The Circulatory System
● The circulatory system is responsible for transporting fluids throughout the body
● It is composed of two main components:
1. Cardiovascular system
2. Lymphatic system
Cardiovascular System S
● The cardiovascular system forms the blood transportation network
● Composed of:
○ The heart
○ The blood vessels (arteries, capillaries, veins)
● Function:
○ The heart pumps blood through the body's extensive vascular network
○ Blood serves as a transport medium for:
■ Nutrients
■ Oxygen
■ Waste products (e.g., carbon dioxide, metabolic waste)
■ All to and from the body’s cells
Vascular Circuits
● The heart is made up of two muscular pumps:
1. Right heart (right atrium + right ventricle)
2. Left heart (left atrium + left ventricle)
● Although located side by side, the two pumps function in series, forming two main
circulatory circuits:
1. Pulmonary circulation & O2
lungs
to get
to
2. Systemic circulation 7 to cells to remove O2
Pulmonary Circulation
● Begins in the right ventricle
● Function:
○ Propels low-oxygen blood (returning from systemic circulation)
○ Into the lungs via the pulmonary arteries
● In the lung capillaries:
○ Carbon dioxide is exchanged for oxygen
● Oxygen-rich blood is returned to the left atrium via the pulmonary veins
● This complete loop—right ventricle → lungs → left atrium—defines the pulmonary
circulation
,Systemic Circulation
● Begins in the left ventricle
● Function:
○ Propels oxygen-rich blood (received from pulmonary circulation)
○ Into the systemic arteries (including the aorta and its branches)
● In systemic capillaries throughout the body:
○ Oxygen and nutrients are exchanged for carbon dioxide and metabolic
waste
● Low-oxygen blood returns to the right atrium of the heart via:
○ The systemic veins, which are tributaries of the:
■ Superior vena cava
■ Inferior vena cava
● This circuit—left ventricle → body tissues → right atrium—defines the systemic
circulation
Systemic Circulation – Parallel Circuits
● The systemic circulation is not a single loop but rather:
○ A collection of multiple parallel circuits
○ These circuits supply different regions and organ systems of the body
individually
Blood Vessels
There are three primary types of blood vessels in the circulatory system:
● Arteries
● Veins
● Capillaries
Arteries and Arterioles
● Blood under high pressure exits the heart through a branching system of
thick-walled arteries
● These arteries distribute oxygen-rich blood to various regions of the body
● The terminal branches of arteries are called arterioles
○ Arterioles deliver oxygenated blood to the capillary networks
Capillaries
● Capillaries form a capillary bed
● This is the site of interchange between blood and extracellular fluid, including:
○ Oxygen
, ○ Nutrients
○ Waste products
○ Other substances
Venules and Veins
● Blood from the capillary beds drains into venules, which:
○ Are thin-walled
○ Resemble wide capillaries
● Venules merge into small veins, which in turn open into larger veins
● The largest veins in the body are:
○ The superior vena cava
○ The inferior vena cava
● These major veins return low-oxygen blood to the heart
Structure of Blood Vessels
Most blood vessels of the circulatory system are composed of three concentric layers, called
tunics:
1. Tunica intima
○ Innermost layer
○ Composed of a single layer of extremely flattened epithelial cells (known as
the endothelium)
○ Supported by delicate connective tissue
○ Capillaries consist only of this layer
■ In blood capillaries, the tunica intima is also supported by a basement
membrane
2. Tunica media
○ Middle layer
○ Composed primarily of smooth muscle
○ Most variable of the three layers
○ In arteries, may contain variable amounts of elastic fibers
3. Tunica adventitia
○ Outermost layer
○ Made of connective tissue
○ Functions as a sheath providing external support and anchoring
Distinguishing Features
● Arteries, veins, and lymphatic ducts can be differentiated based on:
○ The relative thickness of the tunica media
○ The size of the lumen
○ The organization of the layers
, ○ In arteries, the presence of elastic fibers in the tunica media
ARTERIES
● Arteries are blood vessels that carry blood under relatively high pressure (compared
to corresponding veins) from the heart and distribute it throughout the body
● As blood flows through the arterial system, it moves through arteries of progressively
decreasing caliber
Classification of Arteries
Arteries are categorized based on:
● Overall size
● Relative amounts of elastic tissue or smooth muscle in the tunica media
● Wall thickness and structure, which gradually changes from one type of artery to
another (forming a morphological continuum)
There are three main types of arteries:
1. Large Elastic Arteries (Conducting Arteries)
● Contain numerous elastic layers (sheets of elastic fibers) in their walls
● These arteries are the first to receive cardiac output directly from the heart
Function:
● Their elasticity allows them to:
○ Expand when they receive high-pressure blood from ventricular contraction
○ Recoil between contractions to maintain forward flow
● This function:
○ Minimizes pressure changes during the cardiac cycle
○ Maintains arterial pressure between heartbeats
○ Reduces the ebb in pressure as the heart alternately contracts and relaxes
Examples:
● Aorta
● Arteries originating from the arch of the aorta:
○ Brachiocephalic trunk
○ Subclavian arteries
○ Carotid arteries
● The circulatory system is responsible for transporting fluids throughout the body
● It is composed of two main components:
1. Cardiovascular system
2. Lymphatic system
Cardiovascular System S
● The cardiovascular system forms the blood transportation network
● Composed of:
○ The heart
○ The blood vessels (arteries, capillaries, veins)
● Function:
○ The heart pumps blood through the body's extensive vascular network
○ Blood serves as a transport medium for:
■ Nutrients
■ Oxygen
■ Waste products (e.g., carbon dioxide, metabolic waste)
■ All to and from the body’s cells
Vascular Circuits
● The heart is made up of two muscular pumps:
1. Right heart (right atrium + right ventricle)
2. Left heart (left atrium + left ventricle)
● Although located side by side, the two pumps function in series, forming two main
circulatory circuits:
1. Pulmonary circulation & O2
lungs
to get
to
2. Systemic circulation 7 to cells to remove O2
Pulmonary Circulation
● Begins in the right ventricle
● Function:
○ Propels low-oxygen blood (returning from systemic circulation)
○ Into the lungs via the pulmonary arteries
● In the lung capillaries:
○ Carbon dioxide is exchanged for oxygen
● Oxygen-rich blood is returned to the left atrium via the pulmonary veins
● This complete loop—right ventricle → lungs → left atrium—defines the pulmonary
circulation
,Systemic Circulation
● Begins in the left ventricle
● Function:
○ Propels oxygen-rich blood (received from pulmonary circulation)
○ Into the systemic arteries (including the aorta and its branches)
● In systemic capillaries throughout the body:
○ Oxygen and nutrients are exchanged for carbon dioxide and metabolic
waste
● Low-oxygen blood returns to the right atrium of the heart via:
○ The systemic veins, which are tributaries of the:
■ Superior vena cava
■ Inferior vena cava
● This circuit—left ventricle → body tissues → right atrium—defines the systemic
circulation
Systemic Circulation – Parallel Circuits
● The systemic circulation is not a single loop but rather:
○ A collection of multiple parallel circuits
○ These circuits supply different regions and organ systems of the body
individually
Blood Vessels
There are three primary types of blood vessels in the circulatory system:
● Arteries
● Veins
● Capillaries
Arteries and Arterioles
● Blood under high pressure exits the heart through a branching system of
thick-walled arteries
● These arteries distribute oxygen-rich blood to various regions of the body
● The terminal branches of arteries are called arterioles
○ Arterioles deliver oxygenated blood to the capillary networks
Capillaries
● Capillaries form a capillary bed
● This is the site of interchange between blood and extracellular fluid, including:
○ Oxygen
, ○ Nutrients
○ Waste products
○ Other substances
Venules and Veins
● Blood from the capillary beds drains into venules, which:
○ Are thin-walled
○ Resemble wide capillaries
● Venules merge into small veins, which in turn open into larger veins
● The largest veins in the body are:
○ The superior vena cava
○ The inferior vena cava
● These major veins return low-oxygen blood to the heart
Structure of Blood Vessels
Most blood vessels of the circulatory system are composed of three concentric layers, called
tunics:
1. Tunica intima
○ Innermost layer
○ Composed of a single layer of extremely flattened epithelial cells (known as
the endothelium)
○ Supported by delicate connective tissue
○ Capillaries consist only of this layer
■ In blood capillaries, the tunica intima is also supported by a basement
membrane
2. Tunica media
○ Middle layer
○ Composed primarily of smooth muscle
○ Most variable of the three layers
○ In arteries, may contain variable amounts of elastic fibers
3. Tunica adventitia
○ Outermost layer
○ Made of connective tissue
○ Functions as a sheath providing external support and anchoring
Distinguishing Features
● Arteries, veins, and lymphatic ducts can be differentiated based on:
○ The relative thickness of the tunica media
○ The size of the lumen
○ The organization of the layers
, ○ In arteries, the presence of elastic fibers in the tunica media
ARTERIES
● Arteries are blood vessels that carry blood under relatively high pressure (compared
to corresponding veins) from the heart and distribute it throughout the body
● As blood flows through the arterial system, it moves through arteries of progressively
decreasing caliber
Classification of Arteries
Arteries are categorized based on:
● Overall size
● Relative amounts of elastic tissue or smooth muscle in the tunica media
● Wall thickness and structure, which gradually changes from one type of artery to
another (forming a morphological continuum)
There are three main types of arteries:
1. Large Elastic Arteries (Conducting Arteries)
● Contain numerous elastic layers (sheets of elastic fibers) in their walls
● These arteries are the first to receive cardiac output directly from the heart
Function:
● Their elasticity allows them to:
○ Expand when they receive high-pressure blood from ventricular contraction
○ Recoil between contractions to maintain forward flow
● This function:
○ Minimizes pressure changes during the cardiac cycle
○ Maintains arterial pressure between heartbeats
○ Reduces the ebb in pressure as the heart alternately contracts and relaxes
Examples:
● Aorta
● Arteries originating from the arch of the aorta:
○ Brachiocephalic trunk
○ Subclavian arteries
○ Carotid arteries
