Presión Arterial y su Regulación | Fisiología | FCM UNR

Understanding Blood Pressure and Its Physiological Regulation

Introduction to Blood Circulation

• The class focuses on blood pressure and its physiological regulation, emphasizing that the concepts will be built gradually.

• Blood circulation begins from the left ventricle, where blood is expelled into the aorta, creating pressure that opens aortic valves at approximately 70-80 mmHg, peaking at 120 mmHg before declining.

Key Concepts of Blood Pressure

• Blood pressure is defined as the force exerted by blood per unit area against vessel walls, ranging from 70-80 mmHg to a maximum of about 120 mmHg.

• Cardiac output (volume minute cardiac) represents the total volume of blood ejected by the heart per minute; it is crucial for understanding overall body flow.

Relationship Between Flow and Resistance

• The relationship between flow (cardiac output), arterial pressure in the left ventricle, and right atrium pressure can be expressed through resistance.

• Mean arterial pressure (MAP) is calculated as systolic plus twice diastolic pressures divided by three due to longer diastolic duration.

Peripheral Resistance Factors

• Peripheral resistance opposes blood flow and is primarily regulated at arterioles; increased resistance leads to decreased cardiac output.

• MAP can be expressed as cardiac output multiplied by peripheral resistance, highlighting their direct proportionality.

Determinants of Cardiac Output

• Two main factors influence cardiac output: stroke volume (amount of blood ejected per heartbeat) and heart rate (beats per minute).

• Stroke volume depends on preload (the volume before contraction), contractility (force during contraction), and afterload (resistance against which the heart must pump).

Understanding Preload and Afterload

• Preload relates to muscle fiber tension at end-diastole; greater filling leads to stronger contractions due to elastic properties.

• Afterload correlates with aortic pressure; conditions like valvular diseases increase afterload due to higher forces needed for valve opening.

Impact of Ventricular Conditions on Function

• A dilated ventricular wall increases afterload while affecting stroke volume negatively if residual volumes are too high.

• Post-load influences how much effort is required for valve opening; factors such as valve stiffness or narrowing significantly affect this dynamic.

Role of Venous Return in Cardiac Filling

• The filling volume at end-diastole relies on venous return; adequate blood volume is essential for effective heart function.

Physiological Mechanisms of Blood Volume and Pressure Regulation

Relationship Between Body Water and Blood Volume

• The total body water is directly related to blood volume, while extracellular fluid volume correlates with normal sodium concentration. Dehydrated patients or those with low sodium levels experience decreased venous return and reduced filling volume, which can lead to lowered blood pressure.

Factors Influencing Venous Return

• Venous return is significantly affected by venous tone and the pumping action of veins. The basal tone in vein walls prevents blood stagnation due to their high distensibility.

• Muscle contractions in deep veins of the lower limbs create pressure that aids venous return, aided by one-way valves preventing backflow.

• Thoracoabdominal pumping occurs through pressure changes during breathing; abdominal pressure increases while thoracic pressure decreases, facilitating blood flow towards the heart.

Filling Dynamics During Diastole

• Approximately two-thirds of ventricular filling occurs passively from the inertia of incoming blood from the vena cavae and pulmonary veins, while one-third results from atrial contraction.

• Atrial contractility influences this final third of filling; impaired compliance in ventricular walls (e.g., due to hypertrophy or fibrosis) limits effective filling.

Peripheral Resistance and Its Impact on Blood Pressure

• Peripheral resistance is directly proportional to blood pressure; increased resistance leads to higher pressures or reduced flow. Key factors include vessel length, viscosity, and radius—where increased radius significantly reduces resistance due to its fourth power relationship.

• For example, doubling a vessel's radius decreases resistance by 16 times rather than half. Thus, vessel radius is crucial for regulating peripheral resistance.

Autonomic Nervous System Regulation

• Blood pressure regulation involves physiological variations in diastolic and systolic pressures based on bodily demands while striving for homeostasis.

• The sympathetic nervous system plays a major role by releasing norepinephrine at nerve terminals, stimulating alpha receptors leading to vasoconstriction in arterioles.

• Continuous stimulation via adrenaline release activates both alpha and beta receptors causing vasodilation in areas like coronary arteries during physical activity while constricting vessels supplying skin and digestive organs.

Cardiac Output Adjustments During Exercise

• Initially during exercise, diastolic pressure rises but may decrease over time as cardiac output adjusts through sympathetic stimulation increasing heart rate (beta 1 receptor activation).

• Increased venous return combined with enhanced myocardial contractility raises cardiac output until rapid heart rates limit optimal ventricular filling times.

• At extreme heart rates (e.g., 180 bpm), diastole shortens significantly affecting adequate ventricular filling—a critical factor for maintaining efficient arterial pressure regulation.

Role of Parasympathetic Nervous System

Hormonal Effects on Resistance and Blood Pressure Regulation

Hormonal Influence on Vascular Resistance

• Hormones play a significant role in vascular resistance, with cortisol stimulating vasoconstriction and enhancing adrenergic receptor sensitivity, leading to intensified sympathetic responses.

• Angiotensin II is highlighted as a crucial hormone for arterial vasoconstriction; it also promotes renal sodium and water reabsorption while stimulating aldosterone release from the adrenal cortex, further increasing blood volume.

Impact on Cardiac Function

• The increase in total body water due to hormonal actions results in enhanced venous return, preload, cardiac output, and ultimately affects blood pressure regulation.

Baroreceptor Mechanisms