Blood Flow and the
Control of Blood
Pressure

Arteriolar Resistance
• Arteriolar resistance is influenced by both local and systemic control mechanisms
• Local control
• Sympathetic reflexes
• Hormones

Arteriolar Resistance
Table 15-2

Arteriolar Resistance
• Myogenic autoregulation
• Paracrines
• Active hyperemia
• Reactive hyperemia
• Sympathetic control
• SNS: norepinephrine
• Adrenal medulla: epinephrine

Hyperemia is a Locally Mediated Increase in Blood
Flow
Figure 15-11a

Hyperemia is a Locally Mediated Increase in Blood
Flow
Figure 15-11b

Norepinephrine
• Tonic control of arteriolar diameter
Figure 15-12

Distribution of Blood
• Distribution of blood in the body at rest
Figure 15-14

Blood Flow
• Blood flow through individual blood vessels is determined by vessel’s resistance to flow
Figure 15-15a

Blood Flow
• Flow

1/resistance
Figure 15-15b

Precapillary Sphincters
Figure 15-16a

Precapillary Sphincters
Figure 15-16b

Capillaries: Exchange
• Plasma and cells exchange materials across thin capillary wall
• Capillary density is related to metabolic activity of cells
• Capillaries have the thinnest walls
• Single layer of flattened endothelial cells
• Supported by basal lamina
• Bone marrow, liver and spleen do not have typical capillaries but sinusoids

Two Types of Capillaries
Nucleus
Endothelial cells
Endothelial cell junctions
Basement membrane
Transcytosis vesicles
(a)
Continuous capillaries have leaky junctions.
Figure 15-17a

Two Types of Capillaries
Fenestrated pores
Basement membrane (cut)
Transcytosis vesicles
Fenestrations or pores
Endothelial cell
Basement membrane
(b)
Fenestrated capillaries have large pores.
Figure 15-17b

Velocity of Blood Flow
• Velocity of flow depends on total crosssectional area of the vessels
Figure 15-18

Capillary Exchange
• Exchange between plasma and interstitial fluid occurs by paracellular pathway or endothelial transport
• Small dissolved solutes and gasses move by diffusion
• Larger solutes and proteins move by vesicular transport
• In most capillaries, large proteins are transported by transcytosis

Capillary Exchange
• Bulk flow
• Mass movement as a result of hydrostatic or osmotic pressure gradients
• Absorption : fluid movement into capillaries
• Net absorption at venous end
• Filtration : fluid movement out of capillaries
• Caused by hydrostatic pressure
• Net filtration at arterial end

Fluid Exchange at a Capillary
• Hydrostatic pressure and osmotic pressure regulate bulk flow
Figure 15-19a

Autoregulation and Capillary Dynamics
PLAY
Interactive Physiology® Animation: Cardiovascular
System: Autoregulation and Capillary Dynamics

Lymphatic System
• Returning fluid and proteins to circulatory system
• Picking up fat absorbed and transferring it to circulatory system
• Serving as filter for pathogens

Fluid Exchange at a Capillary
Arteriole
Net filtration
Net absorption
Lymph vessels
(b) Relationship between capillaries and lymph vessels
Venule
Figure 15-19b

Lymphatic System
Cervical lymph nodes
Right lymph duct
Thymus
Thoracic duct
Lumbar lymph nodes
Thoracic (left lymph) duct
Lymphatics of upper limb
Axillary lymph nodes
Lymphatics of mammary gland
Spleen
Pelvic lymph nodes
Inguinal lymph nodes
Lymphatics of lower limb
Blind-end lymph capillaries in the tissues remove fluid and filtered proteins.
Lymph fluid empties into the venous circulation.
Figure 15-20

Edema
• Two causes
• Inadequate drainage of lymph
• Filtration far greater than absorption
• Disruption of balance between filtration and absorption
• Increase in hydrostatic pressure
• Decrease in plasma protein concentration
• Increase in interstitial proteins

Blood Pressure
• Components of the baroreceptor reflex
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Change in blood pressure
Parasympathetic neurons
Carotid and aortic baroreceptors
Sympathetic neurons SA node
Ventricles
Veins
Arterioles
Figure 15-22

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Change in blood pressure
Figure 15-22 (1 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (2 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (3 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (4 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (5 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons SA node
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (6 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons SA node
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (7 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons SA node
Ventricles
Change in blood pressure
Carotid and aortic baroreceptors
Figure 15-22 (8 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons SA node
Ventricles
Change in blood pressure
Carotid and aortic baroreceptors
Arterioles
Figure 15-22 (9 of 10)

Blood Pressure
KEY
Stimulus
Sensory receptor
Integrating center
Efferent path
Effector
Medullary cardiovascular control center
Parasympathetic neurons
Sympathetic neurons SA node
Ventricles
Change in blood pressure
Carotid and aortic baroreceptors
Veins
Arterioles
Figure 15-22 (10 of 10)

Blood Pressure
• The baroreceptor reflex: the response to increased blood pressure
Figure 15-23

Blood Pressure
• The baroreceptor reflex: the response to orthostatic hypotension
Figure 15-24

Blood Pressure Regulation
PLAY
Interactive Physiology® Animation: Cardiovascular
System: Blood Pressure Regulation

CVD: Risk Factors
• Not controllable
• Sex
• Age
• Family history
• Controllable
• Smoking
• Obesity
• Sedentary lifestyle
• Untreated hypertension

CVD: Risk Factors
• Uncontrollable genetic but modifiable lifestyle
• Blood lipids
• Leads to atherosclerosis
• HDL-C versus LDL-C
• Diabetes mellitus
• Metabolic disorder contributes to development of atherosclerosis

LDL and Plaque
• The development of atherosclerotic plaques
Endothelial cells
Elastic connective tissue
Smooth muscle cells
(a) Normal arterial wall
LDL cholesterol accumulates
Macrophages
Smooth muscle cells
(b) Fatty streak
A lipid core accumulates
Fibrous scar tissue
Smooth muscle cells
Calcifications are deposited
(c) Stable fibrous plaque within the plaque.
Platelets
Macrophages
(d) Vulnerable plaque
Figure 15-25

LDL and Plaque
(a) Normal arterial wall
Endothelial cells
Elastic connective tissue
Smooth muscle cells
Figure 15-25a

LDL and Plaque
(b) Fatty streak
LDL cholesterol accumulates
Macrophages
Smooth muscle cells
Figure 15-25b

LDL and Plaque
(c) Stable fibrous plaque
A lipid core accumulates
Fibrous scar tissue
Smooth muscle cells
Calcifications are deposited within the plaque.
Figure 15-25c

LDL and Plaque
(d) Vulnerable plaque
Platelets
Macrophages
Figure 15-25d

Hypertension
• The risk of developing cardiovascular disease doubles with each 20/10 mm Hg increase in blood pressure
• Essential hypertension has no clear cause other than hereditary
Figure 15-26

Hypertension
• Carotid and aortic baroreceptors adapt
• Risk factor for atherosclerosis
• Heart muscle hypertrophies
• Pulmonary edema
• Congestive heart failure
• Treatment
• Calcium channel blockers, diuretics, betablocking drugs, and ACE inhibitors

Summary
• Blood vessels
• Vascular smooth muscle, metarterioles, venules, and angiogenesis
• Measuring blood pressure
• Systolic pressure, diastolic pressure, pulse pressure, MAP, and Korotkoff sounds
• Resistance in the arterioles
• Myogenic autoregulation, active hyperemia, and reactive hyperemia

Summary
• Distribution of blood
• Capillary exchange
• Continuous capillaries, fenestrated capillaries, bulk flow, filtration, absorption, and colloid osmotic pressure
• Lymphatic system
• Blood pressure regulation
• Baroreceptors, baroreceptor reflex, and cardiovascular control center
• Cardiovascular disease