BIOD 152 · A&P II W/LAB · MODULE 3 LECTURE STUDY GUIDE
Objective 3 — Capillary Dynamics & Tissue Exchange
OBJECTIVE: Explain the dynamics of blood flow through capillaries, including capillary dynamics, types of
capillaries, and their roles in tissue exchange and regulation. Sources used: Lecture page 3.1 (Capillaries;
Types of Capillaries; Capillary Beds) · Lecture page 3.3 – Systemic Blood Pressure (Capillary Blood
Pressure; Blood Flow through Capillaries and Capillary Dynamics; Hydrostatic Pressure; Osmotic Pressure;
Interaction of HP and OP; Role of Lymphatic Capillaries) · R&R Summary Statements 4, 8 · Study Strategy 5 ·
Figures 3.4, 3.5, 3.6, 3.14, 3.15, 3.16, 3.17 · Table 3.3.
3.1 Capillaries: Structure and Perfusion
• Capillaries are the smallest vessels — microscopic channels that supply blood to the tissues.
• Perfusion is the process in which an exchange of gases and other substances occurs between the blood
in capillaries and the surrounding cells and their tissue fluid (interstitial fluid).
• Capillary lumen diameter = 5–10 µm; the smallest are just barely wide enough for a single
erythrocyte to squeeze through.
• Flow through capillaries is often described as microcirculation.
• Wall structure: the endothelial layer surrounded by a basement membrane with occasional smooth
muscle fibers. In a large capillary, several endothelial cells bordering each other may line the lumen; in
a small capillary, there may be only a single cell layer that wraps around to contact itself.
• For capillaries to function, their walls must be leaky, allowing substances to pass through. Capillary cells
contain many endosomes — membrane-bound compartments involved in sorting, trafficking, and
processing internalized materials (nutrients, receptors, other molecules) that enter the cell through
endocytosis.
3.2 The Three Types of Capillaries (Figure 3.4)
The three types differ according to their degree of "leakiness." Ranked least → most permeable:
continuous → fenestrated → sinusoid.
Type Permeability Structure Locations
Continuous Least Complete endothelial lining with tight junctions Almost all vascularized
permeable between endothelial cells. Tight junctions are usually tissues; the blood-brain
(most common impermeable and allow only water and ions, but in barrier
type) capillaries they are often incomplete, leaving
intercellular clefts that allow exchange of water and
other very small molecules between blood plasma and
interstitial fluid. Substances that can pass between
cells: metabolic products such as glucose, water, small
hydrophobic molecules like gases and hormones, and
various leukocytes. Those not in the brain are rich in
transport vesicles (endosomes) for endocytosis/
exocytosis.
Fenestrated Moderately Has pores (fenestrations) in addition to tight junctions Small intestine (primary
permeable in the endothelial lining, making the capillary site of nutrient absorption),
permeable to larger molecules. The number of kidneys (filter blood),
fenestrations and degree of permeability vary by choroid plexus of the brain,
location. and many endocrine
structures including the
hypothalamus, pituitary,
pineal, and thyroid glands
BIOD 152 · A&P II · Module 3 Study Guide · page 1
, Type Permeability Structure Locations
Sinusoid Most Flattened, with extensive intercellular gaps and Liver and spleen, bone
permeable incomplete basement membranes, in addition to marrow, lymph nodes
(least common intercellular clefts and fenestrations. These very large (where they carry lymph,
type) openings allow passage of the largest molecules, not blood), and many
including plasma proteins and even whole cells. Blood endocrine glands including
flow through sinusoids is very slow, allowing more time the pituitary and adrenal
for exchange of gases, nutrients, and wastes. glands
Figure 3.4 — Structure of capillaries: continuous, fenestrated, and sinusoidal. Identify by the
signature feature: continuous = complete lining with intercellular clefts; fenestrated = pores in the
endothelium; sinusoid = flattened, with large intercellular gaps and an incomplete basement
membrane. Clinical Connection — why sinusoids are indispensable Without these specialized
capillaries, certain organs could not perform their critical functions. When bone marrow forms new blood
cells, those cells must enter the blood supply — and can only do so through the large openings of a
sinusoid capillary; cells cannot pass through the small openings of continuous or fenestrated capillaries.
The liver also requires extensive sinusoid capillaries to process materials brought by the hepatic portal
vein from the digestive tract and spleen, and to release plasma proteins into circulation.
Watch Out — the blood-brain barrier is a SPECIAL continuous capillary Continuous capillaries in
the brain differ from continuous capillaries elsewhere: at the blood-brain barrier the tight junctions
have no intercellular clefts, there is a thick basement membrane, and astrocyte extensions wrap
around the capillaries to prevent movement of nearly all substances into the CNS. Continuous capillaries
not associated with the brain are rich in transport vesicles. Also note the brain contains both — the
choroid plexus is fenestrated, not continuous.
BIOD 152 · A&P II · Module 3 Study Guide · page 2
Objective 3 — Capillary Dynamics & Tissue Exchange
OBJECTIVE: Explain the dynamics of blood flow through capillaries, including capillary dynamics, types of
capillaries, and their roles in tissue exchange and regulation. Sources used: Lecture page 3.1 (Capillaries;
Types of Capillaries; Capillary Beds) · Lecture page 3.3 – Systemic Blood Pressure (Capillary Blood
Pressure; Blood Flow through Capillaries and Capillary Dynamics; Hydrostatic Pressure; Osmotic Pressure;
Interaction of HP and OP; Role of Lymphatic Capillaries) · R&R Summary Statements 4, 8 · Study Strategy 5 ·
Figures 3.4, 3.5, 3.6, 3.14, 3.15, 3.16, 3.17 · Table 3.3.
3.1 Capillaries: Structure and Perfusion
• Capillaries are the smallest vessels — microscopic channels that supply blood to the tissues.
• Perfusion is the process in which an exchange of gases and other substances occurs between the blood
in capillaries and the surrounding cells and their tissue fluid (interstitial fluid).
• Capillary lumen diameter = 5–10 µm; the smallest are just barely wide enough for a single
erythrocyte to squeeze through.
• Flow through capillaries is often described as microcirculation.
• Wall structure: the endothelial layer surrounded by a basement membrane with occasional smooth
muscle fibers. In a large capillary, several endothelial cells bordering each other may line the lumen; in
a small capillary, there may be only a single cell layer that wraps around to contact itself.
• For capillaries to function, their walls must be leaky, allowing substances to pass through. Capillary cells
contain many endosomes — membrane-bound compartments involved in sorting, trafficking, and
processing internalized materials (nutrients, receptors, other molecules) that enter the cell through
endocytosis.
3.2 The Three Types of Capillaries (Figure 3.4)
The three types differ according to their degree of "leakiness." Ranked least → most permeable:
continuous → fenestrated → sinusoid.
Type Permeability Structure Locations
Continuous Least Complete endothelial lining with tight junctions Almost all vascularized
permeable between endothelial cells. Tight junctions are usually tissues; the blood-brain
(most common impermeable and allow only water and ions, but in barrier
type) capillaries they are often incomplete, leaving
intercellular clefts that allow exchange of water and
other very small molecules between blood plasma and
interstitial fluid. Substances that can pass between
cells: metabolic products such as glucose, water, small
hydrophobic molecules like gases and hormones, and
various leukocytes. Those not in the brain are rich in
transport vesicles (endosomes) for endocytosis/
exocytosis.
Fenestrated Moderately Has pores (fenestrations) in addition to tight junctions Small intestine (primary
permeable in the endothelial lining, making the capillary site of nutrient absorption),
permeable to larger molecules. The number of kidneys (filter blood),
fenestrations and degree of permeability vary by choroid plexus of the brain,
location. and many endocrine
structures including the
hypothalamus, pituitary,
pineal, and thyroid glands
BIOD 152 · A&P II · Module 3 Study Guide · page 1
, Type Permeability Structure Locations
Sinusoid Most Flattened, with extensive intercellular gaps and Liver and spleen, bone
permeable incomplete basement membranes, in addition to marrow, lymph nodes
(least common intercellular clefts and fenestrations. These very large (where they carry lymph,
type) openings allow passage of the largest molecules, not blood), and many
including plasma proteins and even whole cells. Blood endocrine glands including
flow through sinusoids is very slow, allowing more time the pituitary and adrenal
for exchange of gases, nutrients, and wastes. glands
Figure 3.4 — Structure of capillaries: continuous, fenestrated, and sinusoidal. Identify by the
signature feature: continuous = complete lining with intercellular clefts; fenestrated = pores in the
endothelium; sinusoid = flattened, with large intercellular gaps and an incomplete basement
membrane. Clinical Connection — why sinusoids are indispensable Without these specialized
capillaries, certain organs could not perform their critical functions. When bone marrow forms new blood
cells, those cells must enter the blood supply — and can only do so through the large openings of a
sinusoid capillary; cells cannot pass through the small openings of continuous or fenestrated capillaries.
The liver also requires extensive sinusoid capillaries to process materials brought by the hepatic portal
vein from the digestive tract and spleen, and to release plasma proteins into circulation.
Watch Out — the blood-brain barrier is a SPECIAL continuous capillary Continuous capillaries in
the brain differ from continuous capillaries elsewhere: at the blood-brain barrier the tight junctions
have no intercellular clefts, there is a thick basement membrane, and astrocyte extensions wrap
around the capillaries to prevent movement of nearly all substances into the CNS. Continuous capillaries
not associated with the brain are rich in transport vesicles. Also note the brain contains both — the
choroid plexus is fenestrated, not continuous.
BIOD 152 · A&P II · Module 3 Study Guide · page 2
