Introduction to Human Physiology

• The learning objectives include understanding physiology and its importance in medicine, body fluids (ICF and ECF), homeostasis, and water balance.

Physiology Defined

• Physiology stems from the Greek words "physis" (nature) and "logos" (study).
• Physiology is defined as both the study of nature and life processes.
• Physiology is defined as the scientific study of how the human body functions.
• Physiology examines the complex mechanisms that sustain life at scales from molecular to systemic.
• Examples of complex mechanisms examined include gene expression, enzymatic reactions, and signal transduction.
• Physiology studies how tissues, organs, and systems interact in a coordinated manner under normal conditions.

Physiology: Life in Action

• Remaining alive results from complex control systems related to hunger, fear, and temperature.
• Physiology is used when you exercise, read, breathe, eat, sleep, or move.
• In short, physiology is life in action: the study of how every bodily function operates to keep us alive and well.

Physiology Questions and Examples

• Physiology answers "what, why, and how" questions about bodily functions.
• Shivering and sweating is an example of a physiological response.
• Anatomy of the respiratory membrane is related to diffusion.
• Red blood cells (RBCs) lack organelles, which enables oxygen transport.
• The pancreas has abundant RER and Golgi, related to secretion.
• Cardiac myocytes are rich in mitochondria, which provide energy.

Cells and Functions

• The entire body contains about 100 trillion cells, each adapted to perform special functions.
• RBCs are the most numerous cells (25 trillion) and transport blood gases.
• Microbiota in the body outnumber human cells and form another trillion-cell ecosystem.
• Bacteria, viruses, fungi, and archaea play a critical role in digestive processes like carbohydrates digestion, vitamin synthesis (K and B), and immune system regulation.

Cell Specialization and Common Characteristics

• Although the body's many cells have specialized functions, they share certain basic characteristics such as energy production and utilization.
• Oxygen combines with end products of fats, carbohydrates, or proteins to release energy necessary for cell function.
• Most cells can reproduce when cells are destroyed, until the appropriate number is restored.
• Cells are surrounded by extracellular fluid, whose composition is precisely controlled.

Physiology: Integration and Experimentation

• Physiology is the most fascinating and ancient branch of science.
• A distinguishing feature of physiology is that it seeks to integrate the functions of all of the parts of the body to understand the function of the entire human body.
• Physiology research ranges from the discovery of blood circulation to the modern study of molecular biology
• Physiology is an experimental science with contributions from numerous scientists.

History of Physiology

• Hippocrates (ca. 460–377 b.c.e.) emphasized the "healing power of nature," closely associating the field with medicine.
• Aristotle (384–322 B.C.) was the first person to coin the term physiology, defined as the study of nature.
• Galen (130–201 A.D.) was the most outstanding physician and began modern experimental physiology by studying body parts of animals
• William Harvey (1578–1657) contributed to physiology becoming a fully experimental science and demostrated that the heart pumps blood through a closed system of vessels

Modern Physiology Pioneers

• Claude Bernard (1813–1878) is considered to be the father of modern physiology.
• Claude Bernard observed that the "milieu interieur" (internal environment) remains remarkably constant despite changing external conditions.
• Walter Cannon (1871–1945) coined the term "homeostasis" to describe the internal constancy of the body.
• Ivan Pavlov (1904) was famous for his work on digestive physiology.
• Ivan Pavlov was famous for his work on the concept of conditioned reflexes in the nervous system.

Physiology: 20th and 21st Century

• Most of our present knowledge of physiology was gained in the 20th century.
• New physiological knowledge in the 21st century is being added rapidly.

Fields of Physiology

• Physiology has many subfields including Human/Medical, Cellular, Viral, Bacterial, and Plant physiology.
• Car physiology is not a subfield since a car doesn't have biological processes like metabolism, growth, or homeostasis

Importance of Physiology

• A deep understanding of normal physiology is necessary to recognize pathophysiology.
• This insight is crucial for diagnosing conditions and developing treatments.
• Studying physiology fosters analytical and critical thinking skills.
• Physiology links disciplines such as biochemistry, molecular biology, pharmacology, pathology, and medicine.

Body Fluid Overview

• About 60% of the adult human body weight is fluid.
• Of the fluid, 2/3 is Intracellular fluid (ICF) and 1/3 is Extracellular fluid (ECF).

Intracellular Fluid (ICF)

• ICF comprises 2/3 of total body water.
• It is found inside the cells.
• ICF is high in potassium, magnesium, and phosphate.
• The ionic composition and osmolarity of the ICF are tightly regulated to support biochemical reactions and energy production.

Extracellular Fluid (ECF)

• ECF comprises 1/3 of total body water (TBW).
• It is the fluid outside of the cells.
• Interstitial Fluid (IF) makes up 80% of the ECF.
• IF surrounds tissue cells in the spaces between them.
• Interstitial fluid acts as the immediate environment for cells, providing nutrients and removing waste products.
• Interstitial fluid is an ultra-filtrate of plasma, serving as a reservoir for plasma.
• Interstitial fluid has little, if any, protein due to the impermeability of capillary walls to large molecules

Plasma

• Plasma comprises 20% of the ECF.
• Plasma is the liquid component of blood.
• Plasma transports nutrients, hormones, and waste products throughout the body, maintaining blood pressure and volume.

Fluid Compartment Separation and Motion

• ICF and ECF are separated by the cell membranes.
• Plasma and interstitial fluid are separated by the capillary wall.
• The ECF is in constant motion throughout the body.
• The first stage is the movement of blood throughout the circulatory system.
• The second stage is the movement of fluid between blood capillaries and cells.

Transcellular Fluid

• Transcellular fluid: is a fluid within epithelium and lined space.
• Examples of transcellular fluid include: Pleura, pericardium, peritoneum, Perilymph and endolymph, Lymph in the lymphatic vessels, CSF in the brain and spinal cord, Synovial fluid in joints, Intraocular fluid, and Glomerular filter in the body.

Fluid Compartments: Infants

• Infants are approximately 75% water.
• Infants are more susceptible to dehydration.
• Infants have more ICF for proper cellular function, supporting biochemical reactions, and ensuring overall homeostasis.

Water and Its Role in the Body

• The human body is approximately 60% water.
• Water has a high specific heat, making it difficult to boil or freeze; this provides temperature stability for the body.
• Water exhibits capillary action; it can move in a thin blood vessel.
• Water is an excellent solvent for solutes.
• Water has Redox reaction capability to remove ROS (reactive oxygen species).

Ionic Differences Between ICF and ECF

• ECF contains large amounts of Sodium, Bicarbonate, Calcium, and Chloride ions.
• ICF contains large amounts of Potassium, Proteins, Magnesium, and Phosphate ions.

Internal and External Environment

• Cells are capable of living and performing their special functions as long as the proper concentrations of: oxygen, glucose, different ions, amino acids, and fatty substances are available in the ECF
• The components above create the internal environment.

Origin of Nutrients in the ECF

• The respiratory system supplies O2 and removes CO2.
• The gastrointestinal system absorbs nutrients like carbohydrates, fatty acids, and amino acids into the ECF.
• The MSK (musculoskeletal system) helps obtain food, protects internal organs, supports the body, and aids in respiration and movement.
• The liver converts many absorbed substances into more usable forms and helps remove metabolic waste products.
• The alveolar membrane is only 0.4 to 2.0 micrometers thick, facilitating gas exchange.

Quantitative Science in Physiology

• Physiology is a quantitative science and almost all physiological parameters are measurable.
• Blood pressure (120/80 mm Hg) and Blood glucose levels (75-110 mg/dl) serve as examples.

Homeostasis Definition

• Homeostasis is a regulated process by which a biological system maintains dynamic but relatively consistent conditions in the body's internal environment despite wide internal and external variations.
• It ensures that the body functions remain within optimal ranges.
• The term "homeostasis" was coined by American physiologist Walter Cannon.

Homeostasis: Organ Functions

• All organs of the body perform their functions to maintain constant conditions in the ECF.
• This is the golden goal of every organ : to maintain homeostasis.
• The lung maintains respiratory gases in the blood (normal range).
• The CVS transports required substances and removes waste products.
• The kidneys maintain relatively constant H2O volume and ionic concentration.

Dynamic Mechanisms and Range

• Homeostasis involves dynamic mechanisms that detect and respond to deviations in physiological variables from their "set point" values.
• Parameters are regulated within a range of values, rather than at fixed values.

The Role of Insulin and Glucagon In Homeostasis

• If blood glucose concentration is too low, the hormone glucagon, from alpha cells of the pancreas will increase it.
• If blood glucose concentration is too high, the hormone insulin from the beta cells of the pancreas will lower it by enhancing cellular uptake, storage, and metabolism.

Homeostatically Regulated Factors

• Factors regulated include: Nutrients, Gases, Waste products, pH, Water, Salt, electrolyte, Volume and pressure, and Temperature.

Set Points

• These are established during embryonic development
• These depend on multiple factors, such as age
• These are optional, normal levels of a variable

Fundamentals of Homeostasis: Pacemaker

• Every set point has a pacemaker in organ systems.
• It regulates the set point of given physiological activity.

Fundamentals of Homeostasis: Time

• Different regulatory processes need different durations of time such as Seconds, Minutes, Days, and Years.
• Seconds (e.g. Acute hemorrhage and reflexes).
• Minutes (e.g. Osmotic auto regulation).
• Days (e.g. auto regulation of enzymes of energy metabolism).
• Years (Long-term physiological changes (growth, aging, reproduction).
• Long-term adjustments are under hormonal control (minutes - years).

Disruption of Homeostasis: Ranges

• Optimal range: Is where body function most efficiently.
• Range of tolerance: Is where the body can still function, but not optimal (Min and Max).
• Beyond Min or Max: Leads to Illness, disease, and Death.

Regulatory Systems of Homeostasis: Nervous and Edocrine

• The regulatory systems of homeostasis are the nervous and endocrine systems.

Nervous Regulation

• The nervous system regulatory mechanism has Three major components (sensory, integrative and motor).
• It comprises Sensory receptor, CNS, and effectors.
• The sensory receptor detects any change in the body (blood glucose concentration, body temperature, BP, and pain etc. ). Then send impulse to CNS.
• The CNS associate the information, store, generate thought and send appropriate response to the effectors (muscle + glands).

Hormonal Regulation

• Hormones are chemical messengers secreted by endocrine glands, and transported in blood to the target tissue
• The hormonal regulation has a longer duration of action.
• PTH- to the kidney, bone, intestine = ↑[Ca2+].
• Aldosterone to the kidney, intestine =↑ [Na+].
• ADH → controls water & electrolyte balance.
• Thyroid hormone increases metabolism in all cells.
• Insulin controls glucose metabolism.

Regulatory Systems: Speed

• The nervous system regulates mainly muscular and secretory activities of the body, very rapid
• Hormonal system regulates many metabolic functions, slower than the nervous system.

Classes of Control Systems

• Control systems are grouped into 2 classes based on location: Intrinsic/local controls and Extrinsic controls.

Intrinsic Controls

• Intrinsic/local controls usually involve only one organ or tissue.
• when muscles use more O2, Also produce more CO2, it causes local vasodilatation.

Extrinsic Controls

• Extrinsic controls involve regulatory mechanisms initiated outside an organ and it is Accomplished by nervous and endocrine systems.
• Regulatory mechanisms regulate BT, and ABP

Control Mechanisms

• Feed forward learned response for stimulus before the change occurred
• Feed-back response made after initial change is detected such as Negative feedback and Positive feedback.
• Behavioral responses.

Feed-Forward Control

• Total control of a particular body function may be more complex than can be accounted for by simple negative feedback
• It enables Responses made in anticipation of a change meaning Prepare the body for the change.
• It is Usually acts in combination with NFM.
• Shivering before diving into the cold water is a good example.
• Smell of food can trigger digestive juice secretion is also an example.
• Increase insulin secretion while meal is still in GIT is also a good example."

Feed- Forward, Rapid Movement and Exercise

• Feed forward control causes required muscle contractions during rapid movement.
• Sensory nerve signals from the moving parts tells about whether the movement is performed correctly.
• Adaptive control and delayed negative feedback can be viewed as the same thing.
• Heart rate and breathing increase even before a person begins to exercise.

Feedback System

• Involves responses made after a triggered change has been detected.
• It alters the function of organs.

The Negative Feedback Mechanism (NFM)

• Causes the variable to change in a direction opposite to that of the initial change, returning it to its "ideal" value.
• The output shuts off the original stimulus or reduces its intensity this way.
• Most homeostatic mechanisms of the body are NFM.
• If PCO2 increase in the blood, the NFM stimulates pulmonary ventillation rate which has an effect of reducing PCO₂ to normal

Examples of Negative Feedback (Closed loop)

• Regulation includes: Temperature, PaCO2, Blood pressure, Plasma volume, Acute hemorrhage, Hormonal secretion, and Blood glucose level.

Regulation of Blood Pressure (BP): Steps

• Increased BP is sensed by baroreceptors.
• Baroreceptors send signal to brain
• Brain sends command to effectors (heart, blood vessels).
• Heart rate decreases and blood vessels dilate.
• BP decreases to the normal.
• Decreased BP inhibits the stimulation of baroreceptors .

Positive Feedback Loops

• These are less common in physiology
• These enhance the stimulus so that activity is accelarated in the same direction from the normal set point
• Positve feedback cycles can cause viscous cycles, instability, or death
• Examples include: Uterine contractions during labor, Urination and defecation reflex, LH surge during the follicular phase, Upstroke of the action potential, Calcium-induced calcium release., and Blood clotting

Action of High Degree of Blood Loss

• There is High degree of blood loss, leading to a Weakening of the heart, further diminished pumping, a further decrease in coronary blood flow, and still more weakness of the heart.
• The cycle repeats itself until death occurs.
• A mild degree of positive feedback can be overcome by the negative feedback

Behavioral Responses

• The responses complement physiological mechanisms in maintaining homeostasis.
• Low blood glucose levels activate feeding centers in the brain, leading to hunger and food intake to restore balance.
• These Behavioral responses are relevant to Cold / hot, and Thirst.
• Behavior includes Resting when fatigued to conserve energy.

Aging and Homeostasis

• Homeostatic mechanisms gradually decline with age, leading to reduced adaptability.
• Older adults may struggle more with physiological stressors such as exercise, temperature changes, or illness.
• Aging also leads to slower recovery from disturbances in homeostasis.

Water Homeostasis

• Water homeostasis refers to the balance of water intake and output in the body.

Importance of Water Homeostasis

• Cellular Function maintains environment for processes, enables temp regulation, digestion and waste removal.
• Electrolyte Balance regulates concentration of electrolytes (e.g., sodium, potassium) which support nerve function and muscle contraction.

Water Intake Sources

• Water intake sources include Ingested liquids, Solid foods, and Metabolic water or water of oxidation
• Called preformed water (amounts 2300ml/day).

Water output

• Water outputs are Kidney excretion (urine), Skin, Lungs, and GIT. water

Mechanisms of Regulation

• Kidneys: Filter blood to remove, water, electrolytes, and waste products.
• Kidneys Adjust concentration of urine Antidiuretic Hormone (ADH): helps to prevent water loss
• ADH Is released by the pituitary gland in response to dehydration. ,Increased water reabsorption in the kidneys, reducing urine output.
• Thirst Mechanism: Trigerred by the hypothalamus when body water levels.
• Body responds by water intake.

Regulation of ADH Release

• Water reabsorption in collecting ducts is proportional to ADH release.
• Hypothalamic osmoreceptors trigger or inhibit ADH release.
• Factors that specifically trigger ADH release include prolonged fever, excessive sweating, vomiting (or diarrhea), severe blood loss, and traumatic burns

Disturbances of Water Balance

• Dehydration involves reduced total body water when the body loses more fluids than it takes in.
• Leads to reduced blood volume, cellular dysfunction, and can cause symptoms like dizziness, fatigue, and confusion.
• Over hydration (Hyponatremia) involvesExcessive water and can dilute the body electrolytes especially sodium leading to brain swelling.
• Edema is the accumulation of excess fluid in the body's tissues, leading to swelling.
• Peripherial Edema, Pulmonary Edema, and Cerebral Edema are all forms of edema.

Solutions and Osmolality

• Tonicity refers to the ability of a solution to cause a cell to gain or lose water
• This is determined by the relative concentration of solutes
• A isotonic solution has the same osmolality as plasma (0.9% NaCl)
• A hypertonic solution has higher osmolality (9% NaCl)
• A hypotonic solution has a lower osmolality (0.6% NaCl)

Explore human physiology and its significance in medicine, understanding body fluids (ICF and ECF), homeostasis, and water balance. Learn how the human body functions, from molecular to systemic levels, examining complex mechanisms that sustain life. Discover the interactions between tissues, organs, and systems under normal conditions.