SILVERTHORN
CHAPTER 15 – BLOOD FLOW AND BLOOD
PRESSURE
THE BLOOD VESSELS
The walls of the blood vessels are composed of layers of smooth muscle, elastic connective
tissue, and fibrous connective tissue. The inner lining of all blood vessels is a thin layer of
endothelium. These endothelial cells secrete many
paracrine signals and play important roles in the
regulation of blood pressure, blood vessels growth,
and absorption of materials. The endothelium and its
adjacent elastic connective tissue together make up
the tunica intima.
Smooth muscle layer
The smooth muscle of blood vessels is known as
vascular smooth muscle, arranged in circular or
spiral layers. Smooth muscle cells maintain a state of
partial contraction at all times, creating the condition
known as muscle tone. Contraction of smooth
muscle depends on the entry of Ca from the
extracellular fluid through Ca-channels. Signal
molecules, including neurotransmitters, hormones
and paracrine signals, influence vascular smooth
muscle tone. These signals are secreted by
endothelial lining cells or by tissues surrounding the
vessels.
Arteries / arterioles
The aorta and major arteries are characterized by
walls that are both stiff and springy. The arteries and
arterioles are characterized by a divergent pattern of
blood flow. As major arteries divide into smaller arteries, the character of the wall changes,
becoming less elastic and more muscular. The walls of the arterioles contain several layers of
smooth muscle that contract and relax under the influence of various chemical signals. Arterioles,
along with capillaries and venules, form the microcirculation. Some arterioles branch into vessels
known as metarterioles, which are only partly surrounded with a layer of smooth muscle. Blood
flowing through these arterioles can take one of two paths. If muscle rings called pre-capillary
sphincters are relaxed, blood flowing into a metarteriole is directed into adjoining capillary beds.
Constriction causes blood to go directly to the venous circulation instead of the capillary bed.
Capillaries
Capillaries play an important role in the exchange between the blood and the interstitial fluid. To
facilitate exchange of materials, capillaries lack smooth muscle and elastic or fibrous tissue
reinforcement. Their walls consist of a single layer endothelium, supported by the basal lamina.
Many capillaries are closely associated with cells called pericytes. These cells secrete factors that
Silverthorn – chapter 15: Blood flow & blood pressure Page 1 of 10
CHAPTER 15 – BLOOD FLOW AND BLOOD
PRESSURE
THE BLOOD VESSELS
The walls of the blood vessels are composed of layers of smooth muscle, elastic connective
tissue, and fibrous connective tissue. The inner lining of all blood vessels is a thin layer of
endothelium. These endothelial cells secrete many
paracrine signals and play important roles in the
regulation of blood pressure, blood vessels growth,
and absorption of materials. The endothelium and its
adjacent elastic connective tissue together make up
the tunica intima.
Smooth muscle layer
The smooth muscle of blood vessels is known as
vascular smooth muscle, arranged in circular or
spiral layers. Smooth muscle cells maintain a state of
partial contraction at all times, creating the condition
known as muscle tone. Contraction of smooth
muscle depends on the entry of Ca from the
extracellular fluid through Ca-channels. Signal
molecules, including neurotransmitters, hormones
and paracrine signals, influence vascular smooth
muscle tone. These signals are secreted by
endothelial lining cells or by tissues surrounding the
vessels.
Arteries / arterioles
The aorta and major arteries are characterized by
walls that are both stiff and springy. The arteries and
arterioles are characterized by a divergent pattern of
blood flow. As major arteries divide into smaller arteries, the character of the wall changes,
becoming less elastic and more muscular. The walls of the arterioles contain several layers of
smooth muscle that contract and relax under the influence of various chemical signals. Arterioles,
along with capillaries and venules, form the microcirculation. Some arterioles branch into vessels
known as metarterioles, which are only partly surrounded with a layer of smooth muscle. Blood
flowing through these arterioles can take one of two paths. If muscle rings called pre-capillary
sphincters are relaxed, blood flowing into a metarteriole is directed into adjoining capillary beds.
Constriction causes blood to go directly to the venous circulation instead of the capillary bed.
Capillaries
Capillaries play an important role in the exchange between the blood and the interstitial fluid. To
facilitate exchange of materials, capillaries lack smooth muscle and elastic or fibrous tissue
reinforcement. Their walls consist of a single layer endothelium, supported by the basal lamina.
Many capillaries are closely associated with cells called pericytes. These cells secrete factors that
Silverthorn – chapter 15: Blood flow & blood pressure Page 1 of 10
