Blood Vessels — Comprehensive Study Notes Summary & Study Notes

🩸 Capillary Exchange

Capillary exchange is how substances move between blood and interstitial fluid. The three main mechanisms are diffusion, transcytosis, and bulk flow. Short distances and thin walls favor rapid exchange.

Diffusion is the most important method for small, lipid-soluble or small water-soluble molecules. Gases like $O_2$ and $CO_2$, nutrients such as glucose and amino acids, and many hormones move down their concentration gradients. Most plasma solutes except large proteins cross via the lipid bilayer, fenestrations, or intercellular clefts. The blood–brain barrier restricts diffusion of many water-soluble substances because of tight junctions.

Transcytosis moves larger, lipid-insoluble molecules across the endothelium using vesicles formed by endocytosis and released by exocytosis. Examples include insulin and maternal antibodies crossing the placenta.

🔁 Bulk Flow

Bulk flow is the movement of large amounts of fluid and solutes in one direction in response to pressure differences. It controls relative volumes of blood and interstitial fluid faster than diffusion and is critical for maintaining fluid balance between compartments.

Two opposing bulk-flow processes occur across capillary walls: filtration (fluid leaving capillaries into interstitial fluid) and reabsorption (fluid returning to capillaries). The balance of these determines net fluid movement.

💉 Pressures in Capillaries

Important pressures that drive filtration and reabsorption:

• Blood Hydrostatic Pressure (BHP) — the pressure exerted by blood on vessel walls due to cardiac contraction and blood volume. Typical values: arteriole end ≈ 35 mm Hg; venule end ≈ 16 mm Hg.

• Blood Colloid Osmotic Pressure (BCOP) — the osmotic “pull” from plasma proteins (mainly albumin) that tends to draw water into capillaries. Typical value ≈ 26 mm Hg.

• Interstitial Fluid Osmotic Pressure (IFOP) — the pull from solutes in the interstitial fluid. Often small; assume ≈ 1 mm Hg.

• Interstitial Fluid Hydrostatic Pressure (IFHP) — back-pressure from interstitial fluid; usually close to zero because lymphatics drain fluid quickly, so assume ≈ 0 mm Hg.

➗ Net Filtration Pressure (Starling Forces)

The net driving pressure for fluid movement is given by the Starling equation:

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

At the arterial end of a capillary bed, BHP is higher, producing a positive $NFP$ (net filtration) of about 10 mm Hg outward. At the venous end, BHP falls, and $NFP$ becomes negative (net reabsorption) of about 8–9 mm Hg inward.

About 85% of filtered fluid is reabsorbed by capillaries. The remainder (≈ 3 L/day) is returned to circulation via lymphatic capillaries. Starling's law of the capillaries states that reabsorbed volume is nearly equal to filtered volume under normal conditions.

🔄 Blood Flow & Velocity

Blood flow is the volume of blood moving through a tissue per unit time. Flow velocity is inversely related to the total cross-sectional area of vessels: velocity is slowest where cross-sectional area is greatest (capillaries). For example, flow in the aorta can be ≈ 40 cm/s while capillary flow is ≈ 0.1 cm/s, which facilitates exchange.

Circulation time is the time for a drop of blood to travel from the right atrium back to the right atrium. Venous return depends on the pressure difference from venules (~16 mm Hg) to the right atrium (~0 mm Hg).

🫁 Mechanisms Aiding Venous Return

• Respiratory pump: during inhalation, decreased thoracic pressure and increased abdominal pressure propel blood into thoracic veins and the right atrium.

• Skeletal muscle pump: muscle contractions compress veins; venous valves ensure one-way flow toward the heart.

• Venous valves and the siphon effect (pressure gradient) also assist return.

🩺 Blood Pressure (BP)

Blood pressure is the force exerted by blood on vessel walls, generated by ventricular contraction. Typical arterial pressures: systolic ≈ 120 mm Hg, diastolic ≈ 80 mm Hg. Pressure falls with distance from the left ventricle: about 35 mm Hg entering capillaries and ~0 mm Hg at the right atrium.

Factors affecting BP include cardiac output (heart rate and contractility), blood volume (water retention raises BP; loss >10% lowers BP), and arterial elasticity.

⚙️ Vascular Resistance

Vascular resistance opposes blood flow and arises from friction between blood and vessel walls. Major determinants:

• Vessel radius: the most powerful moment-to-moment regulator; smaller radii dramatically increase resistance.

• Blood viscosity: proportion of red blood cells to plasma. Increased hematocrit (e.g., polycythemia) or dehydration raises viscosity and resistance.

• Total vessel length: more length increases resistance (e.g., obesity adds vessel length and raises BP).

Most systemic resistance (total peripheral resistance) is in arterioles, capillaries, and venules due to small diameters. Arterioles play a key role in controlling BP by changing diameter.

🧠 Nervous Control of Blood Pressure & Flow

The cardiovascular center (CV) in the medulla integrates inputs and adjusts heart rate, contractility, and vessel diameter via autonomic output.

Inputs include: higher brain centers (cortex, limbic system, hypothalamus), proprioceptors (activity), baroreceptors (blood pressure changes), and chemoreceptors (blood chemistry).

Outputs travel via parasympathetic fibers (vagus nerve) to decrease heart rate and via sympathetic fibers to increase heart rate, contractility, and vasomotor tone. Sympathetic activity maintains a baseline vasomotor tone (partial contraction).

Baroreceptor reflexes

• Carotid sinus reflex (via glossopharyngeal nerve) protects cerebral perfusion and helps maintain stable brain BP.
• Aortic reflex (via vagus nerve) helps maintain systemic BP.

🧪 Hormonal Control of Blood Pressure

Key hormonal systems:

• Renin–Angiotensin–Aldosterone System (RAAS): kidney release of renin converts liver-produced angiotensinogen to angiotensin I, which is converted to angiotensin II (powerful vasoconstrictor) in the lungs. Angiotensin II stimulates aldosterone release, promoting renal retention of $Na^+$ and $H_2O$, increasing blood volume and BP. ACE inhibitors ("-prils") block conversion to angiotensin II; ARBs (e.g., losartan) block angiotensin II receptors; aldosterone antagonists (e.g., spironolactone) block aldosterone effects.

• Epinephrine & Norepinephrine increase heart rate and contractility, constrict vessels in skin/abdominal organs, and dilate vessels in heart and skeletal muscle, raising BP overall.

• Antidiuretic hormone (ADH) causes vasoconstriction and promotes $H_2O$ retention, increasing blood volume and pressure.

• Atrial natriuretic peptide (ANP) lowers BP by causing vasodilation and promoting salt and water loss in the urine, reducing blood volume.

🚨 Shock — Failure of Perfusion

Shock occurs when cardiac output cannot deliver adequate oxygen and nutrients to meet cellular needs. There are three stages:

• Nonprogressive (compensated): homeostatic mechanisms can restore balance if the cause is removed.
• Progressive (decompensated): shock worsens and tissue damage occurs; recovery possible but with injury.
• Irreversible: prolonged inadequate perfusion leads to cell death; death is likely even if pressures are temporarily restored.

Types of shock

• Hypovolemic: decreased blood volume from hemorrhage, dehydration, vomiting, diarrhea, burns, or excessive sweating.

• Cardiogenic: decreased cardiac output due to heart failure (e.g., myocardial infarction, valve dysfunction, arrhythmias). High mortality (≈85% in severe cardiogenic shock).

• Vascular (distributive): abnormal vasodilation increases vascular capacity so blood cannot adequately fill the system. Subtypes:

  • Neurogenic: loss of vasomotor tone (e.g., spinal anesthesia, brain damage) causes venous pooling.
  • Anaphylactic: severe allergic reaction with massive histamine release, causing widespread vasodilation.
  • Septic: systemic infection leads to widespread inflammation, vasodilation, and tissue damage; a common and serious hospital cause of death.

• Obstructive: physical blockage of blood flow (e.g., pulmonary embolus) causes upstream pressure rise and plasma/protein leakage into tissues, reducing effective circulating volume.

Signs and symptoms of shock

Early and common signs: tachycardia, weak rapid pulse, hypotension, cool clammy pale skin, sweating, altered mental state, decreased urine output, thirst, acidosis, and nausea.

Keep these core concepts in mind: capillary exchange is governed by diffusion and Starling forces; blood flow depends on vessel area and resistance; arterioles and autonomic/hormonal systems regulate BP; and prompt recognition of shock type is critical for appropriate treatment.