Definition

Blood vessels

Portal system

Valves

Anastomoses

Collateral circulation

lymphatic

Term angiology derived from Greek word angeion=vessel, tube

The science of blood vessels is termed as angiology

Blood vessels form a system of closed tubes

Along the route is a muscular pump, the heart

Three types of blood vessels

Arteries,

Capillaries

Veins

Are vessels carrying blood away from the heart and distribute it to the body

when cut bleed in spurts

have no valves

Their walls have three coats or tunics

Tunica adventitia (outer coat)

Tunica media (middle coat)

Tunica intima (inner coat)

Three types of arteries based on their coats, especially the tunica media’s;

thickness, and

Constituents

Elastic arteries (conducting arteries) Muscular arteries (distributing arteries) Arterioles

Elastic arteries (conducting arteries)

Largest type e.g. the Aorta

High degree of elasticity;

permits considerable expansion when the heart contracts and return to normal between contractions

Muscular arteries (distributing arteries)

Medium sized arteries e.g. Femoral artery

Their walls chiefly consist of circular smooth muscle fibers

contract under autonomic stimuli to diminish the lumen of the artery.

Arterioles

Smallest arteries with a diameter of less than 0.1 mm

Have relatively thick walls in relation to their lumen

The walls are almost wholly smooth muscle

The degree of arterial pressure within vascular system is regulated mainly by degree of tonus (firmness) in the smooth muscle in the arteriolar walls

If the tonus is above normal, hypertension results

Elasticity decreases with the size of the arteries while the muscularity increases.

Are microscopic vessels with walls only one cell layer thick (endothelium)

Cells are flat, making the wall a semi permeable membrane which allows water, certain plasma proteins to pass through

Connect the arterial and venous sides of the circulation

Capillaries form an anastomosing (network) which is rich in active tissues e.g. liver, muscles, kidneys

In less active tissues like tendons and ligaments, capillaries are very few

No capillaries are found in

Cornea and lens of the eye

Epithelium and hyaline cartilage

Hair and nails

Capillaries unite and form venules (small veins)

More numerous than arteries

Return blood from capillary bed to the heart

Have thinner walls and larger diameters than the corresponding arteries due to low blood pressure in the venous system

Medium-sized veins in the limbs where blood flow is against gravity have flap valves that permit blood flow only towards the heart

Medium sized deep arteries especially in the limbs are often flanked by two veins with numerous intercommunications (venae comitantes)

Systemic veins are more variable than arteries and anastomoses between two veins occur more often

System of vessels interposed between two capillary beds

Thus blood from one set of capillaries passes through a second set before entering the systemic veins

E.G. Hepatic and hypophyseal portal systems

Present in many veins allowing flow in one direction (prevent reverse blood flow)

Absent in large veins of the trunk

e.g. S.V.C proximal to the internal jugular and subclavian veins

Absent in portal and vertebral veins

Prevent the return of the blood to the veins of the head and limbs when the pressure in the abdomen is increased as in defecation

When one artery is directly continuous with another artery or vein (mouth to mouth)

Anastomoses can be arterial, venous and arteriovenous

Alternative blood channels which divert blood to the distal end when a certain area of artery is obstructed

Can be established through capillaries

Anatomical end arteries – vessels whose terminal branches do not anastomose with branches of arteries supplying adjacent areas e.g. central artery of retina

Functional end arteries – vessels whose terminal branches do anastomose with those of adjacent arteries, but the calibre of anastomosis is insufficient to keep the tissue alive should one of the arteries become occluded

The diameter of the lumen of the blood vessel can be controlled by

Nervous

Humoral

Efferent fibres from the spinal cord release noradrenalin at their nerve endings in the walls of the vessel and this causes contraction of the smooth muscle

The adrenalin in the blood causes contraction of the smooth muscle of the vessel walls

Humoral control

Lymph – clear watery fluid with the same constituents as blood plasma

Lymphatic system collects surplus tissue fluid (lymph) and conveys it into the venous system by lymphatic trunks uniting to form either:

Right lymphatic duct which drains lymph from right upper quadrant (right side of head and neck, right upper limb and right half of thoracic cavity)

Thoracic duct which drains the rest of the body

Lymphatic system consists of

Lymph nodes

Lymphatic (plexuses) vessels

Aggregation of lymphoid tissue in spleen, GIT

Circulating lymphocytes

Abnormal enlargement of lymph nodes