Basic Physiology for High School Students

Organ Systems

Physiology pertains to the study of biological functions. To facilitate the comprehension of physiology, it is often beneficial to initially categorize the subject into organ systems. This method of categorization proves useful across various scientific disciplines that students may encounter during their high school and college education. After segmenting into organ systems, it is imperative to understand the communication and interactions among these systems. Presented below are some of the organ systems within the scope of human physiology and their intercommunication.

Skin

Due to its visibility and extensive surface area, the skin is one organ system that readily comes to mind. Its functions and interactions with other systems include:

• The skin acts as a cushion and protective barrier for underlying tissues.
• It serves as a defense against infections.
• It regulates body temperature through perspiration and sweat evaporation.
• In conjunction with sunlight, the skin synthesizes vitamin D, which is subsequently modified in the liver and plays a crucial role in calcium absorption from food in the intestines. Calcium is vital for several biological functions, including maintaining bone health.
• The skin provides a foundation for hair growth, adding an additional layer of protection for the head and helping to retain warmth in cold weather.
• It contains nerve endings that enable sensations of touch, heat, cold, and pain.

The skin is characterized by its continuous self-renewal. This regenerative process initiates in the deeper layers of the skin, while the outer layer is consistently shed through exfoliation. As an example of epithelial tissue, similar to the lining of the intestine, the skin exhibits a high rate of renewal, turnover, and reproduction. However, this high turnover rate renders the skin and intestines susceptible to mutations, which may pose problems such as cancer. The probability of mutations occurring in this renewal process increases with age.

Heart

The heart’s primary function is to circulate blood throughout the body, including the skin. Single-cell organisms do not require a circulatory system due to their small size, as simple diffusion can adequately bring nutrients to their cell surface. Simple diffusion, driven by random molecular motion associated with heat, is effective over short distances. However, in larger organisms such as mammals, blood circulation is necessary to transport nutrients close to individual cells. Blood, propelled by the heart through vessels, arterioles, and capillaries, delivers nutrients near cells, where simple diffusion then takes over. Essential nutrients delivered include amino acids, sugar, and fats.

The heart consists of four chambers divided into left and right sides. The left side pumps blood at high pressure to all parts of the body except the lungs, requiring a larger muscle mass for this task. The systemic arteries’ arterioles present resistance to blood flow from the high-pressure left side. The right side of the heart pumps blood to the lungs at lower pressure, which is suited for the delicate pulmonary tissues. Valves between the heart’s chambers ensure unidirectional blood flow.

Heart muscle cells are one of three muscle tissue types, alongside skeletal and smooth muscle. Heart cells are connected electrically through tight junctions, enabling coordinated contraction triggered by periodic electrical activity originating in the pacemaker region. This activity quickly spreads via a network of Purkinje cells and through cell-to-cell electrical tight junctions.

The heart rate is regulated by the autonomic nervous system, specifically the parasympathetic and sympathetic branches. The sympathetic system increases heart rate, while the parasympathetic system decreases it, both influencing the pacemaker region of the heart.

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Lungs

Oxygen makes up 20% of the air we breathe and is essential for cells with mitochondria. Lungs facilitate oxygen delivery to cells via blood. Protected by our ribs, lungs transfer oxygen to the blood over a short distance in tiny air pockets called alveoli. Lung capillaries surround each alveolus, allowing oxygen to transfer through simple diffusion.

Our lungs have about 500 million alveoli, where gas exchange occurs. Oxygen diffuses from the alveoli into the blood while carbon dioxide, a metabolic byproduct, moves from the blood to the alveoli and is exhaled. When we inhale, fresh air with low carbon dioxide and 20% oxygen enters the alveoli, partially replenishing them. Exhaling removes air that contains about 5% carbon dioxide. This cycle happens with each breath.

The “Conditioning” of blood involves adding oxygen and removing carbon dioxide, as well as providing nutrients, preparing it for delivery to cells.

The Digestive System

Cells need various nutrients such as glucose for energy, amino acids for proteins, and fat for long-term energy storage. The digestive system extracts these nutrients and water from food.

Food is chewed into small particles and mixed with saliva to begin digestion. Swallowed food is moved to the stomach by esophageal muscles. In the stomach, it’s mixed with acid to aid digestion and kill bacteria. As food enters the small intestines, bicarbonate neutralizes stomach acid, enzymes break down proteins, and bile emulsifies fats for absorption.

The liver, a significant organ in the human body, is connected to the intestine through the hepatic portal system, which consists of veins that transport blood from the intestines to the liver before it returns to the heart. The term “hepatic” pertains to the liver. A key function of the hepatic system is the production of bile, an emulsifying agent essential for fat absorption. Bile is stored in the gallbladder and released into the small intestine as needed to facilitate this task.

Additionally, the hepatic system plays a crucial role in detoxifying harmful chemicals that may enter the bloodstream. For instance, while excessive alcohol consumption can damage liver function, the liver metabolizes it into non-toxic substances. Fortunately, the liver possesses regenerative capabilities, allowing it to recover if the damage is limited and ceases.

Production of Urine

The kidneys remove unwanted water-soluble metabolic products from the blood. Each kidney contains about a million nephrons, which first filter blood by removing red and white blood cells, leaving only plasma fluid passing through to the next part of the nephrons. This initial filtration happens at the beginning of each nephron in a structure called Bowman’s capsule. The filtered plasma then moves through nephron tubules, where over 99% of desirable molecules and water are reabsorbed into the blood, concentrating the urine. Nephrons vary in length and ability to concentrate. The concentrated urine, with waste products and regulated water content, empties into the bladder for storage until elimination.

When the concentration of a substance in the urine is high relative to its concentration in blood plasma, it indicates that the kidneys have effectively cleared or removed that substance from the blood. High clearance occurs when the substance passes through the kidney tubules without being reabsorbed into the bloodstream. In some cases, high clearance also involves the active transport of a substance from the blood into the tubules. Conversely, if the urine concentration of a substance is low compared to its blood plasma concentration, this signifies low clearance by the kidneys. For instance, glucose, an essential nutrient, typically has a clearance value of zero.

Unwanted or harmful substances removed from the blood are excreted in the urine. These substances may include hemoglobin breakdown products, urobilin (which imparts the yellow color to urine), urea, uric acid, ammonia from protein metabolism, and creatinine, a byproduct of normal muscle activity. The distinctive ammonia odor in cat urine can be attributed to their high-protein diet. Analyzing the presence and concentration of these substances in urine is useful for assessing overall health and particularly the health of the kidneys.

It is possible to live a healthy life with only one kidney. However, when both kidneys fail, medical interventions such as dialysis or a kidney transplant become necessary to sustain life.

Hormones

Organ systems must communicate effectively, and hormones facilitate this by traveling through the blood to different body parts. Unlike neurotransmitters, which act between nearby neurons, hormones operate over large distances. The human body has over fifty hormones, many of which are small peptides, with numerous ones acting in the hypothalamus and pituitary. Detailed explanations of this system exceed the scope of this essay.

Insulin, a peptide hormone produced in the pancreas’ beta cells, helps move glucose into muscle, liver, and fat cells, preventing high blood sugar levels.

Glucagon, made by alpha cells in the pancreas, stops blood sugar from getting too low by releasing glucose from the liver.

The pancreas also produces enzymes to digest fats, sugars, and proteins for absorption in the small intestine.

Most hormones are peptides, but catecholamines such as adrenaline (epinephrine in the US) are not. Adrenaline, released from the adrenal gland atop the kidneys during stress, acts as both a hormone and a neurotransmitter. It triggers the “fight or flight” response, essential for survival in stressful situations. However, prolonged stress can make this response harmful.

This text is designed for high school students; thus, it omits much physiological detail. For instance, the nervous system, immune system, and reproduction are not covered. Even the included physiology is simplified and incomplete. However, it aims to encourage interested students to explore these topics deeply. Learning thrives with curious, self-motivated, and active students. If you’re reading this, you likely belong to that group.

Enrichment Problems for Students of Physiology

Explain the evolutionary advantage of sexual versus asexual reproduction.

Draw a side view of the head and identify the locations of the thalamus, hypothalamus, and pituitary.

Select one process regulated by hormones involved in the hypothalamus-pituitary axis and describe this regulation in detail.

Define neurotransmitters, hormones, and vitamins, providing examples and their functions.

Describe the types of sensory receptors found in the skin.

Explain the generation of electrical activity in the heart and how it is detected by electrodes on the skin surface. Identify what the visual tracing of this electrical activity is called.

List the sections of the small intestine and describe their functions.

Describe the amounts and directions of water movements across the large and small intestinal walls.

Define kidney stones and discuss methods of removal.

Identify the causes of gallstone formation.

Explain the role of gravity in the varying sizes of alveolar air sacs within the lungs in an upright person.

Provide detailed descriptions of the functions of the gallbladder, pancreas, and liver.