Lecture note: General anatomy of cardiovascular system

 

Cardiovascular system

It is a transport system that carries blood and lymph to and from the tissues of the body It consists of the heart, blood vessels, and blood which circulates oxygen and nutrients and removes carbon dioxide and the waste products of metabolism  

Types of circulation

 Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body and returns deoxygenated blood back to the heart.

Flow chart of systemic circulation

Left ventricle of heart > artery > capillary > venule > vein > right atrium of heart > right ventricle > pulmonary artery (carry deoxygenated blood ) > lung >  four pulmonary vein (carry oxygenated blood ) > left atrium > left ventricle

 

Pulmonary circulation: it  is the portion of the cardiovascular system which carries deoxygenated blood away from the heart, to the lungs, and returns oxygenated (oxygen-rich) blood back to the heart.

Flow chart of pulmonary circulation

Right ventricle > pulmonary artery (carry deoxygenated blood) > lung >  four pulmonary vein (carry oxygenated blood ) > left atrium

Portal circulation: it is the part of the systemic circulation  in  which blood collected from one set of capillaries passes through a large vessel or vessels, and drain into another set of capillaries before draining into the systemic vein.

Classification of portal circulation

1. Arterial  portal system, EX: renal portal system

 

Portal circulation	Systemic artery	1st set capillary	Portal Vessel/ vessels	2nd set capillary	Draining Veins
Renal portal system	Afferent arteriole branch of renal artery	Glomerular capillaries	Efferent arterioles	Peritubular capillaries & vasa recta	Renal vein drain into IVC

 

2. Venous portal system : EX: hepatic portal system, pituitary portal system

 

Portal circulation	Systemic artery	1st set capillary	Portal Vessel/ vessels	2nd set capillary	Draining Veins
a) Hepatic portal system	Coeliac trunk Superior & inferior mesenteric artery	Intestinal capillaries	Portal vein	Hepatic sinusoid	Hepatic vein drain into IVC

Portal circulation	Systemic artery	1st set capillary	Portal Vessel/ vessels	2nd set capillary	Draining Veins
b)Pituitary portal system	Superior & inferior hypophyseal arteries	Hypothalamic capillary & Posterior pituitary capillaries	Long & short portal vessels	Capillaries of anterior pituitary	Hypophyseal vein drain into the cavernous sinus

 

 Summary diagram of all cardiovascular circulation

Sequence of blood vessel in circulatory system

SN:  heart > elastic artery(aorta) > medium sized artery > small sized artery > arteriole >

terminal arteriole > meta-arteriole > pre-capillary sphincter > capillary >venule –vein ---heart

Blood vessels, the components of the circulatory system which is a tubular structure carrying blood through the tissues and organ. e.g. a vein, artery, or capillary.

Functions: They transport the blood for nutrition, respiration and excretion of the waste products of the body

Classification of blood vessels Arteries: three types of artery large (elastic) artery, medium (muscular) sized artery, small artery Arterioles Capillaries Venules and  veins

Functional classification of blood vessels

1. Conducting vessels: large arteries e.g. aorta

2. Distributing vessel: muscular arteries e.g. brachial artery, femoral artery

3. Resistance vessels: Small artery, arterioles etc.

4. Exchange/metabolic vessels: capillaries

5. Capacitance vessels: veins

Artery is a blood vessel takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; except  pulmonary and the umbilical arteries, which carry deoxygenated blood to the organs that oxygenate it (lungs and placenta, respectively).

Types of artery :1. Elastic/ large artery 2. muscular / medium sized artey  3. small sized artery

Generalized structure of blood vessels

From within outwards the artery presents three coats –

1.      Tunica intima: is consists of -

·         Endothelium with its basement membrane

·         subendothelial connective tissue

·         internal elastic lamina

2.       Tunica media :it is the thickest layer in case artery. This layer consist of --

 circumferentially arranged smooth muscle cells with  elastic & collagen fiber. External elastic lamina separate tunica media from tunica adventitia.  

Tunica adventitia:  it is the thickest layer in case vein, it is formed by connective tissue rich in collagen and elastic fibers.

 

Histology of Elastic artery

Tunica intima consist of *endothelium with its basal lamina *subendothelial connective tissue is present (contain smooth muscle, collagen and elastic fibres) *an inconspicuous internal elastic membrane (internal elastic membrane present but difficult to visualized in histological slide due to large amount of elastic fibres present in elastic artery).

Tunica media consist of large amount of elastic lamellae and smooth muscle cells present in between the elastic lamella.

Tunica adventitia Relatively thinner, form by connective tissue rich in collagen & elastic fibres Example:  Aorta/Pulmonary arteries,Brachiocephalic arteries / Common carotid artery.

 

What is the funtional importance of elastic artery ?

The walls of large elastic arteries (e.g. aorta, common carotid, subclavian, and pulmonary arteries and their larger branches) contain elastic fibers which distend  during systole and recoil when the blood pressure falls during diastole which prevent the fluctuation of  blood pressure (pulse pressure) over the cardiac cycle and maintain the organ perfusion during diastole when cardiac ejection ceases. 

Why large artery also known as elastic artery?

Large arteries are those nearest the heart (aorta and pulmonary arteries) that contain much more elastic tissue in the tunica media than muscular arteries. So it is also known as elastic artery.

Histology of medium sized artery

Tunica intima consist of  *endothelium with its basal lamina *subendothelial connective tissue is present  (contain smooth muscle, collagen and elastic fibres) and *Prominent internal elastic membrane 

Tunica media consist of circumferentially arranged smooth muscle cells,

 with little elastic, collagen and reticular fibers 

Tunica adventitia Relatively thicker form by connective tissue rich in collagen & elastic fibres ,

external elastic membrane separated it from the tunica media

Example Brachial artery, femoral artery, external carotid artery

 

Why medium sized arteries, also known as muscular artery ?

Ans: the medium sized artery includes the anatomically named arteries like the brachial artery, the radial artery, and the femoral artery, for example. Medium sized arteries contain more smooth muscle cells in the tunica media layer than the elastic arteries, so it is also known as muscular artery.

Microcirculation  is the blood flow through the smallest vessels in the circulatory system, i.e. arterioles, venules, shunts and capillaries. Microcirculation exists in all tissues and organs except for the cornea.  

Example Splenic microcirculation, Renal microcirculation involves two capillary systems: the glomerular capillary system and peritubular capillary system

 

Small artery / Arterioles:  they are the smallest divisions of the muscular arteries possessing three coats. They successively form terminal arteriole and meta-arterioles.

 Arterioles do not have an external elastic lamina, whereas the tunica adventitia is scant and composed by fibroelastic connective tissue housing a few fibroblasts. 

Histology of small artery or arterioles

Topics	Small arteries	Arterioles
Tunica media	Up to eight layers	Only one or two layers
Internal elastic membrane	Present	May or may not present
External elastic membrane	Present	Absent
Tunica adventitia	Thin, ill defined	Thin, ill defined

Functions of arterioles:

1. Regulate the amount of blood entering into the capillaries (by pre-capillary sphincter)
2. They offer peripheral resistance and regulate systolic arterial pressure

Meta arteriole: it is a short micro vessel in the microcirculation that links arterioles and capillaries. Meta –arteriole terminate as capillary. The smooth muscle layer of meta-arterioles is not continuous; rather, individual muscle cells are spaced apart, and each encircles the endothelium of a capillary arising from the meta-arteriole, allowing them to act as a sphincter upon contraction, controlling blood flow into the capillary bed.

Why small diameter blood vessel in the microcirculation known as resistance vessels?

Small arteries,  arterioles &  precapillary sphincters are known as resistance vessels .

These arteries contain  relatively thick muscular walls and narrow lumen which contribute the most  the resistance to blood flow. So it is known as resistance vessels.

 

Pre-capillary sphincter: is the slight thickening of the smooth muscle at the junction of meta- arteriole and the origin of a capillary bed that adjusts blood flow into capillaries.

Thoroughfare channel  

: it  connects the metarteriole to a venule.

Arteriovenous anastomoses : they are low-resistance connections between the arterioles (smaller arteries) and venules (smaller veins). They shunt blood directly into the venous plexus of the skin, bypassing the capillaries.

Location: They are plentiful in the skin and. mostly found in the tip of the nose, lips, lobule of the ear, finger tip, nail bed etc.

Glomera: the arteriole continuous with venule and lose the internal elastic membrane and smooth muscle cells layer is thick.. Example: in fingerpads, fingernail beds and ears.                                 

Function: they have a role in temperature regulation

Venous system (reservoir vessels)

 There are four vein in  venous system

1.      Caval (drain blood into right atrium of heart)

2.      Portat system ( connect two sets of capillary)

3.      Azygous system (straight an paravertebral in position. drain blood in to caval system)

4.      Para vertebral vein (valveless vein lying within vertebral canal)

Large vein

It  has three layers- from within outward

Tunica intima is consist of  endothelial with its basal lamina, a small amount of sub-endothelial connective tissue and some smooth muscle cells but no internal elastic lamina.  

Tunica media is relatively thin and contain circumferentially arranged smooth muscle cells, collagen and elastic fibers.   

Tunica adventitia: is the thickest layer, formed by connctive tissue with collagen, elastic fibers, fibroblast and longitudinally arranged smooth muscle cells.

Valves of vein histologically it is formed by reduplication of tunica intima extend into the lumen of the vein.

 It  consist of collagen fibers, elastic fibers, smooth muscles cells, and covering endothelium.^.  

Smooth muscle cells exist only in the thick part of the valve leaflet. Vein of the extremities contain valves but trunk of body devoid of valve.

Function of valves: it permits blood flow in one direction so blood goes toward heart from periphery.

Histological Difference between artery and vein

Topics	Artery	Vein
Tunica intima	contain prominent internal elastic lamina	internal elastic lamina is absent
Tunica media	Thickest layer of artery , contain many layers of smooth muscle	Thinner then the artery, contain few layers of smooth muscle cells
Tunica adventitia	Thinner then the tunica media of artery, formed by collagenous connective tissue	Thickest layer of vein , formed by collagenous connective tissue
Wall	thicker	thinner
Location	deeper part of body	more superficial part of body
Lumen	narrow	wide
Valve	absent	present
Carry	Oxygenated blood away from the heart (except pulmonary artery)	Deoxygenated blood towards the heart (Except pulmonary veins).
After death, lumen is	empty	contain blood

Features and Difference between elastic artery , medium sized artery and vein

Topics	Features of Elastic artery	Features of  Medium sized artery	Vein
Tunica intima	consist of  *endothelium with its basal lamina *subendothelial connective tissue is present (contain smooth muscle, collagen and elastic fibres) *an inconspicuous internal elastic membrane  ( internal elastic membrane present but difficult to visualized in histological slide due to large amount of elastic fibres present in elastic artery)	*endothelium with its basal lamina *subendothelial connective tissue is present  (contain smooth muscle, collagen and elastic fibres) and *Prominent internal elastic membrane	endothelium with its basal lamina *subendothelial connective tissue is present  (contain smooth muscle, collagen and elastic fibres) and no internal elastic lamina
Tunica media	consist of large amount of elastic lamellae and smooth muscle cells present in between the elastic lamella	circumferentially arranged smooth muscle cells,  with little elastic, collagen and reticular fibers	circumferentially arranged smooth muscle cells,  with little elastic, collagen and reticular fibers
Tunica adventitia	Relatively thinner form by connective tissue rich in collagen & elastic fibres	Relatively thicker form by connective tissue rich in collagen & elastic fibres , external elastic membrane separated it from the tunica media	Thickest layer of vein Contain fibroblast and fibrocyte collagen & elastic fibres ,
Example	•       Aorta/Pulmonary arteries •       Brachiocephalic arteries / Common carotid artery	Brachial artery, femoral artery, external carotid artery	Inferior vena cava, brachiocephalic vein

Vascular anastomosis

Anastomoses between arteries

In Actual anastomoses arteries meet end to end. e.g. Labial branches of two facial arteries.

Potential  Anastomoses is occurred by terminal arterioles. It needs sufficient time for dilatation, sudden occlusion of main artery, this anastomoses is failed to nourish the affected part. e.g. coronary arteries.  

 End arteries, the arteries which do not form any pre-capillary anastomoses.

They are two types

Functional end artery contain insufficient pre-capillary anastomosis  e.g. coronary arteries.	True end artery  no pre-capillary anastomosis  e.g. Central artery of retina.

 Vasa venorum:  It (Latin, "the vessels of the vessels") is a network of small blood vessels  that supply the walls of large blood vessels. such as elastic arteries (e.g., the aorta) and large veins (e.g., the venae cavae).

Function : The vasa vasorum are found in large veins and arteries such as the aorta and its branches. These small vessels serve to provide blood supply and nourishment for tunica adventitia and outer parts of tunica media of large vessels.The  coronary arteries of the heart is the vasa vesorum of heart

Cavernous tissues  are blood filled spaces lined by endothelium & surrounded by traeculae (formed by smooth muscle cells ). The arteriole and venule directly open into it. e.g. penis, clitoris.

Blood flow through cavernous tissue during erection of penis

Cavnous artery> helical artery > sinusoidal space fills by arterial blood > no venous return

Blood flow through cavernous tissue during flaccid state of penis

Cavnous artery> helical artery > sinusoidal space > emissary vein > vein  

 Capillary is formed by a single layer of flattened endothelium resting on a basal lamina

 Function   

• It provides nutrition and oxygen to cells
• Helps to removal of waste product from cells like carbondioxide . 
• Anti-thrombogenic
• Metabolic functions

   Classification of capillaries

Name of capillaries	Features &example	Figure
Continuous capillary	The endothelial cells  & basal lamina (non-cellular protein-polysaccharide rich layer) form a continuous uninterrupted layer e.g. In muscle, lung & CNS
Fenestrated capillaries with diaphragm	The fenestrated, or visceral, capillaries are characterized by the presence of several circular transcellular openings in the endothelium membrane called fenestrae which  is obliterated by a diaphragm that is thinner than a cell membrane. The hydrophobic barrier may be absent in these diaphragms. The basal lamina of the fenestrated capillaries is continuous. e.g. Kidney, intestine and endocrine glands
Fenestrated capillaries without diaphragm	Fenestrated endothelium but no diaphragm & a very thick basal lamina. e.g. The renal glomerulus
Discontinuous capillaries (sinusoidal capillaries)	The endothelial cells & their basal lamina discontinuously arranged e.g. Liver, the bone marrow, spleen and adrenal medulla

        Circulation of lymph is the part of circulatory system which is discuss in lymphatic system.