Blood and Capillaries

What are capillaries?

Smallest blood vessels with thin walls that allow exchange of gases/nutrients/waste

What is the structure of capillary walls?

One layer of endothelial cells, 5-10 µm in diameter

What is the function of capillaries?

Exchange oxygen/nutrients with tissues and remove CO₂/waste

What are the three types of capillaries?

Continuous, fenestrated, sinusoidal

Where are continuous capillaries found?

Muscle, skin, brain (including blood-brain barrier)

Where are fenestrated capillaries found?

Kidneys, small intestine, endocrine glands

Where are sinusoidal capillaries found?

Liver, spleen, bone marrow

What type of molecules move by diffusion in capillaries?

Oxygen, carbon dioxide, glucose, small ions

What is filtration in capillary exchange?

Movement of fluid out of capillaries due to hydrostatic pressure

What is osmosis in capillary exchange?

Movement of water into capillaries due to plasma protein oncotic pressure

What drives filtration at the arterial end of capillaries?

High hydrostatic pressure (~35 mmHg)

What drives reabsorption at the venous end of capillaries?

Higher oncotic pressure (~25 mmHg) than hydrostatic (~18 mmHg)

What is bulk flow?

Mass movement of fluid by filtration and reabsorption

What is hydrostatic pressure?

The pressure exerted by blood on capillary walls, pushing fluid out

What is oncotic pressure (colloid osmotic pressure)?

The pulling pressure due to plasma proteins, drawing fluid into capillaries

What are normal values for capillary hydrostatic and oncotic pressures?

CHP: 35 mmHg (arterial), 18 mmHg (venous); Oncotic: ~25 mmHg

What is net filtration pressure (NFP)?

CHP minus BCOP; positive = filtration, negative = reabsorption

What happens at the midpoint of the capillary?

CHP = BCOP, so no net fluid movement

What happens to excess fluid not reabsorbed?

Picked up by lymphatic capillaries and returned to bloodstream

What happens if blood pressure increases?

Raises CHP → more filtration → risk of edema

What happens if plasma protein levels drop?

Lowers oncotic pressure → less reabsorption → edema

What increases capillary permeability?

Inflammation, allowing proteins to leak out and raise tissue oncotic pressure

What effect does high tissue osmotic pressure have?

Pulls water from capillaries into interstitial space

What does the lymphatic system do?

Removes excess interstitial fluid and returns it to circulation

What causes inflammation-related edema?

Increased permeability → proteins & fluid leak into tissue

How does liver disease affect capillary exchange?

Reduces albumin → decreases oncotic pressure → ascites, edema

What is capillary leak syndrome?

Sudden capillary permeability increase → fluid shift → low blood pressure

What happens in heart failure?

Venous pressure rises → higher CHP → increased filtration → edema

What is ascites?

Abdominal fluid buildup due to liver disease and low plasma proteins

What is lymphedema?

Swelling due to blocked or impaired lymphatic drainage