Blood Vessels

Chapter 19Biology 2122

Blood Vessels Three types of blood vessels: arteries, veins and capillaries

Arteries -- arterioles --- capillary beds -- venules ---- veins

Structure - TunicsTunica interna

– contains endothelium (simple squamous) which is in direct contact with the blood

Tunica media – mostly circular, smooth muscle and

elastin sheets

Tunica externa – Collagen fibers

– Contains nerve fibers, lymph vessels, and in large veins elastin fibers

– Very large vessels contain vasa vasorum which are small blood vessels that nourish the external cell wall

Types of Arteries Elastic arteries

• have thick walls and are located close to the heart

– Aorta and branches of aorta • Diameters from 2.5 cm to 1.0 cm

– ‘Conducting arteries’ • Elastin

– Arteriosclerosis

Muscular arteries – ‘distributing arteries’ which move

blood into organs • Diameter 1.0 – 0.3 cm

– Thickest tunica media

Arteries-CapillariesArterioles

– smallest diameters (0.3 cm or less)– Large - all three tunica’s– Smaller - lead to capillary beds

Capillaries – thin walls- only tunica intima

• Exchange – Endothelial cells – Pericytes – Diameter is 8-10 microns

Capillary Types

1. Continuous – skin and muscles– uninterrupted endothelial lining– joined by tight junctions – incomplete

2. Fenestrated – ‘fenestrations’

3. Sinusoids – leaky – found in liver, bone marrow, lymph

tissue and some endocrine organs– irregular and have fenestrations– fewer tight junctions– allows for blood cells and larger

particles to pass

Capillary Beds

Flow from arterioles to venules -‘microcirculation’

Blood will pass through either: – Metarteriole straight through to the

venule via a thoroughfare channel (shunt) – True capillaries to outer tissue areas

Percapillary sphincters (smooth muscle) – When opened, blood flows through the true

capillaries; blood exchange at the tissue site

Structure of Veins and Venous System Venules

– small (8-100 microns in diameter) – Postcapillary venules

Veins have three tissues but the tunica media contains very little smooth muscle and elastin

• Externa

• Vena cava

• 65% of blood supply

• Blood pressure in the veins is lower than in the arteries

• Contain ‘valves’ to help move blood back to the heart

Varicose veins Venous sinuses

Anastomoses Merging of vascular channels

– Organs or body regions receive blood from more than one artery

Form ‘anastomoses’ which form alternate pathways called collateral channels for blood to reach a region of the body

– Collateral pathways

Carbon Dioxide Angiogram

Venous Anastomosis

Kidney Stenosis

Physiology of Circulation

Dynamics of Blood Circulation:

1. Relationship between flow, pressure and resistance

2. Blood Pressure

3. Regulation of Blood Pressure

4. Imbalances

Blood Flow, Pressure and Resistance – Important Terminology

Blood Flow – volume of blood flowing through a vessel, organ or entire

circulation at a given time (equal to CO)

Blood pressure (BP) – force per unit area exerted on a vessel wall by blood (expressed in

mmHg); refers to ‘systemic BP’ near the aorta and large arteries near the heart

Resistance – opposition to blood flow– Caused by friction in vessels and most is encountered away from

the heart in the peripheral parts of the body

Blood viscosity: – Viscosity is proportional to resistance – Greater the viscosity the greater the resistance

Blood vessel length– the greater the length the greater the resistance (constant)

Blood vessel diameter– the smaller the diameter the greater the resistance

Relationship

Flow = ΔP/R

Systemic Blood

Pressure

• The closer to the ‘pump’ the greater the BP – Aorta > Veins

• Blood flow is ‘immediately’ opposed by resistance.

• Steepest Drop = Arterioles – Highest resistance

Arterial Blood PressureDetermined by the elastic properties of BV in this circuit

Blood Flow – from left side of heart to the aorta, this is the peak of pressure called ‘systolic

pressure’ (120 mmHg)

Closing of the aortic semilunar valve •this prevents blood from reentering the heart •the walls of the aorta recoil which helps to maintain adequate pressure on the reducing blood volume to keep blood flowing forward in small vessels

– diastolic pressure (70-80 mmHg)

Pulse Pressure– Difference between systolic and diastolic pressures

Mean Arterial Pressure– MAP = diastolic + pulse pressure/3– Important: moves blood to and through the tissues!

Capillary and Venous Blood Pressures BP drops in the capillaries to 40-20 mmHg

– Desirable because they are thin and fragile and very permeable so low pressures are adequate for forcing fluids out

Venous pressure is constant (around 20 mmHg)

Blood flow is increased due to large lumens, and valves

Even with the two factors above, this is not enough to cause proper flow

– Respiratory pump (pressure changes in ventral body cavity) in which breathing squeezes the BV’s and causes blood flow

– Muscular pump and the contraction of muscles

Short-term control of Blood PressureNeural Controls - Counteract immediate flucuations in BP

– Maintain MAP– Alters distribution of blood to tissues – operate via reflex arcs (baroreceptors and chemoreceptors)– Changes in BP diameter – vasomotor centers (medulla)

• Cardiac center (cardiovascular center) controls CO and BV diameter – Vasomotor tone

1.Baroreceptors – carotid sinuses-internal carotid arteries; aortic arch; walls of large

arteries of neck and thorax – Arterial BP rises------- stretched (impulses to vasomotor center)

• Promotes reduction in BP

2. Chemoreceptors – Aortic arch; large arteries of neck – CO2 rise --- pH falls ----- cardioacceleratory center -- CO

increased

Neural Controls

Chemoreceptors

Controls of BP1. Adrenal medulla

– releases NE and E which enhance the Sympathetic response via vasoconstriction of BVs

2. Antidiuretic hormone or ADH – is produced by the hypothalamus and stimulates the kidney to conserve

water. When BP falls, more ADH is released and causes strong vasoconstriction action

3. Angiotensin II – renin an enzyme released by the kidneys which causes the release of

Angiotensin II which causes vasoconstriction increasing BP.

Long Term – Renal Control – Control via altering blood volume – Direct renal mechanism – Indirect renal mechanism

• Renin-angiotension mechanism

Role of the Kidneys

Hypertension• Transient – normal increases during illness, physical exertion

• Persistant – caused by obesity, genetics– Obese people have longer BVs compared to thinner

• Can be ‘asymptomatic’ for years and can lead to heart failure, vascular disease, renal failure, stroke

– Heart is forced to pump against greater resistance which will enlarge the myocardium which will eventually weaken the heart (becomes flabby and ineffective

– Causes small tears in the endothelium and accelerates atherosclerosis

• Primary hypertension (90% of people) caused by the factors on page 732; Secondary hypertension (10%) caused by identified causes like renal failure, excessive renin, athrosclerosis

– Animation– Stroke – Stroke II

• Hypotension or low blood pressure – In elderly people when standing or sitting due to slow response of sympathertic nervous system– Chronic situations may be caused by poor diets, anemia causing low blood volume

Difference in blood flow at rest compared to exercise

Blood Flow- Tissue Perfusion• Flow of blood to tissues is an exact amount to that

particular tissue (gas exchange, body tissues, digestion) dependent on the function

• At rest, brain (13%); heart (4%); kidneys (20%); abdominal organs (24%); skeletal muscles (20%)

• During exercise more is shunted to skeletal muscles and shunted away from digestive organs

Capillary Exchange Diffusion of Molecules Across Capillary Membranes

– Lipid-soluble (gases) through lipid bilayer of plasma membranes– Small water-soluble (sugars, amino acids)

• Intercellular capillary clefts• Fenestrations

– Larger molecules (proteins)• Pinocytotic vesicles

Fluids move via Bulk Flow – Hydrostatic pressure – Colloid osmotic pressure– Hydrostatic – Osmotic pressure interactions

Capillary Exchange

1. Nutrient Exchange - Animation

2. Fluid Exchange

Capillary Exchange of Gases and Nutrients

Circulatory Shock