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Bio/Biochem 3B: Structure and Integrative Function of the Main Organ Systems

Eleven organ systems taken one at a time and then put back together, because in a living body no system works alone.

44 topics · 201 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

3B. Structure of the Lungs and Alveoli

  1. How it worksWalk me through air travelling trachea to bronchi to bronchioles to one single alveolus.
  2. How it worksHow does surfactant stop a tiny alveolus from collapsing into the bigger one next door?
  3. Worked exampleFind which of two connected bubbles empties into the other when their surface tensions match.
  4. Common mistakeWhy isn't the alveolar wall a barrier that gases have to be forced across?
  5. Going furtherWhat happens to gas exchange when alveolar walls break down and merge into large sacs?
Your turnhard1 of 3
A lung holds 4 hundred million alveoli, each a sphere of radius 132 micrometres. Taking the surface of a sphere as 4·π·r2, what total alveolar surface area does that give, in square metres?

3B. Breathing Mechanisms and Differential Pressure

  1. How it worksWalk me through the diaphragm dropping, the thorax widening, and pressure falling below atmospheric.
  2. How it worksHow does the pleural space couple the lungs to a rib cage they are not attached to?
  3. How it worksHow do nasal hairs and the mucus-cilia escalator strip particles out before air reaches an alveolus?
  4. Common mistakeWhy isn't quiet exhalation an active push by muscles?
  5. Going furtherWhat happens to a lung when air leaks into the pleural space through a hole in the chest?
Your turneasy1 of 3
With the airway clamped, the lungs hold 2479 mL of air at 761 mmHg. The diaphragm drops and thoracic volume rises to 3073 mL. What is the new intrapulmonary pressure, by Boyle's law?

3B. Alveolar Gas Exchange and Partial Pressures

  1. How it worksHow does a partial pressure difference drive each gas completely independently of the other?
  2. How it worksWalk me through Henry's law setting how much gas dissolves in plasma at a given pressure.
  3. Worked exampleFind the direction and driving pressure for oxygen with the alveolus at 100 and blood at 40 mmHg.
  4. Common mistakeWhy isn't the oxygen a blood sample carries proportional to its partial pressure?
Your turnmedium1 of 3
Tidal volume is 609 mL, respiratory rate is 16 breaths per minute, and anatomical dead space is 176 mL. Find the alveolar ventilation in litres per minute.

3B. Control of Breathing, pH, and Thermoregulation

  1. How it worksHow do the medullary centers read the pH of cerebrospinal fluid and set a breathing rate from it?
  2. How it worksWalk me through the CO2, carbonic acid, and bicarbonate equilibrium tying breathing to blood pH.
  3. Worked exampleTrace blood pH and breathing drive through one minute of deliberate hyperventilation.
  4. Going furtherWhat happens to breathing control in someone whose carbon dioxide has been high for years?
  5. Common mistakeWhy isn't panting a cooling strategy that also improves gas exchange?
Your turnhard1 of 3
Plasma bicarbonate is 26 mM and dissolved carbon dioxide contributes 1.389 mM of carbonic acid. Using pH = 6.1 + log10 of bicarbonate over carbonic acid, find the blood pH.

3B. The Four-Chambered Heart and the Cardiac Cycle

  1. How it worksWalk me through systole and diastole with each valve opening and closing in its turn.
  2. How it worksHow do the atrioventricular and semilunar valves make flow one-way with nothing but flaps?
  3. Worked exampleTrace one red blood cell from the vena cava through all four chambers and out the aorta.
  4. Common mistakeWhy isn't the lub-dub sound made by heart muscle contracting?
  5. Going furtherWhat happens to output when one valve leaks backwards during ventricular contraction?
Your turneasy1 of 3
A ventricle holds 138 mL at the end of diastole and 59 mL at the end of systole. What is the stroke volume?

3B. The Cardiac Conduction System

  1. How it worksWalk me through the SA node firing, the AV node stalling, and the Purkinje fibers spreading it.
  2. How it worksHow does the AV node's delay let the atria finish emptying before the ventricles squeeze?
  3. Common mistakeWhy isn't the nervous system what starts each individual heartbeat?
  4. Going furtherWhat happens to the rhythm when the AV node stops passing signals from the atria?
Your turneasy1 of 3
The SA node fires at 87 per minute, the AV node at 50 per minute, and the Purkinje fibres at 34 per minute. If the SA node fails completely and the AV node takes over, by how many beats per minute does the heart rate fall?

3B. Pulmonary and Systemic Circulation

  1. How it worksWalk me through the two circuits and exactly where oxygen is picked up and dropped off.
  2. How it worksHow does the pulmonary circuit run at a quarter of the systemic circuit's pressure?
  3. Common mistakeWhy aren't the pulmonary arteries carrying oxygenated blood like every other artery?
  4. Ties togetherHow does the portal system between gut and liver break the usual one-capillary-bed rule?
Your turneasy1 of 3
The systemic circuit drops a mean pressure of 95 mmHg across it and the pulmonary circuit drops 12 mmHg, and both carry the same cardiac output. How many times greater is systemic vascular resistance than pulmonary?

3B. Arteries, Veins, and Peripheral Resistance

  1. How it worksHow do arterioles set peripheral resistance and therefore blood pressure for the whole body?
  2. How it worksWalk me through pressure falling from 120 in the aorta to almost nothing in the vena cava.
  3. Worked exampleFind what happens to flow through a vessel when its radius is cut in half.
  4. Common mistakeWhy isn't blood pressure highest in the narrowest vessels of all?
  5. Going furtherWhat happens to venous return when the skeletal muscles around the veins stop working?
Your turneasy1 of 3
Systolic pressure is 143 mmHg and diastolic pressure is 83 mmHg. Using mean arterial pressure = diastolic plus one third of the pulse pressure, find the mean arterial pressure.

3B. Capillary Beds, Solute Exchange, and Heat Exchange

  1. How it worksHow do hydrostatic and osmotic pressure push fluid out at one end and pull it back at the other?
  2. How it worksWalk me through gas, solute, and heat all leaving a capillary by three different routes.
  3. Worked exampleTrace fluid movement across a capillary when plasma protein concentration drops by half.
  4. Common mistakeWhy isn't the total cross-section of all capillaries smaller than the aorta's?
  5. Going furtherWhat happens to a vessel when its endothelial cells release nitric oxide into the muscle around them?
Your turnhard1 of 3
At the arterial end of a capillary the hydrostatic pressure is 38 mmHg and the plasma oncotic pressure is 27 mmHg, while the interstitial hydrostatic pressure is 3 mmHg and the interstitial oncotic pressure is 5 mmHg. Find the net filtration pressure.

3B. Composition of Blood

  1. How it worksWalk me through plasma, erythrocytes, leukocytes, and platelets and the job each one holds.
  2. How it worksHow do bone marrow making red cells and the spleen destroying them stay in balance?
  3. Worked exampleFind the hematocrit when packed red cells fill 45 of a 100 millimeter column.
  4. Common mistakeWhy isn't a mature red blood cell able to repair itself or divide?
  5. Going furtherWhat happens to erythropoietin and red cell count after a month living at high altitude?
Your turneasy1 of 3
A spun tube of blood stands 104 mm tall in total, and the packed red cell layer at the bottom measures 45 mm. What is the haematocrit, as a percentage?

3B. Oxygen Transport and the Hemoglobin Dissociation Curve

  1. How it worksHow does cooperative binding across four subunits bend the dissociation curve into an S?
  2. How it worksWalk me through the curve sliding right under high CO2, low pH, and high temperature.
  3. Worked exampleFind hemoglobin saturation at a tissue pressure of 40 mmHg, then again after a right shift.
  4. Common mistakeWhy isn't a left-shifted curve always better for getting oxygen into tissue?
  5. Going furtherWhat happens to the curve when fetal hemoglobin is laid over the adult version?
Your turnmedium1 of 3
Arterial blood is 98 percent saturated and the venous blood returning from a tissue is 74 percent saturated. Haemoglobin is 15 g/dL and each gram carries 1.34 mL of oxygen when fully saturated. How many millilitres of oxygen does each decilitre of blood hand over to that tissue?

3B. Carbon Dioxide Transport and Blood pH

  1. How it worksWalk me through carbonic anhydrase turning CO2 into bicarbonate inside a red blood cell.
  2. How it worksHow does the chloride shift keep that red cell balanced while bicarbonate walks out?
  3. Worked exampleTrace one CO2 molecule from a working muscle to the alveolus, naming its form at each step.
  4. Common mistakeWhy isn't carbon dioxide carried mostly bound to hemoglobin the way oxygen is?
Your turneasy1 of 3
Of the carbon dioxide leaving a tissue, 72 percent travels as bicarbonate and 23 percent as carbamino compounds on haemoglobin, with the remainder simply dissolved. What percentage travels dissolved?

3B. Coagulation and Clotting Mechanisms

  1. How it worksWalk me through platelets plugging a hole and the cascade laying fibrin mesh over them.
  2. How it worksHow does a chain of inactive enzymes switching each other on amplify one tiny injury signal?
  3. Common mistakeWhy isn't a platelet plug enough on its own to stop bleeding from a real wound?
  4. Worked exampleTrace where a drug that blocks vitamin K interrupts the clotting cascade.
  5. Going furtherWhat happens when that same cascade fires inside an undamaged vessel?
Your turnmedium1 of 3
In the coagulation cascade each activated enzyme switches on 10 molecules of the next factor, and the chain runs 4 steps below the trigger. How many molecules of the final factor does a single trigger molecule activate?

3B. The Lymphatic System

  1. How it worksHow does lymph collect the fluid capillaries leave behind and hand it back to the blood?
  2. How it worksWalk me through a digested fat entering a lacteal instead of a blood capillary in the gut.
  3. Common mistakeWhy isn't the lymphatic system driven by a pump of its own?
  4. Going furtherWhat happens to a limb when its lymphatic drainage is blocked?
Your turneasy1 of 3
Capillaries filter 21 litres of fluid into the tissues each day and reabsorb 17 litres of it at their venous ends. How many litres must the lymphatics return to the bloodstream daily?

3B. Innate and Adaptive Immunity

  1. How it worksWalk me through skin, phagocytes, and inflammation stopping most invaders before day two.
  2. How it worksHow does the adaptive system trade days of delay for precision and a memory of the attacker?
  3. Going furtherWhat happens to infection risk when the barriers hold but adaptive cells are missing?
  4. Common mistakeWhy isn't a fever a malfunction of the body's temperature control?
Your turnhard1 of 3
Bacteria entering a wound double every 49 minutes. Starting from 4 thousand organisms, how many million are present after 8 hours if nothing stops them?

3B. T Lymphocytes, Antigen Presentation, and the MHC

  1. How it worksWalk me through a macrophage digesting a pathogen and displaying a fragment on MHC class II.
  2. How it worksHow do MHC I and MHC II report different news: what is inside a cell versus what was outside?
  3. Worked exampleTrace a virus-infected cell being identified and killed by a cytotoxic T cell.
  4. Common mistakeWhy isn't a helper T cell able to kill an infected cell by itself?
  5. Going furtherWhat happens to the whole immune response when helper T cells are selectively destroyed?
Your turneasy1 of 3
A macrophage displays 154 thousand MHC class II molecules on its surface, and 6 percent of them carry a peptide from the pathogen it has just digested. How many pathogen-derived peptide-MHC complexes does the cell display?

3B. B Lymphocytes, Antibodies, and Clonal Selection

  1. How it worksWalk me through clonal selection: one matching B cell picked out, then copied into an army.
  2. How it worksHow does an antibody's Y shape let one molecule grab two antigens and flag them for killing?
  3. Worked exampleTrace one antigen from first binding to a plasma cell pumping out thousands of antibodies a second.
  4. Common mistakeWhy isn't an antibody able to destroy a pathogen on its own?
Your turnhard1 of 3
A plasma cell secretes 2393 antibody molecules every second and survives 6 days. How many billion antibody molecules does it release over its working life?

3B. The Primary and Secondary Immune Response

  1. How it worksHow do memory cells left over from the first response cut the lag from ten days to two?
  2. How it worksWalk me through antibody levels rising, fading, and then spiking far higher on re-exposure.
  3. Worked exampleTrace the antibody curve for a vaccine given at week zero and boosted at week eight.
  4. Common mistakeWhy isn't lifelong protection guaranteed once memory cells for a pathogen exist?
  5. Ties togetherHow does clonal selection explain why the second response is both faster and larger?
Your turneasy1 of 3
A primary response peaks at an antibody titre of 154 units, and the secondary response after a booster peaks at 5894 units. How many times higher is the secondary peak?

3B. Self Versus Nonself, Autoimmunity, and Immune Tissues

  1. How it worksWalk me through the thymus killing off T cells that react against the body's own proteins.
  2. How it worksHow do bone marrow, thymus, spleen, and lymph nodes divide the immune system's labor?
  3. Common mistakeWhy isn't an autoimmune disease simply an immune system that is too strong?
  4. Going furtherWhat happens to a transplanted organ whose MHC molecules do not match the recipient's?
Your turneasy1 of 3
Of the 68 million thymocytes entering the thymus each day, 80 percent fail positive selection and a further 14 percent of the original number are deleted by negative selection. How many million leave as mature T cells?

3B. Ingestion, Saliva, and the Esophagus

  1. How it worksWalk me through salivary amylase working on starch while the teeth are still grinding.
  2. How it worksHow does the esophagus push a bolus downward even when you are hanging upside down?
  3. Common mistakeWhy isn't swallowing just gravity carrying food down an open tube?
  4. Going furtherWhat happens when the sphincter at the bottom of the esophagus fails to stay shut?
Your turneasy1 of 3
Salivary amylase cleaves 154 glycosidic bonds per second per enzyme molecule. Saliva delivers 7 million enzyme molecules to a bolus that is chewed for 29 seconds. How many billion bonds are cleaved before swallowing?

3B. The Stomach

  1. How it worksWalk me through parietal cells making hydrochloric acid and chief cells making pepsinogen.
  2. How it worksHow does pepsinogen stay harmless until the acid around it switches it on?
  3. Worked exampleTrace a protein through churning, acid denaturation, and pepsin cleavage into chyme.
  4. Common mistakeWhy isn't the stomach where most nutrient absorption happens?
  5. Going furtherWhat happens to the stomach lining when its mucus layer is broken by an infection?
Your turnhard1 of 3
Gastric juice has a pH of 2. What is the hydrogen ion concentration, in millimoles per litre?

3B. The Small Intestine and Absorption

  1. How it worksHow do villi and microvilli multiply absorptive surface hundreds of times over?
  2. How it worksWalk me through duodenum, jejunum, and ileum each taking a different share of the work.
  3. Worked exampleTrace a glucose, an amino acid, and a fatty acid across the intestinal wall by three routes.
  4. Common mistakeWhy isn't fat absorbed straight into the bloodstream the way sugar is?
Your turnmedium1 of 3
A smooth tube of intestine would offer 0.679 square metres of surface. Circular folds multiply that 3 times, villi a further 12 times, and microvilli a further 18 times. What total absorptive area results?

3B. The Pancreas, the Liver, and Bile

  1. How it worksWalk me through pancreatic bicarbonate and enzymes arriving in the duodenum together.
  2. How it worksHow does bile break fat into droplets small enough for lipase to finally reach?
  3. How it worksShow me the liver taking in gut blood first and deciding what reaches the rest of the body.
  4. Worked exampleTrace a swallowed drug's route before it ever reaches the general circulation.
  5. Common mistakeWhy isn't bile a digestive enzyme?
Your turnhard1 of 3
Bile breaks a fat droplet of radius 779 micrometres into droplets of radius 4 micrometres with no loss of total volume. By what factor does the total surface area available to lipase increase?

3B. The Large Intestine, Bacterial Flora, and Elimination

  1. How it worksHow does the colon pull water back out and compact what is left into feces?
  2. How it worksWalk me through gut bacteria making vitamin K and fermenting what you could not digest.
  3. Common mistakeWhy isn't the appendix a useless leftover with no job at all?
  4. Going furtherWhat happens to water balance when material races through the colon too fast?
Your turneasy1 of 3
1618 mL of fluid enters the colon each day and only 165 mL leaves in the faeces. How many millilitres of water does the colon reclaim daily?

3B. Peristalsis and the Hormonal and Neural Control of Digestion

  1. How it worksWalk me through a peristaltic wave with circular muscle squeezing behind and relaxing ahead.
  2. How it worksHow do gastrin, secretin, and cholecystokinin each time one separate step of digestion?
  3. Worked exampleTrace the hormones fired the moment fatty chyme first reaches the duodenum.
  4. Common mistakeWhy isn't the enteric nervous system just a branch taking orders from the brain?
  5. Going furtherWhat happens to stomach function when the vagus nerve supplying it is cut?
Your turneasy1 of 3
Fatty chyme entering the duodenum lifts plasma cholecystokinin from 2 pmol/L to 28 pmol/L over 29 minutes. What is the mean rate of rise, in pmol/L per minute?

3B. Kidney Structure and the Nephron

  1. How it worksWalk me through cortex and medulla and where each piece of a nephron sits between them.
  2. How it worksHow do glomerulus, capsule, proximal tubule, loop, distal tubule, and duct line up in order?
  3. Worked exampleTrace one drop of filtrate the whole length of a nephron, naming each segment it crosses.
  4. Common mistakeWhy isn't urine simply blood waste strained out in a single step?
  5. Going furtherWhat happens to urine between the collecting duct and the sphincter that finally lets it out?
Your turneasy1 of 3
The kidneys filter 184 litres of plasma per day and produce 1.97 litres of urine. What percentage of the filtrate escapes reabsorption and leaves the body?

3B. Glomerular Filtration

  1. How it worksHow does the filtration barrier pass water and small solutes while holding every protein back?
  2. How it worksWalk me through afferent and efferent arterioles setting the pressure inside the glomerulus.
  3. Worked exampleFind what happens to filtration rate when the efferent arteriole constricts.
  4. Common mistakeWhy isn't glomerular filtration a selective process that picks which molecules to remove?
  5. Going furtherWhat happens to filtration when whole-body blood pressure falls by a third?
Your turneasy1 of 3
Glomerular capillary hydrostatic pressure is 58 mmHg, Bowman's capsule pressure is 15 mmHg, and plasma oncotic pressure is 30 mmHg. With a filtration coefficient of 11 mL per minute per mmHg, what is the glomerular filtration rate?

3B. Reabsorption and Secretion Along the Tubule

  1. How it worksWalk me through the proximal tubule reclaiming glucose, amino acids, and most of the sodium.
  2. How it worksHow does secretion add to the filtrate things that filtration alone missed?
  3. Worked exampleFind the blood glucose at which tubule transporters saturate and sugar starts appearing in urine.
  4. Common mistakeWhy isn't the distal tubule doing the same job as the proximal one?
Your turnmedium1 of 3
Plasma sodium is 142 mM and the glomerular filtration rate is 126 mL per minute. If 99.49 percent of the filtered sodium is reabsorbed along the tubule, how many millimoles of sodium are excreted per minute?

3B. The Countercurrent Multiplier and Urine Concentration

  1. How it worksWalk me through the descending limb losing water and the ascending limb pumping salt out.
  2. How it worksHow do two limbs running in opposite directions multiply a small gradient into a huge one?
  3. Worked exampleTrace filtrate osmolarity from 300 at the top of the loop to 1200 at the bend and back up.
  4. Common mistakeWhy isn't the ascending limb of the loop permeable to water?
  5. Going furtherWhat happens to concentrating power in a desert animal with a much longer loop?
Your turnmedium1 of 3
Filtrate enters the descending limb at 304 mOsm/L and 49 mL per minute. By the bend of the loop it is 1160 mOsm/L, with no solute added or removed. What volume per minute remains at the bend?

3B. Osmoregulation, Blood Pressure, and Acid-Base Balance

  1. How it worksWalk me through ADH making the collecting duct leaky to water and concentrating the urine.
  2. How it worksHow does the renin-angiotensin-aldosterone system raise pressure by hoarding sodium?
  3. Common mistakeWhy isn't aldosterone doing the same job as ADH by a different route?
  4. Worked exampleTrace ADH, aldosterone, and urine volume across six hours of dehydration.
  5. Going furtherWhat happens to blood pH when the kidney changes how much bicarbonate it reabsorbs?
Your turnhard1 of 3
A patient's plasma holds 144 mM sodium, 223 mg/dL glucose and 24 mg/dL urea. Using osmolarity = 2 times sodium, plus glucose over 18, plus urea over 2.8, find the plasma osmolarity in mOsm/L.

3B. Male and Female Reproductive Structures

  1. How it worksWalk me through sperm from seminiferous tubule to epididymis to vas deferens to urethra.
  2. How it worksHow do ovary, fallopian tube, and uterus each own a different stage of the same journey?
  3. Common mistakeWhy isn't male and female anatomy built from different embryonic starting material?
  4. Going furtherWhat happens to those shared embryonic structures without testosterone?
Your turneasy1 of 3
The testes produce 204 million sperm per day, and maturation and storage in the epididymis take 14 days. How many billion sperm are in transit through that system at any moment?

3B. Hormonal Control of the Female Reproductive Cycle

  1. How it worksWalk me through FSH, LH, estrogen, and progesterone across a 28-day cycle.
  2. How it worksHow does the LH surge trigger ovulation and turn the empty follicle into a corpus luteum?
  3. Worked exampleTrace the four hormone curves and the uterine lining together from day 1 to day 28.
  4. Common mistakeWhy isn't menstruation triggered by hormones rising rather than falling?
  5. Going furtherWhat happens to the corpus luteum when implantation happens and hCG appears?
Your turneasy1 of 3
A menstrual cycle lasts 32 days, and the luteal phase is fixed at 14 days regardless of cycle length. On which day of the cycle does ovulation occur?

3B. Sexual Development, Pregnancy, Parturition, and Lactation

  1. How it worksHow does the hypothalamus restarting pulsed GnRH release set the whole cascade going?
  2. How it worksWalk me through pregnancy's hormone supply handing off from corpus luteum to placenta.
  3. How it worksShow me oxytocin's positive feedback loop driving labor to its own conclusion.
  4. Common mistakeWhy isn't milk released during pregnancy even though the breast is ready to make it?
Your turnhard1 of 3
Plasma hCG is 154 IU/L in early pregnancy and doubles every 2.47 days. What concentration is reached 13 days later?

3B. The Three Muscle Types

  1. How it worksHow do skeletal, cardiac, and smooth muscle differ in striation, nuclei, and control?
  2. How it worksWalk me through why cardiac cells are branched and wired end to end through intercalated discs.
  3. Common mistakeWhy isn't smooth muscle simply a weaker version of skeletal muscle?
  4. Going furtherWhat happens to contraction speed moving from smooth to cardiac to skeletal muscle?
Your turneasy1 of 3
A smooth muscle cell takes 5 seconds to reach peak tension, a cardiac cell 247 milliseconds, and a skeletal fibre 57 milliseconds. How many times faster than the smooth muscle does the skeletal fibre reach its peak?

3B. The Sarcomere: Bands, Lines, and Zones

  1. How it worksWalk me through the A band, I band, H zone, M line, and Z line in one sarcomere.
  2. How it worksHow does the overlap between thick and thin filaments create each of those bands?
  3. Worked exampleFind which bands narrow and which hold their width when a sarcomere shortens by 20 percent.
  4. Common mistakeWhy isn't the A band shrinking during contraction when everything around it does?
Your turneasy1 of 3
A relaxed sarcomere measures 2.9 micrometres from Z line to Z line, and its A band is 1.63 micrometres wide. How wide is the half-I band at each end?

3B. The Sliding Filament Model and Crossbridge Cycling

  1. How it worksWalk me through one crossbridge cycle: bind, power stroke, release, re-cock.
  2. How it worksHow does ATP power the re-cocking step rather than the pulling stroke itself?
  3. Worked exampleTrace the Z lines drawing together as thin filaments slide past thick ones over one cycle.
  4. Common mistakeWhy isn't rigor mortis caused by muscle running out of calcium?
  5. Going furtherWhat happens to force when a sarcomere is stretched until the filaments barely overlap?
Your turneasy1 of 3
Each power stroke slides a thin filament 10 nanometres past the thick filament, and a half-sarcomere shortens by 460 nanometres during one contraction. How many crossbridge cycles must each myosin head complete?

3B. Excitation-Contraction Coupling, Calcium, and Troponin

  1. How it worksWalk me through the T-tubule carrying depolarization in and the sarcoplasmic reticulum releasing calcium.
  2. How it worksHow does calcium binding troponin swing tropomyosin off the myosin binding sites?
  3. Worked exampleTrace calcium concentration inside a fiber from the nerve signal through to full relaxation.
  4. Going furtherWhat happens to a muscle when the sarcoplasmic reticulum cannot pump calcium back in?
  5. Common mistakeWhy isn't relaxation just the muscle stopping what it was doing?
Your turnmedium1 of 3
Resting cytosolic calcium is 99 nanomolar and rises to 7 micromolar at the peak of a twitch. By what factor does the concentration rise?

3B. Motor Units, the Neuromuscular Junction, Twitch, and Tetanus

  1. How it worksWalk me through acetylcholine crossing the neuromuscular junction and depolarizing the end plate.
  2. How it worksHow do recruiting more motor units and firing them faster grade force smoothly?
  3. Worked exampleTrace the tension record from a single twitch through summation into full tetanus.
  4. Common mistakeWhy isn't a single muscle fiber able to contract with different strengths?
Your turneasy1 of 3
A muscle contains 479 motor units of 619 fibres each, and every fibre develops 4 millinewtons of force. What total force, in newtons, does full recruitment produce?

3B. Fiber Types, Fatigue, and Oxygen Debt

  1. How it worksHow do slow oxidative and fast glycolytic fibers pay for contraction in different currencies?
  2. How it worksWalk me through lactate piling up when glycolysis outruns the oxygen arriving.
  3. Worked exampleFind how long stored ATP and creatine phosphate can last in an all-out sprint.
  4. Common mistakeWhy isn't next-day muscle soreness caused by lactate still sitting in the muscle?
  5. Going furtherWhat happens to fiber composition after months of endurance training?
Your turnmedium1 of 3
A muscle holds 6 millimoles of ATP and 24 millimoles of creatine phosphate per kilogram, and an all-out sprint burns 5 millimoles of ATP per kilogram per second. How many seconds can those stores alone sustain it?

3B. Bone Structure and Cellular Composition

  1. How it worksHow do collagen fibers and calcium mineral combine into something stiff that will not shatter?
  2. How it worksWalk me through osteoblasts building, osteoclasts dissolving, and osteocytes keeping watch.
  3. Common mistakeWhy isn't bone an inert mineral scaffold that stops changing once growth ends?
  4. Worked exampleTrace the rings of an osteon outward from its central canal.
  5. Going furtherWhat happens to bone density in a limb kept weightless for months?
Your turnhard1 of 3
A limb held weightless loses 2 percent of its remaining bone mineral density every month. After 9 months, what percentage of the original density is left?

3B. Cartilage, Ligaments, Tendons, and Joints

  1. How it worksWalk me through a synovial joint: cartilage, capsule, fluid, and the ligaments binding it.
  2. How it worksHow do a tendon and a ligament differ in what they join and how they behave under load?
  3. Common mistakeWhy isn't cartilage able to heal itself the way bone does after a break?
  4. Going furtherWhat happens to the range of motion as hinge, ball-and-socket, and pivot joints are compared?
Your turneasy1 of 3
A healing bone fracture regains 87 percent of its original strength within 15 weeks, while an articular cartilage defect of similar size regains 7 percent over the same period. How many times better is the bone's recovery?

3B. Bone Growth, Calcium Storage, and Endocrine Control

  1. How it worksWalk me through endochondral ossification replacing a cartilage model with real bone.
  2. How it worksHow does intramembranous ossification build a skull plate with no cartilage step at all?
  3. Worked exampleTrace blood calcium, parathyroid hormone, and bone resorption over a day of low calcium intake.
  4. Common mistakeWhy isn't bone lengthening at the growth plate the same process as bone thickening?
  5. Ties togetherHow do kidney, gut, and skeleton work together to hold blood calcium at one number?
Your turnmedium1 of 3
The skeleton holds 1109 grams of calcium while the plasma holds only 3 grams. A day of low intake requires 573 milligrams to be drawn out of bone. What percentage of the skeleton's store is that?

3B. Skin Structure and Physical Protection

  1. How it worksWalk me through epidermis, dermis, and hypodermis and the cell types living in each.
  2. How it worksHow does keratin plus tight junctions make a layer nearly impermeable to water?
  3. Common mistakeWhy isn't skin a passive barrier with no part in immunity?
  4. Going furtherWhat happens to fluid balance when a large area of skin is burned away?
Your turnmedium1 of 3
Intact skin loses 17 mL of water per square metre per hour by evaporation, and burned skin loses 16 times as much. How many extra millilitres per hour are lost from 1.146 square metres of burn?

3B. Skin in Thermoregulation and Osmoregulation

  1. How it worksHow do vasodilation and vasoconstriction in surface capillaries dump heat or trap it?
  2. How it worksWalk me through sweat evaporating and carrying off far more heat than warming it ever could.
  3. Worked exampleTrace skin blood flow, sweat rate, and core temperature over 30 minutes of exercise in heat.
  4. Common mistakeWhy isn't sweat that drips off your skin cooling you?
  5. Ties togetherHow do skin, circulation, and the hypothalamus close one loop that holds core temperature?
Your turnmedium1 of 3
Evaporating one gram of sweat removes 2439 joules of heat. A runner is generating 989 watts of excess heat and sweating 34 grams per minute, all of which evaporates. How many watts does evaporation remove?

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