1. Cardiovascular system. General microscopic, ultramicroscopic and functional
characteristics of its organs. Major (or systematic) circulation and lesser (or pulmonary)
circulation. Embryonic development of major blood vessels. Fetal circulation.
The circulatory system, also known as the cardiovascular system, is a complex network responsible for the transportation of blood,
nutrients, oxygen, carbon dioxide, hormones, and waste products to and from the cells of the body. It is essential for maintaining
homeostasis, regulating body temperature, pH levels, and protecting the body through the immune response. The system ensures that
tissues receive adequate oxygen and nutrients while removing metabolic waste. The circulatory system consists of the heart, blood vessels,
and blood:
1. Heart: A muscular organ located in the thoracic cavity, the heart functions as the pump that propels blood throughout the body. It is
divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The right side of the heart deals with
deoxygenated blood, while the left side handles oxygenated blood.
2. Blood Vessels: These are the conduits through which blood flows. They include:
- Arteries: Thick-walled vessels that carry oxygen-rich blood away from the heart (with the exception of the pulmonary artery).
- Veins: Thinner-walled vessels that return oxygen-depleted blood back to the heart (with the exception of the pulmonary veins).
- Capillaries: Microscopic vessels where the exchange of gases, nutrients, and waste occurs between blood and tissues.
3. Blood: A fluid tissue composed of plasma (the liquid component) and formed elements (red blood cells, white blood cells, and platelets).
Blood serves as the transport medium for gases, nutrients, hormones, and waste products.
However, even if this system may be seen strictly as a blood distribution network, some consider the circulatory system as composed of the
cardiovascular system, which distributes blood, and the lymphatic system, which distributes lymph.
• The blood, heart, and blood vessels form the cardiovascular system. Humans, as well as other vertebrates, have a closed
cardiovascular system (blood never leaves the network of arteries, veins and capillaries); but some invertebrates have an open
cardiovascular system. The most primitive animal phyla lack circulatory systems.
• The lymph, lymph nodes, and lymph vessels form the lymphatic system which, on the other hand, is an open system.
In summary, two types of fluids move through the circulatory system: blood and lymph, so, the cardiovascular system and the lymphatic
system collectively make up the circulatory system.
,Microscopic Structure of Cardiovascular Organs
1. The Heart
• Endocardium: Inner lining composed of simple squamous epithelium (endothelium), subendothelial connective tissue, and Purkinje
fibers.
• Myocardium: Thick middle layer made of cardiac muscle fibers with branching cells, central nuclei, and intercalated discs for electrical
conduction.
• Epicardium: Outer layer composed of mesothelium (visceral pericardium), connective tissue, fat, and blood vessels.
2. Arteries and Veins
• Tunica intima: Inner layer: endothelium + subendothelial connective tissue.
• Tunica media: Middle layer: smooth muscle cells, elastic fibers (prominent in arteries).
• Tunica adventitia: Outer layer: connective tissue, collagen, vasa vasorum (small vessels supplying vessel walls in large vessels).
Arteries have thicker tunica media than veins, which have larger lumens and thinner walls.
3. Capillaries: Single layer of endothelial cells on a basal lamina. Types:
• Continuous – most common; found in muscle, brain.
• Fenestrated – found in kidneys, intestines; allow filtration.
• Sinusoidal – found in liver, bone marrow; large openings for cells and molecules.
Ultramicroscopic Characteristics (Electron Microscopy)
1. Endothelial Cells
Possess tight junctions, pinocytotic vesicles, and Weibel-Palade bodies (which store von Willebrand factor).
Regulate permeability, blood flow, coagulation, and inflammation.
2. Cardiac Myocytes
Display well-defined sarcomeres, abundant mitochondria, and intercalated discs. Intercalated discs consist of:
• Desmosomes for mechanical strength
• Fascia adherens for anchoring actin filaments
• Gap junctions for electrical coupling
3. Smooth Muscle Cells (in vessels)
Spindle-shaped with central nuclei.
Produce extracellular matrix components like collagen and elastin.
Functional Characteristics
The heart acts as a rhythmic pump with automaticity and conducts impulses via the SA node, AV node, and the His-Purkinje system.
Arteries transport blood under high pressure and regulate flow through vasoconstriction and vasodilation.
Capillaries are sites of gas, nutrient, and waste exchange between blood and tissues.
Veins return blood to the heart under low pressure and are equipped with valves to prevent backflow.
Embryonic Development of Major Blood Vessels
The vascular system originates from mesodermal tissue, developing early as blood islands in the yolk sac, connecting stalk, and embryo
proper. These merge to form a primitive vascular network that remodels extensively.
A. Aortic Arch Derivatives: There are six pairs of embryonic aortic arches. Although not all persist, each contributes to major arterial
structures:
• 1st aortic arch: Forms part of the maxillary artery.
• 2nd aortic arch: Transiently contributes to the stapedial artery.
• 3rd aortic arch: Gives rise to the common carotid artery and the proximal internal carotid artery.
• 4th aortic arch: Right side: Forms part of the right subclavian artery. Left side: Forms part of the arch of the aorta.
• 5th aortic arch: Either rudimentary or absent.
• 6th aortic arch: Proximally: Contributes to the pulmonary arteries. Distally (left side): Forms the ductus arteriosus, which becomes the
ligamentum arteriosum after birth.
B. Development of Major Veins: The early embryonic venous system consists of three paired veins:
1. Vitelline veins: Drain the yolk sac and contribute to the formation of the portal vein, hepatic sinusoids, and parts of the inferior vena
cava (IVC).
2. Umbilical veins: Carry oxygenated blood from the placenta. The left umbilical vein persists and later becomes the ligamentum teres
hepatis after birth.
3. Cardinal veins: Drain the body of the embryo and evolve into the systemic venous system:
• Anterior cardinal veins become part of the internal jugular veins and the superior vena cava (SVC).
• Posterior cardinal veins contribute to the iliac veins and parts of the IVC.
• Subcardinal and supracardinal veins give rise to the renal, gonadal, and azygos venous systems.
C. Formation of the Aorta and Its Branches
The paired dorsal aortae fuse to form the descending aorta.
Intersegmental arteries sprout from the dorsal aorta and become the vertebral and intercostal arteries.
Lateral branches of the aorta form arteries to kidneys, gonads, and adrenal glands.
Ventral branches form the three major unpaired arteries of the abdominal aorta: Celiac trunk (foregut), Superior mesenteric artery (SMA)
(midgut), Inferior mesenteric artery (IMA) (hindgut).
Clinical Correlations
A patent ductus arteriosus (PDA) results from the failure of the ductus arteriosus to close after birth, allowing blood to shunt from the
aorta to the pulmonary artery.
Coarctation of the aorta is a congenital narrowing of the aortic arch, often distal to the left subclavian artery.
A double aortic arch or right-sided aortic arch may occur if both 4th aortic arches persist or regress abnormally, possibly leading to
vascular rings compressing the trachea or esophagus.
, MAJOR (SYSTEMIC) CIRCULATION: This circuit carries oxygenated blood from the left ventricle of the heart through the aorta and to
the rest of the body. Oxygen and nutrients are delivered to tissues via capillaries, and waste products and carbon dioxide are collected.
Deoxygenated blood returns to the heart through veins, entering the right atrium.
Oxygenated blood coming from pulmonary circulation enters the heart through pulmonary vein directly into the left atrium. From the left
atrium it reaches the left ventricle and, passing through the aortic semilunar valve, enters the systemic circulation. The first part of the
systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and branches into major arteries to the upper body
before passing through the diaphragm, where it branches further into arteries which supply the lower parts of the body. After their passage
through body tissues, capillaries merge once again into venules, which continue to merge into veins. The venous system finally coalesces
into two major veins: the superior vena cava (roughly speaking draining the areas above the heart) and the inferior vena cava (roughly
speaking from areas below the heart). These two great vessels empty into the right atrium of the heart.
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation - coronary circulation.
In summary, arteries from the heart branch into capillaries, which collect into veins leading back to the heart. Portal venous system is an
exception: the hepatic portal vein (which combines from capillaries around the gut where the blood absorbs the various products of
digestion) rather than leading directly back to the heart, branches into a second capillary system in the liver.
LESSER (PULMONARY) CIRCULATION: This circuit carries deoxygenated blood
from the right ventricle of the heart through the pulmonary arteries to the lungs. In the
lungs, carbon dioxide is exchanged for oxygen. Oxygenated blood then returns to the left
atrium of the heart through the pulmonary veins.
Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-
depleted blood away from the heart, to the lungs, and returns oxygenated blood back to
the heart. A separate system known as the bronchial circulation supplies blood to the
tissue of the larger airways of the lung. In the pulmonary circulation, deoxygenated
blood leaves the right ventricle of the heart through the pulmonary artery, enters the
lungs and oxygenated blood comes through the pulmonary veins. The blood then
moves to the left atrium of the heart then to the left ventricle where the blood is
pumped through the semilunar valve into the aorta.
De-02-blood → right atrium → right
·
ventricle → pulmonary artery →
lungs → 02-blood → pulmonary vein
→ Left atrium → left ventricle →
aortic semilunar valve → aorta
AORTA → aortic arch → aortic branches
e
for UPPER BODY → diaphragm → aortic
branches for LOWER BODY → arterial
capillaries → venules → veins → SUP.
VENA CAVA & INF. VENA CAVA → Right
heart (atrium)
Fetal circulation is unique because the fetus relies on the mother for oxygen and nutrient exchange through the placenta, bypassing the
lungs and liver to some extent.
1. Placental Exchange: Oxygenated blood from the placenta enters the fetus through the umbilical vein. This blood bypasses the liver via
the ductus venosus and enters the inferior vena cava, mixing with deoxygenated blood.
2. Bypassing the Lung and the pulmonary circulation: Blood enters the right atrium and is shunted to the left atrium through the
foramen ovale, bypassing the non-functional fetal lungs. From the left atrium, blood moves to the left ventricle and is pumped into the
systemic circulation via the aorta.
3. Ductus Arteriosus: Some blood from the right ventricle still enters the pulmonary artery, but is diverted to the aorta through the
ductus arteriosus, bypassing the lungs.
4. Returning Deoxygenated Blood: Deoxygenated blood returns to the placenta through umbilical arteries, where gas exchange occurs.
Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right
ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta,
called the ductus arteriosus, which directs most of this blood away from the lungs (which aren't being used for respiration at this point as
the fetus is suspended in amniotic fluid): the pulmonary circulation loop is virtually bypassed in fetal circulation. The fetal lungs are
collapsed, and blood passes from:
1. the right atrium directly into the left atrium through the foramen ovale, an open passage between the two atria;
2. through ductus arteriosus, a vessel communication between pulmonary artery and aorta.
At birth, when the infant breathes for the first time, there is a decrease in the resistance in the pulmonary vasculature, which causes the
pressure in the left atrium to increase relative to the pressure in the right atrium. This leads to the closure of the foramen ovale, which is
then referred to as the fossa ovalis. Additionally, the increase in the concentration of oxygen in the blood leads to a decrease in
prostaglandins, causing closure of the ductus arteriosus which remains as ligamentum arteriosum in the adult heart . These closures
prevent blood from bypassing pulmonary circulation, and therefore allow the neonate's blood to become oxygenated in the newly
operational lungs.
characteristics of its organs. Major (or systematic) circulation and lesser (or pulmonary)
circulation. Embryonic development of major blood vessels. Fetal circulation.
The circulatory system, also known as the cardiovascular system, is a complex network responsible for the transportation of blood,
nutrients, oxygen, carbon dioxide, hormones, and waste products to and from the cells of the body. It is essential for maintaining
homeostasis, regulating body temperature, pH levels, and protecting the body through the immune response. The system ensures that
tissues receive adequate oxygen and nutrients while removing metabolic waste. The circulatory system consists of the heart, blood vessels,
and blood:
1. Heart: A muscular organ located in the thoracic cavity, the heart functions as the pump that propels blood throughout the body. It is
divided into four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The right side of the heart deals with
deoxygenated blood, while the left side handles oxygenated blood.
2. Blood Vessels: These are the conduits through which blood flows. They include:
- Arteries: Thick-walled vessels that carry oxygen-rich blood away from the heart (with the exception of the pulmonary artery).
- Veins: Thinner-walled vessels that return oxygen-depleted blood back to the heart (with the exception of the pulmonary veins).
- Capillaries: Microscopic vessels where the exchange of gases, nutrients, and waste occurs between blood and tissues.
3. Blood: A fluid tissue composed of plasma (the liquid component) and formed elements (red blood cells, white blood cells, and platelets).
Blood serves as the transport medium for gases, nutrients, hormones, and waste products.
However, even if this system may be seen strictly as a blood distribution network, some consider the circulatory system as composed of the
cardiovascular system, which distributes blood, and the lymphatic system, which distributes lymph.
• The blood, heart, and blood vessels form the cardiovascular system. Humans, as well as other vertebrates, have a closed
cardiovascular system (blood never leaves the network of arteries, veins and capillaries); but some invertebrates have an open
cardiovascular system. The most primitive animal phyla lack circulatory systems.
• The lymph, lymph nodes, and lymph vessels form the lymphatic system which, on the other hand, is an open system.
In summary, two types of fluids move through the circulatory system: blood and lymph, so, the cardiovascular system and the lymphatic
system collectively make up the circulatory system.
,Microscopic Structure of Cardiovascular Organs
1. The Heart
• Endocardium: Inner lining composed of simple squamous epithelium (endothelium), subendothelial connective tissue, and Purkinje
fibers.
• Myocardium: Thick middle layer made of cardiac muscle fibers with branching cells, central nuclei, and intercalated discs for electrical
conduction.
• Epicardium: Outer layer composed of mesothelium (visceral pericardium), connective tissue, fat, and blood vessels.
2. Arteries and Veins
• Tunica intima: Inner layer: endothelium + subendothelial connective tissue.
• Tunica media: Middle layer: smooth muscle cells, elastic fibers (prominent in arteries).
• Tunica adventitia: Outer layer: connective tissue, collagen, vasa vasorum (small vessels supplying vessel walls in large vessels).
Arteries have thicker tunica media than veins, which have larger lumens and thinner walls.
3. Capillaries: Single layer of endothelial cells on a basal lamina. Types:
• Continuous – most common; found in muscle, brain.
• Fenestrated – found in kidneys, intestines; allow filtration.
• Sinusoidal – found in liver, bone marrow; large openings for cells and molecules.
Ultramicroscopic Characteristics (Electron Microscopy)
1. Endothelial Cells
Possess tight junctions, pinocytotic vesicles, and Weibel-Palade bodies (which store von Willebrand factor).
Regulate permeability, blood flow, coagulation, and inflammation.
2. Cardiac Myocytes
Display well-defined sarcomeres, abundant mitochondria, and intercalated discs. Intercalated discs consist of:
• Desmosomes for mechanical strength
• Fascia adherens for anchoring actin filaments
• Gap junctions for electrical coupling
3. Smooth Muscle Cells (in vessels)
Spindle-shaped with central nuclei.
Produce extracellular matrix components like collagen and elastin.
Functional Characteristics
The heart acts as a rhythmic pump with automaticity and conducts impulses via the SA node, AV node, and the His-Purkinje system.
Arteries transport blood under high pressure and regulate flow through vasoconstriction and vasodilation.
Capillaries are sites of gas, nutrient, and waste exchange between blood and tissues.
Veins return blood to the heart under low pressure and are equipped with valves to prevent backflow.
Embryonic Development of Major Blood Vessels
The vascular system originates from mesodermal tissue, developing early as blood islands in the yolk sac, connecting stalk, and embryo
proper. These merge to form a primitive vascular network that remodels extensively.
A. Aortic Arch Derivatives: There are six pairs of embryonic aortic arches. Although not all persist, each contributes to major arterial
structures:
• 1st aortic arch: Forms part of the maxillary artery.
• 2nd aortic arch: Transiently contributes to the stapedial artery.
• 3rd aortic arch: Gives rise to the common carotid artery and the proximal internal carotid artery.
• 4th aortic arch: Right side: Forms part of the right subclavian artery. Left side: Forms part of the arch of the aorta.
• 5th aortic arch: Either rudimentary or absent.
• 6th aortic arch: Proximally: Contributes to the pulmonary arteries. Distally (left side): Forms the ductus arteriosus, which becomes the
ligamentum arteriosum after birth.
B. Development of Major Veins: The early embryonic venous system consists of three paired veins:
1. Vitelline veins: Drain the yolk sac and contribute to the formation of the portal vein, hepatic sinusoids, and parts of the inferior vena
cava (IVC).
2. Umbilical veins: Carry oxygenated blood from the placenta. The left umbilical vein persists and later becomes the ligamentum teres
hepatis after birth.
3. Cardinal veins: Drain the body of the embryo and evolve into the systemic venous system:
• Anterior cardinal veins become part of the internal jugular veins and the superior vena cava (SVC).
• Posterior cardinal veins contribute to the iliac veins and parts of the IVC.
• Subcardinal and supracardinal veins give rise to the renal, gonadal, and azygos venous systems.
C. Formation of the Aorta and Its Branches
The paired dorsal aortae fuse to form the descending aorta.
Intersegmental arteries sprout from the dorsal aorta and become the vertebral and intercostal arteries.
Lateral branches of the aorta form arteries to kidneys, gonads, and adrenal glands.
Ventral branches form the three major unpaired arteries of the abdominal aorta: Celiac trunk (foregut), Superior mesenteric artery (SMA)
(midgut), Inferior mesenteric artery (IMA) (hindgut).
Clinical Correlations
A patent ductus arteriosus (PDA) results from the failure of the ductus arteriosus to close after birth, allowing blood to shunt from the
aorta to the pulmonary artery.
Coarctation of the aorta is a congenital narrowing of the aortic arch, often distal to the left subclavian artery.
A double aortic arch or right-sided aortic arch may occur if both 4th aortic arches persist or regress abnormally, possibly leading to
vascular rings compressing the trachea or esophagus.
, MAJOR (SYSTEMIC) CIRCULATION: This circuit carries oxygenated blood from the left ventricle of the heart through the aorta and to
the rest of the body. Oxygen and nutrients are delivered to tissues via capillaries, and waste products and carbon dioxide are collected.
Deoxygenated blood returns to the heart through veins, entering the right atrium.
Oxygenated blood coming from pulmonary circulation enters the heart through pulmonary vein directly into the left atrium. From the left
atrium it reaches the left ventricle and, passing through the aortic semilunar valve, enters the systemic circulation. The first part of the
systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and branches into major arteries to the upper body
before passing through the diaphragm, where it branches further into arteries which supply the lower parts of the body. After their passage
through body tissues, capillaries merge once again into venules, which continue to merge into veins. The venous system finally coalesces
into two major veins: the superior vena cava (roughly speaking draining the areas above the heart) and the inferior vena cava (roughly
speaking from areas below the heart). These two great vessels empty into the right atrium of the heart.
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation - coronary circulation.
In summary, arteries from the heart branch into capillaries, which collect into veins leading back to the heart. Portal venous system is an
exception: the hepatic portal vein (which combines from capillaries around the gut where the blood absorbs the various products of
digestion) rather than leading directly back to the heart, branches into a second capillary system in the liver.
LESSER (PULMONARY) CIRCULATION: This circuit carries deoxygenated blood
from the right ventricle of the heart through the pulmonary arteries to the lungs. In the
lungs, carbon dioxide is exchanged for oxygen. Oxygenated blood then returns to the left
atrium of the heart through the pulmonary veins.
Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-
depleted blood away from the heart, to the lungs, and returns oxygenated blood back to
the heart. A separate system known as the bronchial circulation supplies blood to the
tissue of the larger airways of the lung. In the pulmonary circulation, deoxygenated
blood leaves the right ventricle of the heart through the pulmonary artery, enters the
lungs and oxygenated blood comes through the pulmonary veins. The blood then
moves to the left atrium of the heart then to the left ventricle where the blood is
pumped through the semilunar valve into the aorta.
De-02-blood → right atrium → right
·
ventricle → pulmonary artery →
lungs → 02-blood → pulmonary vein
→ Left atrium → left ventricle →
aortic semilunar valve → aorta
AORTA → aortic arch → aortic branches
e
for UPPER BODY → diaphragm → aortic
branches for LOWER BODY → arterial
capillaries → venules → veins → SUP.
VENA CAVA & INF. VENA CAVA → Right
heart (atrium)
Fetal circulation is unique because the fetus relies on the mother for oxygen and nutrient exchange through the placenta, bypassing the
lungs and liver to some extent.
1. Placental Exchange: Oxygenated blood from the placenta enters the fetus through the umbilical vein. This blood bypasses the liver via
the ductus venosus and enters the inferior vena cava, mixing with deoxygenated blood.
2. Bypassing the Lung and the pulmonary circulation: Blood enters the right atrium and is shunted to the left atrium through the
foramen ovale, bypassing the non-functional fetal lungs. From the left atrium, blood moves to the left ventricle and is pumped into the
systemic circulation via the aorta.
3. Ductus Arteriosus: Some blood from the right ventricle still enters the pulmonary artery, but is diverted to the aorta through the
ductus arteriosus, bypassing the lungs.
4. Returning Deoxygenated Blood: Deoxygenated blood returns to the placenta through umbilical arteries, where gas exchange occurs.
Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right
ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta,
called the ductus arteriosus, which directs most of this blood away from the lungs (which aren't being used for respiration at this point as
the fetus is suspended in amniotic fluid): the pulmonary circulation loop is virtually bypassed in fetal circulation. The fetal lungs are
collapsed, and blood passes from:
1. the right atrium directly into the left atrium through the foramen ovale, an open passage between the two atria;
2. through ductus arteriosus, a vessel communication between pulmonary artery and aorta.
At birth, when the infant breathes for the first time, there is a decrease in the resistance in the pulmonary vasculature, which causes the
pressure in the left atrium to increase relative to the pressure in the right atrium. This leads to the closure of the foramen ovale, which is
then referred to as the fossa ovalis. Additionally, the increase in the concentration of oxygen in the blood leads to a decrease in
prostaglandins, causing closure of the ductus arteriosus which remains as ligamentum arteriosum in the adult heart . These closures
prevent blood from bypassing pulmonary circulation, and therefore allow the neonate's blood to become oxygenated in the newly
operational lungs.
