Capillary Exchange, Hemodynamics, and BP Control

Capillary Exchange

Movement of substances between blood and interstitial fluid occurs via three basic mechanisms:

Diffusion

Simple diffusion is the most important method of capillary exchange. Substances like O2, CO2, glucose, amino acids, and hormones enter and leave capillaries by diffusing down their concentration gradients. For example, pO2 is higher in blood than tissues, so O2 diffuses out; CO2 does the opposite.

Transcytosis

Substances within blood plasma become enclosed by pinocyte vesicles and move across endothelial cells. This mechanism handles large, lipid-insoluble molecules that cannot cross capillary walls by other means.

Bulk Flow

A passive process in which large numbers of ions, molecules, or particles in a fluid move together from an area of higher pressure to lower pressure.

Starling's Law of the Capillaries

The volume of fluid and solutes reabsorbed is normally almost as large as the volume filtered — this near equilibrium is Starling's Law. It governs bulk flow and depends on four pressures:

Net Filtration Pressure (NFP)

NFP = (BHP + IFOP) - (BCOP + IFHP)

• Pressures promoting filtration (out): BHP + IFOP
• Pressures promoting reabsorption (in): BCOP + IFHP
• Positive NFP = net filtration; negative NFP = net reabsorption

Hemodynamics: Factors Affecting Blood Flow

Blood flow is the volume of blood that flows through any tissue in a given period (mL/min). Total blood flow is cardiac output (CO).

Blood flows from regions of higher pressure to lower pressure — the greater the pressure difference, the greater the flow.

Blood Pressure (BP)

Determined by:

• Cardiac output
• Blood volume
• Vascular resistance

BP is highest in the aorta and large systemic arteries (~110/70 mmHg). It decreases as it approaches arterioles and reaches ~0 mmHg at the right ventricle.

Mean Arterial Pressure (MAP)

MAP = Diastolic BP + 1/3 (Systolic BP - Diastolic BP)

Blood Volume

Normal adult blood volume is ~5 liters. Compensation for blood loss can occur, but loss greater than 10% will drop blood pressure.

Vascular Resistance

The opposition to blood flow due to friction between blood and vessel walls.

Systemic Vascular Resistance (SVR)

Total peripheral resistance caused by vessels throughout the cardiovascular system. Diameters of the vasculature (primarily arterioles) regulate total system pressure, controlled by the vasomotor center.

Venous Return

Volume of blood returning to the heart through systemic veins. Pressure difference between venules (~16 mmHg) and the right ventricle (~0 mmHg) drives return.

Skeletal Muscle Pump: Contraction of leg muscles compresses veins, pushing blood through proximal valves ("milking"). When muscle relaxes, pressure falls, and blood is drawn from distal areas.

Respiratory Pump: During inhalation, the diaphragm descends, decreasing thoracic pressure and increasing abdominal pressure. Abdominal veins compress; thoracic veins decompress, forcing blood toward the heart.

Control of Blood Pressure and Blood Flow

Several interconnected negative feedback systems control BP by adjusting heart rate, stroke volume, systemic vascular resistance, and blood volume.

Cardiovascular Center (CV)

Located in the medulla oblongata, the CV center regulates heart rate, stroke volume, and coordinates neural, hormonal, and local negative feedback systems.

Sensory Receptors

Neural Regulation

Sympathetic stimulation (via cardiac accelerator nerves):

• Increases heart rate, force of contraction, and CO
• Vasoconstriction of most veins increases blood pressure

Parasympathetic stimulation (via vagus nerves):

• Decreases heart rate
• Opposes sympathetic influence

Vasomotor Tone: Smooth muscle around vessels is continuously stimulated by sympathetic neurons, maintaining a general state of vasoconstriction.

Baroreceptor Reflexes

If BP falls: Baroreceptors are stretched less → slower nerve impulses to CV center → CV decreases parasympathetic response and increases sympathetic stimulation → heart beats faster and more forcefully, SVR increases → BP rises.

If BP rises: Baroreceptors send impulses faster → CV increases parasympathetic and decreases sympathetic stimulation → HR and contractility decrease, vasodilation occurs → BP falls.

Chemoreceptor Regulation

Chemoreceptors detect changes in pO2 (hypoxia), pCO2 (hypercapnia), and H+ (acidosis). These stimuli cause direct sympathetic stimulation to arterioles and veins, increasing cardiac output.

Hormonal Regulation

Autoregulation

The ability of a tissue to automatically adjust its blood flow to match metabolic demands.

Physical changes: Warming causes vasodilation; cooling causes vasoconstriction. The myogenic response — vessels contract more forcefully when stretched and relax when stretching less.

Chemical regulation: Vasodilating substances (histamines, nitric oxide) and vasoconstricting substances (serotonin) regulate local blood flow.

Shock and Homeostasis

Shock is a failure of the cardiovascular system to deliver enough O2 and nutrients to meet cellular metabolic needs. All forms are characterized by inadequate blood flow to body tissues.

Types of Shock

Homeostatic Response to Shock

The body responds to low BP from shock via the same mechanisms of control:

• Increased sympathetic response
• Increased hormonal response (causing vasoconstriction)
• Increased local vasodilators