Which layer of the heart contains the cardiac muscle responsible for contraction?

A. Left atrium:
The left atrium receives oxygenated blood from the pulmonary veins but does not generate enough force to propel blood into systemic circulation. Its main function is atrial filling of the left ventricle.

B. Left ventricle:
The left ventricle is the main pumping chamber for systemic circulation, sending oxygenated blood into the aorta. Its thick muscular wall generates high pressure necessary to perfuse the entire body, making it essential for maintaining blood pressure and organ perfusion.

C. Right ventricle:
The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery and does not contribute to systemic circulation.

D. Right atrium:
The right atrium receives systemic venous blood and delivers it to the right ventricle; it does not pump blood into the systemic circulation.

A. Baroreceptor reflex:
Baroreceptors in the carotid sinus and aortic arch detect sudden changes in blood pressure due to posture changes. They initiate autonomic responses, increasing heart rate and vasoconstriction to maintain cerebral perfusion, preventing dizziness or syncope.

B. Chemoreceptor reflex:
Chemoreceptors respond to blood oxygen, carbon dioxide, and pH, regulating ventilation more than immediate blood pressure changes.

C. Release of antidiuretic hormone:
ADH acts more slowly to conserve water and increase blood volume; it is not responsible for immediate posture-related blood pressure changes.

D. Renin-angiotensin-aldosterone system:
RAAS regulates blood pressure over minutes to hours via vasoconstriction and fluid retention; it does not act immediately during rapid postural changes.