Cardiovascular System

8. The Cardiovascular System

Functions:

o distribution of oxygen and nutrients and thermoregulation

Most circulatory vessels contain 3 tunics (coats):

• tunica intima: thin endothelial lining

• tunica media: middle smooth muscle layer

• tunica adventitia: outer connective tissue coat

3 main types of blood vessels:

Arteries: carry blood away from the heart and distribute it to the body; 3 types:

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1. large elastic (e.g. aorta and its arch branches): many elastic layers allow

expansion when the heart contracts.

2. medium muscular (distributing; e.g. femoral artery): ability to vasoconstrict

regulates blood flow to different parts of the body as required.

3. small arteries and arterioles: narrow lumina and thick muscular walls

Veins: return poorly oxygenated blood to the heart from the capillary beds.

• Have thinner walls compared to companion arteries due to lower blood

pressure in the venous system; variable and frequently anastomose.

• Smallest veins and venules form plexuses.

• Medium veins in limbs: valves permit blood flow toward the heart; another

mechanism is the musculovenous pump.

• Large veins (e.g. SVC and IVC): wide bundles of longitudinal smooth muscle

and a well-developed tunica adventitia

• Varicose veins: abnormally weak and dilated veins as a result of loss of wall

elasticity; > normal caliber and incompetent valves with cusps that do not

meet or have been destroyed by inflammation; most often seen in the legs

Capillaries

• Simple endothelial tubes: connection between arterial and venous sides of

circulation

• Arranged in networks: capillary beds between arterioles and venules

• Allow the exchange of oxygen, nutrients, and other cellular materials with

surrounding tissues.

• Arteriovenous anastomoses (AV) shunts permit blood to bypass the

capillaries; numerous in the skin – conserve body heat.

Major Vascular Systems

Pulmonary circulation

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This system carries deoxygenated blood from the heart to the respiratory surfaces of

the lungs for oxygenation and back into the heart for distribution to the body.

right ventricle  pulmonary trunk (arteries)  lungs  pulmonary veins  left

atrium

Systemic circulation

All systemic arteries arise from the aorta. The aorta can be divided into 4 main parts:

ascending aorta, aortic arch, descending thoracic aorta, and abdominal aorta.

Each of these sections has important branches:

1. Ascending aorta

• right and left coronary arteries (supply the heart)

2. Aortic arch

• arteries to head, neck, upper limbs

3. Descending thoracic and abdominal aorta

• arteries to thorax, abdomen, pelvis, lower limbs

Deoxygenated blood is carried toward the heart by 3 major veins:

1. Superior vena cava (SVC)

• drains the head and upper limbs

2. Inferior vena cava (IVC)

• drains the thorax, abdomen, and lower limbs

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3. Coronary sinus

• drains the wall of the heart

Azygos venous system

This system drains blood from the thoracic wall and some thoracic viscera into the

SVC and includes 3 unpaired vessels: azygos, hemiazygos, and accessory

hemiazygos veins. These veins are highly variable among individuals.

Hepatic portal venous system

• Relays digestive products from the stomach and intestines to the liver for

processing.

• Formed by the superior mesenteric, splenic, and inferior mesenteric

veins.

• This system drains deoxygenated, but nutrient-rich blood to the liver via the

hepatic portal vein for processing, detoxification, and storage.

• Processed blood is conveyed into the systemic venous circuit by the hepatic

veins that drain into the IVC.

Heart

A muscular double pump with two functions:

1. right side – receives oxygen-poor blood from body tissues and pumps it to the lungs

for gas exchange; blood vessels that carry blood to and from the lungs form the

pulmonary circuit.

2. left side – receives oxygenated blood and pumps it throughout the body to supply

nutrients to the body tissues; vessels that transport blood to and from all body tissues

form the systemic circuit.

Chambers of the heart

2 receiving chambers:

o right atrium and left atrium – receive blood from the systemic and pulmonary circuits.

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2 pumping chambers:

o right ventricle and left ventricle – pump blood around the two circuits.

Position of the heart within the thorax:

o posterior to the sternum and costal cartilages.

o rests on the superior surface of the diaphragm.

o assumes an oblique position within the thorax, its apex lies to the left of the midline

and anterior to the rest of the heart.

Coverings of the heart: pericardium – a triple-layered sac that encloses the heart.

Layers of the pericardium:

o fibrous pericardium – the outer sac – a strong layer of dense connective tissue that

holds the heart in place and keeps it from overfilling with blood.

o serous pericardium – deep to the fibrous pericardium, formed from two layers:

1. parietal layer – adheres to the inner surface of the fibrous pericardium.

2. visceral layer – also called epicardium, and considered the outer layer of the heart

wall.

Layers of the heart wall:

o epicardium – superficial.

o myocardium – the bulk of the heart, consists of cardiac muscle – muscle bundles

arranged in circular and spiral patterns.

o endocardium – deep layer, lines the heart chambers and valves.

Great vessels of the heart

o superior vena cava (SVC) – drains all body regions superior to the diaphragm (except

the heart wall).

o aorta – largest artery of the body.

o pulmonary trunk – carries blood from the heart to the lungs for oxygenation.

o inferior vena cava (IVC) – returns blood to the heart from all areas of the body inferior

to the diaphragm.

Heart chambers

o internal divisions – between the atria and ventricles → interatrial and interventricular

septa.

o external divisions – boundaries between the four chambers are marked by grooves:

1. coronary sulcus – divides the atria from the ventricles.

2. anterior and posterior interventricular sulci – separate the ventricles.

Right atrium

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o receives oxygen-poor blood from the systemic circuit: superior and inferior

vena cavae and the coronary sinus (drains the heart itself).

o right auricle.

o pectinate muscles – rough anterior portion of the muscular wall, (posterior

atrial wall is smooth).

o crista terminalis – a muscular ridge separating the anterior and posterior

parts of the atrium.

o fossa ovalis – a depression in the interatrial septum, remnant of the foramen

ovale.

o opens into the right ventricle through the tricuspid valve (right atrioventricular

valve).

Right ventricle

o receives blood from the right atrium and pumps it into the pulmonary circuit via the

pulmonary trunk.

o trabeculae carneae – ventricular muscle ridges.

o papillary muscles – project from the walls into the ventricular cavity.

o chordae tendineae – strong bands that project superiorly from the papillary muscles

and anchor the cusps of the tricuspid valve.

o pulmonary semilunar valve – at the opening between the right ventricle and

pulmonary trunk.

Left atrium

o receives oxygen-rich blood returning from the lungs via the four pulmonary veins.

o smooth interior, pectinate muscles only in the left auricle

o pumps blood through the bicuspid valve (mitral, or left atrioventricular) into left

ventricle.

Left ventricle

o similar to the right ventricle, but its walls are three times thicker and the structures are

larger.

o pumps blood through the aortic semilunar valve to the aorta.

Pathway of blood through the heart

Pulmonary circuit: superior and inferior vena cava, coronary sinus → right atrium → right

ventricle → pulmonary trunk → lungs

Systemic circuit: lungs → pulmonary veins → left atrium → left ventricle → aorta → body

Heart valve structure

o atrioventricular (AV) valves – between the atria and ventricles.

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1. right atrioventricular valve – 3 cusps (tricuspid)

2. left atrioventricular valve – 2 cusps (bicuspid, mitral)

o aortic and pulmonary valves – at the junctions of the ventricles and great arteries

(aorta and pulmonary trunk); also called semilunar due to the shape of their cusps.

Heart valves open and close in response to differences in blood pressure on each side of the

valves.

o the two atrioventricular valves prevent the backflow of blood into the atria during

contraction of the ventricles.

o the semilunar valves prevent backflow from the great arteries into the ventricles.

Conducting system of the heart

Cardiac muscle cells have the intrinsic ability to:

o generate and conduct impulses.

o signal these same cardiac muscle cells to contract rhythmically.

o conducting system – a series of specialized cardiac muscle cells.

o sinoatrial (SA) node (pacemaker) sets the inherent rate of contractions (in the wall of

the right atrium).

o from the SA node, impulses spread to the atria, signaling the atria to contract,

followed by a shortly delayed contraction of the ventricles.

Blood Supply of the Heart

Arterial supply: coronary arteries are the first two branches of the aorta:

o right coronary artery – runs in the coronary sulcus.

o left coronary artery – splits into 2 branches: anterior interventricular and circumflex

arteries.

Venous Drainage:

o great, middle, and small cardiac veins converge into the coronary sinus that empties

into the right atrium.

Pulmonary circulation – carries blood to and from the lungs for oxygenation.

Systemic circulation – supplies blood to all parts of the body except the lungs.

Azygos system

o this venous system drains blood from the thoracic wall and some thoracic viscera into

the superior vena cava.

o consists of 3 unpaired vessels: the azygos, hemiazygos, and accessory hemiazygos

veins.

o these vessels are highly variable among individuals.

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Hepatic portal system

o relays digestive products from the stomach and intestines, and red blood cell

products to the liver for processing.

o formed by the superior mesenteric, splenic, and inferior mesenteric veins.

Development of the Cardiovascular System

Mesoderm gives rise to the vascular system: heart, arteries, veins, lymph vessels, and all

blood and lymph cells.

A. Blood vessels development

The cardiovascular system is the 1st major system to function in the embryo. The

primordial heart and vascular system appear in the middle of the 3rd week of embryonic

development. The earliest blood vessels begin to develop as condensations of

mesodermal mesenchyme called blood islands surrounding the wall of the yolk sac in

the 3-week embryo. Initially, the vessels only consist of endothelium, but adjacent

mesenchyme cells soon surround these tubes and form the muscular and fibrous tunics

of the vessel walls. The blood islands will give rise to blood cells, vessels, and the heart.

B. Heart development

The blood islands that would eventually become the heart form in the splanchnic

mesoderm around the future head and neck of the embryonic disc (a horseshoe-shaped

cardiogenic region). Initially, the central portion of the cardiogenic area is located

anterior to the neural plate. As the flat trilaminar disc begins to fold as a result of neural

tube closure and rapid CNS growth in a cranial direction, the heart (which at this point is

a pair of tubular structures) moves into the cervical and then finally the future thoracic

area.

➢ ~Day 20, the 2 endothelial heart tubes fuse together to create a single tube.

➢ ~Day 22, the single heart tube is now able to pump; it develops a sequence of bulges

in a caudal to cranial direction – the earliest unpaired heart chambers, including:

1. sinus venosus: initially receives all blood from the embryonic veins; destined

to become the smooth-walled part of the right atrium, coronary sinus, and SA

node; also contributes to the posterior wall of the left atrium.

2. atrium: forms pectinate muscled walls of the right and left atria.

3. ventricle: forms the left ventricle; this is the strongest pumping chamber of

the early heart.

4. bulbus cordis: this chamber and its most cranial extension, the truncus

arteriosus , give rise to the right ventricle, pulmonary trunk, and 1st part of the

aorta.

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5. ~Day 23-28, the heart tube begins to bend into an S-shape (cardiac looping),

eventually bringing the ventricles and atria to their respective adult caudal and

cranial positions.

6. During month 2 of development, the embryonic heart starts to undergo a

series of changes that will result in the formation of septa and valves that

establish the 4 definitive heart chambers. Endocardial cushions are regional

thickenings of the endocardium that help to from the heart’s midline septum

and valves. Most of the interatrial septum forms by growing caudally from the

heart’s roof, while most of the interventricular septum forms by growing

cranially from the heart’s apex. In addition, neural crest cells migrate into the

area where atrium meets ventricle and contribute to the developing heart

valves and the bases of the pulmonary trunk and ascending aorta – the great

arteries that form by the splitting of the bulbus cordis.

C. Circulatory system development

All major vessels are in place by the 3rd month of development and blood is flowing

through these vessels in the same direction as in the adult. Two significant differences

exist between fetal and postnatal circulation. During pregnancy, the placenta provides

both oxygen and nutrients to the fetus. The fetus must supply blood to the placenta.

Furthermore, fetal circulation is different in that it bypasses the lungs and most of the

liver.

During fetal development, 3 early vascular systems are established: intra-embryonic –

aorta and cardinal veins in the embryo, vitelline – paired vitelline arteries and veins to

the yolk sac, and placental – umbilical arteries and vein to the placenta.

1. Embryonic system: vessels supply blood to and drain blood from the developing

embryonic tissues.

2. Vitelline system: vessels supply blood to and drain blood from the yolk sac; this

system contributes to the blood supply of the gastrointestinal tract.

3. Placental (umbilical) system: includes paired umbilical arteries that deliver

deoxygenated blood and fetal wastes to the placenta and umbilical vein that

transports highly oxygenated, nutrient-rich blood from the placenta.

D. Adult remnants of fetal structures

Several structures in the adult are remnants of functional circulatory components of the fetus:

1. Umbilical vein: oxygenated blood returns to the fetus from the placenta through

this vein. The remnant of this structure is the ligamentum teres (round ligament of

the liver).

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2. Ductus venosus: shunts some oxygenated blood away from the liver to the IVC in

the fetus. The remnant of this structure is the ligamentum venosum, which

passes through the liver from the left branch of the portal vein to the IVC, to which

it is attached.

3. Umbilical arteries: carry oxygen-poor blood from the fetus to the placenta;

become the medial umbilical ligaments.

4. Ductus arteriosus: shunts blood from the pulmonary trunk into the ascending

aorta in the fetus, bypassing the lungs; becomes the ligamentum arteriosum in

the adult, a structure that connects the left pulmonary artery to the aortic arch.

5. Foramen ovale: an opening in the interatrial wall that shunts blood away from the

non-functional lungs; becomes the fossa ovalis.