Physio. Ch.14 Part 2 Capillaries

Capillaries

• Site where gas, nutrients, waste are exchanged

• Only 5–10 µm in diameter

• Very thin walls: Only a single layer of squamous epithelium and a basement membrane

• Types

- Continuous

- Fenestrated

- Discontinuous/sinusoids

• Blood flow regulators

- Vasoconstriction and vasodilation of arterioles & metarterioles

- Precapillary sphincters

Material Exchange Across Capillary Walls

• Types

- Simple diffusion: Lipophilic substances cross through the epithelial cells directly

- Transcytosis: Vesicles transport materials through endocytosis and exocytosis

- Mediated transport: Co-trasnport or pumps are used

Bulk flow

• Movement of large amounts of water and small solutes through capillaries

- Done through simple diffusion

• Maintains balance between interstitial fluid and plasma

• Divided into two parts

- Filtration

- Absorption / reabsorption

Filtration

• Movement of fluid from the blood to the interstitial fluid (out of the capillaries)

Absorption / reabsorption

• Movement of fluid from the interstitial fluid to the blood (into capillary)

Starling forces

• Hydrostatic and osmotic pressures that exist across capillary walls and determine the direction of fluid movement

• Subdivided

- Capillary hydrostatic pressure (PCAP)

- Interstitial fluid hydrostatic pressure (PIF)

- Capillary osmotic pressure (πCAP)

- Interstitial fluid osmotic pressure (πIF)

Capillary hydrostatic pressure (PCAP)

• One of the four starling forces

• Hydrostatic pressure of fluid inside the capillary

• Promotes filtration

• Decreases as blood flows flows from the arteriole end to the venule end

- Usually moves from 38mmHg to 16mmHg

Interstitial fluid hydrostatic pressure (PIF)

• One of the four starling forces

• Hydrostatic pressure of fluid outside the capillary

• Promotes absorption

• Constant along length of capillaries

- Usually a low value like 1mmHg

Capillary osmotic pressure (πCAP)

• One of the four starling forces

• Due to nonpermeating solutes inside the capillary

• Promotes absorption

• Constant, about 25mmHg

Interstitial fluid osmotic pressure (πIF)

• One of the four starling forces

• Due to nonpermeating solutes outside the capillary

• Promotes filtration, about 0mmHg

Net Filtration Pressure

• Sum of Starling forces that determines the direction of fluid movement across capillary walls

• Difference in the filtration pressures and absorption pressures, if positive filtration will occur

- Calculation: (PCAP + πIF) - (PIF + πCAP)

Arteriole end of capillary

• Net filtration pressure

• Pushes fluid out of the capillaries and into the interstitial space

Venule end of capillary

• Net absorption pressure

• Pushes fluid into the capillaries and out of the interstitial space

Net movement across capillary beds

• Filtration is larger the reabsorption

• Lymphatic system picks up the leftover fluid in the interstitial tissue

Factors Affecting Filtration and Reabsorption

• Standing on feet: Increases capillary hydrostatic pressure

• Injuries

- Damaged capillaries that leak fluid and proteins

- Histamine increases capillary permeability to proteins

• Liver disease: decreased plasma proteins → decreases capillary osmotic pressure

• Kidney disease: increased blood volume and BP → increases capillary hydrostatic

pressure

• Sometimes decreases plasma proteins → decreases capillary osmotic pressure

• Heart disease: pulmonary edema

Systemic veins

• Hold at least 60% of the body’s blood

Driving force for venous return

• Pressure gradient between the peripheral veins & right atrium

Venous pressure

• Typically 15mmHg

• Factors that cause a variance

- Skeletal muscle pump

- Respiratory pump

- Blood volume

- Venomotor tone

Skeletal muscle pump

• Factor that cause a variance in venous pressure

• Contraction and relaxation of muscles helps drive blood toward the heart

- Valves in peripheral veins prevent blood from moving backward

Respiratory pump

• Factor that cause a variance in venous pressure

• Inhalation increases pressure in abdominal cavity & decreases pressure in thoracic cavity → pressure gradient promotes movement of blood into central veins

Blood volume

• Factor that cause a variance in venous pressure

• Increased blood volume → increased venous pressure

Venomotor tone

• Factor that cause a variance in venous pressure

• Sympathetic activity stimulates contractile activity in the smooth muscle in the walls of veins

- NE binds to α1 adrenergic receptors to cause more contraction

Venous pooling

• Accumulation of blood in veins cause a reduction in arterial pressure

Lymphatic System

• Returns interstitial fluid to the circulatory system that does not return via blood capillary beds

- Average of 3L per day

• Inspects the fluid for immune purposes when carrying it back to the heart

Arterial baroreceptors

• Receptors that detect stretch in arterial walls

• Located in the aortic arch and carotid sinuses of the carotid arteries

• Send AP to medulla oblongata

- AP frequency increases with amount of stretch

Cardiovascular Control Center

• Controlled by several nuclei in the medulla oblongata

- Act as the control center in this process

Low pressure baroreceptors in right atria & large systemic veins

• Detect changes in blood volume and sent message to medulla oblongata

Major Autonomic Innervations to Cardiovascular Effectors

• Sympathetic and parasympathetic nerves to SA node (heart rate)

• Sympathetic nerves to ventricular myocardium (ventricular

contractility)

• Sympathetic nerves to arterioles and other resistance vessels (vascular resistance)

• Sympathetic nerves to veins (control of venomotor tone)

Regulation of Blood Pressure: Atrial Stretch Reflexes

• Activated by increased venous return

• Activates atrial natriuretic peptide (ANP) hormone

- Promotes salt and water excretion in urine

- Inhibits ADH secretion → excretion of more urine

- Physiological antagonist of aldosterone

Hypertension

• Elevated resting blood pressure: greater than 120 mm Hg / 80 mm Hg

• Effects

- Atherosclerosis

- Vascular damage (especially dangerous in the cerebral vessels – risk of stroke)

- Can lead to organ damage (ex: kidney failure, loss of vision)

- Increases workload on the heart → heart attack

Atherosclerosis

• Results from hypertension

• Hardening of the arteries due to fatty plaque build up

- Decreased elasticity, narrower lumen, and increased resistance)

Hypertension

• High blood pressure

• Subdivided

- Primary/Essential hypertension

- Secondary hypertension

Primary/Essential hypertension

• No known secondary cause / disease

• 90-95% of cases of hypertension

Secondary hypertension

• Hypertension caused by a second disease process

• only about 5-10% of cases

Hypotension

• low arterial pressure, reduces blood flow to all systemic organs

• Effects

- Compromised organ function

- Can cause permanent damage

- Can be deadly in minutes

- Baroreceptor reflex acts to maintain blood flow to heart and brain – at the expense of other organs

Circulatory Shock

• Prolonged deprivation of adequate blood supply to the body’s tissues causes local smooth muscle relaxation in the vasculature

• Local influences override sympathetic vasoconstrictor nerves → arterial pressure drops → decrease in blood flow to heart and brain