ANATOMY

Detailed explanation-1: -Left ventricular stroke volume also falls because of reduced pulmonary venous return (decreased left ventricular preload). This causes cardiac output (CO) and mean arterial pressure (MAP) to fall. If arterial pressure falls appreciably upon standing, this is termed orthostatic or postural hypotension.

Detailed explanation-2: -Balance is achieved by the Frank-Starling mechanism. For example, if systemic venous return is suddenly increased (e.g., changing from upright to supine position), right ventricular preload increases, leading to an increase in stroke volume and pulmonary blood flow.

Detailed explanation-3: -Limiting venous return leads to reduced cardiac output and mean arterial blood pressure. An inactive person standing still is subject to the full hydrostatic pressure gradient in the venous system, and pressure in the foot veins will be  90 mmHg.

Detailed explanation-4: -An increase in venous return to the left ventricle via the left atrium will result in greater end-diastolic stretch of the ventricle walls and an increase in stroke volume at the next beat; conversely, stroke volume will be reduced if cardiac return falls.

Detailed explanation-5: -The heart creates circulatory flow by lowering the right atrial pressure and allowing the recoil pressure in veins and venules to drain blood back to the heart. The heart then puts the volume back into the systemic circulation so that stroke return equals stroke volume.