Human Physiology: Urine Formation, Endocrine & Nervous Systems

Posted on Mar 10, 2026 in Biology

Physiology of Urine Formation

Physiology of urine formation: Kidneys form urine by filtration and secretion of waste materials from the blood. In addition, selective reabsorption by tubular cells contributes to the maintenance of homeostasis and the regulatory activities of the kidneys.

Formation of Urine

The urinary system: The nephrons of the kidney perform this function. Primarily, three processes are involved in the formation of urine: glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular Filtration

This occurs in Bowman’s (glomerular) capsule. The barrier between the internal portion of Bowman’s capsule and the blood in the glomerular capillaries acts as an ultrafilter. This barrier is composed of endothelial cells of capillaries, their basement membrane, and the podocytes that form the inner portion of Bowman’s capsule.

Glomerular Filtration Rate (GFR)

The amount of filtrate formed in both kidneys per minute is called the GFR. It is approximately 120 mL/min (around 180 L per day). GFR depends on net filtration pressure (NFP), which in turn depends on capillary hydrostatic pressure (capillary blood pressure).

The capillary hydrostatic pressure mainly depends on the difference in diameter of the afferent and efferent arterioles and on systemic blood pressure. Although systemic blood pressure may vary, the pressure in the glomerulus is maintained by alterations in the lumen (diameter) of the efferent arteriole compared with the afferent arteriole. Thus, even in hypotension filtration continues, and in hypertension filtration can occur at a near-normal rate. This ability of the kidneys to maintain a relatively constant glomerular pressure and GFR irrespective of changes in systemic blood pressure is called autoregulation.

Tubular Reabsorption

It is the process by which the composition and volume of glomerular filtrate are altered during its passage through the tubule. The purpose of this process is to reabsorb constituents of the filtrate that are essential to the body, to maintain fluid and electrolyte balance, and to preserve the alkalinity of the blood.

Tubular Secretion (Active Secretion)

Tubular cells actively secrete certain substances into the filtrate. This secretion is aimed at eliminating materials and controlling blood pH. The substances secreted by tubular cells include potassium (K+), hydrogen ions (H+), ammonium ions (NH4+), and creatinine.

Male Reproductive System

Male reproductive system: The male reproductive system plays a crucial role in producing and delivering sperm, which are essential for sexual reproduction. Key components:

  1. Testes: Primary male reproductive organs; produce spermatozoa and secrete male sex hormones (such as testosterone).
  2. Scrotum: A sac of skin that surrounds and protects the testes and helps regulate testicular temperature for optimal sperm production.
  3. Spermatic ducts:
    • Epididymis: A coiled tube where immature sperm cells mature and gain motility.
    • Vas (ductus) deferens: A muscular tube that transports mature sperm from the epididymis to the urethra.
  4. Male accessory glands:
    • Prostate gland: Produces a milky fluid that nourishes and activates sperm.
    • Seminal vesicles: Secrete a fructose-rich fluid that provides energy for sperm.
    • Bulbourethral glands: Release a lubricating fluid to facilitate sperm movement.
  5. Penis: The male external genitalia, involved in both reproduction and urination.

Spermatogenesis: The Journey of Sperm Cells

  1. Stem cells and mitosis: Spermatogonial stem cells near the basement membrane of the seminiferous tubules multiply by mitosis. Half of the new cells become future sperm cells; the other half remain as stem cells to maintain a supply of germ cells.
  2. Primary spermatocytes: Spermatogonia destined for sperm development become primary spermatocytes and move toward the lumen while attaching around Sertoli cells (which support and nourish them).
  3. Meiosis I and II: Primary spermatocytes undergo meiosis I to yield two secondary spermatocytes. Each secondary spermatocyte undergoes meiosis II to yield two haploid spermatids; in total, four spermatids arise from one primary spermatocyte.
  4. Spermiogenesis: Spermatids transform into mature spermatozoa: they condense chromatin, form a flagellum, develop mitochondria in the midpiece, and shed excess cytoplasm.

Hormones Released from the Pituitary Gland and Their Roles

Hormones secreted by the pituitary gland: These hormones regulate many physiological functions.

  1. Thyroid-stimulating hormone (TSH)

    Function: Stimulates the thyroid gland to produce thyroid hormones (T3 and T4). Role: These hormones regulate metabolism, energy utilization, and growth.

  2. Adrenocorticotropic hormone (ACTH)

    Function: Stimulates the adrenal cortex to release glucocorticoids (e.g., cortisol). Role: Cortisol helps manage stress responses, immune function, and metabolism.

  3. Follicle-stimulating hormone (FSH)

    Function: Promotes growth of ovarian follicles in females and spermatogenesis in males. Role: Essential for reproduction.

  4. Luteinizing hormone (LH)

    Function: Triggers ovulation in females and stimulates testosterone production in males. Role: Crucial for reproductive health.

  5. Growth hormone (GH or STH)

    Function: Promotes growth, cell division, and protein synthesis. Role: Vital for childhood growth and tissue repair.

  6. Prolactin

    Function: Stimulates milk production in mammary glands. Role: Essential for lactation.

  7. Oxytocin (stored in posterior pituitary)

    Function: Induces uterine contractions during childbirth and facilitates milk ejection during breastfeeding. Role: Important for reproductive processes and bonding.

  8. Antidiuretic hormone (ADH or vasopressin; stored in posterior pituitary)

    Function: Regulates water balance by increasing water reabsorption in the kidneys. Role: Helps maintain blood pressure and prevent dehydration.

Transmission of Nerve Impulses

Transmission of nerve impulses: Neurons communicate using electrical and chemical signals.

  1. Resting membrane potential: Neurons have a charged membrane due to differences in ion concentrations inside and outside the cell. Ion channels regulate ion flow; the baseline charge is called the resting membrane potential.
  2. Action potential: When a neuron receives a sufficient signal, voltage-gated ion channels open, causing a brief reversal of the membrane potential that propagates along the axon.
  3. Chemical synapses: Communication between neurons typically occurs at synapses: neurotransmitters released from the presynaptic terminal cross the synaptic cleft and bind receptors on the postsynaptic cell, causing excitatory or inhibitory postsynaptic potentials.
  4. Propagation of the signal: If the postsynaptic potential reaches threshold, an action potential is generated and travels along the axon to release neurotransmitter onto the next cell.
  5. Neurotransmitters: Examples include acetylcholine (muscle contraction and autonomic functions), dopamine (reward, motivation, movement), serotonin (mood, sleep, appetite), glutamate (major excitatory transmitter), and GABA (primary inhibitory transmitter).

Oogenesis

Oogenesis: The formation of female gametes

  1. Pre-natal stage: Oogenesis begins before birth. Oogonia undergo mitosis to increase in number and then differentiate into primary oocytes that arrest in meiosis I. Granulosa cells form a stratified epithelium around the primary oocyte and secrete glycoproteins to create the zona pellucida.
  2. Antral stage: Fluid-filled spaces develop between granulosa cells, forming the antrum; these follicles are called secondary follicles. Under FSH and LH influence, secondary follicles mature during the monthly cycle.
  3. Pre-ovulatory stage: An LH surge triggers completion of meiosis I; the primary oocyte divides asymmetrically, producing a polar body and a secondary oocyte arrested in meiosis II.
  4. Ovulation: The mature secondary oocyte is released from the ovary and enters the fallopian tube awaiting fertilization.
  5. Fertilization: If a sperm penetrates the secondary oocyte, meiosis II completes, and a haploid ovum is ready for fusion with the sperm nucleus.

Role of the Kidney in Acid-Base Balance

Role of the kidney in acid-base balance: The kidneys maintain acid-base homeostasis by:

  1. Reabsorption of bicarbonate (HCO3-): Kidneys reabsorb filtered bicarbonate back into the bloodstream; bicarbonate is a major buffer that neutralizes excess acid.
  2. Excretion of hydrogen ions (H+): The kidneys secrete H+ into urine to eliminate acid produced during metabolism.
  3. Ammoniagenesis: The kidneys produce ammonia (NH3), which combines with H+ to form ammonium (NH4+), facilitating H+ excretion and new bicarbonate generation.

Compensation Mechanism

When blood becomes too acidic (acidosis), the kidneys respond by reabsorbing more bicarbonate, secreting more hydrogen ions in the collecting ducts (thereby generating additional bicarbonate), and increasing ammoniagenesis. Conversely, during alkalosis, the kidneys excrete more bicarbonate and reduce hydrogen-ion secretion; more ammonium is excreted.

Process of Absorption of Nutrients from the Small Intestine

Process of absorption of nutrients from the small intestine: The small intestine is the primary site for nutrient absorption and achieves this through structural and functional adaptations.

Structural adaptations

The mucosa is lined with villi—finger-like projections that increase surface area. Each villus contains capillaries and lacteals (lymphatic vessels) to transport absorbed nutrients.

Digestion and absorption

As chyme enters the small intestine from the stomach, nutrients are digested and absorbed:

  1. Carbohydrates: Enzymes break complex carbohydrates into monosaccharides (glucose, fructose, galactose), which are absorbed through epithelial cells into the bloodstream.
  2. Proteins: Proteases break proteins into amino acids, which are actively transported across epithelial cells into the bloodstream.
  3. Lipids (fats): Bile salts emulsify fats; pancreatic lipase breaks them into fatty acids and monoglycerides. These are absorbed into epithelial cells, reassembled into triglycerides, packaged as chylomicrons, and enter lacteals to reach the bloodstream via the lymphatic system.
  4. Vitamins and minerals: Water-soluble vitamins (e.g., vitamin C, B vitamins) and many minerals (iron, calcium) are absorbed directly into the blood; fat-soluble vitamins (A, D, E, K) are absorbed with lipids into chylomicrons.

Role of RAS in the Kidney

Role of the renin-angiotensin system (RAS): Also known as the renin-angiotensin-aldosterone system (RAAS), this hormone system regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance.

  1. Activation: Reduced renal blood flow, low blood pressure, or sympathetic stimulation causes juxtaglomerular cells to release renin. Renin converts angiotensinogen (from the liver) into angiotensin I.
  2. Angiotensin II formation: Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE), primarily on pulmonary vascular endothelium. Angiotensin II is rapidly active and subsequently degraded to angiotensin III.
  3. Functions of angiotensin II:
    • Vasoconstriction: increases blood pressure.
    • Stimulates aldosterone secretion from the adrenal cortex, which increases sodium and water reabsorption and causes potassium excretion.
  4. Clinical significance: Abnormal RAS activation can lead to hypertension. ACE inhibitors, angiotensin II receptor blockers (ARBs), and renin inhibitors target this system and are used to treat hypertension, heart failure, and some kidney diseases.

Basal Metabolic Rate (BMR)

BMR: Basal metabolic rate (BMR), sometimes called resting metabolic rate (RMR), is the number of calories the body needs to perform essential life-sustaining functions at rest, such as circulation, breathing, cell maintenance, and ion transport.

Definition and calculation

BMR is the minimum number of calories required for basic functions at rest. A commonly used estimate is the Harris-Benedict equation:

  • For men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age in years).
  • For women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age in years).

RMR and BMR are sometimes used interchangeably; RMR is measured under slightly less strict conditions (e.g., after restful sleep but not necessarily fasted). While no method is 100% accurate, calculating BMR provides a baseline for dietary planning and weight management.

Hormones Released from the Pancreas and Blood Glucose Control

Pancreatic hormones and blood glucose regulation:

  1. Insulin (beta cells):

    Function: Promotes glucose uptake into cells for energy, stimulates glycogen synthesis in liver and muscle, supports protein synthesis, and promotes lipid storage. Effect on blood glucose: Lowers blood glucose after meals.

  2. Glucagon (alpha cells):

    Function: Stimulates hepatic glycogen breakdown (glycogenolysis) and gluconeogenesis (glucose synthesis from non-carbohydrate sources). Effect on blood glucose: Raises blood glucose during fasting or between meals.

  3. Balancing act: Insulin and glucagon act antagonistically to maintain blood glucose within a narrow range.

Reflex Action

Reflex action: A reflex action is an automatic, rapid, involuntary response to a stimulus that occurs without conscious thought.

  1. Definition: A sudden and immediate response to a specific stimulus that helps protect the body.
  2. Examples: Knee-jerk reflex, blinking reflex, and withdrawal reflex (e.g., pulling away from a hot object).
  3. Neural pathway (reflex arc):
    • Receptor: detects the stimulus.
    • Sensory neuron: transmits the signal to the spinal cord or brainstem.
    • Integration center: processes the signal in the spinal cord or brainstem.
    • Motor neuron: carries the response away.
    • Effector: muscle or gland that produces the response.
  4. Speed: Reflexes are rapid because they often involve only spinal or brainstem circuits rather than routing signals to the conscious brain.

Cardiopulmonary Resuscitation (CPR)

Cardiopulmonary resuscitation (CPR): CPR is an emergency procedure combining chest compressions with artificial ventilation (often mouth-to-mouth) to maintain circulation and oxygenation of vital organs, particularly the brain, until spontaneous blood circulation and breathing can be restored.

CPR basics

Purpose: Maintain blood flow to vital organs during cardiac arrest. Components: chest compressions (to pump blood) and artificial ventilation (to provide oxygen to lungs).

Why is it called the “kiss of life”?

The phrase “kiss of life” refers to mouth-to-mouth resuscitation—the act of breathing air into a victim’s lungs. It symbolizes giving the gift of oxygen and life during a critical emergency.

Hypothalamus

Hypothalamus: the brain’s maestro

The hypothalamus is a small structure, roughly the size of an almond, located near the base of the brain adjacent to the pituitary gland. It can be divided into three main regions:

  1. Anterior (supraoptic) region: Regulates body temperature and circadian rhythm. Important nuclei include the supraoptic and paraventricular nuclei. Hormones and releasing factors include corticotropin-releasing hormone, thyrotropin-releasing hormone, gonadotropin-releasing hormone, oxytocin, vasopressin, and somatostatin (as regulatory factors).
  2. Middle (tuberal) region: Contains ventromedial and arcuate nuclei. The ventromedial nucleus is involved in appetite control; the arcuate nucleus contributes to regulation of growth hormone release and metabolic signals.
  3. Posterior (mammillary) region: Includes posterior hypothalamic nucleus and mammillary nuclei; involved in thermoregulation (shivering, heat conservation) and memory-related functions.

Functions: The hypothalamus maintains homeostasis and integrates the endocrine and nervous systems. It influences childbirth, emotions, sleep cycles, fluid balance, appetite, thirst, blood pressure, and heart rate.

Liver

Liver structure: The liver is the largest solid organ, weighing around 1.4 kg (approximately 3 lb) in an adult, located in the upper right abdomen beneath the diaphragm. It consists of a larger right lobe and a smaller left lobe and is covered externally by Glisson’s capsule and the peritoneum. Blood supply comes from the portal vein (nutrient-rich blood from the digestive tract) and the hepatic artery (oxygenated blood from the heart). The liver’s functional units are lobules composed of hepatocytes.

Liver functions

  1. Bile production: Hepatocytes produce bile, which helps emulsify fats and facilitate absorption of fat-soluble vitamins in the small intestine.
  2. Detoxification: The liver filters toxins, drugs, and metabolic waste from the blood and converts them into less harmful products for excretion.
  3. Metabolism: The liver metabolizes carbohydrates, proteins, and fats; stores and releases glucose; and synthesizes plasma proteins including clotting factors and albumin.
  4. Storage: Stores vitamins (A, D, E, K, B12), minerals (iron), and glycogen (a glucose storage form).
  5. Immune function: Kupffer cells in the liver remove bacteria and foreign particles from the blood.

Common liver diseases

  1. Hepatitis: Inflammation caused by viral infections (A, B, C, D, E) or other causes; symptoms include jaundice, fatigue, and abdominal pain.
  2. Fatty liver disease: Accumulation of fat in hepatocytes, including nonalcoholic fatty liver disease (NAFLD), often related to obesity and metabolic syndrome.
  3. Cirrhosis: Chronic liver damage and scarring due to alcohol abuse, viral hepatitis, or NAFLD.
  4. Liver cancer: Primary hepatocellular carcinoma often arises in cirrhotic livers; secondary (metastatic) liver cancer spreads from other organs.

Action Potential and Generation of Nerve Impulse

Action potential: An action potential (nerve impulse) is a rapid, transient change in membrane potential used by neurons to transmit signals along axons.

Generation of the nerve impulse

  1. Resting membrane potential: About −70 mV due to unequal ion distributions (mainly Na+ and K+) and the Na+/K+ pump (3 Na+ out, 2 K+ in), plus potassium leak channels.
  2. Stimulus and depolarization: A stimulus opens ligand-gated Na+ channels, allowing Na+ influx; if depolarization reaches threshold (around −50 to −55 mV), an action potential is triggered.
  3. All-or-none law: Subthreshold stimuli elicit no action potential; stimuli at or above threshold produce a full action potential.
  4. Rapid depolarization (upstroke): Voltage-gated Na+ channels open, Na+ rushes in, and the inside of the cell becomes positive.
  5. Overshoot and peak: Membrane potential can reach approximately +30 mV briefly.
  6. Repolarization: Voltage-gated Na+ channels inactivate and voltage-gated K+ channels open, allowing K+ efflux to restore negative membrane potential.

Functional Organization of the Cerebral Cortex

The cerebral cortex: The cortical mantle is the outer layer of the cerebrum and is central to perception, cognition, memory, and voluntary motor control.

Functional areas

  1. Motor areas: Located in the frontal lobe.
    • Primary motor cortex (precentral gyrus): initiates voluntary movements; organized somatotopically (motor homunculus).
    • Premotor cortex: plans and coordinates complex movements.
    • Broca’s area (usually left frontal lobe): essential for speech production; damage causes expressive aphasia.
  2. Sensory areas: Parietal, temporal, and occipital lobes receive and process sensory inputs.
    • Primary somatosensory cortex (postcentral gyrus): receives tactile and proprioceptive input (sensory homunculus).
    • Primary visual cortex (occipital lobe): processes visual information.
    • Primary auditory cortex (superior temporal gyrus): processes auditory information.
  3. Association areas: Integrate sensory data and mediate higher cognitive functions.
    • Prefrontal cortex: decision-making, planning, personality, social behavior.
    • Wernicke’s area (usually left temporal lobe): language comprehension; damage causes receptive aphasia.
  4. Other areas: Olfactory cortex (smell), gustatory cortex (taste), and the insular cortex (interoception and integration of visceral states).

External Respiration and Factors Affecting Gas Exchange

External respiration: The exchange of oxygen (O2) and carbon dioxide (CO2) between alveolar air and pulmonary capillary blood.

Mechanism

  1. Ventilation: Breathing moves air in and out of the alveoli; inhalation expands the thorax (diaphragm contracts), exhalation relaxes it.
  2. Alveolar surface area: Large total alveolar surface area (about 70 m²) promotes efficient diffusion; each alveolus is closely associated with capillaries.
  3. Partial pressure gradients: Gases diffuse from high to low partial pressure (O2 from alveoli to blood; CO2 from blood to alveoli).
  4. Ventilation–perfusion matching: Efficient gas exchange requires adequate airflow to alveoli (ventilation) and blood flow to corresponding capillaries (perfusion).

Factors affecting gaseous exchange

  • Surface area (e.g., emphysema reduces surface area).
  • Thickness of respiratory membrane (inflammation or edema increases thickness and reduces diffusion).
  • Partial pressure gradients (altitude reduces O2 partial pressure).
  • Solubility of gases (CO2 is more soluble than O2 and diffuses faster).

Pituitary Gland

Pituitary gland: Also called the hypophysis, it is a pea-sized endocrine gland at the base of the brain and is often called the “master gland” because it secretes hormones that regulate other endocrine glands.

Anatomy and hormones

The pituitary lies in the sella turcica (pituitary fossa) below the hypothalamus and near the optic chiasm. Its hormones include:

  • Human growth hormone (HGH): Promotes growth and tissue repair.
  • Thyroid-stimulating hormone (TSH): Stimulates thyroid hormone release.
  • Adrenocorticotropic hormone (ACTH): Stimulates cortisol release from the adrenal cortex.
  • Luteinizing hormone (LH) & follicle-stimulating hormone (FSH): Control gonadal function and reproduction.
  • Prolactin (PRL): Stimulates milk production.
  • Antidiuretic hormone (ADH): Controls water balance by affecting renal water reabsorption.
  • Oxytocin: Regulates childbirth and milk ejection.

Pituitary disorders (e.g., pituitary adenomas) may cause hormone imbalances and a wide range of clinical effects.

Anatomy of the Thyroid Gland and Hormones

Anatomy: The thyroid gland sits in the anterior neck below the larynx and around the trachea. It normally has two lobes connected by an isthmus and may have a pyramidal lobe. The functional unit is the thyroid follicle, with thyrocytes producing thyroglobulin and storing it in colloid. Parafollicular (C) cells secrete calcitonin.

Thyroid hormones and functions

  1. Thyroxine (T4): Major product of the thyroid; regulates metabolic rate and is converted in peripheral tissues to T3.
  2. Triiodothyronine (T3): More active than T4; increases basal metabolic rate, heat production, and affects cardiovascular and neuromuscular function.
  3. Calcitonin: Secreted by parafollicular cells; lowers blood calcium by inhibiting bone resorption and promoting calcium deposition in bone.

Thyroid function is regulated by TRH (hypothalamus) → TSH (pituitary) → T3/T4 (thyroid) with negative feedback control.

Neuron: Structure

Neuron: Neurons consist of three main parts: cell body (soma), dendrites, and axon. The cell body contains the nucleus and cytoplasm with organelles (mitochondria, Golgi complex). Characteristic structures include chromatophilic substance (Nissl bodies) and neurofibrils.

Processes: Dendrites are the receiving, usually branched, nonmyelinated processes containing organelles. The axon is a long, thin cylindrical projection that propagates nerve impulses to other neurons, muscle fibers, or glands. The axon contains mitochondria, microtubules, and neurofibrils but lacks rough endoplasmic reticulum; it is surrounded by a plasma membrane called the axolemma. Axons branch into collaterals and terminate in axon terminals (synaptic end bulbs) that contain synaptic vesicles holding specific neurotransmitters.

Organs of the Digestive System

Organs of the digestive system: The digestive tract and accessory organs process food, extract nutrients, and eliminate waste. Main organs include:

  1. Mouth (oral cavity): Mechanical digestion by teeth and chemical digestion by salivary enzymes (amylase).
  2. Esophagus: Muscular tube that propels food to the stomach via peristalsis.
  3. Stomach: Stores and churns food; gastric juice (HCl and pepsin) initiates protein digestion; chyme results.
  4. Small intestine: Duodenum, jejunum, ileum—site of most digestion and nutrient absorption.
  5. Large intestine (colon): Absorbs water and electrolytes; forms and stores feces.
  6. Liver: Produces bile; detoxifies; metabolizes nutrients; stores glycogen and vitamins.
  7. Gallbladder: Stores and concentrates bile; releases it into the duodenum to emulsify fats.
  8. Pancreas: Exocrine pancreas secretes digestive enzymes (amylase, lipase, proteases); endocrine pancreas secretes insulin and glucagon to regulate blood sugar.
  9. Anus: Terminal opening for fecal elimination.

Small intestine anatomy

  1. Duodenum: Receives chyme, bile, and pancreatic juice; Brunner’s glands secrete alkaline mucus.
  2. Jejunum: Main site of nutrient absorption; rich blood supply.
  3. Ileum: Absorbs remaining nutrients and contains Peyer’s patches (lymphoid tissue) for immune defense.

Neuron and Neurophysiology

Neurophysiology: The study of nervous system function, including neurons, glial cells, and neural circuits.

Key aspects

  1. Ion channels: Proteins that allow ions to cross membranes. Types include voltage-gated, chemically gated, and mechanically gated channels. They provide electrical excitability.
  2. Resting membrane potential and action potentials: Result from ion gradients and selective membrane permeability. Action potentials arise from voltage-gated channel activation and propagate signals.
  3. Neurotransmitters and synapses: Chemical messengers (acetylcholine, dopamine, serotonin, glutamate, GABA) mediate communication at synapses.
  4. Neuronal circuits: Networks of neurons process information to control movement, sensation, emotion, and cognition.

ATP

ATP: the energy currency of cells ATP stores and transfers energy for cellular processes by releasing energy when its high-energy phosphate bonds are broken.

Roles of ATP

  • Energy transfer: Provides energy for mechanical work (muscle contraction), transport work (active transport of ions and molecules), and chemical work (biosynthesis).
  • Enzyme activation: Phosphorylation of enzymes can alter activity and conformation.
  • Metabolism: Central to glycolysis, the citric acid cycle, and oxidative phosphorylation for ATP production.
  • Signal transduction: ATP participates in cellular signaling, e.g., as a substrate for kinases that phosphorylate proteins.

Posterior Pituitary Gland

Posterior pituitary: The posterior lobe stores and releases neurohormones produced in the hypothalamus.

  1. Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding; associated with bonding and social behavior.
  2. Vasopressin (ADH): Regulates water balance by increasing renal water reabsorption and can increase blood pressure by vasoconstriction.

Meninges

Meninges: Three membranous layers that protect the brain and spinal cord.

  1. Dura mater: Outermost, tough connective tissue. In the skull it has two layers (periosteal and meningeal) and forms dural septa such as the falx cerebri.
  2. Arachnoid mater: Middle, web-like layer; contains the subarachnoid space filled with cerebrospinal fluid and arachnoid granulations that drain CSF into dural venous sinuses.
  3. Pia mater: Delicate, vascular layer closely adherent to the brain and spinal cord, following gyri and sulci.

Meningeal spaces: Epidural (between dura and bone), subdural (between dura and arachnoid), and subarachnoid (between arachnoid and pia; contains CSF).

RAAS Pathway

Renin–angiotensin–aldosterone system (RAAS): A hormonal cascade that controls blood pressure, fluid balance, and electrolytes.

  1. Renin: Released by juxtaglomerular cells in response to low blood pressure, low blood volume, or sympathetic stimulation; converts angiotensinogen to angiotensin I.
  2. Angiotensin I: Inactive precursor converted by ACE to angiotensin II.
  3. ACE (angiotensin-converting enzyme): Converts angiotensin I to angiotensin II (mainly in the lungs).
  4. Angiotensin II: Potent vasoconstrictor; stimulates aldosterone release, promotes thirst, and modulates ADH release.
  5. Aldosterone: Mineralocorticoid from the adrenal cortex that increases renal sodium reabsorption and potassium excretion, thereby increasing extracellular fluid volume and blood pressure.

Clinical implications: Dysregulation can cause hypertension; ACE inhibitors and ARBs target steps of this pathway.

Phases of the Menstrual Cycle

Phases of the menstrual cycle: The monthly ovarian and uterine cycle prepares the body for potential pregnancy.

  1. Menstrual phase (days 1–~5): Shedding of the uterine lining due to a drop in progesterone; blood and tissue are expelled. Symptoms often include low energy and tiredness. Action: rest and gentle movement.
  2. Follicular phase (approx. days 1–13): FSH stimulates follicular growth; estrogen rises; follicles mature.
  3. Ovulatory phase (around day 14): LH surge triggers ovulation; a mature oocyte is released into the fallopian tube.
  4. Luteal phase (approx. days 15–28): Corpus luteum forms and secretes progesterone; if fertilization does not occur, the corpus luteum degenerates and progesterone falls, leading to menstruation.

Blood Glucose Regulation

Blood glucose regulation: Maintaining blood glucose is essential because glucose is a primary energy source, especially for the brain. The pancreas and hormones like insulin and glucagon regulate glucose homeostasis (normal range ~4–5 mmol/L when fasting in many references).

  1. Insulin: Lowers blood glucose by promoting uptake into cells and glycogen synthesis.
  2. Glucagon: Raises blood glucose by stimulating glycogenolysis and gluconeogenesis.
  3. Imbalance consequences: Hypoglycemia (dizziness, confusion, seizures) or hyperglycemia (polyuria, dehydration, long-term complications).
  4. Storage: Excess glucose is stored as glycogen in liver and muscle or as triglycerides in adipose tissue.

Lung Volumes and Capacities

Lung volumes:

  1. Tidal volume (TV): Air exchanged in quiet breathing (~500 mL).
  2. Inspiratory reserve volume (IRV): Extra air inspired above TV (approx. 2,500 mL).
  3. Expiratory reserve volume (ERV): Extra air expired below TV (approx. 1,500 mL).
  4. Residual volume (RV): Air remaining after maximal expiration (approx. 1,500 mL); cannot be measured directly by spirometry.

Lung capacities:

  • Vital capacity (VC): IRV + TV + ERV; maximum exhaled after maximal inspiration.
  • Inspiratory capacity (IC): TV + IRV (approx. 3,000 mL).
  • Functional residual capacity (FRC): ERV + RV; air remaining after quiet expiration.
  • Total lung capacity (TLC): VC + RV; total volume after maximal inspiration.

Values vary with height, sex, posture, and disease. Restrictive or obstructive patterns alter these measurements clinically.

Translation Processes

Translation: Translation decodes messenger RNA (mRNA) to build a polypeptide (protein). Key points:

  1. Genetic code and codons: mRNA codons are triplets; AUG is the start codon (codes for methionine). Sixty-one codons specify amino acids; three are stop codons.
  2. Stages of translation:
    • Initiation: Ribosome assembles at the start codon and the initiator tRNA binds.
    • Elongation: The ribosome moves along mRNA; tRNAs bring amino acids and the polypeptide chain grows.
    • Termination: A stop codon signals release factors to release the completed polypeptide and dissociate the ribosome.