Blood Pressure, Lymphatic System, and Respiratory Physiology: ANP Study Guide

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Blood Pressure & Flow

Venous Pressure: Why Is It Low?

Venous pressure is significantly lower than arterial pressure due to structural and functional differences in the circulatory system.

Arteries: Thick, elastic walls; directly receive force from heart contraction.
Veins: Thinner walls; blood has lost most pressure after passing through arterioles and capillaries.
By the time blood reaches veins, pressure is very low and must overcome gravity, especially from the legs.
Skeletal muscle pump: Muscle contractions squeeze veins, pushing blood toward the heart.
Respiratory pump: Inhalation decreases thoracic cavity pressure, pulling blood toward the heart.
Valves: Prevent backflow of blood in veins.
Immobility can cause blood pooling and increase risk of deep vein thrombosis (DVT).

Resistance: Why Vessel Diameter Matters Most

Vascular resistance is primarily determined by vessel diameter, especially in arterioles.

Resistance is inversely related to the fourth power of radius:
If vessel diameter decreases slightly, resistance increases dramatically.
Example: Cutting radius in half increases resistance 16-fold.
Arterioles: Known as "resistance vessels" because they regulate blood pressure by constricting or dilating.

Formula:

Where = resistance, = viscosity, = length, = radius.

Blood Flow Velocity: Why It Is Slow in Capillaries

Blood flow slows in capillaries due to their large total surface area, not because of low pressure.

Capillaries have a huge combined surface area.
Increased surface area leads to decreased velocity.
This allows time for oxygen and nutrient exchange with tissues.
If blood moved too quickly, tissues would not receive adequate oxygen.

Pulse Pressure: What It Indicates

Pulse pressure is the difference between systolic and diastolic blood pressure and reflects the force of heart contraction.

Pulse Pressure Formula:

Wide pulse pressure may indicate stiff arteries, aging vessels, or high stroke volume.

Hypovolemic Shock: Step-by-Step Changes

Hypovolemic shock occurs when blood volume drops, leading to decreased tissue perfusion and organ failure.

Blood volume decreases
Venous return decreases
Stroke volume decreases
Cardiac output decreases
Blood pressure drops
Organs receive less oxygen
Organ failure may occur

Key term: Tissue perfusion decreases.

First change: Decreased blood volume leads to decreased venous return.

Exercise: Cardiovascular Changes

During exercise, the cardiovascular system adapts to meet increased oxygen demands of muscles.

Heart rate increases
Stroke volume increases
Cardiac output increases
Systolic pressure increases
Vessels in muscles dilate
Vessels in digestive organs constrict
Diastolic pressure may remain unchanged

Example: Athletes often have higher systolic blood pressure during activity.

Lymphatic System (Deeper Physiology)

Purpose of the Lymphatic System

The lymphatic system maintains fluid balance by returning leaked fluid from capillaries to the bloodstream.

Capillaries leak fluid constantly (~3 liters/day).
Lymphatic vessels collect excess interstitial fluid and return it to the blood.
Prevents swelling (edema).
Blocked lymphatic drainage leads to lymphedema.

Spleen vs Thymus: Key Differences

The spleen and thymus are distinct lymphatic organs with different functions.

Organ	Main Function	Additional Features
Spleen	Filters blood, removes old RBCs, stores platelets	Immune surveillance
Thymus	Site of T-cell maturation	Large in childhood, shrinks after puberty

If thymus is dysfunctional, T-cell immunity is impaired.

T-Cell Activation

T-cells require specific steps for activation before mounting an immune response.

Antigen presentation
Binding to specific T-cell receptor
Co-stimulation

Activated T-cells multiply, attack infected cells, and form memory cells.

Vaccines: Mechanism of Action

Vaccines stimulate the immune system to develop immunity without causing disease.

Introduce antigen (weakened or inactive)
Immune system recognizes antigen
B-cells produce antibodies
Memory cells form

Subsequent exposure leads to faster, stronger immune response.

Inflammation: Purpose and Signs

Inflammation is a protective response to injury or infection, facilitating healing.

Increases blood flow, vessel permeability, and immune cell migration.
Signs: Redness (vasodilation), heat (increased blood), swelling (fluid leakage), pain (nerve irritation).
Initially protective, but can become harmful if prolonged.

Respiratory System (Deeper Physiology)

Intrapleural Pressure: Why It Is Negative

Negative intrapleural pressure is essential for keeping the lungs inflated.

Lungs tend to collapse due to elastic recoil.
Chest wall tends to expand outward.
Opposing forces create negative pressure in the pleural cavity.
If air enters pleural cavity (pneumothorax), pressure equalizes and lung collapses.

External vs Internal Respiration

Respiration involves gas exchange at two levels: lungs and tissues.

Type	Location	Oxygen Movement	CO2 Movement
External Respiration	Alveoli → Blood	O2 diffuses into blood	CO2 diffuses into alveoli
Internal Respiration	Blood → Tissues	O2 leaves blood	CO2 enters blood

Professors may reverse wording to test understanding.

Residual Volume: Importance

Residual volume is the air remaining in lungs after maximal exhalation, preventing lung collapse.

~1200 mL remains after forced exhale.
Prevents alveoli from sticking together and keeps lungs open.

Respiratory Control Center

The respiratory control center in the brain regulates breathing rate based on CO2 levels.

Located in medulla and pons.
Monitors CO2 levels primarily; increased CO2 stimulates increased breathing rate.
Oxygen plays a smaller role in regulation.

Additional info: The respiratory control center also responds to pH changes in blood, as CO2 is converted to carbonic acid.