Doctor-Reviewed Health Guides: Conditions, Procedures & Health News

Polyclonal antibodies are a collection of antibodies made by different B-cell lineages. Each antibody can recognize a different epitope on the same antigen, giving the immune system multiple ways to bind and identify that antigen. They help the immune system respond to infections and help detect antigens in tests and research.

Pulmonary veins are blood vessels that return oxygen-rich blood from the lungs to the left atrium of the heart. They travel toward the heart like other veins, but they are unique because they carry oxygen-rich rather than oxygen-poor blood. Most people (60%–70%) have four; others have three or five.

The placenta is a temporary pregnancy organ attached to the uterus. Through the umbilical cord, it supports the fetus with oxygen and nutrients, removes waste, produces pregnancy-related hormones and transfers antibodies. It forms during pregnancy, attaches to the uterine wall and connects to the fetus through the umbilical cord.

The pancreas has two main jobs: it helps digest food and helps regulate blood sugar. It sends digestive juice through ducts into the duodenum and makes hormones such as insulin and glucagon. Insulin helps lower blood sugar when it is too high, while glucagon helps raise it when it is too low.

Peritubular capillaries are microscopic blood vessels around the kidney tubules inside the nephrons. They help the urinary system remove wastes and extra water from the blood, reabsorb nutrients the body needs and supply blood and oxygen to the cells of the kidney's tubular system. Damage to them can contribute to chronic kidney disease and high blood pressure.

The pituitary gland is a small endocrine gland below the hypothalamus that coordinates hormone signals involved in growth, metabolism, reproduction, stress response, lactation and water balance. It communicates with the hypothalamus through the pituitary stalk and with other glands through hormones released into the bloodstream.

The parotid gland is one of the major salivary glands, with one on each side of the face just below and in front of the ears. It makes saliva that supports chewing, digestion and protection from cavities. Swelling can come from duct stones, infection or tumors. Get advice for persistent swelling, a new lump, facial pain or fever.

The ovaries have two main jobs: they produce and store eggs, and they make hormones. Each menstrual cycle, one ovary usually releases an egg during ovulation. The ovaries also make estrogen and progesterone, hormones that help control the menstrual cycle and support pregnancy.

The penis is an external sex organ that belongs to the male urinary and reproductive systems. It allows urine to leave the body, transports semen, contributes to sexual pleasure and can contribute to reproduction. Its tissues include blood vessels and spongy chambers that can fill with blood during an erection.

The peritoneum is a sheet of smooth tissue that lines the abdomen and pelvis and covers many abdominal organs, with a parietal layer, a visceral layer and a cavity between them. It provides padding, lubrication, structure and immune protection. Conditions affecting it include peritonitis, ascites, hernia, adhesions and cancer, most commonly causing abdominal pain and swelling.

The phrenic nerve is the main motor nerve for the diaphragm and an essential part of the respiratory system. It carries signals that change the diaphragm's shape, allowing the lungs to expand for inhalation and return toward their starting position for exhalation.

The pelvis is a bony basin at the base of the spine. It sits inside the hips, buttocks and pubic region, supports the trunk and connects the upper body to the legs. Its cavity holds urinary and reproductive organs, and its central opening serves as the birth canal during vaginal delivery.

The parathyroid glands are small glands, usually four behind the thyroid, that make parathyroid hormone (PTH) to tightly control blood calcium, which nerves, muscles, clotting and the heart depend on. Too much PTH raises blood calcium, while too little lowers it. Persistent symptoms of high or low calcium, especially after neck surgery, warrant blood tests.

Photoreceptors are light-sensitive neurons in the retina. Rods respond especially well to very low light, while cones require more light and support color and detailed central vision. Together, they begin the process that turns light entering the eye into coded nerve signals the brain uses for sight.

Osteoblasts build new bone. Osteoclasts break down old or damaged bone. Together, they support bone formation and remodeling. Osteoblasts lay down bone matrix made from proteins such as collagen mixed with calcium, phosphate and other minerals. Osteoclasts release enzymes that break down hardened bone matrix in specific areas, creating space for osteoblasts to add newer tissue.

The pectineus is a small, flat muscle at the front of the thigh near the pelvis. It works with other hip flexors to move and rotate the hip. Its two main actions are hip adduction, moving the hip inward toward the center of the body, and hip flexion, lifting the knee toward the chest.

The perineum is the patch of skin between the genitals and anus, along with deeper tissues at the bottom of the pelvic cavity. It contains nerves, muscles and connective tissue, supports the pelvic floor and contributes to sexual sensation, urination and bowel movements. Pain is the most common sign of a problem, along with itching, a lump or swelling.

The parasympathetic nervous system is a network of nerves that relaxes or reduces body activity during calm periods, often called rest and digest. It controls heart rate, digestion, pupils, saliva and bladder function, and the vagus nerve makes up about 75% of it. Conditions such as uncontrolled type 2 diabetes can disrupt it, causing constipation or a fast heart rate.

An accessory navicular bone is an extra bone that forms near the navicular bone in the arch of the foot when extra cartilage hardens into bone during growth. Most cause no pain and need no treatment. It can be seen on an X-ray, and painful cases may be treated with supportive shoes, orthotics, immobilization, physical therapy or surgery.

The abducens nerve, also known as the sixth cranial nerve or CN VI, sends motor signals to the lateral rectus muscle, which moves the eye outward. Damage may cause difficulty moving the eyes left or right, double vision and eye misalignment. See a healthcare provider right away when abducens nerve palsy is suspected.

The chest, or thorax, is the body region between the neck and abdomen, extending down to the diaphragm. It consists of the sternum, 12 thoracic vertebrae, 12 pairs of ribs, cartilage and muscles, and it protects the heart and lungs while acting as a respiratory pump. Conditions can affect the chest wall, such as fractures, or the organs inside.

The deltoid muscles are the large, triangular muscles covering the top of the shoulder joint, divided into anterior, lateral and posterior heads that each move the arm in a different direction. They power movements like reaching overhead or pitching a ball and stabilize the shoulder to help prevent dislocations. Healthy deltoids are vital for upper-body mobility and strength.

Vestigial structures, also known as vestigial organs, are anatomical features that no longer serve the primary function for which they originally evolved in human ancestors. They persist as remnants of evolutionary history, often with minor or secondary roles. Examples such as the appendix and tailbone reflect how humans adapted to changes in diet and movement.

Vocal cords are two flexible bands of muscle and tissue inside the larynx, or voice box, at the top of the trachea. They produce the voice by vibrating, regulate airflow during breathing and protect the lungs by closing during swallowing. Their layered structure changes in size and thickness throughout life, shaping vocal pitch and quality.