Blood Pressure Regulation and Vascular Physiology

Core Parameters of Blood Pressure Control

Blood pressure is regulated through three primary parameters that the brain and body use minute-to-minute to ensure tissues receive adequate blood flow. Without correct blood pressure, tissues begin to die, leading to systemic failure. The parameters include:

• Cardiac Output (The Pump): The volume of blood the heart pumps per minute.

• Peripheral Resistance (The Pipes): The opposition to blood flow provided by the vessel walls.

• Blood Volume (The Fluid): The total amount of fluid within the circulatory system.

Blood Viscosity

A fourth parameter is blood viscosity. Under normal circumstances, viscosity does not change rapidly; it is difficult to effectively turn blood into "molasses." However, if viscosity increases (e.g., via blood doping or extreme dehydration), resistance increases, forcing the heart to work harder. Due to its stability in healthy individuals, it is often excluded from the minute-to-minute control discussion.

Cardiac Output (CO) and the Heart as a Pump

The effectiveness of the heart is measured by cardiac output. The formula for cardiac output is:

$CO = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}$

Heart Rate Regulation

• Sympathetic Nervous System (SNS): The SNS uses neurotransmitters like epinephrine and norepinephrine. These bind to Beta receptors in the heart to increase heart rate (chronotropic effect). An increased HR increases CO, which in turn increases blood pressure.

• Parasympathetic Nervous System (Vagal System): The Vagus Nerve (Cranial Nerve X) is the primary parasympathetic nerve. Stimulating the vagus system decreases the heart rate, decreasing CO and lowering blood pressure.

Stroke Volume (SV) Regulation

• Preload: This is also known as End Diastolic Volume (EDV) or venous return. It represents the amount of blood returning to the heart. Increased preload increases stroke volume, CO, and blood pressure.

• Contractility (Inotropes): This refers to how hard the ventricle contracts. If the ventricle contracts harder, stroke volume increases, leading to higher CO and blood pressure.

• Afterload: This is the resistance the ventricle must overcome to eject blood. Increasing afterload typically decreases stroke volume initially.

Peripheral Resistance and Vessel Characteristics

Peripheral resistance refers to the friction that hinders fluid movement through the "pipes" (vessels).

Vessel Diameter

The primary minute-to-minute controller of resistance is the diameter of the arterioles.

• Vasoconstriction: Decreasing diameter increases peripheral resistance and blood pressure. The SNS triggers this via Alpha receptors in the arterioles.

• Vasodilation: Increasing diameter decreases resistance and blood pressure.

Vessel Length

The length of the pipes also affects resistance. While length does not change minute-to-minute, it is determined by body size.

• Weight Gain and Hypertension: Adding body mass requires growing new blood vessels (miles of extra vessels for every 5–10 pounds). The heart must pump harder to overcome the resistance of these extra hoses, leading to hypertension. This is why weight loss is often the first clinical recommendation for high blood pressure.

Clinical Standards and Blood Pressure Dynamics

Normal Values

• Standard Reading: While textbooks often cite $120/80\text{ mmHg}$, a more accurate physiological "normal" for most healthy adults is approximately $120/70\text{ mmHg}$.

• Hypotension: A blood pressure lower than normal. It is clinically significant only if tissues are not being perfused.

• Hypertension Definition: Consistently running a pressure of $140/90\text{ mmHg}$ or higher.

Age and Blood Pressure

There is a common misconception that high blood pressure is "normal" for elderly patients (e.g., "Granny at 102 having 150/95"). While common due to vessel stiffness (arteriosclerosis) or weight gain, it is not technically normal. A healthy 102-year-old should ideally have the same blood pressure as they did at 35.

Components of the Reading

• Systolic Blood Pressure: The top number. It reflects the force the left ventricle generates when actively shoving blood into the arteries. It is a direct reflection of ventricular work.

• Diastolic Blood Pressure: The bottom number. It represents the pressure in the vessels when the ventricle is relaxed, maintained by the elastic recoil of the large arteries (e.g., aorta). Clinically, elevated diastolic pressure is often more concerning because it indicates a high afterload that will eventually wear out the left ventricle.

Vascular Dynamics and Vessel Types

Distribution of Blood

Blood is not distributed equally. The venous system acts as capacitance vessels, holding a huge volume of blood moving slowly.

• Venous Reserve: If there is an emergency (e.g., running from an axe murderer), veins constrict to increase preload and CO.

• Thermoregulation: Most blood is in the venous system. The body sends blood to superficial veins near the skin's surface to dissipate heat.

Flow Equations

Physiologists view blood movement through the lens of flow:

$\text{Flow} = \frac{\text{Change in Pressure}}{\text{Resistance}}$

Pressure is determined by CO and blood volume, while resistance is determined by vessel length, diameter, and blood viscosity.

Capillary Dynamics and Fluid Exchange

Capillaries are exchange vessels composed of simple squamous epithelium. They are thin enough for red blood cells (approx. $7\text{ μm}$) to pass through single file. All capillaries leak fluid.

Starling Forces and Net Filtration Pressure

Fluid movement is governed by the balance of hydrostatic and osmotic pressures:

1. Blood Hydrostatic Pressure: Pushes fluid out of the capillary into the tissue (filtration).

2. Interstitial Fluid Hydrostatic Pressure: The pressure of fluid already in the tissue pushing back against the capillary.

3. Blood Oncotic (Osmotic) Pressure: Large proteins like Albumin pull fluid back into the vessel. The "quick and dirty" rule of osmosis is: Water follows the particles.

4. Interstitial Fluid Oncotic Pressure: Proteins in the tissue space pulling fluid out of the vessel.

The "Cookie" Analogy

On the arterial end, more fluid is pushed out than is pulled back. On the venous end, more is pulled back, but a deficit remains.

• If 10 "cookies" (units of fluid) are pushed out and only 9 are reclaimed, 1 remains in the tissue space.

• This occurs in every capillary, 24/7. Without a return system, the vascular system would empty and the tissues would swell.

The Lymphatic System and Edema

Lymphatic Function

The lymphatic system picks up the leftover fluid (lymph) from the tissue spaces and returns it to the cardiovascular system, maintaining blood volume and pressure.

Edema (Swelling)

Edema is an excess of fluid in the tissue spaces. Causes include:

• Liver Disease/Starvation: Decreased albumin production reduces the "pull" needed to reabsorb fluid.

• High Blood Pressure: Increased hydrostatic pressure shoves more fluid out than can be reabsorbed.

• Lymphatic Blockage: Damage to vessels or removal of lymph nodes prevents fluid return.

Endocrine Regulation of Blood Pressure

Hormones interact with volume and diameter to control pressure:

• Antidiuretic Hormone (ADH): Increases blood volume by preventing fluid loss in urine, thereby increasing BP.

• Atrial Natriuretic Peptide (ANP): Released by the right atrium when volume is too high. It causes the kidneys to dump sodium. Since water follows particles (sodium), water is lost, decreasing blood volume and BP.

• Aldosterone: Released by the adrenal gland. It is "sodium-sparing," causing the kidneys to hang onto sodium, which increases water retention and BP.

• Renin-Angiotensin System: The kidneys secrete Renin if BP is too low. This leads to the production of Angiotensin II, a potent vasoconstrictor (Angio = vessel, Tensin = constriction), which raises BP.

Clinical Assessment and Pathology

Measuring BP

Blood pressure is measured by compressing the brachial artery and listening for Korotkoff sounds with a stethoscope.

• First sound: Systolic pressure.

• Silence/Change in pitch: Diastolic pressure.

Mean Arterial Pressure (MAP)

MAP is the average pressure necessary to perfuse organs.

$MAP = \text{Diastolic BP} + \frac{1}{3}(\text{Pulse Pressure})$

Note: Pulse Pressure = Systolic - Diastolic.

Capillary Refill

A "quick and dirty" check for perfusion. Pressing a fingernail until it turns white; it should "pink up" immediately upon release. If refill is slow, the brain and kidneys may not be getting enough blood.

Shock and Vasodilation

• Anaphylactic Shock: Histamine release causes massive vasodilation, dropping BP.

• Septic Shock: Gram-negative bacterial infections cause vasodilation and BP drops.

• Treatment: Epinephrine is administered to stimulate alpha receptors, causing vasoconstriction and raising BP.

Questions & Discussion

Student Question: Is that why when people have lymph nodes removed, they have super swollen areas?

Professor Response: Yes, that is one reason. When lymphatic vessels are damaged or nodes are removed, the system cannot pick up the fluid that naturally leaks from the capillaries, leading to localized edema.