🔶 A myocardial infarction (heart attack) occurs when there is a blockage in the coronary arteries, which supply oxygen-rich blood to the heart muscle (myocardium).

🔶 Without adequate blood flow, the heart tissue becomes ischemic and damaged, leading to infarction.

🔶 Coronary arteries branch off the aorta and are critical for nourishing the heart itself.

🔶 The pulmonary artery, aorta, and vena cava are major vessels involved in systemic and pulmonary circulation but do not directly nourish the heart muscle.

🔹 The pituitary gland, located at the base of the brain (label A in the image), is a major endocrine organ that regulates milk production through the secretion of prolactin.

🔹 Prolactin stimulates the mammary glands to produce milk after childbirth.

🔹 The thyroid gland (label B) regulates metabolism, the adrenal glands (label C) manage stress and metabolism, and the ovaries (label D) produce sex hormones and eggs.

🔹 Thus, only the pituitary gland is directly responsible for initiating lactation.

🔷 Nitrogen gas (N₂) is composed of two nitrogen atoms joined by a triple covalent bond—one of the strongest types of chemical bonds.

🔷 This bond includes one sigma bond and two pi bonds, which require a large amount of energy to break.

🔷 The strength and stability of the triple bond make nitrogen gas chemically inert under normal conditions.

🔷 This is why atmospheric nitrogen must be fixed by certain bacteria or industrial processes before it can be used by most organisms.

✔️ The corpus luteum is a temporary endocrine structure formed in the ovary after ovulation from the remains of the ruptured follicle.

✔️ It secretes progesterone, and to a lesser extent estrogen, which help maintain the uterine lining (endometrium) to support a potential pregnancy.

✔️ If fertilization does not occur, the corpus luteum degenerates; if pregnancy occurs, it continues functioning until the placenta takes over hormone production.

✔️ It is not the oocyte, which is the egg cell, nor the oviduct or umbilical cord, which serve different roles.

🔷 Erythropoietin (EPO) is a hormone primarily produced by the kidneys in response to low oxygen levels in the blood.

🔷 It stimulates the bone marrow to produce more red blood cells (erythrocytes), which increases the hemoglobin content of the blood.

🔷 Hemoglobin is the oxygen-carrying protein in red blood cells, so higher levels improve the oxygen-carrying capacity of the blood.

🔷 This is a key mechanism in oxygen homeostasis, especially at high altitudes or during anemia.

✔️ Water's ability to act as a universal solvent is due to its polar nature.

✔️ Each water molecule has a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

✔️ This polarity allows water to surround and interact with other polar molecules and ions, effectively dissolving them by forming hydration shells.

✔️ Nonpolar substances do not dissolve well in water, highlighting the role of molecular polarity in solubility.

🔶 The pleura is a double-layered connective tissue membrane that surrounds and protects the lungs.

🔶 It consists of two layers: the visceral pleura, which directly covers the lung surface, and the parietal pleura, which lines the thoracic cavity.

🔶 The pleural cavity between these layers contains pleural fluid, which reduces friction during breathing.

🔶 This structure is essential for lung expansion and contraction within the chest cavity.

🔹 Calcium ions (Ca²⁺) are essential for initiating muscle contraction.

🔹 When a muscle cell is stimulated, calcium is released from the sarcoplasmic reticulum into the cytoplasm.

🔹 Calcium then binds to the troponin complex on actin filaments, causing a conformational change that shifts tropomyosin and exposes binding sites for myosin.

🔹 This allows the cross-bridge cycle to begin, resulting in muscle contraction through the sliding filament mechanism.