Bone Growth and Epiphyseal Plate

Bone Growth

• The proliferating zone of the epiphyseal plate is where chondrocytes produce new cartilage cells
• The degenerating zone of the epiphyseal plate is where cartilage cells die off
• The ossification zone of the epiphyseal plate is where bone forms, which increases the length of the bone
• When the epiphyseal plates close, bone lengthening can no longer occur
• Important hormones in bone growth include vitamin D, growth hormone, thyroid hormone, and sex hormones
• Vitamin D is formed in the skin after exposure to sunlight and is converted to a hormone necessary for calcium absorption from food
  • Low vitamin D levels in children leads to rickets with bone deformities
• Growth hormone (GH) stimulates bone growth and requires concurrent action of thyroid hormones for metabolism
  • Dwarfism is caused by insufficient GH in childhood
  • Gigantism is caused by excessive GH in childhood
  • Acromegaly is caused by excessive GH in adults which leads to excessive growth in the hands and face
• Sex hormones fuel a growth spurt during adolescence

Bone Remodeling and Calcium Homeostasis

• Bone remodeling is a continuous process of bone breakdown by osteoclasts and bone buildup by osteoblasts
  • 18% of bone is recycled each year
• Paget disease causes new bone to be generated at a faster than normal rate which produces softer and weaker bones
  • Can cause bone pain, deformities, and fractures
• If blood calcium levels rise, excess calcium is deposited in bones

Heartbeat and Electrocardiogram

• The SA node in the right atrium initiates the heartbeat, sending electrical signals causing the atria to contract
  • The SA node is considered the pacemaker of the heart
• The impulse travels to the AV node, also in the right atrium, which sends a signal down the AV bundle and Purkinje fibers which trigger ventricular contraction
• The cardiac control center of the brain increases or decreases heart rate based on the body's needs in external control
• Some hormones also increase heart rate
• An electrocardiogram (ECG) measures electrical changes in the heart muscle during the cardiac cycle
  • The atria create a P wave when stimulated by the SA node
  • The QRS complex, reflecting a wave of electrical current passing through the ventricles, signals the ventricles' impending contraction
  • The T wave represents the recovery of the ventricles
• Ventricular fibrillation is caused by uncoordinated, irregular electrical signals in the ventricles
  • This results in the heart's inability to pump blood, leading to oxygen deprivation in tissues
• Defibrillation involves applying a strong electrical signal to reset the heart
  • This allows the SA node to resume firing hopefully

Blood Flow

• The pulmonary circuit carries deoxygenated blood from the right atrium, through the right ventricle, into the pulmonary trunk
  • The trunk divides into right and left pulmonary arteries, supplying the lungs
  • Pulmonary arteries branch into arterioles, leading to capillaries, where gas exchange takes place
• Pulmonary capillaries connect to venules, merging into pulmonary veins
• Four pulmonary veins empty into the left atrium
  • Pulmonary arteries carry oxygen-poor blood, while pulmonary veins carry oxygen-rich blood
• The systemic circuit carries oxygenated blood from the left ventricle to the aorta
  • The aorta branches, supplying tissues throughout the body
  • Branches further into arterioles, which connect to capillaries, where exchanges with interstitial fluid occur
  • Capillaries lead to venules, merging into veins, which then connect to the superior and inferior vena cavae
  • The vena cavae empty into the right atrium
• The hepatic portal system carries nutrient-rich blood from the digestive tract to the liver via the hepatic portal vein
  • The liver synthesizes blood proteins from amino acids in the portal vein and stores glucose as glycogen
  • It also removes toxins and pathogens from the blood that entered through the digestive system
  • Blood is then drained from the liver into the hepatic veins, which flow into the inferior vena cava

Cardiovascular System

• The cardiovascular system consists of the heart and blood vessels
  • The heart pumps blood through blood vessels
• Essential functions of the cardiovascular system include:
  • Transport: delivers oxygen, carbon dioxide, waste products, nutrients, and hormones
  • Protection: transports immune cells for infection defense
  • Regulation: maintains homeostasis of pH balance, electrolyte levels, and temperature
• The lymphatic system collects excess tissue fluid and returns it to the blood
  • Fluid entering the lymphatic vessels is called lymph