Blood Vessels & Circulation

Blood Vessels & Circulation

Heart --> Arteries--> Arterioles ---> Capillaries---> Venules ---> Veins ---> Heart

Vessel Wall Anatomy: 3 layers (except capillaries)

Tunica intima (interna) - endothelial lining
Tunica media - smooth muscle innervated by vasomotor fibers.
Tunica adventitia (externa) - outermost, loose fibrous connective tissue

Arteries: Carry blood away from heart.

Elastic Arteries: have elastic connective tissue in t. media. Withstand large pressure changes. Elastic recoil is important in smooth blood flow.

Histology of Elastic Artery (Aorta)

Muscular Arteries: smaller arteries near organs. More smooth muscle in tunica media.

Muscular vs. Elastic Artery

Arterioles: Smallest, thinnest arterial vessels. Feed capillary beds and major site of blood flow regulation in tissues.

Capillaries: Most numerous (1.2 billion). 1,000 square miles of potential surface area. But have only 4-5% of body's blood at rest.

Thin walled - only have tunica intima. Site of gas and nutrient exchange with tissues.

Continuous or fenestrated.
Thoroughfare capillaries or channels - direct link between arteriole and venule.
True capillaries - Flow into them is regulated by pre-capillary sphincters.
Sinusoids - large, distended capillaries found in lymphatic system (spleen), bone marrow, pituitary, and liver. Sites where phagocytosis and secretion is important.

Venules: Smallest venous vessels. Smallest ones have no tunica media.

Micrograph: venules indicated (in adipose tissue). Note thin walls & RBCs in lumen.
Micrograph: Arteriole and Venule compared

Veins: Three distinct tunics. Low pressure system. Blood flow to heart aided by skeletal muscle pump, respiratory pump, and one-way valves.

Histology: Vein with valve.
Histology of Vena Cava (largest vein)

Contain 65% of body's blood at rest.

Varicose veins -- due to stretching of veins and weakening of valves.

Histology of Arteries, Veins and Capillaries (Virtual Hospital)

Historical Note: William Harvey's Demonstration of Valve Function

Physiology of Circulation

Blood Flow = amount of blood flowing thru a vessel or organ. Varies according to local factors and neural (ANS) regulation.

Blood Pressure = force per unit area exerted on vessel wall. Expressed in mmHg.

Resistance = opposition to flow; friction.

Major determinants of resistance are:

blood vessel diameter
vessel length
viscosity of blood.

Resistance is proportional to 1/(radius)4 -- i.e., if radius is doubled or halved, resistance is decreased or increased 16 fold.

Viscosity and vessel length don't change much in the short term, so vessel diameter is the major factor determining resistance.

Flow = D Pressure/Resistance

Systemic Blood Pressure

Systolic arterial pressure: maximum pressure exerted by left ventricle.

Diastolic arterial pressure: minimum pressure in vessels.

Mean arterial pressure = Diastolic pressure + 1/3(SP - DP). Note, this is not a straight average since diastole lasts longer than systole.

Blood pressure drops primarily in the arterioles because of the numerous fine branches which increase the total cross-sectional area of vascular bed.

Blood Pressure = cardiac output X TPR

Anything that influences cardiac output or resistance will affect blood pressure.

Regulation of Blood Pressure

Nervous System Control:

Vasomotor fibers of SNS
Vasomotor center of medulla oblongata
Pressoreceptors (Baroreceptors) in aortic and carotid sinuses
- Location of Baroreceptors
Chemoreceptors (oxygen and pH)
Hypothalamic

Cortical: Cerebral cortex ("higher thought processes") can affect blood pressure

Chemical Control:

Epinephrine and Norepinephrine from adrenal medulla.
Atrial natriuretic factor - increase in BP promotes reduction of blood volume and pressure by increasing secretion of sodium.
Antidiuretic hormone
Kidney's renin-angiotensinogen aldosterone system (RAAS)
- RAAS Illustration
Aldosterone - adrenal cortex. Promotes sodium retention.
- Hyperaldosteronism

Variations in Blood Pressure

Measurement of blood pressure is usually by ausculatory methods. These methods rely on turbulence of blood flow causing detectable sounds.

Classical (using the Sounds of Korotkoff) method is use of sphygmomanometer. Inflated cuff blocks blood flow thru brachial artery. When external cuff pressure is high enough to just stop flow --> systolic arterial pressure. When cuff deflated, blood begins to flow thru artery, but flow is turbulent and can be detected with stethoscope. When blood flow just becomes quiet (laminar flow of blood) --> cuff pressure equals resting pressure in artery --> diastolic arterial pressure.

Nicolai Sergevich Korotkoff: The man behind the sounds!

Normal Range for Blood Pressure:

110-140 mmHg systolic

75-80 mmHg diastolic.

Hypotension: systolic pressure < 100 mmHg. Very relative. Some chronic conditions lead to hypotension.

Orthostatic hypotension: sudden change in posture -- blood wells up in lower extremities --> blood pressure drops --> blood flow to brain slows --> dizziness.

Age & medications (esp. beta blockers) affect orthostatic hypotension.

Hypertension: Persistent systolic pressure > 150 mmHg and/or diastolic > 90 at rest. Damaging to vessels, kidneys in particular.

Multiple causes.

Obesity: increase in vessel lengths (to serve fat) --> increase in resistance.
Atherosclerosis -- decrease in vessel diameter due to fat deposits.
- Figure: normal (left) and atherosclerosis of coronary artery (right)
Arteriosclerosis -- hardening of arteries (less elastic recoil)
Sodium retention. Increases blood volume. Genetic factors/race differences -- relates to gender and racial differences in responses to various hypertension medications.
Stress/Lack of exercise.
Diet

Hypertension Fast-Stats

The Puzzle of Hypertesion in African-Amercians: Scientific American Article

Blood Flow

Delivery of blood depends on metabolic "needs" of tissue.

How is "need" recognized by body?

Flow is proportional to Pressure/Resistance

Change in cross sectional area is the major determinant

Capillary Dynamics

Exchange of nutrients relies primary on diffusion.

Rate of diffusion will depend on permeability of capillaries and capillary pressures.

Permeability differences

Presence or absence of fenestrations.

Blood-brain barriers

Two Key Pressures:

Hydrostatic pressure or filtration pressure:
Osmotic or Oncotic Pressure

Normal pressure difference results in net fluid movement into capillary.

For delivery of nutrients to tissues, there must be net fluid movement out of capillary, so net hydrostatic pressure must be greater than net osmotic pressure.

Excess interstitial fluid is recovered by lymphatic system.

Edema:.

Circulatory Shock: inadequate blood flow to meet metabolic needs of tissues.

From:

Blood or fluid loss (hypovolemic)
Excessive vasodilation
Heart failure (cardiogenic)

Blood Pressure & Capillary Hemodynamics

Pathophysiology of Circulatory Shock

Hemodynamics Lecture

Capillary Exchange Online Quiz

Pulmonary Circulation

Low pressure system

Blood flow must be slower and exert less pressure on fine capillaries.

Right Ventricle (low oxygen blood) --> Pulmonary Trunk --> Pulmonary arteries --> lobar arteries --> arterioles --> capillaries surrounding alveolar sacs --> pulmonary venules and veins (high oxygen blood) --> left atrium --> systemic circulation.

Hepatic Portal System

Transports blood from visceral (spleen, stomach and intestines) organs to liver--> blood then goes to heart.

Principal Systemic Arteries

Ascending Aorta