Comprehensive Study Notes: Physiology, Clinical Pathology, and General Pathology

Cellular Electrophysiology and Muscular Contraction

The fundamental electrical properties of excitable cells are characterized by two types of potentials: the graded potential and the action potential ($PdA$). A graded potential is proportioned to the intensity of the stimulus and decreases in amplitude as it moves away from its site of origin. In contrast, an action potential in a motor neuron is a "all-or-nothing" event, meaning it does not vary in amplitude regardless of stimulus intensity and maintains a constant amplitude and duration during propagation; it constitutes the primary nerve impulse. Ion channels are defined as integral membrane proteins that allow the passage of specific ions, such as $Na^+$, $K^+$, $Ca^{2+}$, and $Cl^-$.

In chemical synapses, the signal is transmitted from the pre-synaptic neuron to the post-synaptic neuron via the release of neurotransmitters. For an excitatory chemical synapse, the neurotransmitter typically causes the opening of channels for $Na^+$ or $Ca^{2+}$, leading to depolarization. Conversely, inhibitory signals may involve the opening of $K^+$ or $Cl^-$ channels. The neuromuscular junction specifically utilizes acetylcholine as its primary neurotransmitter. In skeletal muscle contraction, the process begins when a motoneurone releases acetylcholine, leading to a plaque potential (a type of graded potential). The action potential then travels along the T-tubules, where voltage-dependent $DHPR$ receptors induce the opening of $RyR1$ calcium channels on the sarcoplasmic reticulum. The subsequent increase in intracellular $Ca^{2+}$ allows the ion to bind to troponin, which shifts tropomyosin and enables the binding between actin and myosin. In smooth muscle, $Ca^{2+}$ binds instead to calmodulin.

Cardiovascular System and Hemodynamics

The cardiac cycle consists of several distinct phases: atrial systole (depolarization/contraction marked by the P wave on an ECG), isovolumetric ventricular contraction (where ventricular pressure rises to approximately $80\,mmHg$ and all valves are closed), isovolumetric relaxation, and ventricular filling. The QRS complex on an ECG represents ventricular depolarization, while the T wave represents ventricular repolarization (diastole). During ventricular systole, blood is pushed from the ventricles into the arteries, reaching an ejection pressure of approximately $120\,mmHg$ in the aorta. The mitral valve (bicuspid) is located between the left atrium and left ventricle and closes at the start of isovolumetric contraction; it opens at the end of isovolumetric relaxation when atrial pressure exceeds ventricular pressure.

Cardiac output ($CO$) is calculated as the product of heart rate ($HR$) and stroke volume ($SV$): $CO = HR \times SV$. At rest, the heart rate is approximately $70\,bpm$, the stroke volume is $70\,ml$, and the total cardiac output is approximately $5\,L/min$. During intense exercise, cardiac output can drastically increase. Blood volume in the average human is approximately $5\,L$. Arteries conduct blood from the heart to the tissues, while veins (which contain $60\text{--}80\%$ of total blood volume) return blood to the heart. Veins are characterized as being distensible and operating under low pressure. Blood pressure ($PA$) is the force exerted by blood on arterial walls, with normal values typically around $120/80\,mmHg$. Mean arterial pressure is the integral average between maximum (systolic) and minimum (diastolic) pressures. Resistance to blood flow is inversely proportional to the fourth power of the vessel radius, according to Poiseuille's Law.

Regulating heart function involve opposing actions from the nervous system. Parasympathetic (vagal) stimulation reduces the heart rate by activating receptors linked to $K^+$ channels, while sympathetic stimulation increases heart rate, conduction velocity, and myocardial contractility (inotropism) by increasing $Ca^{2+}$ influx.

Respiratory System and Gas Exchange

The respiratory system includes the upper airways (which warm, humidify, and filter air), the pleura (composed of parietal and visceral layers), and the lungs with their alveoli. Inspiration occurs when lung volume increases, causing intrapolmonar pressure to decrease below atmospheric pressure ($760\,mmHg$), following Boyle's Law. The intrapleural pressure is approximately $-4\,mmHg$ before inspiration and drops to $-6$ to $-8\,mmHg$ during the process. Surfactant, a chemical substance secreted by type 2 pneumocytes, reduces the surface tension of the fluid lining the alveoli, preventing the collapse of smaller alveoli and increasing stability.

Gas exchange at the alveolar-capillary level is driven by the difference in gas partial pressures. In dry air at $760\,mmHg$ with $21\%$ oxygen, the $pO_2$ is approximately $160\,mmHg$. In venous blood, $pO_2$ is typically $40\,mmHg$ and $pCO_2$ is $46\,mmHg$, whereas in the alveoli, they are $100\,mmHg$ and $40\,mmHg$ respectively. Oxygen is primarily transported bound to Hemoglobin ($Hb$) ($97\text{--}98\%$), with a small amount physically dissolved in plasma. Factors that decrease the affinity of $Hb$ for oxygen (increasing release) include decreased $pH$, increased temperature, increased $CO_2$ concentration, and increased $2,3\text{-}biphosphoglycerate$ ($DPG$). Carbon dioxide is transported mainly as bicarbonate ions ($70\%$), bound to $Hb$ ($23\%$), or dissolved in plasma ($7\%$). Alveolar ventilation refers to the volume of air that reaches the alveoli and participates in gas exchange per minute.

Renal Physiology and Metabolism

The kidneys are responsible for regulating volume, osmolarity, $pH$, and the ionic balance of body fluids, as well as excreting waste. The basic unit is the nephron, where processes occur in the order: filtration, reabsorption, secretion, and excretion. The glomerular filtration rate ($VFG$) is approximately $125\,ml/min$, resulting in about $180\,L$ of ultrafiltrate produced in $24$ hours. Ultratfiltered fluid does not normally contain red or white blood cells, platelets, fibrinogen, or large plasma proteins. Filtration is favored by high hydrostatic pressure in the glomerular capillaries (approx. $60\,mmHg$) and hindered by hydrostatic pressure in Bowman's capsule and oncotic pressure from plasma proteins ($6\text{--}8\,g/dl$).

In the proximal convoluted tubule ($TCP$), about $70\%$ of water and solutes (like glucose) are reabsorbed. Glucose is completely reabsorbed unless the plasma concentration exceeds the renal threshold of approximately $200\,mg/100\,ml$ ($2\,mg/ml$). The descending limb of the Loop of Henle is responsible for water reabsorption, while the ascending limb creates an osmotic gradient in the medulla. Hormonal regulation includes Vasopressin ($ADH$), which inserts aquaporins into the collecting duct to increase water reabsorption, and Aldosterone, which stimulates $Na^+$ reabsorption and $K^+$ secretion in the distal nephron.

Digestive and Endocrine Systems

Digestion begins in the mouth with salivary amylase breaking down carbohydrates. In the stomach, parietal cells produce hydrochloric acid ($HCl$) and intrinsic factor (necessary for Vitamin $B_{12}$ absorption), while principal cells secrete pepsinogen (converted to pepsin by acid). Gastrin and the vagus nerve stimulate acid secretion, while somatostatin inhibits it. In the small intestine, disaccharidases like maltase and sucrase are localized to the brush border to break down sugars into simple monosaccharides. Proteins are digested starting in the stomach and continuing in the intestine with pancreatic enzymes like trypsin and chymotrypsin. Lipids are emulsified by bile salts (produced by hepatocytes, stored in the gallbladder, and synthesized from cholesterol) and absorbed via micelles/chylomicrons into the lymphatic system.

The endocrine system involves the hypothalamus-pituitary axis. The neurohypophysis (posterior pituitary) releases hormones synthesized in the hypothalamus (like $ADH$ and oxytocin), while the adenohypophysis (anterior pituitary) produces its own hormones under the influence of hypothalamic releasing or inhibiting factors. Steroid hormones are synthesized from cholesterol, are lipid-soluble, and often interact with intracellular receptors to induce genomic changes.

Clinical Pathology and Transfusion Medicine

Blood is a connective tissue composed of plasma ($55\%$) and a corpusculated portion ($45\%$), which includes red blood cells ($GR$), white blood cells ($GB$), and platelets. There are over $600$ known antigens on red blood cells, which define blood group systems such as $AB0$ and $Rh$. A person with group A blood has anti-B antibodies; those with the Bombay phenotype have anti-A, anti-B, and anti-H antibodies. Transfusion law regulates the safe collection, storage, and distribution of blood components. Human error remains the most frequent cause of transfusion injury. Common blood components include erythrocyte concentrates (washed to remove plasma or filtered to remove the "buffy coat" leukocyte-platelet layer), fresh frozen plasma, and platelets. Risks of chronic transfusion treatment include iron overload (hemosiderosis). Fetal hemoglobin ($HbF$) consists of two alpha and two gamma chains ($\alpha_2 \gamma_2$).

Diagnostic markers for biological safety include $NAT$ (Nucleic Acid Testing) for viral genomes. The diagnostic sequence for $AIDS$ (caused by $HIV$) involves $ELISA\,1$, $ELISA\,2$, $RIBA\, (WB)$, and $PCR$. The window period for $HIV$ is $3\text{--}4$ weeks. For Hepatitis B ($HBV$), markers appear in the order: $HBsAg$, $HBeAg$, and $anti\text{-}HBc\,IgM$. The Philadelphia chromosome, a reciprocal translocation between chromosomes $9$ and $22$ ($t(9;22)$), producing the $BCR\text{-}ABL$ fusion gene, is diagnostic for Chronic Myeloid Leukemia ($LMC$).

General Pathology, Oncology, and Immunology

General pathology examines the mechanisms of disease (pathogenesis) and cell death. Necrosis is unplanned cell death; coagulative necrosis is typical of ischemic/hypoxic damage (e.g., myocardial infarction), while liquefactive necrosis involves lysosomal enzyme release, often from infections. Apoptosis is a programmed, energy-dependent cell death involving the caspases (proteolytic enzymes). Acute inflammation (angioflogosi) is characterized by redness (rubor), swelling (tumor), heat (calor), pain (dolor), and loss of function. Mediators like histamine (released by mast cells) and cytokines like $TNF\text{-}\alpha$ and $IL\text{-}1$ drive vascular permeability and leukocyte migration. Chronic inflammation can result in the formation of a granuloma (a cluster of immune cells seen in tuberculosis or Crohn's disease).

Oncology distinguishes between benign tumors (e.g., adenoma, angioma) and malignant tumors (e.g., adenocarcinoma, angiosarcoma, melanoma). Malignant tumors are characterized by autonomous replication, local invasion, and metastasis. Oncogenes like $Ras$ (a $GTPase$) promote cell growth, while tumor suppressors like $Rb$ (Retinoblastoma protein) inhibit the cell cycle by sequestering the transcription factor $E2F$. Immunology involves the study of the immune system. Passive immunization provides immediate but temporary protection (e.g., maternal antibodies crossing the placenta), while active immunization is specific and provides long-term protection. Antibodies (Immunoglobulins) include classes like $IgG$ (most abundant in serum), $IgM$ (early response), $IgA$ (present in colostrum and secretions), and $IgE$ (involved in allergies and parasites). $MHC$ class I molecules consist of an alpha chain and a $\beta_2\text{-}macroglobulin$ chain, found on all nucleated cells.