loss of useful substances. 00:22:23.267 --> 00:22:27.567 Dialysis requires repeated sessions and careful fluid control. 00:22:27.717 --> 00:22:30.895 A kidney transplant can restore continuous function, 00:22:31.045 --> 00:22:34.927 but needs a suitable donor and carries surgical and rejection risks; 00:22:35.077 --> 00:22:39.512 immunosuppressant drugs reduce rejection but increase infection risk. 00:22:42.167 --> 00:22:46.287 That completes Animal coordination, control and homeostasis. 00:22:46.437 --> 00:22:49.783 Revisit the notes and test yourself on the revision website.