Comprehensive Human Physiology and Cellular Biology Study Guide

Classification of Cells and Internal Structures

Cell biology distinguishes between two primary types of cells. Prokaryotic cells are characterized by the absence of a defined nucleus and membrane-bound organelles. In contrast, eukaryotic cells, which comprise the building blocks of plants, fungi, and animals, contain a distinct nucleus and various membrane-bound organelles. Within the human body, cells differentiate into specific types to perform specialized functions, including epithelial cells, muscle cells, nerve cells, connective tissue cells, and blood cells. Collectively, cells serve as the fundamental structural and functional units of living organisms, and their proper operation is vital for maintaining the homeostasis of the entire body.

Every human cell includes several core components. The cell membrane acts as a selective barrier between the internal and external environments. The cytoplasm represents the internal environment containing metabolic structures. Organelles are specialized structures within the cell that carry out specific tasks, while the nucleus serves as the control center containing genetic information. The cell membrane itself is composed of a phospholipid bilayer, featuring hydrophilic heads facing outward and hydrophobic tails facing inward. This membrane also incorporates membrane proteins for transport, reception, and enzymatic activity; cholesterol to provide strength and flexibility; and glycoproteins and glycolipids that facilitate cellular communication.

Cellular Transport Mechanisms and Metabolism

Cells maintain homeostasis through the regulated transport of substances across the membrane. Transport is categorized into three types. Passive transport does not require adenosine triphosphate ($ATP$) and includes simple diffusion (movement from higher to lower concentration), facilitated diffusion (using membrane proteins), and osmosis (water movement across a semipermeable membrane). Active transport requires $ATP$ and is subdivided into primary active transport, such as the sodium-potassium pump ($\text{Na}^+/\text{K}^+\text{-ATPáza}$), and secondary active transport, which utilizes ionic gradients for substance transfer. Vesicular transport consists of endocytosis (engulfing particles) and exocytosis (secreting substances).

Cellular metabolism encompasses chemical reactions for energy production and the synthesis of molecules. Key processes include glycolysis, an anaerobic process occurring in the cytoplasm that breaks down glucose into pyruvate and releases $2$ molecules of $ATP$. The Krebs cycle occurs in the mitochondria and represents a shared metabolic pathway for the aerobic oxidation of carbohydrates, lipids, and proteins, producing reduction equivalents ($\text{NADH} + \text{H}^+$, $\text{FADH}_2$). These are used in the respiratory chain on the inner mitochondrial membrane for oxidative phosphorylation to synthesize large amounts of $ATP$. Anaerobic metabolism occurs without oxygen and produces significantly less $ATP$ than aerobic metabolism.

Cellular Communication, Cycle, and Death

Cells exchange information via signaling molecules through various pathways. Autocrine signaling involves a cell responding to its own signals. Paracrine signaling affects neighboring cells, while endocrine signaling utilizes hormones traveling through the blood to distant targets. Juxtacrine signaling requires direct contact between cells. These signals are received by membrane receptors, which activate intracellular pathways, or intracellular receptors in the cytoplasm or nucleus that influence gene expression. Typical signaling molecules include neurotransmitters, hormones, and growth factors.

Stem cells are the primary cells arising from a fertilized zygote and form the foundation of all organs. In animals, they allow for the repair and replacement of damaged or non-dividing cells throughout life. Cell division follows a regulated cycle consisting of the $G1$ phase (cell growth and $RNA$/protein synthesis), the $S$ phase ($DNA$ replication), the $G2$ phase (preparation for mitosis), and the $M$ phase (mitosis/division). Cells can enter a resting phase known as $G0$, typical for adult neurons. The cycle is regulated by cyclins and cyclin-dependent kinases ($CDK$).

Cell death occurs via two distinct mechanisms. Apoptosis is a programmed, actively regulated physiological process for removing unnecessary or damaged cells, involving the activation of caspases, $DNA$ fragmentation, and removal of debris by macrophages. It is essential for embryogenesis and immune function. In contrast, necrosis is an unmanaged, passive process resulting from severe damage like ischemia or toxins. Necrosis is characterized by cell swelling (edema), plasma membrane disintegration, and the release of cellular contents, often triggering a significant inflammatory response involving neutrophils and cytokines.

Homeostasis and Internal Environment Parameters

Homeostasis is the ability of an organism to maintain a stable internal environment despite external changes, regulated via nervous and hormonal systems using negative feedback loops. Examples include thermogenesis and vasoconstriction during cold exposure. Key components of homeostasis include isoionia (mineral balance), isoosmia/isotonia (osmotic pressure balance), isohydria ($pH$ stability), isovolemia (fluid volume stability), and isotermia (body temperature stability). The body also focuses on removing non-protein nitrogen.

Standard physiological parameters for the internal environment are strictly defined. The blood $pH$ is normally between $7.35\text{--}7.45$. Body temperature is approximately $37\text{--}^{\circ}\text{C}$. The osmolarity of body fluids is roughly $290\,\text{mOsm/l}$. Specific ion levels must be maintained for sodium ($\text{Na}^+$) and potassium ($\text{K}^+$). Fasting blood glucose levels range from $3.6\text{--}5.5\,\text{mmol/l}$. Partial pressures of respiratory gases are maintained at $p\text{O}_2 \thickapprox 100\,\text{mmHg}$ and $p\text{CO}_2 \thickapprox 40\,\text{mmHg}$.

Acid-Base Balance and Body Fluids

Acid-base balance ($ABR$) represents the equilibrium between the production and excretion of acidic and basic substances. The bicarbonate pufrační systém (bicarbonate buffer) is the most critical immediate regulator. Long-term compensation involves the lungs, kidneys, liver, and heart. Metabolic disturbances are characterized by changes in $pH$ and Base Excess ($BE$), with physiological $BE$ values between $-2$ to $+2\,\text{mmol/l}$. Metabolic alkalosis can be caused by vomiting ($BE \thickapprox +10\,\text{mmol/l}$), while metabolic acidosis occurs in shock or diabetic coma ($BE \thickapprox -20\,\text{mmol/l}$). Respiratory disturbances involve changes in $p\text{CO}_2$, with a normal average of $5.3\,\text{kPa}$ ($40\,\text{mmHg}$). Hyperventilation leads to respiratory alkalosis, while hypoventilation leads to respiratory acidosis. Kussmaul breathing is a specific compensatory hyperpnea seen in metabolic acidosis.

Body fluids constitute approximately $60\%$ of an adult male's weight, or roughly $42\,\text{liters}$ for a $70\,\text{kg}$ man. This is divided into intracellular fluid ($ICT$), which accounts for $2/3$ of total fluids ($40\%$ of body weight or $28\,\text{liters}$), and extracellular fluid ($ECT$), which accounts for $1/3$ of total fluids ($20\%$ of body weight or $14\,\text{liters}$). $ICT$ has high concentrations of $\text{K}^+$ ($140\,\text{mmol/l}$), magnesium, phosphates, and proteins. $ECT$ has high concentrations of $\text{Na}^+$ ($135\text{--}145\,\text{mmol/l}$), chloride ($\text{Cl}^-$), and bicarbonate ($\text{HCO}_3^-$). $ECT$ is further divided into intravascular fluid (plasma, $3.5\,\text{liters}$), interstitial fluid (tissue fluid, $10.5\,\text{liters}$), and transcellular fluid (e.g., cerebrospinal fluid, synovial fluid, roughly $1\,\text{liter}$). Newborns have higher water content ($70\text{--}80\%$) with a predominance of $ECT$ ($44\%$) over $ICT$ ($33\%$), making them susceptible to rapid dehydration.

Blood Composition and Functions

Blood is a suspension of cellular elements (platelets, red blood cells, and white blood cells) in plasma, making up about $7\%$ of body weight ($4.5\text{--}6\,\text{liters}$). Its functions include transport (gases, nutrients, hormones), regulation ($pH$, temperature, ions), defense (immune response), and hemostasis (clotting). Blood plasma consists of water ($90\text{--}92\%$), proteins ($7\text{--}8\%$ including albumins for oncotic pressure, globulins for immunity/transport, and fibrinogen for clotting), electrolytes, nutrients, and waste products like urea and creatinine. Oncotic pressure caused by proteins is approximately $25\,\text{mmHg}$.

Erythrocytes (red blood cells) are the most numerous elements, numbering $4.3\text{--}5.7 \times 10^{12}\text{/l}$ in men and $3.8\text{--}4.9 \times 10^{12}\text{/l}$ in women. They lack a nucleus and mitochondria to maximize space for hemoglobin ($Hb$). Hemoglobin consists of four subunits with heme (containing iron) and globin chains; $1\,\text{g}$ of $Hb$ binds $1.34\,\text{ml}$ of $\text{O}_2$. Forms include $HbA$ (adult) and $HbF$ (fetal, with higher oxygen affinity). Erythropoiesis occurs in the bone marrow, regulated by erythropoietin ($EPO$) from the kidneys in response to hypoxia. Reticulocytes are the stage just before maturity. Erythrocytes live for approximately 120\,\text{# days} before being degraded in the spleen.

Hemostasis, Leukocytes, and Blood Groups

Thrombocytes (platelets) are fragments of megakaryocytes ($150\text{--}400 \times 10^{9}\text{/l}$) that live for $9\text{--}12\,\text{days}$. Hemostasis involves vasoconstriction, the formation of a temporary platelet plug, blood coagulation (hemocoagulation), and finally fibrinolysis to remove the clot. Leukocytes (white blood cells) function in defense ($4\text{--}9 \times 10^{9}\text{/l}$). They are divided into granulocytes (neutrophils for bacteria, eosinophils for allergies/parasites, and basophils for histamine release) and agranulocytes (monocytes which become macrophages, and lymphocytes including $T$-cells for cellular immunity, $B$-cells for antibodies, and $NK$ cells for tumors/viruses).

Blood groups, discovered by Jan Janský and Karl Landsteiner, are determined by antigens (agglutinogens) on erythrocytes and antibodies (agglutinins) in the plasma. The $Rh$ system classifies blood based on the $D$ antigen; $85\%$ of the population is $Rh+$. In pregnancy, if an $Rh-$ mother carries an $Rh+$ fetus, she may develop anti-$D$ antibodies, leading to hemolytic disease of the newborn (erythroblastosis) in subsequent pregnancies. Prevention involves administering anti-$D$ antibodies to the mother after the first birth. Compatibility is verified via a cross-match test before transfusions.

Cardiac Physiology and Conduction

The heart is a muscular pump with four chambers: the right atrium ($PS$), right ventricle ($PK$), left atrium ($LS$), and left ventricle ($LK$). The $LK$ is $4\text{--}5$ times thicker than the $PK$ because it pumps against higher resistance in the systemic circulation. Valves include the tricuspid, mitral, pulmonary, and aortic. The cardiac cycle consists of systole (contraction) and diastole (relaxation). Heart muscle properties include automatie (self-generation of impulses), excitability, contractility, and conductivity. The conduction system includes the sinoatrial ($SA$) node (primary pacemaker, $60\text{--}100\,\text{bpm}$), atrioventricular ($AV$) node, His bundle, Tawara branches, and Purkinje fibers.

Electrical phenomena are measured by the Resting Membrane Potential ($-70$ to $-90\,\text{mV}$). Depolarization involves a shift toward $0\,\text{mV}$ or $+20\,\text{mV}$ as $\text{Na}^+$ and $\text{K}^+$ gradients shift. The Electrocardiogram ($EKG$) records these events: the $P$ wave represents atrial depolarization, the $QRS$ complex represents ventricular depolarization, and the $T$ wave represents ventricular repolarization. Intervals such as $PQ$ ($0.12\text{--}0.2\,\text{s}$) and $QT$ (frequency-dependent) are clinically significant. The $EKG$ is used to diagnose arrhythmias, ischemia, and myocardial infarction.

Cardiac Mechanics and Hemodynamics

Cardiac output ($MV$) is the volume of blood pumped per minute: $MV = \text{systolic volume} \times \text{heart rate}$. At rest, this is approximately $5\,\text{l/min}$, rising to $35\,\text{l/min}$ during exercise. Systolic (stroke) volume is about $70\,\text{ml}$ from an end-diastolic volume of $120\,\text{ml}$, leaving a residual end-systolic volume of $50\,\text{ml}$. Starling's Law states that increased filling (within limits) increases contraction force. Blood is distributed according to organ importance: the heart receives $5\%$, kidneys $20\%$, and the brain $15\%$. Coronary circulation supplies the myocardium, utilizing $250\,\text{ml/min}$ at rest.

Blood pressure ($TK$) is measured as systolic ($120\,\text{mmHg}$) and diastolic ($80\,\text{mmHg}$). It depends on cardiac output, peripheral resistance, and blood volume. Peripheral resistance is influenced by vessel diameter (vasoconstriction increases resistance/pressure), blood viscosity, and the autonomic nervous system. Regulation is achieved through baroreceptors (aortic arch/carotid sinus), chemoreceptors (sensing $\text{O}_2$, $\text{CO}_2$, $pH$), and the Renin-Angiotensin-Aldosterone System ($RAAS$). Adrenaline increases output, while Antidiuretic Hormone ($ADH$) promotes water retention to raise pressure.

Respiratory Physiology

Ventilation involves inspiration (active) and expiration (passive at rest). Respiration refers to gas exchange between alveoli and blood, and blood and tissues. Protective reflexes include sneezing (trigeminal/olfactory nerves), coughing (vagus nerve), the Kratschmer apnea reflex, and epiglottis closure. Anatomical dead space is approximately $150\,\text{ml}$, where no gas exchange occurs, but air is humidified and warmed. Lung mechanics depend on compliance (the ease of expansion) and elasticity (the ability to recoil). Pulmonary surfactant is a substance that reduces surface tension in alveoli to prevent their collapse (atelectasis) during expiration.

Gas exchange occurs via diffusion across the alveolocapillary membrane based on pressure gradients ($p\text{O}_2 \thickapprox 100\,\text{mmHg}$ in alveoli vs $40\,\text{mmHg}$ in blood). The ventilačně-perfuzní poměr ($V/Q$ ratio) is ideally $0.8\text{--}1$, though it varies from lung apex to base. Oxygen is transport primarily bound to hemoglobin ($98\%$), with $2\%$ dissolved in plasma. Carbon dioxide is transported as bicarbonate ($70\%$), bound to hemoglobin ($20\%$), or dissolved ($10\%$). Breathing is controlled by the medullary respiratory center, influenced by central chemoreceptors (sensing $p\text{CO}_2$ and $pH$) and peripheral chemoreceptors (sensing $p\text{O}_2$ and $pH$).

Metabolic Pathways and Energy Balance

Metabolism is the sum of all chemical reactions in the body. Anabolism involves synthetic reactions requiring energy ($ATP$), while catabolism involves degradation reactions releasing energy. Aerobic processes are significantly more efficient than anaerobic ones: $1\,\text{mol}$ of glucose provides $2800\,\text{kJ}$ aerobically versus only $150\,\text{kJ}$ anaerobically. Macronutrients include carbohydrates ($16.4\,\text{kJ/g}$), proteins ($16.4\,\text{kJ/g}$), and lipids ($34\,\text{kJ/g}$). Energy is measured in calories and joules ($1\,\text{cal} = 4.18\,\text{J}$). In the body, $40\%$ of food energy is used for $ATP$ production, while $60\%$ is released as heat.

Basal Metabolism ($BM$) is the minimum energy required to maintain life at rest. It is influenced by gender (men $5\text{--}10\%$ higher), age, weight, and hormones (thyroxine). Approximate values are $7000\text{--}7500\,\text{kJ/day}$ for men and $5500\text{--}6000\,\text{kJ/day}$ for women. A $1\,^{\circ}\text{C}$ increase in body temperature raises $BM$ by approximately $14\%$. Carbohydrates provide $50\text{--}60\%$ of energy needs; glucose is regulated between $3.6\text{--}5.5\,\text{mmol/l}$ by insulin (lowering glucose) and glucagon/adrenaline (raising glucose via glycogenolysis and gluconeogenesis).

Protein and Lipid Metabolism

Proteins are essential for tissue repair, immune function, and enzyme production. The daily requirement is $0.9\,\text{g/kg}$. There is no storage form for proteins; prolonged fasting leads to proteolysis of one's own tissues. Amino acids are classified as essential (must be ingested), semi-essential (required during growth/stress), and non-essential. Simple proteins include albumin and fibrinogen, while complex proteins include glycoproteins and lipoproteins. Lipids serve as the primary energy reserve in adipose tissue. White fat stores energy, whereas brown fat (predominant in newborns) is specialized for thermogenesis.

Adipose tissue makes up $30\%$ of weight in women and $20\%$ in men. Visceral fat in the abdominal cavity is metabolically active and associated with insulin resistance. Steroid substances derived from cholesterol ($0.3\,\text{g}$ daily intake, $1\,\text{g}$ internal production) include hormones and vitamin $D$. Polyunsaturated fatty acids ($PUFA$) like Omega-$3$ and Omega-$6$ are essential. Lipids circulate as lipoproteins: Chylomicrons, $VLDL$, $LDL$, and $HDL$. Lipolysis breaks down triglycerides into fatty acids and glycerol, whereas lipogenesis synthesizes fats when energy intake exceeds demand. Carnitine is required for fatty acid transport into mitochondria.

Liver Function and Gastrointestinal Physiology

The liver is the primary metabolic organ, receiving $1500\,\text{ml/min}$ of blood via the portal vein. It produces bile ($600\,\text{ml/day}$) essential for fat emulsification. Other functions include detoxification (alcohol, ammonia to urea, drugs), heat production (maintaining the "thermal core" at $39\,^{\circ}\text{C}$), storage (glycogen, iron, $B$ vitamins), and synthesis of plasma proteins and coagulation factors. It also degrades hemoglobin into bilirubin.

The gastrointestinal tract ($GIT$) performs digestion, absorption, motility, and secretion. Saliva ($1.5\text{--}2.0\,\text{l/day}$) contains $\text{α-amylase}$ for starch and lysozyme for defense. Swallowing is a reflex that closes the epiglottis. The stomach secretes $2\,\text{liters}$ of gastric juice daily, containing $HCl$ ($pH\thickapprox 2\text{--}3$), pepsinogen (activated to pepsin), and intrinsic factor for $B_{12}$ absorption. Small intestine (duodenum, jejunum, ileum) is the main site of absorption via villi ($200\,\text{m}^2$ surface area). The large intestine absorbs water and electrolytes, houses flora that synthesize vitamins $K$ and $B$, and manages defecation.

Renal Physiology and Excretion

Kidneys filter blood and maintain fluid/electrolyte balance, receiving $20\text{--}25\%$ of cardiac output ($RF \thickapprox 1000\text{--}1300\,\text{ml/min}$). The nephron is the functional unit, consisting of the glomerulus and tubule system. Glomerular filtration rate ($GFR$) is approximately $120\,\text{ml/min}$, producing $180\,\text{liters}$ of primary urine daily. Effective filtration pressure is about $10\,\text{mmHg}$. Clearance, such as creatinine clearance ($110\text{--}150\,\text{ml/min}$), measures the kidneys' cleaning efficiency. The glucose threshold is active at approximately $10\,\text{mmol/l}$; beyond this, glucose appears in the urine (glycosuria).

Tubular function involves reabsorption and secretion. The proximal tubule reabsorbs $65\text{--}70\%$ of the filtrate including glucose and amino acids. The Loop of Henle creates an osmotic gradient for urine concentration. The distal tubule and collecting ducts are regulated by hormones: $ADH$ (vasopressin) increases water reabsorption, and Aldosterone increases $\text{Na}^+$ retention and $\text{K}^+$ excretion. Kidneys also produce erythropoietin ($EPO$) and renin. The micturition reflex is triggered when the bladder fills to $150\text{--}200\,\text{ml}$, involving the detrusor muscle and internal/external sphincters.

Endocrinology and Hormonal Regulation

Hormones are chemical messengers acting via membrane receptors (using second messengers like $cAMP$) or intracellular receptors (influencing gene expression). The hypothalamus-pituitary system is the central regulator. The anterior pituitary ($adenohypofýza$) produces Growth Hormone ($GH$), $ACTH$, $TSH$, $FSH$, $LH$, and Prolaktin. Disorders of $GH$ include nanism (deficiency), gigantism (excess in youth), and acromegaly (excess in adults). The neurohypophysis stores $ADH$ and Oxytocin.

The thyroid gland produces $T_3$ and $T_4$ (requiring iodine) to regulate basal metabolism. Hypothyroidism in early life causes cretinism, while hyperthyroidism (Graves-Basedow disease) causes weight loss and tachycardia. The thyroid also produces Calcitonin, while the parathyroid glands produce $PTH$; both regulate calcium. The adrenal cortex produces mineralocorticoids (aldosterone), glucocorticoids (cortisol), and androgens. The adrenal medulla produces catecholamines (adrenaline/noradrenaline) for the "fight or flight" response. Adipose tissue produced adipokines like Leptin (appetite suppression) and Adiponektin. Stress triggers the $HPA$ axis, increasing cortisol which suppresses the immune system.

Reproduction and Developmental Physiology

Sex is determined at fertilization by $X$ and $Y$ chromosomes ($XX$ for female, $XY$ for male). Male reproduction involves spermatogenesis in the testes ($70\,\text{day}$ cycle), regulated by $FSH$ and $LH$/testosterone. Varlata must descend into the scrotum (lower temperature) to avoid kryptorchizmus. Female reproduction involves oogenesis and the menstrual cycle ($28\,\text{days}$). The ovarian cycle includes follicular, ovulation ($14\,\text{th day}$), and luteal phases. The endometrial cycle includes menstrual, proliferative (estrogen-driven), and secretory (progesterone-driven) phases. Pregnancy lasts roughly $40\,\text{weeks}$. The placenta produces $hCG$, progesterone, and estrogens. Birth occurs in three phases: opening, expulsion, and placental, driven by oxytocin.

Laktace (lactation) is stimulated by prolaktin and the milk-ejection reflex by oxytocin. Kolostrum (first milk) provides critical antibodies ($IgA$). Maternal benefits of breastfeeding include faster uterine involution and reduced risk of breast cancer. Child benefits include immune protection and optimal nutrition. The nervous system manages these and other functions through neurons and neuroglia (astrocytes, microglia, oligodendroglia, ependymal cells). Signal transmission occurs at synapses via neurotransmitters like acetylcholine or glutamate. The autonomic nervous system ($ANS$) is divided into the sympathetic (stress) and parasympathetic (rest/digestion) branches.

Muscle Physiology and Motor Control

Muscle tissue includes skeletal (striated, voluntary), smooth (involuntary/organs), and cardiac (syncytium) types. Skeletal muscle's functional unit is the sarkomera, containing actin and myosin. Contraction requires $Ca^{2+}$ (binding to troponin $C$) and $ATP$. At the neuromuscular junction, acetylcholine ($ACh$) triggers depolarization. Muscle work can be isotonic (length changes) or isometric (tension increases). Energy is provided by $ATP$, creatine phosphate, and glucose. Smooth muscle, found in vessel walls and organs, lacks sarkomeres, is connected by gap junctions, and is controlled by the vegetative nervous system. It is slower but less fatigable than skeletal muscle.

Motor control involves reflexes (automatic responses) and voluntary motor skills. The reflex arc includes a receptor, afferent pathway, center, efferent pathway, and effector. Proprioreceptive reflexes (like the patellar reflex) maintain muscle tone. Voluntary motor functions are managed by the motor cortex, basal ganglia (dysfunction causes Parkinson's or Chorea), and cerebellum (dysfunction causes ataxie). Sleep, essential for regeneration, involves non-$REM$ (slow-wave) and $REM$ (paradoxical) phases. Sleep cycles are governed by circadian rhythms and melatonin produced by the pineal gland ($epifýza$) in response to light levels sensed by the hypothalamus.