Human Physiology
1. Introduction
The scientific study of normal functions and mechanisms that take place in the human body is
known as human physiology. It is concerned with the chemical and physical functions of
maintaining life that are performed by the organs, tissues, cells, and biomolecules. Physiology is
a branch of science that connects anatomy, the study of the structure of the body, and
biochemistry, the study of the mechanism of biological processes in terms of molecular
organizational processes. These disciplines offer a combined view of the functioning of the
human body. Human physiology is an old field that has been developed by the ancient Greek
physicians like Hippocrates and Galen who aimed at explaining the ways the body functions
without superstitions but through observation and reasoning. Technologies have since improved
our knowledge of the physiological processes, both on a cellular and a molecular level, over
time, e.g. the invention of a microscope and modern methods of imaging. Physiology nowadays
is one of the foundations of the medical science as it assists in the explanation of the stability of
the human body, as well as its reaction to the inner or outer changes.
Physiology is fundamental both in the understanding of normal body functions as well as
determining what occurs when the systems fail. Physiological dysfunctions tend to be the causes
of diseases and disorders. As an example, hypertension is caused by the impairment of
cardiovascular physiology, and diabetes is caused by the impairment of glucose metabolism
regulation by endocrine systems. Physiology therefore offers the basis of medical treatment and
preventive health measures. The intricate processes of the human body are linked and delicately
balanced by the complex regulation mechanisms which assure the body of a stable environment
which is needed to sustain life in accordance with homeostasis.
2. Levels of Structural Organization
The human body is structured in a hierarchical level with each level on top of the former so as to
form a functional entity. The most basic level is the chemical level which consists of atoms and
molecules. Carbon, hydrogen, oxygen, and nitrogen are some of the atoms that comprise the
building blocks of life. These atoms form molecules such as water, proteins, lipids,
carbohydrates and nucleic acid which are essential components in cell structure and functioning.
An example of this is the proteins which serve as enzymes and as part of the cell structure
whereas the lipids build cell walls and stores of energy.
The basic unit of life is the cellular level. Cells are the tiniest units that can carry out all the
physiological functions. They differ in size, shape and function as they will have one or more of
their specialization. In this case, an example exists where the contraction of muscle cells results
in movement, whereas the transmission of electrical impulses between body parts occurs through
the neurons. All cells, despite their differences, possess some common characteristics; they all
have a plasma membrane through which the passage of substances in and out is controlled, a
cytoplasm containing organelles, and a nucleus carrying genetic material.
At tissue level, the same types of cells will be assembled in order to undertake particular
functions. It has four major types of tissues which are: epithelial, connective, muscle and nervous
tissues. Epithelial tissue is used to cover the body surfaces and also to line cavities, which serve
as a barrier and helps in absorption or secretion. The connective tissue holds and binds the rest of
the tissues as in tendons, cartilage, and blood. The nervous tissue promotes communication and
coordination whereas the muscle tissue promotes movement.
The structure formed at the organ level contains two or more types of tissue that combine to
create certain functions. These are the kidneys, liver, lungs and the heart. These organs are then
incorporated into organ systems, e.g. cardiovascular or respiratory system which collaborate
together to maintain the body. The organism level is a totality of all systems working together to
sustain life.
Homeostasis is a concept that is very vital in the physiological context and it is defined as the
capability of the body to keep the internal environment constant despite any external changes.
Homeostatic control mechanisms are based on feedback loop- most often negative feedback- that
monitor and correct an imbalance at a set point. As an illustration, body temperature control has
sensors in the hypothalamus that detect changes in temperature and initiate reactions such as
sweating or shivering in review of normalcy. Positive feedback loops are less frequent but they
enhance physiological processes as is the case with uterine contractions during childbirth.
3. The Nervous System
The nervous system is a complex and one of the most important systems in the human
physiology as it is involved in the process of controlling and coordinating the activities in the
body. It interprets information concerning the senses and sends responses as well as controls
communication among body parts. Structurally the nervous system is further partitioned into the
central nervous system (CNS), brain, and the spinal cord; the second is the peripheral nervous
system (PNS); made up of cranial and spinal nerves that are spread throughout the body. It can
also be further divided in functional terms into the somatic nervous system which controls
voluntary movements and the autonomic nervous system (ANS) which controls involuntary
functions like the heart rate, gastrointestinal functions, and glands.
The basic components of the nervous system are neurons which conduct electrical impulses
called action potentials. In every neuron, the cell body, dendrites that receive the signal and axon
that sends the impulse to the other neurons or the effectors make part of the neuron. Interaction
between neurons takes place at the synapses where neurotransmitters are released into the
synaptic cleft and connect with receptor on the postsynaptic cell. Some of the common
neurotransmitters are acetylcholine, dopamine, serotonin, and norepinephrine which are involved
in the mechanisms of regulating mood, movement, and cognition in different ways.
The brain is the command center of the body as it processes and integrates information. It is
further divided into a few parts, each of which do their specialized functions: the cerebrum, the
center of higher functions such as reasoning, emotion, and voluntary movement; the cerebellum,
the centre of balance and motor control; and the brainstem, the centre of vital functions, such as
breath and heart rate. The spinal cord serves as a pathway of communication between the brain
and the peripheral nerves and it all reflexes which are quick automatic responses to stimuli.
The autonomic nervous system is further divided into the sympathetic and parasympathetic
division. The sympathetic division is involved in making the body fight or flight ready with
acceleration in heart rate, opening of pupils and redistributing blood to muscles. On the other
hand, the parasympathetic division is the one that facilitates rest and digest activities, which are
more energy-saving and help to digest and recover. The fine balance between these two segments
will mean that there is appropriate physiological functioning in different conditions.
4. The Endocrine System
The endocrine system is an intricate system of glands which generate and release hormones-
chemical messengers which control metabolism, growth, reproduction and other essential
physiological functions. The endocrine system is a system that is similar to the nervous system,
except that instead of involving electrical impulses to communicate, it involves the use of blood,
with the hormones being able to influence the body of each target organ. This system is
important as it is involved with the homeostasis that makes sure that the internal environment of
the body is stable despite external factors.
Also referred to as the master gland, the pituitary gland is found at the bottom of the brain and
controls the activities of other endocrine glands by releasing tropic hormones. It has two lobes;
the anterior pituitary and secretory of hormones like growth hormone (GH), prolactin (PRL), and
thyroid-stimulating hormone (TSH) and the posterior pituitary secretes antidiuretic hormone
(ADH) and oxytocin. Hypothalamus is a gland that is positioned just above the pituitary gland
and is the connection between the nervous and endocrine system. It regulates the activity of the
pituitary by releasing and inhibiting hormones thereby combining the neural and the hormonal
reactions.
The thyroid gland is a gland in the neck that produces thyroxine (T4) and triiodothyronine (T3),
which are metabolism, growth and development hormones. These hormones affect almost all the
body cells by raising the rate of metabolism and enhancing protein synthesis. Parathyroid
hormone (PTH) secreted by the parathyroid glands before the thyroid controls the balance of
calcium and phosphates. The deficiency of calcium provokes the secretion of PTH that leads to
the intensification of bone and kidney calcium reuptake and the stimulation of intestinal
absorption by activating vitamin D.
The adrenal glands are situated on the top of the kidneys and they are divided into two parts
namely the adrenal cortex and the adrenal medulla. The cortex secretes steroids hormones
including cortisol, aldosterone and androgens. One of the glucocorticoids that is believed to
assist the body in responding to stress is cortisol which raises blood glucose levels and inhibits
inflammation. The blood pressure and fluid balance depends on aldosterone control of sodium
and potassium. Catecholamines, epinephrine and norepinephrine, are secreted by adrenal
medulla and help to trigger the immediate body fight or flight reaction.
Pancreas has got endocrine and exocrine functions. Its endocrine component, the islets of
Langerhans, secret insulin and glucagon, which are the hormones that control the blood sugar
levels. The insulin decreases blood glucose by increasing the uptake of glucose in the cells
whereas glucagon increases blood glucose by increasing liver glycogen breakdown. Imbalance in
this causes a metabolic disorder like diabetes mellitus, which is chronic hyperglycemic and
inability to utilize glucose.
Sex hormones produced by the gonads in males are testosterone, estrogen, and progesterone,
whereas in females they are testosterone, estrogen and progesterone. These hormones regulate
the functions of the reproductive system, sexual maturation and sexual secondary features. Pineal
gland is a secretion that secretes melatonin which is a hormone that controls the circadian
rhythms and sleep. Together these glands make the body physiological processes synchronized
and adaptive to the changing environment and internal conditions.
5. The Musculoskeletal System
The musculoskeletal system gives the human body its structure that enables movement, stability
and protection of internal organs. It consists of cartilage, joints, tendons, ligaments, muscles and
bones. This system aids locomotion besides being central in the storage of minerals, the
formation of blood cells and metabolism of energy.
The physiology of skeletal muscles is the key to movement. The skeletal muscles consist of long
cylindrical structures called muscle fibers that have the myofibrils which are composed of
contractile proteins actin and myosin. The rudimentary functional unit of a muscle cell is the
sarcomere where these filaments slide against each other during contraction. Nerve impulses sent
through the motor neurons trigger muscle contraction. Upon the arrival of an action potential at
the neuromuscular junction, acetylcholine is released, which attaches itself to receptors on the
muscle membrane causing a depolarization of the muscle and release of calcium into the
sarcoplasmic reticulum. The calcium interacts with the troponin and displaces tropomyosin by
blocking actin-binding sites, and allows the formation of cross-bridges between actin and
myosin. Ilsoable energy that is required in contraction is provided by adenosine triphosphate
(ATP), which supplies the sliding filament mechanism.
Physiology of bones is also essential. Bones are dynamic tissues, which constantly remodel
themselves or in other words, are formed by osteoblast and resorbed by osteoclast. Such
equilibrium ensures strength and mineral balance of the bones. The skeleton performs several
functions: it gives the body structure, secures essential organs and serves as a storage of such
minerals as calcium and phosphorus. The development of bone is either through endochondral
ossification of long bones or intramembranous ossification of the flat bones. Such hormones as
growth hormone, estrogen, testosterone, and calcitonin have major influence on bone
metabolism. Bone formation is also stimulated by mechanical stress, which explains why
physical exercises are important to skeletal well-being.
The intersection of two or more bones is known as joints which are categorized by the structure
and functionality of the joints. Knee and shoulder are synovial joints which enable a wide range
of motion because of the existence of the synovial fluid, articular cartilage, and fibrous capsule.
Fibrous joints have no movement whereas cartilaginous joints have some form of limited
movement. The ligaments hold the joints, tendons connect muscles with bones and pass the force
that allow movement.
The musculoskeletal physiology is also important in posture and thermoregulation. During
contraction, muscles produce heat and this helps in maintaining body temperature. The
significance of nutritious diet and physical exercise, as well as hormonal equilibrium, to the
musculoskeletal well-being is emphasized by disorders of such a system as osteoporosis,
arthritis, and muscular dystrophy.
6. The Cardiovascular System
The cardiovascular system has the role of transporting oxygen, nutrients, hormones and waste
products to the body. It comprises of the heart, blood vessels and blood. This system keeps the
tissues perfused and promotes cellular metabolism by ensuring there is enough supply of oxygen
and elimination of carbon dioxide.
The heart is a muscle organ and it is like a two chambered pump, and it has four chambers: two
atria and two ventricles. The heart pumps the deoxygenated blood to the lungs through the
pulmonary circuit and oxygenated blood to the rest of the body through the systemic circuit via
the right and the left sides respectively. The contraction of the heart is triggered by the sinoatrial
(SA) node or pacemaker where electrical impulses are produced that are transmitted in the atria
leading to contraction. This impulse then proceeds to the atrioventricular (AV) node, the bundle
of His, and the Purkinje fibers making sure that the ventricles contract in unison.
The heart has a contraction and relaxation phase known as systole and diastole respectively.
Systole is the process in which blood is pumped out of the ventricles and diastole is the process
in which blood gets pumped into the heart. The lub-dub sound of the heart is associated with the
closing of atrioventricular and semilunar valves which prevents reverse flowing of blood.
Cardiac output is determined as the product of cardiac rate and stroke volume and the measure of
heart efficiency. Intrinsic and extrinsic factors regulate the functioning of the heart, including the
Frank-Starling law and the input of the autonomic nervous system, respectively, and hormones,
respectively.
The blood vessels create a system of circulation. Arteries distribute the blood under high pressure
away the heart, capillaries allow the blood and tissues to exchange, and veins lead the blood back
to the heart. During cardiac cycles, elasticity of arteries is useful in maintaining blood pressure
and venous valves stop the backflow. Neural, renal and hormonal processes are what help to
maintain the blood pressure. The baroreceptor reflex varies the heart rate and vessel diameter
with changing pressure, and the hormones renin, angiotensin II and aldosterone vary the long-
term regulation via changes in blood volume.
Blood, on its part, is a specialized type of connective tissue that consists of plasma and formed
elements, namely, red blood cells (RBCs), white blood cells (WBCs), and platelets. Hemoglobin
is found in the RBCs and is the one that oxygen is bound to and helps in transporting the gases.
WBCs have some immune defense functions and platelets are essential in clotting of blood.
Combining these components will guarantee the tissues get the nutrients and oxygen needed to
be metabolically active and balance is maintained internally.
7. The Respiratory System
Gas exchange is seen to happen in the respiratory system, which brings oxygen to the body and
eliminates carbon dioxide that is a waste product of the metabolic process. This system makes
sure that the tissues get the proper amount of oxygen to enable cellular respiration to produce
ATP-the major source of cellular energy. The respiratory system consists of the nasal cavity,
pharynx, larynx, trachea, bronchi and lungs. All the parts are important in providing effective
ventilation and gaseous exchange.
Air flows in through the nasal cavity to the respiratory system where it gets filtered, warmed and
humidified after which it enters the pharynx and the larynx. The windpipe, known as the trachea,
continues downwards and is then further divided into the right and left bronchi which in turn are
further divided into smaller airways called bronchioles which terminate as the alveoli, which are
small airways where gas exchange takes place. The alveoli walls are very thin and enclosed with
the capillaries whereby oxygen is diffused in the blood and carbon dioxide is diffused out to be
exhaled. The relationship between this exchange is dictated by the partial pressure gradients, and
this exchange occurs on the principle of diffusion between high concentration to low
concentration areas.
The breathing or pulmonary ventilation mechanics consist of two main processes, namely,
inspiration and expiration. When the person inhales, the diaphragm contracts and it moves
downwards as the intercostal muscles widen the rib cage, and thoracic enlargement and decrease
in pressure in the lungs. This suction attracts air into it. As one expires, the thoracic cavity
volume reduces, the diaphragm relaxes and the air is expelled. The respiratory centers in the
medulla oblongata and the pons are the main centers controlling breathing in response to changes
in the concentration of carbon dioxide and the hydrogen ions in the blood. The carotid bodies and
aortic arch chemoreceptors also play a role in regulating the respiratory rate and depth by
monitoring the level of blood gases.
The transport of gases takes place by binding of oxygen with hemoglobin in red blood cells to
form oxyhemoglobin. This is a reversible process, in which oxygen can be released to the tissues
where the oxygen is required. The carbon dioxide is carried to the body in three forms, which
include dissolution in plasma, chemically bonded to hemoglobin, or bicarbonate ions. The latter
process, which is catalyzed by carbonic anhydrase, is critical in the process of achieving acid-
base balance. Respiratory illnesses in the form of asthma, chronic obstructive pulmonary disease
(COPD) and pneumonia may disrupt the gas exchange and therefore the lungs should be kept
healthy by exercising, using clean air and avoiding smoking.
8. The Digestive System
Digestive system carries out the mechanical and chemical decomposition of food, nutrient
absorption and elimination. It assures the body of the much needed nutrients, carbohydrates,
proteins, fats, vitamins and minerals, needed in the body to grow, repair and produce energy. It is
a system that comprises the alimentary canal that comprises the mouth, pharynx, esophagus,
stomach, small intestine, and large intestine, and accessory organs such as the salivary glands,
liver, pancreas, and gallbladder.
The mouth is where the digestion process starts with mechanical, in which the food is chewed
and chemical, which begins with the release of salivary amylase that breaks the starch into
maltose. Food bolus travels through the pharynx and esophagus as a result of peristaltic
movements to the stomach where it is combined with gastric juice that contains hydrochloric
acid (HCl) and pepsin. These secretions start the breakdown of proteins, as well as, create an
acidic environment that kills pathogens. The churning motion of the stomach transforms food
into a semi-liquid food matter known as chyme.
This happens mainly in the small intestine where digestion and absorption occur. It is made up of
three parts- duodenum, jejunum and ileum. Digestive enzymes produced by the pancreas in the
duodenum and bile produced by the liver in the duodenum break the fats, proteins and
carbohydrates down. The pancreatic amylase, lipase and proteins such as trypsin are also
fundamental in the process. The fats are broken down into smaller droplets that are easy to digest
through bile which is held in the gallbladder. Villi and microvilli line the intestine wall making
the surface area of the intestine very large to absorb nutrients. The absorption into the blood is
glucose and amino acids, fatty acids and glycerol into the lymphatic system.
Water, electrolytes and vitamins that are manufactured by the intestinal bacteria are absorbed in
the large intestine. It is also the one that collects the undigested contents into faeces that is
disposed through rectum and anus. The liver is the largest internal organ and is involved in some
critical metabolic processes such as detoxification, glycogen storage, and production of proteins.
It also secretes bile vital in breaking fats. The pancreas is a two-fold gland as it releases digestive
enzymes (exocrine) and the hormones insulin and glucagon (endocrine).
The digestive physiology is the type of physiology that guarantees proper conversion of food into
usable energy. Nutrient intake can be disturbed by disorders like peptic ulcers, irritable bowel
syndrome (IBS), malabsorption syndromes, and so on, which impacts on general health. Proper
diet, water consumption and exercise are associated with good digestion and healthy body.
9. The Urinary System
Urinary system or renal system helps to maintain the fluid balance in the body, eliminate the
metabolic waste and the electrolyte and acid-base levels. It is primarily made up of the kidneys,
the ureters, the bladder of the urine, and the urethra. Kidneys are very important organs which
engage in filtration, reabsorption, secretion and excretion to ensure homeostasis.
There are approximately one million microscopic filtering units in each kidney that are known as
nephrons and form the functional units of the renal system. Irrigation of the nephron occurs by
the blood flow through the afferent arteriole and into the glomerulus which is a bunch of
capillaries encompassed by Bowman capsule. In this instance, filtration takes place, where water
and other smaller solutes like glucose, ions and urea are permitted to be absorbed into the renal
tubule and larger molecules like proteins and blood are retained. That filtrate is then filtered in
the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct
where specific reabsorption and secretion refines the composition of urine.
Electrolyte balance in the body is managed through the kidneys, which balance the levels of
sodium, potassium and chloride in the body. The loop of Henle is important in establishment of a
concentration gradient that facilitates reabsorption of water, which helps the body to conserve the
water in cases of need. Hormones that control the renal functioning include antidiuretic hormone
(ADH), aldosterone and atrial natriuretic peptide (ANP). ADH causes more water to be
reabsorbed in the collecting ducts, aldosterone causes sodium to be reabsorbed and ANP causes
secretion of sodium to reduce blood pressure.
Regulation of blood pH is also achieved through the excretion of the hydrogen ions and
reabsorption of bicarbonate by the kidneys. Moreover, they also synthesize erythropoietin that
stimulates the production of red blood cells in the bone marrow and renin that activates the renin-
angiotensin-aldosterone system (RAAS) to regulate blood pressure. The urine made in the
kidneys moves through the ureters to the urinary bladder where the urine gets stored until it is
discharged voluntarily through the urethra.
The urinary system disorders that may have a dire effect on homeostasis are kidney stones,
infections and chronic kidney disease. The necessary measures to ensure the renal health include
proper hydration, balance in the diet, and management of blood pressure.
10. The Reproductive System
Reproductive system is the one which helps in the continuity of the human race by the means of
production, fertilization and offspring development of the gametes. It also synthesizes the sex
hormones that affect the secondary sexual characteristics and control the reproductive functions.
Although, the reproductive systems of both sexes serve the same purpose of reproduction, the
structure and physiological processes of both sexes differ considerably.
The male reproductive system comprises of the testes, epididymis, vas deferens, seminal
vesicles, prostate gland and penis. Testes are the most important reproductive organs, and they
produce sperms (spermatogenesis) and secrete testosterone. The spermatogenesis process takes
place in the seminiferous tubules of the testes and it starts at puberty under the impact of the
follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH is the hormone that
stimulates sperm production whereas LH stimulates the interstitial cells of Leydig to secrete
testosterone. The mature sperm is then transported to the epididymis where it gets its motility
and is stored until ejaculation. The sperm during ejaculation move in the vas deferens, blending
with the fluids of seminal vesicles, prostate and bulbourethral glands to constitute semen. The
main male sex hormone, testosterone stimulates muscular growth, voice deepening, and hair
growth on the face and the body.
The female reproductive system is composed of the ovary, fallopian tubes, uterus, cervix and
vagina. The major reproductive organs are the ovaries which generate the ova (eggs) and produce
estrogen and progesterone. This begins prior to birth with the immature ova suspended in meiosis
before puberty. With the aid of hormones, one ovum is matured and released during ovulation in
every menstrual period. The ovum released is trapped in the fallopian tubes, and the fertilization
process usually takes place in the ampulla, which is the largest part of the tube. The fertilized egg
or the zygote gets moved to the uterus where it implants to the endometrial lining and starts
developing. Progesterone and estrogen which are produced by the ovaries and subsequently the
placenta sustain the lining of the uterus and control the menstrual cycle.
The hypothalamic-pituitary-gonadal axis is a sensitive feedback system in females between the
hypothalamus, pituitary gland and ovaries. Gonadotropin-releasing hormone (GnRH) is secreted
by the hypothalamus to stimulate the release of FSH and LH by the anterior pituitary. The
hormones control the growth of follicles, ovulation and the production of hormones. Without
fertilization the decreasing levels of progesterone and estrogen initiate menstruation and a new
cycle begins.
Human chorionic gonadotropin (hCG) is a hormone that sustains the corpus luteum in the course
of pregnancy to continue producing progesterone. Later this role is played by the placenta, which
is nourishing the fetus and carrying out the exchange of gases. In the postpartum period the
prolactin triggers the synthesis of milk, whereas the oxytocin stimulates the discharge of milk
and constriction of the uterus. These hormones are synchronized so that they lead to a successful
reproduction, pregnancy and aftercare.
11. The Immune and Lymphatic Systems
The immune and lymphatic systems collaborate to protect the body against any pathogen,
preserve the level of fluids and to absorb the dietary fats. The lymphatic system is composed of
lymph, lymphatic vessels, lymph nodes and such organs as the tonsils, the thymus and the spleen.
Lymph is a transparent fluid that is a derivative of interstitial fluid, which moves in lymphatic
vessels and then back to the bloodstream via the blood vessels, thoracic duct and right lymphatic
duct. In its course, the lymph flows through lymph nodes that trap malevolent materials and store
immune cells such as lymphocytes and macrophages.
There are two big lines of defense and they are innate (nonspecific) immunity and adaptive
(specific) immunity, which are offered through the immune system. Natural immunity is
immediate and it not only has physical barriers such as the skin and mucous membrane but also
physiological processes such as inflammation, phagocytosis, and natural killer (NK) cell activity.
This defense is further augmented by chemical mediators such as lysozyme, complement
proteins and interferons.
Adaptive immunity arises following exposure to certain antigens and entails activation of the
lymphocytes- B cells and T cells. The B cells develop in the bone marrow and they also mediate
the humoral immunity by secretion of antibodies that inactivate pathogens. Cell-mediated
immunity involves T cells which are developed in the thymus and which directly kill infected or
cancerous cells. INS The immune reactions are orchestrated by the Helper T cells which produce
cytokines that stimulate the other immune cells whereas cytotoxic T cells attack and destroy
infected cells. Immune responses to pathogens result in the formation of memory cells that
provide long-term immunity against pathogens that have been previously seen.
The filter of blood is the spleen that eliminates old red blood cells and pathogens, the T-cell
development is necessary and is facilitated by the thymus. Antigens taken in the mouth and the
intestines are trapped by tonsils and Peyer patches in the intestines. Inflammation, where the
body turns red, swollen, hot and painful, is also a part of the immune response and is carried out
to isolate and destroy infectious agents as well as begin repair of the tissue.
Autoimmune diseases arise as a result of an overactive immune response, where the immune
system attacks the body itself as in the case of lupus or rheumatoid arthritis. On the contrary,
infections or diseases which suppress the immune system like HIV/AIDS undermine immune
capability. The interaction and harmony between the inborn and the adaptive processes play a
significant role in ensuring the immune homeostasis and general health.
12. The Integumentary System
The outer protective covering of the body is what is known as the integumentary system and this
includes the skin, the hair, nails and glands. It is the initial point of protection against
environmental risks, controls body heat and supplies the senses. The skin is the greatest body
organ in the human body with a weight of about 16% of the total body weight. In structure, it is
made up of three layers, which include the epidermis, dermis, and the hypodermis.
The epidermis is the most external layer which consists mostly of stratified squamous epithelium
which is keratinized. It has specialized cells like the keratinocytes which produce keratin that
gives it strength and make it waterproof, and the melanocytes that produce melanin that shields
against ultraviolet (UV) radiation. The epidermis maintains its own regeneration with new cells
being formed in the basal layer and moving upwards to replace any dead cells being shed off on
the surface.
Directly below the epidermis is the dermis, which is a thick layer of connective tissue, which has
blood vessels, nerves, hair follicles, and glands. It gives it its elasticity and strength based on
collagen and elastin fibers. The dermis has 2 layers- papillary layer which feeds and reticular
layer which secures more deep structures. The hypodermis or the subcutaneous layer comprises
of adipose tissue which insulates the body, energy storage, and cushions of internal organs.
Sweat glands, sebaceous glands and hair follicles are also part of the integumentary system. The
presence of sweat glands helps in cooling the temperature by evaporation and the presence of
sebaceous glands to lubricate the skin and protect it. Keratin in hair and nails is used in
protection and sensory perception. Sensory receptors can also be found in the skin and they sense
the temperature, pressure, touch, and pain, hence can interact with the environment.
Besides it offers protection and sensation, vitamin D is a product of the skin that plays a vital role
in the metabolism of calcium and bone. The integumentary system also helps in excretion
because it helps in the excretion of small portions of urea, salts, and water in the form of sweat.
Burns, infections, and skin cancers are some of the conditions that indicate the relevance of
ensuring skin integrity and preventing it against injuries.
13. Homeostasis and Integration of Systems
The basis of human physiology is homeostasis. It can be defined as the capacity of the body to
continue having a stable internal environment despite the change in the external environment. All
the physiological systems are involved in the process of homeostasis and interact with each other
in complex ways including feedback, signal molecules as well as neural pathways. Homeostasis
was introduced by Claude Bernard and later elaborated over by Walter Cannon who defined it as
a dynamic balance of the internal environment. To maintain the correct operation of the human
body, the parameters like the temperature, blood pressure, pH, and the concentration of glucose
in the blood should be within a strict range.
Nervous and endocrine systems are the main ones that ensure homeostasis. The nervous system
responds quickly and temporarily, as various electrical impulses, whereas the endocrine system
engages in slower and more permanent responses, as the system responds with the help of
hormones. The two systems interact and synchronize under the hypothalamus which is the center
of control in the body. The hypothalamus controls internal parameters like temperature and
osmolarity and activates the corrective mechanisms in the circumstances of deviation. As an
example, one can discuss thermoregulation where an increase in the body temperature provokes
the generation of sweat and an increase in blood flow to the skin to remove heat, whereas a
decrease in body temperature causes shivering and constriction of blood vessels to preserve it.
The homeostatic control is based on the feedback. The most prevalent one is negative feedback
which quells deviance around a set point to reestablish balance. Regulation of blood glucose by
the insulin and glucagon, and blood pressure by the baroreceptors are examples. Less prevalent
but more amenable to the development of responses is positive feedback, which enhances a
response to reach a particular goal, e.g., the increase of oxytocin in childbirth that increases the
intensity of uterine contractions until delivery takes place.
The cardiovascular, respiratory, and renal systems are interconnected in order to preserve acid-
base balance and oxygen supply. As the amount of carbon dioxide in the blood increases, the
breathing rate accelerates in the respiratory center to clear the excess amount of CO2 causing
acidosis to be avoided. This is enhanced by the kidneys which excrete hydrogen ions and
reabsorb bicarbonate which further stabilize the pH levels. In the same way, the maintenance of
blood pressure entails the liaison behavior between the heart, blood vessels and the kidneys. The
renin-angiotensin-aldosterone system (RAAS) works to raise the volume of blood and tighten
vessels when blood pressure drops initiating normal pressure.
The movement and temperature circulating in the body helps to maintain homeostasis, which is
provided by musculoskeletal systems and integumentary ones. Heat is produced by muscles
during exercise, and heat is lost to the surrounding through the action of sweat glands and blood
vessels on the skin. The digestive system and the urinary system balance out the nutrients and
water essential to the cells to be able to process energy and water. The body maintains
homeostasis through the immune system that detects and removes pathogens, which destabilize
the physiological condition of the body.
When these mechanisms are disrupted or exceeded, this leads to homeostatic imbalance. This
causes morbidity and malfunction. As an example, chronic hyperglycemia in diabetes mellitus is
caused by insulin dysregulation, with consequences on the various organ systems. Similarly, the
long term high blood pressure harms arteries and other body parts like the heart and kidneys.
Homeostasis is essential to survival, health, and longevity, therefore, because it is the capability
of the body to adjust and react to the external and internal alterations.
14. Conclusion
Human physiology is a complex phenomenon that is dynamic and involves a study of the
relationship between the body structures and systems that are in unison to support the body. It
shows how wonderfully all organs, tissues, and cells are coordinated and work together to ensure
a balance and stability of the whole human organism. All the physiological processes are
connected and well controlled, based on the neural networks that pass electrical impulses to the
hormonal systems that govern metabolism and development. The cardiovascular system,
respiratory system, digestive system, and urinary system are integrated to ensure that the cells get
nutrients and oxygen and gets rid of the wastes effectively. In the same way, the immune system
and the integumentary system helps to protect the body against pathogens and environmental
factors, whereas the musculoskeletal system helps the body to move around and provides
structural support.
The maintenance of internal stability in the face of external changes is a major theme in the study
of physiology, and is referred to as homeostasis. This principle is the foundation of all the
processes in the body as it proves how the human body manages to adapt to constant changes by
responds to feedback and inter-system communication. An example of this coordination is
demonstrated by the interaction between the nervous and the endocrine systems, which
collaborates to coordinate functions such as temperature and blood pressure as well as
reproduction and metabolism. Physiological balance is essential to health and survival because
when homeostasis is destabilized it results in disease.
The progress of medical science and biotechnology keeps on increasing our knowledge on the
physiology at molecular, cellular, and systemic levels. Current studies combine genetics,
molecular biology and bioinformatics to reveal the role of physiological mechanisms on health
and disease. Such knowledge does not only improve diagnostic and therapeutic interventions, but
also foster preventive medicine and individualized treatment. An example is the knowledge of
physiology of cardiovascular regulation that has resulted in life-saving interventions in the
cardiovascular disease and the role of understanding the physiology of endocrine regulation that
has transformed the management of diabetes.
Finally, human physiology is not only an academic topic, but the basis of medicine and the key
to the quality of life. Through the study of the body, scientists and other medical practitioners can
be in a better position to tackle the issues over aging, disease, and environmental change. An
example of complex and coordinated behaviour in the living organism is human physiology, the
product of sophisticated design and flexibility of human life.
