Mammalian Cell Biology, Human Anatomy and Physiology: The Cell, Tissues, Homeostasis and the Organ Systems
2. Tissues, organ systems and homeostasis
| Tissue | Features | Examples |
|---|---|---|
| Epithelial | closely packed cells on a basement membrane; covers, lines, secretes, absorbs; avascular | skin epidermis, gut lining, alveolar wall, glands |
| Connective | cells scattered in an extracellular matrix of fibres (collagen, elastin) and ground substance | bone, cartilage, tendon, ligament, adipose tissue, blood |
| Muscle | contractile cells: skeletal (striated, voluntary), cardiac (striated, involuntary, intercalated discs), smooth (non-striated, involuntary) | biceps, myocardium, walls of vessels and gut |
| Nervous | neurons that conduct impulses and glial cells that support them | brain, spinal cord, nerves |
Cells form tissues, tissues form organs, and organs working together form organ systems. Homeostasis is the maintenance of a steady internal environment — temperature, blood glucose, pH, osmolality, blood pressure — against disturbance. A homeostatic loop has a receptor (sensor), a control centre that compares the input with a set point, and an effector that acts, exactly the structure of a feedback control system.
- Negative feedback opposes the change and is the rule: a rise in body temperature, sensed by the hypothalamus, triggers sweating and skin vasodilation; a rise in blood glucose triggers insulin release.
- Positive feedback amplifies the change until an event ends it: oxytocin during labour (contractions stretch the cervix, which releases more oxytocin), the clotting cascade, and the Na⁺ influx of the rising action potential.
3. The musculoskeletal and nervous systems
The adult skeleton has 206 bones, divided into the axial skeleton (skull, vertebral column, ribs, sternum) and the appendicular skeleton (limbs and their girdles). It supports, protects, provides levers for movement, stores calcium and phosphate, and houses the red marrow that makes blood cells. Joints are fibrous (immovable), cartilaginous (slightly movable) or synovial (freely movable, with a capsule, synovial fluid and articular cartilage) — hinge (elbow, knee), ball-and-socket (hip, shoulder), pivot, saddle, condyloid and gliding types.
A skeletal muscle fibre contains myofibrils built from sarcomeres between Z lines: thick myosin filaments form the A band, thin actin filaments reach in from each Z line, and the I band and H zone are where they do not overlap. In excitation–contraction coupling the motor neuron releases acetylcholine at the neuromuscular junction, the muscle action potential travels down the T-tubules, the sarcoplasmic reticulum releases Ca²⁺, Ca²⁺ binds troponin and moves tropomyosin off actin’s binding sites, and myosin heads cycle using ATP. By the sliding-filament mechanism the filaments slide past each other: the I band and H zone shorten while the A band keeps its length. One motor neuron and the fibres it supplies form a motor unit; force is graded by recruiting more units and by raising the firing rate until twitches fuse into tetanus.
The central nervous system is the brain and spinal cord; the peripheral nervous system is the cranial and spinal nerves, divided into the somatic system (voluntary, to skeletal muscle) and the autonomic system. The autonomic system has a sympathetic division (“fight or flight”: raises heart rate and blood pressure, dilates pupils and airways; its postganglionic transmitter is mostly noradrenaline) and a parasympathetic division (“rest and digest”: slows the heart, promotes digestion; transmitter acetylcholine, carried largely by the vagus nerve). A neuron passes its signal to the next across a synapse, where a neurotransmitter released from vesicles binds receptors on the postsynaptic membrane. A reflex arc — receptor, afferent neuron, integrating centre, efferent neuron, effector — produces a rapid automatic response such as the knee jerk.
4. The circulatory and respiratory systems
The heart is two pumps in series. The right side receives deoxygenated blood from the venae cavae into the right atrium and sends it through the tricuspid valve and right ventricle to the lungs via the pulmonary artery; the left side receives oxygenated blood from the pulmonary veins and pumps it through the mitral valve, left ventricle and aortic valve to the body. Excitation starts at the sinoatrial (SA) node, the normal pacemaker, spreads over the atria, is delayed at the atrioventricular (AV) node so the atria finish filling the ventricles, and runs down the bundle of His and the Purkinje fibres. In the ECG the P wave is atrial depolarisation, the QRS complex ventricular depolarisation and the T wave ventricular repolarisation.
- Cardiac output CO = heart rate × stroke volume, about 5 L/min at rest; stroke volume = end-diastolic − end-systolic volume, and ejection fraction = SV/EDV, normally above about 55%.
- Mean arterial pressure ≈ diastolic + (systolic − diastolic)/3, because the heart spends longer in diastole; MAP = CO × total peripheral resistance.
- Poiseuille’s law for steady laminar flow of a Newtonian fluid in a rigid tube: Q = πr⁴ΔP/(8μL), so the resistance is 8μL/(πr⁴). Halving a vessel’s radius raises its resistance sixteen-fold — why arterioles, by small changes in diameter, control the distribution of flow. Blood is non-Newtonian and vessels are elastic, so this is an idealisation, but its r⁴ dependence governs the physiology.
- Blood is plasma plus formed elements — red cells (O₂ transport by haemoglobin), white cells (defence) and platelets (clotting).
Air passes through the nose, pharynx, larynx, trachea, bronchi and bronchioles to about 300 million alveoli, where gas exchange occurs across a very thin membrane. Inspiration is active: the diaphragm and external intercostals enlarge the thorax, intrapleural pressure falls, and by Boyle’s law alveolar pressure drops below atmospheric so air flows in; quiet expiration is passive recoil. Oxygen and CO₂ move by diffusion down partial-pressure gradients (alveolar pO₂ about 100 mmHg against about 40 mmHg in mixed venous blood), and haemoglobin’s sigmoid dissociation curve lets blood load oxygen fully in the lungs and release it in the tissues.
| Quantity | Definition | Measured by spirometry? |
|---|---|---|
| Tidal volume (TV) | volume of a quiet breath, about 500 mL | yes |
| Vital capacity (VC) | IRV + TV + ERV — the largest breath out after the largest breath in | yes |
| Residual volume (RV) | air left after maximal expiration | no — needs gas dilution or body plethysmography |
| Functional residual capacity (FRC) | ERV + RV — the volume at the end of a quiet expiration | no (contains RV) |
| Total lung capacity (TLC) | VC + RV | no (contains RV) |
Minute ventilation is TV × breathing rate, but the first ~150 mL of each breath only fills the conducting airways (the anatomical dead space), so alveolar ventilation = (TV − V_D) × rate — about (500 − 150) × 12 = 4.2 L/min at rest.
5. The excretory, endocrine and gastro-intestinal systems
Each kidney has about a million nephrons. Blood is filtered in the glomerulus into Bowman’s capsule — the glomerular filtration rate (GFR) is about 125 mL/min — and the filtrate is modified along the proximal tubule (bulk reabsorption of glucose, amino acids, Na⁺ and water), the loop of Henle (builds the medullary concentration gradient), the distal tubule and the collecting duct (fine control by hormones). Renal clearance C = UV/P, urine concentration times urine flow over plasma concentration; for inulin, which is filtered and neither reabsorbed nor secreted, clearance equals GFR. ADH (vasopressin) makes the collecting duct permeable to water and concentrates the urine; aldosterone increases Na⁺ reabsorption and K⁺ secretion; the renin–angiotensin–aldosterone system restores blood pressure and volume.
| Gland | Hormone | Main action |
|---|---|---|
| Hypothalamus / posterior pituitary | ADH, oxytocin | water retention; uterine contraction and milk ejection |
| Anterior pituitary | GH, TSH, ACTH, FSH, LH, prolactin | growth; control of the thyroid, adrenal cortex and gonads |
| Thyroid | T₃, T₄; calcitonin | metabolic rate; lowers blood Ca²⁺ |
| Parathyroid | parathyroid hormone | raises blood Ca²⁺ |
| Adrenal cortex / medulla | cortisol, aldosterone / adrenaline, noradrenaline | stress response, Na⁺ balance / fight-or-flight |
| Pancreatic islets | insulin (β cells), glucagon (α cells) | lowers / raises blood glucose |
The gastro-intestinal tract runs mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine, with the salivary glands, liver, gall bladder and pancreas as accessory organs. Digestion is mechanical and chemical: salivary amylase starts on starch; the stomach’s acid and pepsin begin protein digestion; pancreatic juice brings trypsin (proteins), amylase (carbohydrates) and lipase (fats), with bile from the liver emulsifying fats. Nearly all absorption happens in the small intestine, whose folds, villi and microvilli multiply its surface area; the large intestine absorbs water and electrolytes. Peristalsis — waves of smooth-muscle contraction under the enteric nervous system — moves the contents along.
Key takeaways
- Nernst: E = 61.5 log₁₀(C_out/C_in) mV at 37 °C for a monovalent ion; Goldman weights each ion by its permeability, chloride with inside and outside swapped.
- Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in; the action potential is Na⁺ in, then K⁺ out, all-or-none, with refractory periods.
- Homeostasis is negative feedback (receptor, control centre, effector); positive feedback appears in labour, clotting and the rising spike.
- CO = HR × SV, MAP ≈ DBP + (SBP − DBP)/3, Poiseuille Q = πr⁴ΔP/(8μL); alveolar ventilation = (TV − dead space) × rate.
- Clearance C = UV/P and inulin clearance = GFR ≈ 125 mL/min; RV, FRC and TLC cannot be measured by spirometry alone.
Practice questions (15)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Extracellular K⁺ is 4 mM and intracellular K⁺ is 140 mM. Using E = 61.5 log₁₀(C_out/C_in) mV at 37 °C, the K⁺ equilibrium potential (in mV), rounded to the nearest whole number, is ____.
Numerical answer — type the value.
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Answer: -95
E_K = 61.5 log₁₀(4/140) = 61.5 × (−1.544) = −94.96 mV ≈ −95 mV, inside negative because K⁺ tends to leak out. Inverting the ratio gives +95 mV; using log₁₀(140/4) with a positive sign is the same slip. Type the answer as -95.A cell at 37 °C has K⁺ 4 mM outside and 140 mM inside, Na⁺ 145 mM outside and 12 mM inside, Cl⁻ 110 mM outside and 10 mM inside, with relative permeabilities P_K : P_Na : P_Cl = 1 : 0.04 : 0.45. Using the Goldman equation with 61.5 log₁₀, the resting potential (in mV), to one decimal place, is ____.
Numerical answer — type the value.
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Answer: -69.1
Numerator P_K K_o + P_Na Na_o + P_Cl Cl_i = 4 + 5.8 + 4.5 = 14.3; denominator P_K K_i + P_Na Na_i + P_Cl Cl_o = 140 + 0.48 + 49.5 = 189.98. V_m = 61.5 log₁₀(14.3/189.98) = 61.5 × (−1.1234) = −69.1 mV. Treating chloride like a cation (Cl_o on top) gives about −24 mV, which is the usual mistake. Type the answer as -69.1.Per molecule of ATP hydrolysed, the Na⁺/K⁺-ATPase moves
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Answer: A — 3 Na⁺ out and 2 K⁺ in
The pump exports three sodium ions and imports two potassium ions, which keeps Na⁺ high outside and K⁺ high inside and makes the pump itself slightly electrogenic (one net positive charge out per cycle). Reversing either the numbers or the directions describes a pump that would run down the very gradients it exists to build.Which is an example of positive feedback in human physiology?
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Answer: A — oxytocin release during labour
Uterine contractions stretch the cervix, which drives more oxytocin release and stronger contractions, a loop that amplifies until delivery ends it. Sweating, insulin release and the baroreflex each oppose the disturbance that triggered them, which is negative feedback.A heart beats at 72 per minute with an end-diastolic volume of 120 mL and an end-systolic volume of 50 mL. The cardiac output (in L/min), to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 5.04
SV = EDV − ESV = 70 mL, so CO = 72 × 70 = 5040 mL/min = 5.04 L/min; the ejection fraction is 70/120 = 58%. Multiplying by the end-diastolic volume instead gives 8.64 L/min, which counts blood that never leaves the ventricle.A blood pressure of 120/80 mmHg corresponds to an estimated mean arterial pressure (in mmHg), to one decimal place, of ____.
Numerical answer — type the value.
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Answer: 93.3
MAP ≈ DBP + (SBP − DBP)/3 = 80 + 40/3 = 93.3 mmHg. The simple average, 100 mmHg, overweights systole — the heart spends about two-thirds of each cycle in diastole.Blood (viscosity 3.5 × 10⁻³ Pa·s, treated as Newtonian) flows steadily through a rigid tube of radius 2 mm and length 10 cm under a pressure difference of 100 Pa. By Poiseuille’s law the flow rate (in mL/s), to two decimal places, is ____.
Numerical answer — type the value.
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Answer: 1.8
Q = πr⁴ΔP/(8μL) = π × (2 × 10⁻³)⁴ × 100/(8 × 3.5 × 10⁻³ × 0.1) = 5.027 × 10⁻⁹/2.8 × 10⁻³ = 1.795 × 10⁻⁶ m³/s = 1.80 mL/s. Using the diameter in place of the radius multiplies the answer by 16, the r⁴ law at work.A person breathes 12 times a minute with a tidal volume of 500 mL and an anatomical dead space of 150 mL. The alveolar ventilation (in L/min), to one decimal place, is ____.
Numerical answer — type the value.
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Answer: 4.2
V̇_A = (TV − V_D) × rate = (500 − 150) × 12 = 4200 mL/min = 4.2 L/min. 6.0 L/min is the minute ventilation, which counts the air that only reaches the conducting airways and never takes part in gas exchange.Urine inulin is 25 mg/mL, urine flow is 1 mL/min and plasma inulin is 0.2 mg/mL. The glomerular filtration rate (in mL/min) is ____.
Numerical answer — type the value.
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Answer: 125
Inulin is freely filtered and neither reabsorbed nor secreted, so GFR = UV/P = 25 × 1/0.2 = 125 mL/min. Inverting the ratio gives 0.008, and forgetting the urine flow happens to give the same number here only because V = 1 mL/min.Which statements about organ-system physiology are correct?
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Answer: A — insulin lowers blood glucose; B — ADH increases water reabsorption in the collecting ducts; D — the SA node is the normal pacemaker of the heart
Insulin promotes glucose uptake and storage, ADH inserts water channels in the collecting duct, and the SA node has the fastest intrinsic rate and so sets the heartbeat. Pepsin digests proteins; fats are digested mainly by pancreatic lipase after emulsification by bile.Which of these cannot be measured with a simple spirometer?
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Answer: A — residual volume
A spirometer records only air that moves in and out of the mouth. The residual volume stays in the lungs even after maximal expiration, so it — and every capacity containing it (FRC, TLC) — needs gas dilution or body plethysmography.Which statements about the nerve action potential are correct?
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Answer: A — its rising (depolarising) phase is due to Na⁺ influx; B — during the absolute refractory period no second action potential can be fired, however strong the stimulus; C — myelination increases conduction velocity through saltatory conduction
Voltage-gated Na⁺ channels drive the upstroke, their inactivation creates the absolute refractory period, and myelin forces excitation to jump between nodes of Ranvier. The spike is all-or-none: stimulus strength is coded in firing frequency, not in amplitude.Blood is classified as which primary tissue type?
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Answer: A — connective tissue
Blood is a connective tissue: its cells are dispersed in an extracellular matrix, the plasma, as in bone and cartilage where the matrix is solid. It lines nothing and is not contractile or excitable, which rules out the other three.During contraction of a skeletal muscle sarcomere, which band keeps its length?
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Answer: A — the A band
The A band is the length of the myosin filaments, and filaments do not shorten — they slide. As overlap increases the I band (actin only) and H zone (myosin only) narrow and the Z lines move closer, so the sarcomere shortens while the A band stays constant.A subject has total lung capacity 6000 mL, vital capacity 4800 mL and expiratory reserve volume 1200 mL. The functional residual capacity (in mL) is ____.
Numerical answer — type the value.
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Answer: 2400
RV = TLC − VC = 6000 − 4800 = 1200 mL, and FRC = ERV + RV = 1200 + 1200 = 2400 mL. Quoting 1200 mL gives the residual volume alone; TLC − ERV = 4800 mL is the inspiratory capacity plus RV, not FRC.