Human Body Systems: Functions and Interactions

Posted on Dec 12, 2024 in Biology

The Senses

Vision

From the optical point of view, the focus point (where the image is formed) is beyond the eye, rather than on the retina.

Astigmatism: A condition that causes the formation of blurred images due to irregularities in the curvature of the cornea.

All these problems can also be found in any device with artificial lenses. For example, a telescope can have astigmatism.

Hearing

The ear consists of three regions:

Outer ear: The visible part, formed by the outer ear and the ear canal.
Middle ear: Situated below the outer ear. It begins with the eardrum, a membrane that captures vibrations that have entered the ear and transmits them to an assembly of three bones: the hammer, anvil, and stirrup.
Inner ear: The three middle ear bones pass the vibrations to the cochlea, a spiral tube-shaped organ filled with fluid. Inside are cells that detect the vibrations by cilia, which vary depending on the movement of fluid inside the cochlea.

Auricular pavilion, cochlea, tympanic, hammer, anvil, stirrup.

Structures that allow balance are also located in the inner ear. It is a region located on the cochlea, called the vestibule, and three semicircular canals, located just above the vestibule.

The vestibule is divided into two parts (saccule and utricle) and addresses static balance (no movement). It handles standing upright and normal position.
The semicircular canals are responsible for dynamic equilibrium (when moving). In addition, they report on the position of our head during a movement to help us keep posture.

Taste

The sense of taste is responsible for capturing information on the chemical composition of food. Our sense of taste is located in the taste buds, small receptor-filled structures that detect aspects of the composition of substances that reach the mouth. We distinguish five basic tastes: the four traditional (sweet, salty, sour, bitter) plus a fifth, called umami.

Smell

Smell also analyzes chemicals. In this case, those details are mixed with air entering the nose. During feeding, the sense of smell supplements the information gathered by the sense of taste, a notion we think of as flavor. There are only five tastes, but the flavors are much more, because they are combinations of taste and smell.

Smell is one of the oldest human senses; therefore, it is intimately connected to the primitive parts of our brain, allowing the existence of subconscious links between smell and our reactions. Not all scent information is collected and analyzed consciously. In the case of pheromones, for example, the substance is detected and causes a reaction in us, but we are not aware of it.

Touch

Touch is a sense that is distributed across the surface of our skin. It is responsible for reporting when our body makes contact with the surrounding elements. Although there are touch receptors throughout the body, its distribution is not regular. We have more receptors in regions of the body that need to be protected because they are very sensitive (like the head and neck), in regions that normally come into contact with surfaces, or are used to manipulate and understand the environment (such as feet and hands), or in areas related to sexual pleasure.

Pain

Pain receptors are located throughout the body, not only on the surface but also within both muscles and internal organs. Anyway, the area where they are found most abundantly is the skin, where they provide information on elements that are causing or could cause damage to the body. Pain often causes instinctive self-protection reactions.

Temperature

There are two different aspects of temperature that our body is able to detect:

Ambient temperature: This is detected through receptors in our skin (other than pain or touch). These receptors provide conscious information about the temperature of what surrounds us, what we touch, etc.
Internal body temperature: This is detected to keep our temperature around 36.5°C. The information collected by internal temperature sensors is not perceived consciously, but it serves to generate responses and keep our temperature stable.

Proprioception

Proprioception is our unconscious internal perception. It consists of a network of receptors in the joints of the body that are responsible for providing information on our position. This information, combined with the information provided by the sense of balance, gives us accurate information about where each part of the body is at all times (both in absolute terms and relative to other parts). This information is processed unconsciously, like many other pieces of information related to our blood pressure, oxygen requirements, or glucose levels.

Nervous System

The nervous system is responsible for capturing information about the environment through receptors, processing it to prepare a response, and leading the response to the effector organs. Effector organs are those that execute the response, which fall into two basic groups: muscles and glands.

Neurons

The neuron is the fundamental unit of the nervous system. This cell is responsible for building the network that connects the receptors with effectors, as well as circuits that analyze and generate responses. Our nervous system consists of approximately 10¹¹ neurons (100 billion).

Parts of a Neuron

A neuron consists of three basic elements:

Dendrites: One or more branches that capture stimuli, either directly from abroad (in the case of a neuron that acts as a receptor) or in the form of a pulse transmitted from another neuron.
Soma: The “body” of the neuron, where its organelles are located. Impulses captured by the dendrites reach the soma, and it drives them out toward the axon.
Axon: A branch (in this case, always unique) responsible for conducting the impulse from the soma towards its end, to pass it on to another neuron or an effector (in the case of the last neuron in the chain).

Synapse

The synapse is the communication between two neurons. Neurons are not physically connected, but there is a small space between them (approximately 20 nm). The transmission of nerve impulses from one neuron to another occurs through these spaces.

The process begins when a nerve impulse that has made the journey around the axon of a neuron (called presynaptic because it is before the synapse) comes to an end. There are neuron vesicles filled with a substance called a neurotransmitter. There are several molecules that function as neurotransmitters. The type of effect on the neuron that receives the impulse depends on the type of molecule that the neuron uses as a neurotransmitter.
The content of the vesicles is poured into the space between the presynaptic and postsynaptic neuron (the neuron that is after the synapse), called the synaptic gap.
The presence of the neurotransmitter in the synaptic gap causes a change in the membrane of the postsynaptic dendrites. Following that, the nerve impulse is generated in this neuron.
In the next synapse, what was formerly the postsynaptic neuron becomes the presynaptic neuron, as it is before the synapse and shall also forward momentum and release neurotransmitters.

Nerve Impulse

Nerve impulses are conducted through synapses and move through the axons and dendrites of neurons at about 60 centimeters per second. This speed is too slow for our needs. Most of our neural circuits have a system that accelerates the transmission of nerve impulses through the myelin sheath.

The myelin sheaths are produced by Schwann cells, a special cell type that surrounds the axons of many neurons.
Breaks between two segments of myelin sheaths are called nodes of Ranvier.
The pulses in neurons with myelin sheaths move faster because, in reality, the momentum is regenerated in each node of Ranvier. This transmission is called saltatory because the impulse “jumps” from node to node and diffuses passively in the areas covered with myelin.
In unmyelinated neurons, the impulse becomes weaker with distance.
In myelinated neurons, the signal is regenerated and does not lose speed or intensity.

Organization of the Nervous System

The nervous system is basically divided into peripheral and central.

Central Nervous System

The central nervous system is organized along a tube called the spine or spinal cord, which runs through the body from the head to the origin of the lower extremities.

The spinal cord is protected by the spine and three membranes called meninges (which also protect the brain). Nerves that reach every organ in the body leave the spinal cord.
At the head is the brain, which is the most important structure of the central nervous system. The brain consists of billions of neurons connected together and handles the most complex functions of our nervous system. It is protected by the bones of the skull and, in the same way as the spinal cord, by a series of membranes called meninges.
The meninges are responsible for forming a buffer zone between the brain and skull in order to avoid the effect of striking the central nervous system. Between the two innermost meninges is cerebrospinal fluid, which is in charge of cushioning blows.

The brain consists of three main parts:

Medulla: The bottom of the brain, in direct contact with the spinal cord.
It is the passageway for all impulses to or from higher areas to or from the bone.
In addition, the medulla directly controls things like heart rate, breathing, swallowing reflex, our blood pressure, and other basic aspects of working that do not require voluntary control.
Cerebellum: The part that integrates information concerning movement. It is responsible for motor coordination and the acquisition of complex movement patterns.
Brain: The part with the largest size and many more cells and connections. It is formed by a group of structures with a given functional area. The most developed in humans is the neocortex, an area divided into two hemispheres and responsible for higher functions characteristic of the human species (such as abstract thought or language).

Brain, cerebellum, medulla.

Peripheral Nervous System

Within the peripheral nervous system, we distinguish the somatic nervous system (running voluntary movements) and the autonomic nervous system (which controls involuntary movements). The peripheral nervous system consists of sensory nerves (that capture sensory information) and motor nerves (which carry information to the effectors). Nerves are fibers that leave the central nervous system and connect with other parts of the body through nerve impulses. Nerves are composed of several nerve fibers, which in turn are formed by arranged neurons.

Nervous System Function

The nervous system functions by circuits between neurons. These circuits are activated by the type of activity that occurs, the types of neurons, and the types of circuits. Once the information is processed (to varying degrees), it will produce a visible response (such as a motor response, for example) or an invisible one (such as remembering a new word).

Diseases and Disorders of the Nervous System

We distinguish neurological diseases (where there is a specific element of injury) and psychiatric diseases (where there is a problem, but it is not the result of any specific injury). The most important are:

Psychological Disorders

Depression: Deep sadness that prevents a person from getting on with their normal life.
Schizophrenia: Pathological difficulty perceiving or expressing the reality around him. Without treatment, the individual suffers from personality disorders.
Dementia: A process that involves the loss of higher functions of the human brain (mainly memory, attention, language, and problem-solving). In advanced stages, one loses track of time, space, and even identity.

Neurological Diseases

Meningitis: Inflammation, usually infectious, of the meninges (the sheath of the central nervous system).
Epilepsy: A chronic neurological disorder. It provokes an attack in which brain electrical impulses occur in an excessive, abnormal, synchronized manner.
Multiple sclerosis: A degenerative disease caused by progressive loss of the myelin sheath covering the axons of neurons. Depending on the type of neuron affected, the problems presented by the person will be different: mobility issues, vision, speech, etc.
Parkinson’s: A degenerative disease caused by the loss of dopamine-producing neurons in a particular area of the brain responsible for motor control. This causes a person to present motor disorders such as tremors, slowness of movement, or lack of balance. In later stages of the disease, the person may have difficulty performing basic activities such as walking, swallowing, talking, or doing simple tasks.
Alzheimer’s: A particular type of dementia. It is a serious disease that progressively impairs all intellectual functions (memory, reasoning). The cause is unclear, but an active intellectual life seems to have preventive effects.

Musculoskeletal System

The musculoskeletal system has a dual role:

Maintaining the structure of the organism in the environment.
Producing movements and displacements.

Skeletal System

The skeletal system is the basic body holder. It protects against strokes, helps maintain the structure of the body, and serves as an anchor for the muscles that are responsible for producing motion. The skeleton is formed by a set of pieces, most of them very strong and quite rigid, allowing movement in some areas through the articulation of different pieces.

Bones

The units that form the skeleton are the bones. The human body has 206 bones, split between the axial skeleton (the axis that forms the head-spine) and the appendicular skeleton (the limbs and their articulation with the axial skeleton). More than half of the bones of the body correspond to the skeleton that forms the arms and legs. In particular, we have 30 bones in each arm and hand, and another 30 for each leg and foot. That makes a total of 120 bones.

Components of Bone

Bones are made up of cells placed around the Haversian canals. Within the Haversian canals circulate blood vessels and nerves of the bone.
The cells located around the canals are responsible for producing the extracellular matrix, where minerals are deposited (mainly calcium phosphate) that give rigidity to the bone.
The matrix contains other elements, such as proteins, among which are collagen fibers (approximately 90% of the matrix), in charge of providing some degree of elasticity to the bone and preventing it from breaking easily.

Bone Structure

Most of the bones are divided into three layers:

Compact bone tissue: It is on the outside, where most of the phosphate, calcium, and other minerals that give bone its consistency accumulate.
Spongy bone tissue: It is only found in the flat bones and the ends of long bones. The degree of mineralization is much lower. The lower mineralization makes its structure full of holes (hence the name).
Bone marrow: It is found only in the innermost part of the flat bones and long bones. It is a mass of tissue that is responsible for generating all blood cells (red and white).

Classification of Bones

By their shape, bones are classified as:

Long bones: The majority. They are those that make up our limbs, both upper and lower, except the carpal (hand) and tarsal (foot).
Short bones: They are very specific bones of certain joints. This group includes the carpus and tarsus.
Flat bones: Characterized by their surface. They are responsible for forming bones within the body cavities. Examples are the bones of the skull, sternum, and ribs.
Irregular bones: They have shapes conditioned by the role they should perform. The best examples are the vertebrae, which protect the spinal cord and serve as anchors for the muscles. The vertebrae must articulate among them, allow flexibility, and need to join with the limbs.

Cartilage

Cartilage is a type of tissue softer than bone. Its composition is similar to that of a bone, but in this case, we find many more collagen fibers and almost no mineral deposits. That makes the cartilage tissue much more plastic and adaptable than bone.

Cartilage Functions

At points of articulation, it serves to reduce friction and allow the joint to be softer.
In the ears and nasal septum, it is used to thicken the appendices.
In the trachea, between the ribs, it serves to allow flexibility and maintain strength.
At the time of birth, there is a lot more presence of cartilage. This allows the skeleton of the child to grow up to adult height. During the process, the regions of cartilage are replaced by bone tissue until it is reduced to only the three previous areas and at the end of long bones (to allow growth to adult height).

Joints

Joints are where bones are joined together and form a set. Joints are only between bones and may prevent any movement. Depending on the degree of movement, there are three types of joints:

Immovable joints: This does not mean that the bones are fused, but they are strongly united by fibrous tissue or cartilage. Example: skull bones.
Semi-movable joints: The union allows movement but is very limited in space. Example: vertebrae.
Movable joints: They allow large and varied movements. Examples: arms, legs, fingers, hips, etc.

Muscular System

Muscles are fibrous structures that have the ability to contract. They are found anywhere in the body where it needs to produce movement. Not all muscle tissue is linked to the musculoskeletal system. Of the three types of muscle (smooth, cardiac, and striated), only the striated is linked to the musculoskeletal system. The smooth fabric (which is in the gut and causes involuntary movements) and heart tissue (in charge of the movements of the heart) do not belong to the musculoskeletal system.

Muscle Fiber

The basic unit of muscle tissue is the muscle fiber.

A single muscle fiber is a cell, elongated and with many nuclei.
Each muscle fiber is covered by many myofibrils, which are long chains of proteins capable of contracting.
The muscle fibers are bundled and form bundles (also called fascicles).
The union of many bundles is what will result in the muscle.

Our body has about 640 skeletal muscles.

How is Movement Achieved?

Myofibrils that run inside each muscle fiber are formed by repetition of the same structure, called a sarcomere, which is a contraction unit consisting basically of two proteins: actin and myosin. Myosin makes motor movements and causes the movement of actin fibers, thereby shortening occurs, leading to muscle contraction.

The movement occurs when impulses arrive through the axons of motor neurons that reach the muscle fibers and make a synapse with muscle tissue. For the movement not to be limited only to the muscle and to move the whole body, it is necessary that these muscles are linked to elements of our skeleton. There are two types of links between muscle and skeletal elements:

Ligaments: Large arrays of collagen fibers that join the ends of a muscle to elongated bone.
Tendons: Attach muscles to the skeleton. When a muscle contracts, it reduces its size. Since it is attached to the bones, this contraction results in a movement of the skeleton (with all elements of the body that are around).

Functions of Muscle Tissue

Muscle tissue is necessary for locomotion movements, but it also has an important role in other functions, such as:

Maintenance of posture (back muscles and abdominals)
Breathing movements (diaphragm and intercostal muscles)
Muscle gesture
Opening and closing of sphincter muscles

The shape of the muscles is linked to their function.

Most of the limb muscles are fusiform (spindle-shaped).
For coated surfaces, there are flat muscles.
There are orbicular muscles, responsible for forming circular structures such as the mouth and eyes.

Antagonist Muscles

For the movement to run smoothly, the muscles work in pairs.

For each flexor muscle, there is an extensor muscle. When one contracts, the other relaxes.
There will always be active control of the body part that is moving.

Most muscles have one or a group of antagonists. Antagonist muscles are those that deal with conflicting movements.

Physical Activity

We all have the same number of muscles, but their development will be different depending, mainly, on the practice of physical activities. Physical activity has many physiological benefits:

Increases the number of muscle fibers, the strength and flexibility of the tendons, and increases the number of mitochondria in muscle fibers.
Increases the size and strength of the muscles of the heart and lung capacity. These two factors increase the efficiency of pumping blood to the muscles.
Strengthens joints, which prevents and delays the onset of joint problems (such as osteoarthritis).

From the standpoint of muscular work, we can distinguish two types of physical activity:

Aerobic activity: An activity in which the muscles involved have the oxygen supply they need to function. Such activities can be extended for a long time if there are no problems with the availability of other elements (glucose, water, etc.).
Anaerobic activity: An activity that, due to its intensity, requires more oxygen for the muscles than the circulatory system can provide. In this case, the muscle works with low oxygen but can only do so for a limited time.

Musculoskeletal Injuries

The most common injuries of the musculoskeletal system are:

Sprain: Breakdown, damage, or loosening of the ligaments. It is quite common and requires rest and often bandages.
Cramp: Occurs when a contraction is blocked, unable to relax. It is relatively mild and usually occurs when an activity is started without heating or stretches for too long.
Fracture: A broken bone. It is usually treated with immobilization but, if complicated, may need surgery to ensure proper bone position for welding.
Dislocation: The output of a bone from its normal range of joint. There is no broken bone, but the joint does not work. To regain the functionality of the joint, the bone must be moved back into position.

Cramp, sprain, fracture, dislocation.

Endocrine System

The endocrine system consists of glands, which are specialized structures that release substances with a regulatory function. The endocrine system is responsible for:

Maintaining homeostasis
Controlling growth
Controlling sexual development and the reproduction process
Controlling the physiological response to situations such as sleep, stress, etc.

The nervous and endocrine systems operate in a coordinated way. The connection point between the two is established between the hypothalamus (a region of the brain) and the pituitary gland (an endocrine gland located in the center of the regulation of the rest). The other endocrine glands respond to orders that have been sent from the hypothalamus, directly or through other glands that it has stimulated.

Although the substances produced by each gland are distributed throughout the body, they are effective only in tissues that are intended and are sensitive to them; these are the target tissues and are specific to each substance.

Hormones are substances that cause endocrine glands to regulate different aspects. Each gland is responsible for producing and secreting one or more different hormones under the control of the pituitary gland through the pituitary, the hypothalamus.

Hypothalamus-pituitary gland, endocrine pancreas, gonads, thyroid glands, adrenal.

Pituitary Gland

Part of the pituitary gland (the adenohypophysis) secretes various hormones directly:

Growth hormone (GH): Acts in general throughout the body and promotes the growth of the organism. Its main functions are to stimulate the reproduction of cartilage cells from which the bone grows, encourage the accumulation of calcium in the bones, increase protein synthesis, and reduce glucose collection by the liver.
Thyroid-stimulating hormone (TSH): Acts on the thyroid gland.
Adrenocorticotropic hormone (ACTH): Acts on the adrenal glands.
Prolactin: Controls blood levels of sex hormones and decreases them in both sexes. In women, it stimulates the mammary glands (hence the name).
Gonadotropins (FSH and LH): Stimulate the production of sex hormones (testosterone in men, estrogen in women) and sperm production and maturation of oocytes cyclically.

Moreover, a portion of the pituitary, called the neurohypophysis, collects and releases hormones that have actually been produced by the hypothalamus. The hormones released by the neurohypophysis are:

Oxytocin: Stimulates the release of milk, the fruit of the stimulus that causes the baby’s sucking on the mother’s breast. Oxytocin is also responsible for uterine contractions during labor. Oxytocin also acts on the brain and helps to strengthen the emotional bonds between people.
Antidiuretic hormone (or vasopressin): Regulates water levels in the body and is released when the body needs to maintain maximum body water.

Thyroid

The thyroid is a butterfly-shaped gland found at the base of the neck. Its endocrine function is to regulate our energy consumption rate, the rate of production of proteins, and the sensitivity of our body to the presence of other hormones. The thyroid produces thyroxine, triiodothyronine, and calcitonin.

All body tissues are targets of thyroxine and triiodothyronine (all respond to them).
The bones are the target tissue of calcitonin. Calcitonin is responsible for maintaining high levels of calcium in the bone, at the expense of maintaining low blood levels.

Parathyroid Glands

Parathyroid glands are located near the thyroid. They are responsible for producing parathyroid hormone (PTH), which maintains high levels of calcium in the blood at the expense of calcium from the bones. Therefore, calcitonin and parathyroid hormone have antagonistic effects, and because of this opposition, it is possible to maintain a balance between blood calcium and calcium in the bones.

Adrenal Glands

There is an adrenal gland above each kidney, and each is divided into two regions: cortex and medulla.

The adrenal medulla produces adrenaline and noradrenaline, which are responsible for the immediate response to stress, increasing heart rate and blood pressure, and directing the blood supply to the motor areas (preparation for possible reactions from attack or flight).
The adrenal cortex produces aldosterone and glucocorticoids.
Glucocorticoids are hormones that help us adapt to stress in the long term and protect us from dangerous situations such as stress or inflammation.
Aldosterone acts in the kidney and regulates the balance of minerals during filtration by the kidneys.

In stressful situations, adrenaline and glucocorticoids act as antagonists.

Gonads

When receiving a signal that the body has reached the level of growth and maturation adequate, the hypothalamus releases a hormone, gonadotropin, which is the pituitary target tissue.

When there is gonadotropin stimulation from the hypothalamus, the pituitary gland produces two hormones that are common to both sexes: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH and LH act on the female gonads (ovaries) and male (testes) and stimulate them to produce their own hormones.
In the case of girls, estrogen and progesterone are secreted, which are responsible for the appearance of secondary sexual characteristics and the onset of menstrual cycles.
In the case of boys, primarily testosterone is secreted, which causes secondary sexual characteristics to appear and also stimulates sperm production.

Pancreas

Besides its exocrine function during digestion, the pancreas also has an endocrine function and produces two hormones that regulate levels of blood glucose: insulin and glucagon, with opposite effects.

Insulin is responsible for lowering blood glucose in the blood, which stimulates the uptake of it to the tissues and the accumulation of reserves as glycogen in the liver.
Glucagon is responsible for raising blood glucose levels when they are too low, which stimulates the mobilization of these reserves.

Diseases and Disorders of the Endocrine System

The most common conditions affecting the endocrine system are the following:

Gigantism/Dwarfism: Two disorders that are a result of excess growth hormone (GH) in the case of gigantism, or lack thereof, in the case of dwarfism. In both cases, the most apparent symptom is the size of the individual. However, the impact is not limited to being too high or too low; we also find problems with the bone structure and balance of body proportions.
Hyperthyroidism/Hypothyroidism: Hyperthyroidism is the overproduction of thyroid hormones (thyroxine and triiodothyronine), which results, in general, in inflammation of the thyroid (goiter) and disorders of eye pressure, which make them stand out from their sockets. Hypothyroidism is a lack of thyroid hormone production. It can also cause goiter and other problems, such as cretinism (stunted physical and mental development caused by a severe congenital lack of thyroid hormone).
Diabetes mellitus: Diabetes is due to problems producing insulin. Because of that, at times when the level of glucose in the blood increases significantly (soon after eating), the diabetic’s body does not have the ability to lower these levels of glucose and accumulate them as reserves. For that reason, dangerous levels of blood sugar can be reached. There are two types of diabetes:
Type I: Most frequently due to an autoimmune process (lymphocytes attack cells in the pancreas that produce insulin). It usually manifests in youth and usually requires the person who suffers from it to receive periodic doses of insulin to compensate for this deficit.
Type II: Caused by a loss of sensitivity to insulin, often linked to low insulin production. It usually manifests in adulthood.