Tissues

The hypothalamus is a small but master regulatory region located below the thalamus. It maintains the body's internal balance (homeostasis) by linking the nervous and endocrine systems. It controls autonomic functions like temperature, hunger, thirst, and circadian rhythms. By secreting releasing hormones, it directs the pituitary gland, thus governing growth, metabolism, stress responses, and reproductive functions.

The cerebellum, or "little brain," is located at the back of the skull below the cerebrum. Despite its size, it contains over half the brain's neurons. Its primary function is coordinating and fine-tuning voluntary movement, ensuring precision, balance, and posture. It also contributes to motor learning, cognitive functions like attention and language, and regulates emotional responses. It does not initiate movement but refines it.

The whole brain refers to the entire encephalon, encompassing all major divisions: cerebrum (cerebral hemispheres), cerebellum, and brainstem (midbrain, pons, medulla). It is the central organ of the nervous system, responsible for integrating sensory information, generating thoughts and emotions, coordinating all voluntary and involuntary bodily functions, and enabling consciousness. It is protected by the skull and meninges.

The spinal cord is a long, cylindrical bundle of nerve tissue that extends from the medulla oblongata down the vertebral column. It serves as the primary conduit for motor information descending from the brain and sensory information ascending to the brain. It also contains central pattern generators for reflexes and coordinated locomotion. It is protected by the vertebrae, meninges, and cerebrospinal fluid.

The thymus is a primary lymphoid organ located in the upper anterior mediastinum, behind the sternum. It is most active during childhood and adolescence, where it is the site of T lymphocyte (T cell) maturation and selection. Thymic epithelial cells present self-antigens to developing T cells, eliminating self-reactive ones (negative selection) and promoting the survival of useful ones (positive selection). It undergoes involution with age.

Bone marrow is the soft, spongy tissue found within the cavities of bones, primarily in the pelvis, sternum, vertebrae, and ribs. It is the primary site of hematopoiesis—the production of all blood cellular components, including red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. It contains hematopoietic stem cells and a supportive stromal microenvironment.

Peripheral Blood Mononuclear Cells (PBMCs) are a population of blood cells with a single, round nucleus, isolated from peripheral blood. This mixed cell population primarily includes lymphocytes (T cells, B cells, NK cells) and monocytes. They are a critical component of the immune system and are widely used in research and diagnostics for immunophenotyping, functional assays, and genetic studies.

The spleen is the largest secondary lymphoid organ, located in the left upper abdomen. It filters blood, removing old or damaged red blood cells and platelets, and captures blood-borne pathogens. Its white pulp contains immune cells (lymphocytes, macrophages) that mount immune responses. The red pulp is responsible for RBC filtration and storage. It is a key site for antibody production and immune surveillance.

Lymph nodes are small, bean-shaped secondary lymphoid organs distributed along lymphatic vessels throughout the body. They act as filters for lymph fluid, trapping foreign particles and pathogens. They are highly organized structures containing B-cell follicles and T-cell zones, where antigens are presented to lymphocytes, activating adaptive immune responses. Swollen lymph nodes often indicate an active immune response to infection.

The heart is a muscular organ that functions as the central pump of the circulatory system. It has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers). Through coordinated, rhythmic contractions (the cardiac cycle), it pumps deoxygenated blood to the lungs via the pulmonary circuit and oxygenated blood to the rest of the body via the systemic circuit. Its own blood supply comes from the coronary arteries.

The lungs are paired, spongy, air-filled organs located in the thoracic cavity. Their primary function is gas exchange: oxygen from inhaled air diffuses into the blood in the alveoli, while carbon dioxide diffuses out to be exhaled. The branching airway tree (trachea to bronchi to bronchioles) conducts air. They also play roles in pH balance, speech, and host defense.

The liver is the largest internal organ and a metabolic powerhouse located in the upper right abdomen. It performs over 500 vital functions, including detoxification of metabolites, protein synthesis, production of biochemicals for digestion (bile), glycogen storage, regulation of blood glucose, synthesis of plasma proteins (e.g., albumin), and breakdown of old red blood cells. It has a remarkable capacity for regeneration.

The gallbladder is a small, pear-shaped, muscular sac that stores and concentrates bile produced by the liver. It is located on the inferior surface of the liver. In response to cholecystokinin (CCK) released after a meal, it contracts and releases bile via the cystic and common bile ducts into the duodenum, where bile emulsifies fats to aid in their digestion and absorption.

The stomach is a J-shaped muscular sac in the upper abdomen that serves as a temporary reservoir for food. It mechanically churns and chemically digests food via gastric acid (HCl) and enzymes like pepsin. Its lining secretes mucus to protect itself. It regulates the delivery of partially digested food (chyme) into the small intestine. It also produces intrinsic factor, essential for vitamin B12 absorption.

The pancreas is a dual-function gland located behind the stomach. Its exocrine portion (acinar cells) produces digestive enzymes (lipase, amylase, proteases) secreted into the duodenum. Its endocrine portion (islets of Langerhans) secretes hormones directly into the bloodstream, primarily insulin (from beta cells) and glucagon (from alpha cells), which are crucial for blood glucose regulation.

The esophagus is a muscular tube that connects the pharynx to the stomach, passing through the diaphragm. Its primary function is to transport food and liquids from the mouth to the stomach via coordinated, wave-like muscle contractions called peristalsis. The upper and lower esophageal sphincters regulate entry and prevent reflux. Its lining is stratified squamous epithelium for protection against abrasion.

The duodenum is the first and shortest segment (about 25 cm) of the small intestine, immediately following the stomach. It is a C-shaped structure where most chemical digestion occurs. It receives chyme from the stomach, bile from the liver/gallbladder, and digestive enzymes from the pancreas. Its Brunner's glands secrete alkaline mucus to neutralize gastric acid, protecting the intestinal lining and optimizing enzyme activity.

The colon, or large intestine, is the final part of the digestive tract. Its main functions are to absorb water and electrolytes from indigestible food matter, compact the remaining material into feces, and store it until defecation. It houses a vast population of commensal bacteria (gut microbiota) that ferment certain substrates and produce vitamins (like K and some B vitamins). It consists of ascending, transverse, descending, and sigmoid sections.

Salivary glands are exocrine glands that produce saliva, which is secreted into the oral cavity. The three major pairs are the parotid, submandibular, and sublingual glands. Saliva contains water, electrolytes, mucus, antibacterial compounds, and enzymes like amylase (which begins starch digestion). It lubricates food for swallowing, initiates digestion, maintains oral hygiene, and facilitates taste and speech.

The tongue is a muscular organ in the mouth covered with a mucous membrane. Its intrinsic and extrinsic muscles provide precise movement for speech, swallowing, and food manipulation. Its surface is covered with papillae, many of which contain taste buds for gustation (sweet, salty, sour, bitter, umami). It is also crucial for articulating speech sounds and for oral cleansing.

The ovaries are the female primary gonads, located in the pelvic cavity. They have two key functions: gametogenesis, the production and release of oocytes (eggs) through ovulation; and steroidogenesis, the secretion of the sex hormones estrogen and progesterone. These hormones regulate the menstrual cycle, support pregnancy, and maintain female secondary sexual characteristics. The ovaries contain a finite number of follicles at birth.

The Fallopian tubes (uterine tubes or oviducts) are paired tubes that extend from near the ovaries to the uterus. They are not directly attached to the ovaries. Their function is to transport the oocyte from the ovary to the uterus. Fertilization by sperm typically occurs in the ampulla region. Their ciliated lining and muscular contractions facilitate the movement of the oocyte or embryo.

The uterus (womb) is a hollow, muscular, pear-shaped organ located in the female pelvis. It is the site of menstruation, implantation of a fertilized egg, gestation of the fetus, and labor. Its inner lining, the endometrium, cyclically thickens and sheds in response to ovarian hormones. Its thick myometrial wall contracts powerfully during childbirth. The cervix is its lower part, opening into the vagina.

Mammary glands are modified sweat glands located within the breasts. Their primary function is to produce milk (lactation) to nourish an infant. They consist of a network of ducts and lobules containing alveoli, where milk is synthesized and secreted under the influence of prolactin. Myoepithelial cells contract to eject milk in response to oxytocin. They undergo significant development during pregnancy.

The pituitary gland, or hypophysis, is a pea-sized endocrine gland located at the base of the brain in the sella turcica. Often called the "master gland," it regulates other endocrine organs. The anterior pituitary (adenohypophysis) secretes tropic hormones (e.g., TSH, ACTH, FSH, LH, GH, Prolactin). The posterior pituitary (neurohypophysis) stores and releases oxytocin and vasopressin (ADH) produced by the hypothalamus.

The adrenal glands are small, triangular endocrine glands located atop each kidney. Each gland has two distinct parts: the outer adrenal cortex and the inner adrenal medulla. The cortex produces steroid hormones: mineralocorticoids (aldosterone) for salt/water balance, glucocorticoids (cortisol) for stress and metabolism, and androgens. The medulla secretes catecholamines (epinephrine/adrenaline, norepinephrine) for the "fight-or-flight" response.

The thyroid gland is a butterfly-shaped endocrine gland located in the anterior neck, below the larynx. It produces thyroid hormones (thyroxine/T4 and triiodothyronine/T3), which are critical for regulating the body's metabolic rate, growth, development, and temperature. It also produces calcitonin, which helps regulate blood calcium levels. Iodine is an essential component of thyroid hormones.

The kidneys are paired, bean-shaped organs located retroperitoneally in the upper abdominal area. They are the body's primary filtration system, removing waste products and excess ions from blood to form urine. They critically regulate blood volume, pressure, pH, and electrolyte balance (e.g., sodium, potassium). They also produce hormones: erythropoietin (stimulates RBC production) and renin (regulates blood pressure).

The skin is the body's largest organ, forming a protective barrier against the external environment. It has two main layers: the epidermis (outer, keratinized stratified squamous epithelium) and the underlying dermis (dense connective tissue with blood vessels, nerves, appendages). Functions include thermoregulation, sensation (touch, pain, temperature), protection from pathogens and UV radiation, synthesis of vitamin D, and preventing excessive water loss.