Principles of the Circulatory System (Anatomy)

Question 1

Describe the cardiovascular system and basic principles of circulation

Answer 1

The circulatory system typically consists of the cardiovascular system and the lymphatic system which function in parallel to transport fluid around the body

Closed network of blood vessels, connects all organs and tissues with Heart being the pump

Principles of Circulation

Blood flows from a region of high pressure to lower pressure, blood flows in the direction of decreasing pressure gradient - arteries to capillaries to veins

Rate of blood flow to tissue is controlled in relation to tissue need

Arterial pressure is controlled independently of either local blood flow or cardiac output controls

The rate (or velocity) of blood flow varies inversely with the cross-sectional area of the blood vessels. As total cross-sectional area of vessels increases, velocity of flow decreases. Blood flow is thus slowest in the capillaries, allowing time for gas and nutrient exchange

Resistance is a force that opposes the flow of a fluid

In blood vessels, most of the resistance is due to vessel diameter. As vessel diameter decreases, resistance increases and blood flow decreases

Very little pressure remains by the time blood leaves the capillaries and enters the venules

Blood flow through the veins is not the direct result

The Cardiovascular system

Pulmonary circuit:

• Fresh oxygen from breathing enters blood and carbon dioxide is released from the blood
• Pulmonary arteries carry deoxygenated blood
• Pulmonary vein carry oxygenated blood

Systemic Circuit:

• Provide organs, tissues and cells with blood for their oxygen and other vital substances
• Arteries carry oxygenated blood
• Veins carry deoxygenated blood

Question 2

Describe the different types, structure and function of arteries and veins (anastomosis and end arteries)

Answer 2

3 layers

Tunica extema (adventitia):

• Stronger outer covering composed of connective tissue, collagen and elastic fibers
• Separated from the tunica media by the external elastic lamina

Tunica media:

• Composed of smooth muscle and elastic fibers. This layer is thicker in arteries than in veins
• Most of smooth muscles
• Separated from the tunica intima by the internal elastic lamina

Tunica intima - Innermost layer:

• In arteries, composed of elastic membrane lining and smooth endothelium
• In veins, no elastic membrane. Contains valves to keep blood flowing in a single direction

Types of Arteries

Large elastic (conducting)

• Nearest the heart (aorta and pulmonary arteries)
• Contain more elastic tissue in tunica media than muscular artiers
• Help maintain a relative constant pressure gradient despite constant pumping action of the heart by expanding/recoiling during cardiac cycle

Medium Muscular (distributing)

• Contain more smooth muscle cells in tunica media than elastic arteries
• Regulate diameter to control blood flow

Small arteries and arterioles

• Control capillary filling
• contribute to arterial pressure

Anastomosis

Network/connection produced by union of parts or branches (as of blood vessels and streams) providing intercommunication or interconnection

Ensures continues tissue perfusion when one artery is compromised

End artery (Terminal artery)

Artery that suppies a portion of tissue without contribution from other arteries or any significant collateral circulation, and so do not anastomose with their neighbours

Of significant importance as occlusion results in serious supply disturbance resulting in tissue death and loss of function

• True end arteries = no anastomosis
• Functional end artery: Ineffectual anastomoses

Venous System

Veins tend to follow course of arteries

• Transport blood towards heart
• Thin elastic vessels, act as a reservoir of blood: Lack a large amounts of elastin and smooth muscle
• Large lumen, valves (unidirectional) in some veins (not in those in head and neck)

Type of Veins

Superficial:

• Located close to surface of skin, typically not near a corresponding artery

Deep:

• Located deep within muscle tissue and typically near a corresponding artery

Sizes of veins

Large veins:

• Wide bundles of longitudinal smooth muscle
• Well-developed tunica aventitia
• SVC

Medium veins:

• drain venous plexuses
• accompany medium arteries
• venous valves

Venules:

• smallest veins, drain capillary beds

Venous Valves

Inward projection of intima

• cause flow of blood in one direction (towards the heart)
• collagen and elastic fibres
• Semi lunar cusps attached to wall
• Absent in IVC, head & neck
• Dialation of veins due to increased pressure may lead to valves not closing (incompetent valves) resulting in varicose veins

Leg Venous Pump System

• Return of blood is against gravity
• Valves in veins
• Muscular pump compresses veins within a closed compartment

Question 3

Describes the role of capillaries (fenestrated and non-fenestrated capillaries)

Answer 3

Smallest (dedicated) blood vessels

Only possess endothelium

Connect the smallest arteries (arterioles) and the smallest veins (venules)

Transport blood, nutrients and oxygen to cells. Primarily function to facilitate the exchange of various substances (gases, nutrient, waste) between blood stream and tissues

Types

Fenestrated:

• have small openings (fenestrae) that enables the rapid exchange of substances
• Seen in kidneys, small intestine and endocrine glands

Continuous (non-fenestrated)

• have a lining through which only small molecules can pass
• exists in the nervous system as well as fat and muscle tissue

Sinusoidal capillaries:

• have small fenestrae that allow certain substances to pass through
• seen in liver and spleen

Question 4

Outline the role of the lymphatic system and list its components

Answer 4

Complementary to blood vascular system. Transport network of delicate tubes throughout the body

Functions to:

• Convey lymphatic fluid, or lymph, through a network of lymphatic channels
• Filter lymph through lymph nodes
• Helps manage fluid levels in the body by aiding fluid return to the bloodstream (venous system)

Part of immune system

Transport of specific fats

_Components of Lymphatic system

Lymphatic cells (lymphocytes)

Lymphatic fluid (lymph)

Lymphatic vessels and plexuses

Lymph nodes

Lymphatic organs:

• Red bone marrow
• Thymus gland
• Spleen
• Tonsils

Lymphatic trunks are large collecting vessels that receive lymph from multiple lymphatic vessels

Lymphatic Fluid

Originates in tissue capillary beds

Almost identical to interstitial fluid

Contains lymphocytes

Relatively high protein concentration

Normal lymphatic flow is 2L to 3L per day

Chyle:

• Specific type of lymphatic fluid (in GI track)
• Contains fats (cholesterol, glycerol, fatty acids) from intestinal absorption giving it a milk-like appearance
• Transported in vessels known as lacteals

Lymphatic Vessels

Porous blind-ended capillaries

Converge to become larger vessels

Drain into large veins at root of neck at the venous angle

Small valves to prevent backflow

thinner than veins

Major Lymph Vessels

Cisterna Chyli:

Saccular dilation in abdomen

Receives lymph from:

• abdominal viscera
• abdominal walls
• pelvis
• perineum
• lower limbs

Cisterna Chyli to Thoracic Duct

Thoracic duct to Left subclavian vein

Question 5

Describe the various types of lymphatic tissue and its organisation in the human body

Answer 5

Lymphatic organs

Primary lymphoid organs:

Thymus:

• Located in the thorax, posterior to sternum
• Fully formed at birth. Involution (regression) after puberty
• Small and largely atrophied in adults
• Bone marrow

Question 6

Describe the flow of lymph between superficial versus deep lymphatics and to the lymph nodes (fluid transport from the interstitial spaces)

Answer 6

Lymphatic system and fluid transport

Lymphatic system is not a closed system

no pump

lymphatic vessels structurally similar to veins

relatively thin walls and carry lymph under low pressure

large lymphatic vessels have vales to prevent the backflow of lymph

The continued advancement of lymph (lymph flow) is aided by a pressure gradient created by vascular smooth muscle, skeletal muscle contraction, and respiratory movements.

lymph and lymphatics = Chyli

Lymphatic system and fat transport

small intestine

• large chain fatty acids coated with protein and packaged with cholesterol to form chylomicrons
• lymph capillaries here called lacteals
• fluids os opaque and milky and called chyle

Chyle is composed of lymphatic fluid and emulsified fats from the intestinal lymphatic trunk.

Lymph Nodes

• Small bean-shaped tissues along lymphatic vessels
• All lymph passes through lymph nodes​
• Afferent vessels take lymph to node​
• Efferent vessels take lymph away from node​
• Act as elaborate filters that trap and phagocytose particulate matter​
• Detect and defend against foreign antigens

Lymph nodesserve as a filter and function to monitor lymphatic fluid/blood composition,drain excess tissue fluid and leaked plasma proteins, engulf pathogens, augment an immune response, and eradicate infection.

Types of Lymph Vessels and Nodes

Lymph nodes are arranged in chains

Vessels and nodes are grouped:

• Deep
• Superficial

Primary nodes - first node in chain

terminal nodes - last node in chain

sentinel nodes - hypothetical first node draining a cancer

Superficial lymphatic vessels:

• More numerous than veins in subcutaneous tissue
• Converge towards and follow venous drainage

Eventually drain into deep lymphatic vessels

Superficial lymph nodes:

• Superficial to deep fascia
• Associated with superficial veins
• Drain to deep nodes

Deep lymphatic vessels

• Accompany arteries
• Receive drainage of internal organs

Deep Lymph Nodes:

• Closely located to vasculature and viscera

Lymph nodes in disease

Metastases:

• Cancer spread from elsewhere in body
• Spread through lymphatics
• Carcinomas most common
• Nearby or distant spread

Primary Cancer:

• Lymphoma - cancer of lymphocytes (Hodgkin, non-Hodgkin)

Question 7

Lymphadenopathy

Answer 7

• Swelling of lymph nodes

Causes:

• Infection
• Skin wounds
• Metastases
• Primary malignancy (lymphoma)

Inflammation of lymph nodes = lymphadenitis

Infection of lymph vessels = Lymphangitis

Lymphoedema = blockage or damage

Primary lymphoedema = inherited disorder from disrupted development of lymphatic system causing absent or malformed lymphatic tissue

Secondary lymphoedema = Acquired disorder involving lymphatic system dysfunction, resulting from:

• Trauma, including coincidental surgical injury
• Deliberate surgical removal (cancer)
• Infection
• Radiation therapy
• Obstruction