Human Physiology

Introduction to the human body

In terms of structure and function, the human body is organized into six levels: chemical (atoms and molecules), cellular, tissue, organ, organ system and organism. Each level builds upon the previous one, from the most basic chemical components to a fully functioning human being. The human body can also be organized into several systems: circulatory, respiratory, digestive, endocrine, urinary, musculoskeletal, nervous, reproductive, integumentary, immune and lymphatic. Each system plays an important role in maintaining the body's homeostasis and overall health.

Homeostasis and feedback mechanisms are pivotal in maintaining physiological balance. The ability of the body to maintain a stable internal environment is achieved through various negative and positive feedback loops. These loops constantly adjust body processes in response to internal and external changes to maintain systems at a set point.

Another critical aspect of the human body is the management of body fluids and fluid compartments. Water plays a vital role in several physiological processes. It is distributed between intracellular and extracellular fluid compartments, each with distinct compositions crucial for cellular functions.

The functional organization of the human body, homeostatic mechanisms, as well as the management of body fluids and fluid compartments are fundamental in understanding how the body operates and responds to its environment.

The cell and its functions

Cells are the basic living units of the human body. Each organ consists of various cell types, each uniquely adapted to carry out specific functions. 

The cell membrane is an integral component of the cell and is primarily composed of a bilayer of phospholipids, interspersed with proteins, cholesterol and carbohydrates. It regulates the transport of substances in and out of the cell, through various processes such as diffusion, osmosis and active transport. 

The cytoplasm is a jelly-like substance that fills the interior of the cell and is primarily composed of water, salts and organic molecules. It contains various organelles, such as the mitochondria (the powerhouse of the cell), endoplasmic reticulum (involved in protein and lipid synthesis), Golgi apparatus (which modifies, sorts and packages proteins and lipids) and lysosomes (which break down waste materials).

The cytoskeleton of the cell, comprising microtubules, microfilaments and intermediate filaments, provides structural support and facilitates cellular movement. The cell nucleus contains chromatin and chromosomes and is the control center for genetic information and protein synthesis. It regulates gene expression and mediates the replication of DNA through transcription and translation processes, utilizing RNA and ribosomes.

Generally, cells undergo a complex series of stages for growth, DNA replication and division called the cell cycle. This cycle ensures accurate duplication and distribution of genetic material to daughter cells, with checkpoints that help maintain cellular integrity and prevent diseases like cancer.

Overview of muscle tissue

Muscle tissue is a specialized form of tissue characterized by its ability to contract and thereby enable movement. There are three main types of muscle tissue: skeletal, cardiac and smooth. Each type has unique structures and functions, but also shares common properties such as contractility, excitability, conductivity, extensibility and elasticity. These shared characteristics allow muscle cells to effectively respond to stimuli, conduct impulses, stretch and return to their original form.

Skeletal muscles

Skeletal muscle is the most prevalent type of muscle found in the human body and is essential for voluntary movement and posture. It consists of elongated, multinucleated muscle fibers that contain subunits called myofibrils. These myofibrils are composed of two types of protein myofilaments, actin and myosin, which are responsible for muscle contraction. These muscle fibers possess unique electrical properties that are important for muscle activation via action potentials.

The neuromuscular junction serves as the connection point where nerve impulses trigger muscle contraction. The process of skeletal muscle contraction and relaxation is mediated by calcium ions and regulatory proteins, relying mainly on energy sources including adenosine triphosphate (ATP) and glycogen within the muscle. Skeletal muscle fibers are of different types, such as slow-twitch and fast-twitch, each suited for specific activities. 

Smooth and cardiac muscle

Smooth muscle is primarily located in internal organs and body passageways. It is involved in involuntary functions such as controlling blood vessel diameter and facilitating digestion. Smooth muscle is characterized by uninucleated, spindle-shaped cells with dense bodies. It differs from skeletal and cardiac muscles in its structure and slower, sustained contractions. 

Cardiac muscle, as its name suggests, is found only in the heart. It is specialized for continuous, rhythmic contractions without nervous stimulation, pumping blood throughout the body. Cardiac muscle cells are striated and interconnected by intercalated discs. These discs contain gap junctions for rapid electrical impulse transmission and desmosomes that ensure strong physical connections. These unique structures enable synchronized heart contractions which are essential for effective blood circulation. 

Overview of the nervous system

The nervous system is a complex network of nerve cells (neurons) responsible for controlling and coordinating various functions throughout the body. It is broadly categorized into two main functional divisions: the central nervous system (CNS), composed of the brain and spinal cord and the peripheral nervous system (PNS), comprising all neural elements outside the CNS.

The nervous system contains two primary types of cells: neurons and glia. Neurons form the basic functional units of the nervous system and are responsible for transmitting and processing information through electrical and chemical impulses. Glial cells on the other hand, provide support, protection and nutrition to neurons and play a vital role in maintaining the overall health and efficiency of the nervous system.

Action potential and synapses

Action potentials and synapses play a pivotal role in neural communication. An action potential, a swift change in a neuron's membrane charge, enables signal transmission. At neuronal synapses, where neurons connect, the action potential triggers the release of neurotransmitters, facilitating signal transfer between neurons. Neurotransmitters are key in conveying and regulating neural messages between nerves and target tissues throughout the nervous system.

General senses

The general senses of the human body encompass a range of sensations including temperature, pain, touch, stretch, pressure and vibration. These stimuli are detected by specialized sensory receptors distributed throughout the body. These receptors monitor and detect stimuli from both external and internal environments. Touch, stretch, pressure and vibration sensations are categorized under mechanoreceptive somatic senses, while pain and temperature sensations are categorized as nociceptive and thermoreceptive respectively. 

Special senses

Special senses, including smell (olfaction), taste (gustation), vision, hearing and vestibular sensations (equilibrium), are essential for our interaction with the environment. Smell involves detecting airborne chemicals (odorants) by olfactory receptors, taste is mediated by taste buds which house gustatory receptors for different flavors and vision processes light, color and movement via photoreceptors in the retina to form visual images of objects in our environment. Hearing interprets sound waves via the spiral organ, while vestibular sensations in the inner ear are detected by the macula and crista ampullaris, specialized structures which are important for balance and spatial orientation. These senses enrich our experiences and perception of our environment.

Motor control

Motor control is a fundamental aspect of human movement and coordination and involves complex interactions within the nervous system. The spinal cord plays a pivotal role in this process by receiving and transmitting information from the brain to the muscles via motor neurons, but also through reflex arcs, simple pathways that mediate reflex actions, allowing for rapid and involuntary responses to stimuli without the direct involvement of the brain. Beyond these basic mechanisms, motor control is regulated by higher centers in the brain, particularly the cerebral cortex and brainstem. The cortex is responsible for voluntary motor actions, planning and coordination, while the brainstem integrates and relays motor commands between the brain and spinal cord. Together, these systems enable smooth and well coordinated movements. 

Cerebral cortex and higher cognitive functions

The cerebral cortex plays an essential role in higher cognitive functions and is fundamental to processing and integrating sensory information, facilitating higher order thinking, reasoning and problem-solving. Functionally, it is characterized by a complex arrangement of neurons, layers, lobes and areas that perform specific roles. Cognition and language functions are predominantly managed in specific cortical areas such as the Broca and Wernicke areas, enabling abstract thinking, understanding and communication. Learning and memory are also centered in the cortex, where experiences are encoded, stored and retrieved, crucial for knowledge acquisition and recall. Additionally, the cerebral cortex is involved in regulating sleep and wakefulness, as well as balancing brain activity and rest, which is essential for overall cognitive health and functionality. 

Blood

Blood is a vital fluid in the human body that maintains life through its complex and dynamic functions. It is composed of plasma and formed elements (cellular components), which are constantly produced in the bone marrow, through the process of hematopoiesis. These formed elements include erythrocytes (red blood cells), which are responsible for oxygen transport to tissues via hemoglobin, leukocytes (white blood cells), involved in the immune response and platelets which are critical for clotting and wound healing. 

The heart

The heart is the central component of the cardiovascular system. The primary function of this muscular organ is to pump blood throughout the body, delivering oxygen and nutrients to tissues. It is composed of specialized cardiac muscle tissue, which possesses unique electrical properties that allow for automatic and coordinated contractions. With each complete heartbeat, the heart goes through a sequence of filling and pumping phases called the cardiac cycle, that ensure continuous blood flow. The cardiac output, the volume of blood the heart pumps per minute, is a measure of cardiac function and is regulated by both intrinsic cardiac mechanisms and external factors such as hormonal and neural factors.

Blood vessels and circulation

Blood vessels are vascular channels that transport blood throughout the body and are an integral component of the circulatory system. The three main types of blood vessels are arteries, veins and capillaries, each varying in structure and function. Generally, arteries carry oxygenated blood away from the heart, while veins return deoxygenated blood to the heart. Capillaries facilitate the exchange of oxygen, nutrients and waste products between blood and tissues through processes such as diffusion, filtration and osmosis. Blood flow within the vessels is tightly regulated to maintain adequate tissue perfusion and blood pressure.

The circulatory system consists of two circuits; the pulmonary circuit, which moves blood from the right ventricle, through the lungs and back to the left atrium, as well as the systemic circuit, which includes a network of arteries and veins that transport blood from the aorta to all other systems and tissues of the body and back to the heart.

Overview of the lymphatic and immune systems

The lymphatic and immune systems are key components of the body's defense mechanisms. The lymphatic system comprises a network of lymph vessels, nodes and tissues involved in maintaining fluid balance and filtering out harmful pathogens. It transports lymph, a fluid containing white blood cells (lymphocytes) throughout the body. While lymphatic vessels collect and transport lymph from tissues to the venous circulation, lymphoid organs including the spleen, thymus and tonsils are involved in immune surveillance for foreign antigens and the removal of old or damaged blood cells.

On the other hand, the immune system is a complex network of cells, tissues and organs that work together to defend the body against pathogens, such as bacteria, viruses and foreign bodies. This system enables the body to identify its own cells as distinct from foreign cells and substances and to eliminate invaders using various components including white blood cells, antibodies and other substances that identify and attack foreign invaders. 

Immunity

Immunity refers to the ability of the body’s complex defense system to fight against pathogens and foreign substances. It is characterized by both innate and adaptive responses. Innate immunity is the body’s first line of defense with non-specific barriers and cellular responses that act quickly to prevent the spread of infection. Adaptive immune response, on the other hand, is more specialized and involves a cellular response by T lymphocytes and/or a humoral response by B lymphocytes. The adaptive immune system has the ability to learn and therefore, respond more effectively to specific pathogens after initial exposure, leading to stronger and quicker reactions in subsequent encounters.

Overview of the endocrine system

The endocrine system is responsible for the regulation of various physiological processes such as growth, metabolism, reproduction and stress responses through the secretion of chemical messengers called hormones. Hormones have diverse chemical structures ranging from steroids to peptides. They are released directly into the bloodstream and travel to specific target organs or tissues, where they bind to specific receptors, triggering cellular responses to regulate body functions. 

Endocrine glands and organs

The endocrine system is composed of several endocrine glands and organs, each producing hormones that regulate various vital body functions. The pea-sized pituitary gland, often referred to as the "master gland," works in conjunction with the hypothalamus to control several other endocrine glands. The thyroid and parathyroid glands regulate metabolism, calcium and phosphorus levels in the body. The suprarenal (adrenal) glands produce hormones including cortisol and adrenaline, which manage stress responses and metabolic processes. The endocrine pancreas plays an important role in glucose metabolism and regulation by secreting the hormones insulin and glucagon. Additionally, the gonads (ovaries and testes) and the placenta during pregnancy produce hormones that promote reproductive health and fetal development.  

Overview of the respiratory system

The respiratory system is responsible for breathing and gas exchange. Functionally, it is organized into a conducting portion, which transports air and a respiratory portion responsible for gas exchange. The respiratory system begins with the nasal cavity and mouth, leading to the pharynx, larynx and trachea, which further bifurcate into the bronchi and bronchioles within the lungs, forming the conducting portion. The lungs house respiratory bronchioles, alveolar ducts and the alveoli (tiny air sacs) which form the respiratory portion of the system, where oxygen is exchanged for carbon dioxide during respiration. In addition to facilitating the intake of oxygen and the expulsion of carbon dioxide, the respiratory system also plays a role in regulating blood pH and maintaining acid base balance.

Breathing and respiratory gases

Breathing and the management of respiratory gases are fundamental aspects of human physiology, ensuring that body cells receive the oxygen they need for metabolic processes while removing carbon dioxide. The process of breathing involves two cyclic phases: inhalation (inspiration), where oxygen-rich air is drawn into the lungs and exhalation (expiration), where carbon dioxide-rich air is expelled. The entire process of breathing and gaseous exchange is regulated by the brainstem respiratory centers, which adjust the rate and depth of breathing based on the body's needs, as monitored by central and peripheral chemoreceptors that detect changes in blood levels of oxygen, carbon dioxide and pH.

Organs of the digestive system

The digestive system is responsible for the breakdown of food into nutrients that the body can absorb and utilize. It comprises two main groups of organs: digestive organs and accessory digestive organs.

The digestive organs include the various parts of the gastrointestinal tract. It begins in the mouth, where mechanical digestion through chewing and chemical digestion through saliva occur, followed by the transport of food through the pharynx and esophagus to the stomach. The stomach further breaks down food using acid and enzymes into a paste-like chyme. The chyme then moves into the small intestine, the primary site for chemical digestion and nutrient absorption. The large intestine follows, absorbing water and electrolytes, forming feces.

Accessory organs of the digestive system include the salivary glands, liver, pancreas and gallbladder. The salivary glands produce saliva that contains enzymes which initiate the chemical digestion of sugars. The liver produces bile for fat digestion, the pancreas supplies digestive enzymes and the gallbladder stores bile. Together, these components of the digestive system ensure the efficient breakdown of food into essential nutrients.

Metabolism and nutrition

Metabolism and nutrition involve converting the energy in food into fuel for the body's processes through various biochemical reactions. There are two main metabolic pathways: catabolic and anabolic. The catabolic pathway breaks down molecules to release energy, while the anabolic pathway uses energy to build complex molecules. 

Carbohydrate metabolism primarily focuses on the breakdown of sugars into glucose, a primary energy source that is further processed through glycolysis, citric acid cycle (Kreb’s cycle) and electron transport chain to produce ATP, the body’s energy currency. 

Lipid metabolism involves the breakdown and synthesis of fats, which play a key role in energy storage and maintaining cell structure. In times of low carbohydrate availability, the liver produces ketone bodies from fatty acids through the process of ketogenesis, providing an alternative energy source for the body. Additionally, lipogenesis occurs in the liver and adipose tissue, converting precursors such as carbohydrates into fats for efficient energy storage.

Protein metabolism involves the breakdown of proteins into amino acids for energy production or the synthesis of new proteins. Excess amino acids undergo deamination via the urea cycle, converting the resulting ammonia into urea for safe excretion by the kidneys. In the absence of sufficient carbohydrates and fats, proteins can be metabolically converted into energy.

Structure and functions of urinary organs

The urinary system is vital for maintaining water and electrolyte balance and removing waste products from the body through urine. It consists of 4 main organs: kidneys, ureters, urinary bladder and the urethra. The bean-shaped kidneys located in the abdominal cavity are central to this system. They contain numerous nephrons, which are the functional units responsible for filtering blood and producing urine. The ureters carry urine from the kidneys to the urinary bladder, which stores it before it is expelled from the body through the urethra. Urine itself has distinct physical characteristics, including its color, odor, pH and specific gravity, all of which can provide important insights into various metabolic processes and the health of the urinary system.

Renal physiology

Renal physiology involves the study of kidney functions and the processes of urine formation and elimination. Central to this is glomerular filtration, where blood plasma is filtered through the glomeruli of the kidneys, initiating urine formation. Following filtration, tubular reabsorption occurs, a process where needed substances like glucose, ions and water are reabsorbed from the filtrate back into the bloodstream, ensuring the conservation of vital nutrients and maintaining fluid balance. The kidneys also play a pivotal role in the regulation of urine concentration and volume. This process is controlled by the hormones antidiuretic hormone (ADH) and aldosterone, which determine the amount of water reabsorbed and the final concentration of urine. 

Body fluids

Fluids within the human body are distributed across two main compartments: intracellular fluid, contained within cells and extracellular fluid, which includes interstitial fluid, blood plasma and other specialized fluids. Water balance is a key aspect of body fluid regulation, involving the precise control of water intake and loss to maintain the body's fluid equilibrium. This balance is essential for normal functioning, as it ensures that cells and organs have the appropriate environment to carry out metabolic processes, regulates body temperature and facilitates the transportation of nutrients and waste products. 

Electrolytes and acid-base balance

Electrolyte balance refers to the regulation of minerals such as sodium, potassium, calcium and chloride in the body fluids, which are essential for various physiological processes including nerve conduction, muscle contraction and hydration. Maintaining this balance involves complex interactions between different organ systems, particularly the kidneys and endocrine system. Acid-base homeostasis on the other hand, involves the regulation of pH levels in body fluids, ensuring they remain within a narrow range. This pH balance is vital for enzymatic reactions and cellular function and is achieved through buffer systems, respiratory and renal mechanisms. 

Reproductive system

The human reproductive system is essential for the perpetuation of our species and comprises distinct but complementary systems in males and females. The female reproductive system includes the ovaries, which produce eggs/ova (oogenesis) and hormones (estrogen and progesterone), the uterine (fallopian) tubes, where fertilization typically occurs, the uterus, where the developing fetus is nurtured and the vagina, through which childbirth occurs. The female hormonal system involving the hypothalamic-pituitary-ovarian axis plays a critical role in the female reproductive system with regards to the development of female secondary sex characteristics, monthly menstrual cycle (ovarian and uterine cycles), pregnancy and menopause.

In contrast, the male reproductive system comprises the testes, which produce sperm (spermatogenesis) and male hormones (androgens), the epididymis and ductus (vas) deferens, which transport sperm, the seminal vesicles and prostate gland, which contribute fluids to semen and the penis, through which semen is ejaculated. Testosterone, the primary male sex hormone, plays an important role in development of male reproductive tissues such as the testes, prostate gland, seminal vesicles and genital ducts, as well as spermatogenesis and the development of secondary sexual characteristics.

Continuity of life and embryology

The continuity of life and the field of embryology delve into the intricate processes of human development, beginning from a single cell to a fully formed baby. This journey starts with meiosis, a specialized type of cell division that produces gametes (sperm and eggs) with half the usual number of chromosomes, which allows for genetic diversity following the fusion of parental genes and restoration of the full chromosome count.

The pre-embryonic period, the first two weeks post-fertilization, involves the formation of a zygote, its division and implantation into the uterine wall. During this time, the placenta begins to form, establishing a vital connection between the mother and the developing embryo, for nutrient and oxygen transfer and waste removal. The embryonic period follows next, which extends from the third to the eighth week, during which major organs and structures begin to form, marking the most critical phase of embryonic development. The fetal period, starting from the ninth week until birth, is characterized by the growth and maturation of the established structures, preparing the embryo for the transition to extrauterine life at birth.