20.3 Capillaries and Fluid Exchange

Capillary exchange

Two way movement of fluid

What chemicals are given off by and taken up by capillary blood to the perivascular tissue?

Given off: oxygen, glucose, other nutrients, antibodies, hormones

Take up by: carbon dioxide, other waste, glucose and fatty acide released from storage in the liver, and adipose tissue calcium and other minerals released from bones, antibodies secreted by immune systems and hormones secreted by endocrine glands

Chemicals are often two way traffic between blood and connective tissue, leaving capillaries at one point and entering at another

Water going in and out as well

How do chemicals pass through capillary walls?

1) endothelial cell cyptoplasm

2) intercellular clefts between endothelial cells

3) filtration pores of fenestrated capillareis

What is transcytosis?

Process of endothelial cells pick up material on one side of plasma membrane by pinocytosis or receptor-mediated endocytosis, transport the vesicles across the cell and discharge the material on the other side by exocytosis

This helps fatty acids, albumin, and antibodies, and even insulin across the cell well

What is oncotic pressure?

Difference between colloid osmotic pressure of blood and tissue fluid

Tends to draw water into capillary by osmosis opposing outward force of hydrostatic pressure

Net reabsorption pressure

Prevailing force is inward at the venous end because osmotic pressure overrides filtration pressure

Causes the capillary to reabsorb fluid

Why does a capillary give off fluid at one end and reabsorb at another?

Pressure changes significantly from one arterial end to the venous end in capillary blood pressure, responsible shift from filtration to reabsorption

Is capillary BP high or low?

Low, usually capillaries are collapsed

When does capillary BP rise?

In active muscles, capillary pressure rises to the point that filtration over rides reabsorption along the entire length of the capillary, fluid accumulates in the muscles and increases muscular bulk by 25%

What is edema?

Accumulation of excess fluid in tissues, shown in swelling of face, fingers, abdomen, ankles or internal organs where its hidden from view

What are three reasons edema can happen?

1) increased capillary filtration - increase rate of capillary filtration and accumulation of fluid. Capillaries become more permeable in old age, with elderly people at increased risk for edema

2) reduced capillary reabsorption - depends on oncotic pressure, proportional to concentration of blood albumin - deficiency of albumin produces edema by reducing reabsorption of tissue fluid

3) obstructed lymphatic drainage - lymphatic are one way vessels that collect fluid from tissue and return it to the blood stream

What are consequences of edema?

• Tissues become congested with fluid, and oxygen delivery and waste removal are impaired, tissue can begin to die

• Pulmonary edema presents threat of of suffocation as fluid replaces air in lungs

• Cerebral edema can produces headaches, nausea, and sometimes delirium, seizures, and coma

• Severe edema - fluid can transfer from blood vessels to the tissue spaces that blood volume and pressure drop to cause circulatory shock

What is venous return?

Blood flow back to the heart by five mechanisms

Pressure gradient

Gravity

Skeletal muscle pump

Thoracic (respiratory) pump

and cardiac suction

How do pressure gradients for venous return work?

• most important force of venous flow

• wear in veins than in arteries

• pressure gradient/venous return increase when blood volume increases

• Increase in generalized, widespread vasoconstriction because it reduces the volume of circulatory system and raises blood pressure and flow

How does gravity for venous return work?

• When sitting/standing blood from your head and neck returns to the heart by flowing downward through large veins above heart

• Large veins of neck are collapsed or nearly so and their venous pressure is close to zero

• Dural sinuses of brain have more rigid wall and cant collapse - creating risk of air embolism if punctured

How does skeletal pump for venous return work?

• in limbs, veins are surrounded and massaged by muscles

• contracting muscles squeeze blood out of compressed part of the vein

• valves ensure that blood can go to the heart

How does thoracic pump for venous return work?

• aids the flow of venous blood from the abdominal to the thoracic cavity

• when inhaling, your thoracic cavity expands and internal pressure drops while downward movement of the diaphragm raises the pressure of abdominal cavity

• the inferior vena cava (IVC, largest vein) if flexible tube passing through both of these cavities

• In abdominal pressure on the IVC rises while thoracic pressure on it drops, blood is squeezed upward toward the heart

• No forced back into lower limbs because valves prevent it

• Because of thoracic pump central nervous pressure fluctuates from 2mm Hg when you inhale to 6 mm Hg when you exhale, blood flowing quicker when you inhale

How does cardiac suction for venous return work?

• During ventricular systole, tendinous cords pull the AV valve cusps downward, expanding the atrial space

• Creates slight suction that draws blood into the atria from the venae cavae and pulmonary veins

Why does exercise increase venous return?

• Heart beats faster and harder

• Increases cardiac output and blood pressure

• blood vessels of skeletal muscles, lungs and coronary circulation dilate, increasing flow

• Increase in respiratory rate and depth enhance action of thoracic pump

• Muscle contraction increases venous return by means of skeletal output, important in perfusion of muscles

What is venous pooling?

Blood accumulating in the limbs

What is circulatory shock?

Cardiac output is insufficient to meet the body’s metabolic needs, falling into either 1) cardiogenic shock (inadequate pumping of the heart) or 2) low venous return (LVR) shock (cardiac output is low because too little blood is returning to the heart)

What are the principle forms of low venous return shock?

1) hypovolemic shock - most common, due to loss of blood volume as result of hemorrhage, trauma etc. deydration is a big one - water transfer from bloodstream to replace tissue fluid lost in sweat, blood volume may drop too low to maintain circulation adequately

2) obstructed venous return shock - an objects such as tumor or aneurysm compresses a vein and impedes blood flow

3) venous pooling shock - occurs when body has normal total blood volume but too much accumulates in lower body

What is septic shock?

Bacterial toxins trigger vasodilation and increased capillary permeability

What is anaphylatic shock?

Exposure to antigen that the person is allergic (bee venom) antigen-antibody complexes trigger release of histamines, causing generalized vasodilation and increased capillary permeability

what is compensated shock?

several hemostatic mechanisms bring about spontaneous recovery.

• hypotension from low cardiac output triggers sympathetic baroreflex and production of angiotensin II both of which counteract shock by stimulating vasoconstriction

• if a person faints and falls to horizontal position, gravity can restore blood flow to the brain, quicker recovery can be a achieved if a person’s feet are elevated, and the blood can drain from the legs

What is decompensated shock?

If the comensated shock doesn’t work, several life threatening positive feedback loops occur

Poor cardiac output leads to myocardial ischemia and infarction, weakening the heart and reducing output

Slow circulation of blood leads to blood clotting throughout the body (disseminated intravasculr coagulation DIC) as vessels become congested with clotted blood, venous return grows even worse

Ischemia and acidosis of the brain stem depresses vasomotor and cardiac centers, causing loss of vasomotor tone, furthuring vasodilation and further drop in BP and cardiac output.

About half of people who go into decomensated shock, die from it

The Brain Circulatory Route

Total blood flow to the brain fluctuates less than any other organ (700 ML/in at rest) - this is important because a few seconds of oxygen deprivation can cause loss of consciousness - 4 -5 minutes is enough time for irreversible damage

Brain regulates its blood flow in response to changes in BP and chemistry

Cerebral arteries dilate when systemic BP drops and constricts when it rises, minimizing fluctuations in cerebral BP

Main chemical stimulus for cerebral autoregulation is pH.

What is transient ischemic attacks (TIA)?

A brief episode of cerebral ischemia - usually with temporary dizziness, loss of vision or other senses, weakness, paralysis, headache or aphasia

Usually early warning sign of impending stroke

Need prompt medical attention, identify cause using brain imagining and other means

The Skeletal Muscles Circulatory Route

The skeletal muscles receive high variable blood flow depending on state of exertion

At rest, arterioles are constricted, capillary beds are shut down, and total flow through muscular system is 1 liter/minute

During exercise arterioles dilate in response to muscle metabolites such as lactate, nitric oxide (NO), adenosine, CO2, and H+. Blood flow can increase more than 20x in strenuous exercise meaning blood is diverted from other organs in the digestive tract and kidneys

Muscular contraction compresses the blood vessels and impedes flow, isometric contraction cause fatigue more quickly than intermittent isotonic contraction

The Lungs Circulatory Route

Pulmonary circuit is the only route where arteries carry oxygen poor blood and veins carry oxygen rich blood. Pulmonary arteries have thin distensible walls with less elastic tissue than systemic arteries.

Lower BP overall, showing blood flows more slowly through pulmonary capillaries, more time for gas exchange and oncotic pressure overrides hydrostatic pressure, capillaries are engaged throughout almost the whole time in absorption

Fluid is prevented from accumulating in alveolar wall and lumens.

Pulmonary arteries constrict in response to hypoxia - showing the lung is not ventilated well.