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Mucus: The viscous secretion of mucous membranes. It contains mucin, white blood cells, water, inorganic salts, and exfoliated cells.

The Respiratory System

JoVE 10881

The respiratory system is comprised of the organs that enable breathing. Air enters the nostrils and mouth, followed by the pharynx (throat) and larynx (voice box), which lead to the trachea (windpipe). In the thoracic cavity, the trachea splits into two bronchi that allow air to enter the lungs. The bronchi split into progressively smaller bronchioles and terminate in small groups of tiny sacs in the lungs called alveoli, where gas exchange occurs. Air is cleansed in the nasal cavity, but anything that passes those defenses or enters through the mouth can be caught in the lungs. The lungs produce mucus that traps foreign particles, and the bronchi and bronchioles are lined with cilia that beat mucus and debris upward toward the throat for disposal (i.e., swallowing). Smoking damages the cilia, making removal of the excess mucus produced by smoking more difficult. This is one of the reasons smokers are more susceptible to respiratory infections. The trachea is a 10-12 cm long tube located in front of the esophagus that allows air to enter and exit the lungs. Its C-shaped hyaline cartilage keeps the trachea open. When the smooth muscle of the trachea contracts, the diameter of the trachea decreases and exhaled air is pushed out with great force (e.g., coughing). In cases of damage to the throat or mouth that blocks breathing, a tracheostomy, a surgica

 Core: Biology

Allergic Reactions

JoVE 10901

We speak of an allergy when the immune system triggers a response against a benign foreign structure, like food, pollen or pet dander. These elicitors are called allergens. If the immune system of a hypersensitive individual was primed against a specific allergen, it will trigger allergic symptoms during every subsequent encounter of the allergen. Symptoms can be mild, such as hay fever, to severe, such as potentially fatal anaphylactic shock. The immune system is crucial for defending an organism against bacteria, viruses, fungi, toxins, and parasites. However, in a hypersensitive response, it can be triggered by harmless substances and cause unpleasant or potentially life-threatening overreactions, called allergies. The first step toward establishing an allergy is sensitization. For instance, an individual becomes allergic to the pollen of ragweed when, for the first time, immune cells in the respiratory passage take up the pollen and degrade the allergens into fragments. These immune cells are called antigen-presenting cells, or APCs, because they display the degraded allergen fragments on their surface. Examples of APCs are dendritic cells, macrophages and B cells. Subsequently, APCs activate encountered Type 2 helper T cells (Th2). The activated Th2 then release chemical signals (e.g., cytokines) that cause B cells to differen

 Core: Biology

What is Monogastric Digestion?

JoVE 10829

The human body contains a monogastric digestive system. In a monogastric digestive system, the stomach only contains one chamber in which it digests food. Several other animal species also have monogastric digestive systems, including pigs, horses, dogs, and birds. This chapter, however, focuses on the human digestive system.

Saliva is a watery substance secreted by the salivary glands into the mouth. Human saliva contains 99.5% water with electrolytes, mucus, white blood cells, epithelial cells, enzymes, and antimicrobial agents. The enzymes found in saliva are essential in beginning the process of digestion. They also play a role in breaking down food particles trapped around the teeth, protecting them from decay. Saliva is obtained easily, inexpensively, and non-invasively from patients which spurs research interest. Ongoing research identified novel ways of using saliva in molecular diagnostics. DNA, RNA, and proteins found in saliva serve as useful sources of diagnostic information in the early detection of various cancers including oral, pancreatic, and gastric cancer. The primary component of gastric acid is hydrochloric acid. Hydrogen and chloride ions released by parietal cells lining the stomach react in the stomach cavity to form hydrochloric acid. Parietal cells are coupled to feedback systems that increase and decrease acid prod

 Core: Biology

What is the Immune System?

JoVE 10895

The immune system comprises diverse biological structures and processes that protect the body from disease. These processes can be classified into innate and adaptive immunity. To work effectively, the immune system needs to detect pathogens by distinguishing the body’s own structures from foreign elements. If this determination fails, autoimmune diseases occur in which the immune system reacts against the body’s own tissue. The innate immune system is the first line of defense against infection. It comprises physical barriers and a variety of cells that act quickly and non-specifically against elements that are foreign to the host (i.e., non-self). Examples of physical barriers in mammals are skin, the lining of the gastrointestinal tract, and secretions, such as mucus or saliva. Once an invader overcomes physical barriers, cells of the inflammatory response are recruited to the entry site: mast cells release a plethora of chemicals that attract other cells of the innate immune system and activates the adaptive immune system. Phagocytic cells, such as neutrophils and macrophages, ingest and destroy pathogens. Natural killer cells, a special type of white blood cell, destroy virus-infected cells. Together, cells of the innate immune system eradicate the invader or hinder its spread, and activate the adaptive immune system. How can an organism

 Core: Biology

Spermatogenesis

JoVE 10905

Spermatogenesis is the process by which haploid sperm cells are produced in the male testes. It starts with stem cells located close to the outer rim of seminiferous tubules. These spermatogonial stem cells divide asymmetrically to give rise to additional stem cells (meaning that these structures “self-renew”), as well as sperm progenitors, called spermatocytes. Importantly, this method of asymmetric mitotic division maintains a population of spermatogonial stem cells in the male reproductive tract, ensuring that sperm will continue to be produced throughout a man’s lifespan. As spermatogenesis proceeds, spermatocytes embark on meiosis, and each ultimately divides to form four sperm—each with only 23 chromosomes— that are expelled into the male reproductive tract. Interestingly, this is in contrast to oogenesis in women, during which only a single egg is generated for every progenitor cell. At the end of spermatogenesis, sperm demonstrate their characteristic shape: a “head” harboring minimal cytoplasm and a highly condensed nucleus, as well as a motile tail (flagellum). They are small cells, with no organelles such as ribosomes, ER or Golgi, but do have many mitochondria around the flagellum for power. Just below the head is the acrosomal vesicle which contains hydrolytic enzymes to penetrate the egg outer coat—th

 Core: Biology

Antibody Structure

JoVE 10898

Antibodies, also known as immunoglobulins (Ig), are essential players of the adaptive immune system. These antigen-binding proteins are produced by B cells and make up 20 percent of the total blood plasma by weight. In mammals, antibodies fall into five different classes, which each elicits a different biological response upon antigen binding.

Antibodies consist of four polypeptide chains: two identical heavy chains of approximately 440 amino acids each, and two identical light chains composed of roughly 220 amino acids each. These chains are arranged in a Y-shaped structure that is held together by a combination of covalent disulfide bonds and noncovalent bonds. Furthermore, most antibodies carry sugar residues. The process of adding sugar side chains to a protein is called glycosylation. Both the light chain and heavy chain contribute to the antigen binding site at each of the tips of the Y structure. These 110-130 amino acids are highly variable to allow recognition of an almost unlimited number of antigens. This region is also called the variable region and is part of the antigen binding fragment. Each arm of the Y-shaped unit carries an identical antigen binding site. Antibodies can crosslink antigens: when one arm binds to one antigen and the other arm binds to a second, structurally identical antigen. Crosslinking is facilitated by the f

 Core: Biology

Olfaction

JoVE 10852

The sense of smell is achieved through the activities of the olfactory system. It starts when an airborne odorant enters the nasal cavity and reaches olfactory epithelium (OE). The OE is protected by a thin layer of mucus, which also serves the purpose of dissolving more complex compounds into simpler chemical odorants. The size of the OE and the density of sensory neurons varies among species; in humans, the OE is only about 9-10 cm2. The olfactory receptors are embedded in the cilia of the olfactory sensory neurons. Each neuron expresses only one type of olfactory receptor. However, each type of olfactory receptor is broadly tuned and can bind to multiple different odorants. For example, if receptor A binds to odorants 1 and 2, receptor B may bind to odorants 2 and 3, while receptor C binds to odorants 1 and 3. Thus, the detection and identification of an odor depend on the combination of olfactory receptors that recognize the odor; this is called combinatorial diversity. Olfactory sensory neurons are bipolar cells with a single long axon that sends olfactory information up to the olfactory bulb (OB). The OB is a part of the brain that is separated from the nasal cavity by the cribriform plate. Because of this convenient proximity between the nose and brain, the development of nasal drug applications is widely studied, especially in cases

 Core: Biology

Accessory Organs

JoVE 10831

Accessory organs are those that participate in the digestion of food but do not come into direct contact with it like the mouth, stomach, or intestine do. Accessory organs secrete enzymes into the digestive tract to facilitate the breakdown of food.

Salivary glands secrete saliva—a complex liquid containing in part water, mucus, and amylase. Amylase is a digestive enzyme that begins breaking down starches and other carbohydrates even before they reach the stomach. The liver, gallbladder, and pancreas are the other accessory organs involved in digestion. All three secrete enzymes into the duodenum of the small intestine via a series of channels called the biliary tree. The liver and gallbladder work together to release bile into the duodenum. The liver produces bile, but it is stored in the gallbladder for secretion when needed. Bile is a mixture of water, bile salts, cholesterol, and bilirubin. Bile salts contain hydrophobic areas and hydrophilic areas which allows it to engage with both fats and water. Thus it breaks down large fat globules into smaller ones—a process called emulsification. Bilirubin is a waste product that accumulates when the liver breaks hemoglobin from red blood cells. The globin is recycled and the heme, which contains iron, is excreted in the bile. The presence of bilirubin is what gives feces its brown color

 Core: Biology
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