CHAPTER 20

1. Big Picture: What Is the Vasculature?

Vasculature = the entire network of blood vessels in the body.

The cardiovascular system has three major jobs:

1. Transport

   • Oxygen

   • Carbon dioxide

   • Nutrients

   • Hormones

   • Metabolic wastes

2. Regulation

   • Body temperature

   • Blood pressure

   • Fluid balance

   • pH

3. Protection

   • Blood clotting

   • Immune defenses

The basic pathway of blood

Memorize this order:

Heart → arteries → arterioles → capillaries → venules → veins → heart

Think:

A A C V V
Arteries → Arterioles → Capillaries → Venules → Veins

The heart pumps blood into arteries, and veins bring blood back to the heart.

2. Blood Vessel Wall Structure

Most blood vessels have three layers, called tunics.

From inside → outside:

1. Tunica intima

Innermost layer

Contains:

• Endothelium

• Basement membrane

• Small amount of connective tissue

Main function

Provides a smooth inner surface for blood to flow over.

Think:

Intima = Inside

The endothelium also plays an important role in:

• Regulating blood flow

• Controlling exchange between blood and tissues

• Releasing chemicals that affect vessel diameter

• Helping regulate clotting

2. Tunica media

Middle layer

Mostly made of:

• Smooth muscle

• Elastic fibers

Main function

Controls the diameter of the blood vessel.

The smooth muscle can:

Vasoconstriction
→ vessel gets narrower

Vasodilation
→ vessel gets wider

This is extremely important for controlling:

• Blood pressure

• Blood flow

• Resistance

Think:

Media = Muscle

The tunica media is especially thick in arteries because arteries experience high pressure from the heart.

3. Tunica externa

Also called the adventitia.

Outermost layer

Mostly made of:

• Connective tissue

• Collagen fibers

• Elastic fibers

Main functions

• Protects the blood vessel

• Anchors the vessel to surrounding tissues

• Provides structural support

Think:

Externa = Exterior

Easy Memorization

Memory trick:

I-M-E

Intima = Inside
Media = Muscle
Externa = Exterior

3. Types of Blood Vessels

There are five major types you need to know:

1. Arteries

2. Arterioles

3. Capillaries

4. Venules

5. Veins

4. Arteries

Main job

Carry blood away from the heart.

Important:

Artery = Away

Arteries generally carry oxygenated blood, but there is an important exception:

Pulmonary arteries

Carry deoxygenated blood

Right ventricle → lungs

So don't memorize:

"Arteries always carry oxygenated blood."

Instead memorize:

Arteries carry blood AWAY from the heart.

Artery Structure

Arteries have:

• Thick walls

• Thick tunica media

• Lots of smooth muscle

• Elastic tissue

Why?

Because blood is being pushed into them directly from the heart at high pressure.

5. Two Major Types of Arteries

Elastic arteries

These are the largest arteries.

Examples:

• Aorta

• Pulmonary trunk

They contain lots of elastic fibers.

What do they do?

When the heart contracts:

Systole → artery expands

When the heart relaxes:

Diastole → artery recoils

This elastic recoil helps keep blood moving even when the heart is relaxing.

Think:

Elastic arteries = pressure buffers

They help smooth out the pressure changes created by each heartbeat.

6. Muscular Arteries

Muscular arteries are generally smaller than elastic arteries.

They contain a lot of:

• Smooth muscle

• Some elastic tissue

Main function

Distribute blood to specific organs and tissues.

They can:

• Constrict

• Dilate

This allows the body to control how much blood different organs receive.

7. Arterioles

Arterioles are small branches of arteries.

Path:

Artery → arteriole → capillary

They have a very important job:

Control blood flow into capillary beds.

Because arterioles contain smooth muscle, they can change their diameter.

Vasoconstriction

Arteriole becomes smaller.

Result:

↑ Resistance

↓ Blood flow to tissue

Vasodilation

Arteriole becomes wider.

Result:

↓ Resistance

↑ Blood flow to tissue

Why arterioles are important for blood pressure

Arterioles are sometimes called the body's "resistance vessels."

Their diameter has a huge effect on:

• Blood pressure

• Blood flow

• Vascular resistance

Remember:

Smaller vessel = more resistance

Larger vessel = less resistance

8. Capillaries

Capillaries are the smallest blood vessels.

They connect:

Arterioles → capillaries → venules

Their main job is:

Exchange materials between blood and tissues.

Things exchanged include:

• Oxygen

• Carbon dioxide

• Nutrients

• Hormones

• Water

• Wastes

Capillary Structure

Capillaries have extremely thin walls.

They are basically:

One layer of endothelial cells + basement membrane

They do NOT have:

• Thick tunica media

• Thick tunica externa

Why?

Because materials need to cross the wall easily.

Think:

Capillaries = exchange vessels

9. Three Types of Capillaries

You need to know:

1. Continuous

2. Fenestrated

3. Sinusoidal

Continuous Capillaries

These have the least permeable walls.

The endothelial cells are connected closely together.

They allow relatively small substances through.

Common locations:

• Muscle

• Skin

• Lungs

• Brain

Brain

The blood-brain barrier is associated with highly selective continuous capillaries.

Think:

Continuous = controlled/least leaky

10. Fenestrated Capillaries

Fenestrated means "windowed."

These capillaries have tiny pores called fenestrations.

They are more permeable than continuous capillaries.

Found in organs involved in:

• Absorption

• Filtration

• Secretion

Examples:

• Kidneys

• Small intestine

• Endocrine glands

Think:

Fenestrated = windows

11. Sinusoidal Capillaries

Also called sinusoids.

These have the largest openings and are the most permeable.

They allow:

• Large molecules

• Proteins

• Sometimes cells

to pass through.

Found in:

• Liver

• Spleen

• Bone marrow

Think:

Sinusoidal = super leaky

Capillary Comparison

Easy order

Continuous → Fenestrated → Sinusoidal

Least permeable → Most permeable

12. Venules

Venules are small vessels that receive blood from capillaries.

Path:

Capillaries → venules → veins

Their main job is:

Collect blood from capillary beds and move it toward veins.

Small venules are very thin and participate in exchange.

Larger venules contain more connective tissue and smooth muscle.

13. Veins

Veins carry blood:

Toward the heart

Most veins carry deoxygenated blood.

Exception:

Pulmonary veins

Carry oxygenated blood

They travel:

Lungs → left atrium

Again:

Vessel names are based on direction, not oxygen content.

14. Vein Structure

Compared with arteries, veins have:

• Thinner walls

• Less smooth muscle

• Less elastic tissue

• Larger internal diameter

• Lower blood pressure

Veins are sometimes called:

Blood reservoirs

because a large percentage of the body's blood is stored in the venous system.

15. Valves in Veins

Many veins contain one-way valves.

These valves prevent blood from flowing backward.

This is especially important in the limbs because blood has to travel against gravity to get back to the heart.

Important

Valves are particularly associated with veins, not arteries.

Think:

Veins = valves

16. Blood Vessel Comparison

Most important comparisons

Highest pressure: arteries

Lowest pressure: veins/capillaries are much lower than arteries

Thickest walls: arteries

Thinnest walls: capillaries

Smallest diameter: capillaries

Valves: veins

17. Oxygenated vs. Deoxygenated Blood

Do NOT memorize vessels based only on oxygen.

Instead remember:

Systemic circulation

Arteries
→ usually oxygenated

Veins
→ usually deoxygenated

Pulmonary circulation

Pulmonary arteries
→ deoxygenated

Pulmonary veins
→ oxygenated

The key rule

Artery = away from heart
Vein = toward heart

18. Complete Blood Vessel Pathway

Memorize:

Heart

↓

Elastic artery

↓

Muscular artery

↓

Arteriole

↓

Capillary

↓

Venule

↓

Vein

↓

Heart

A more detailed pathway:

Heart → arteries → arterioles → capillary bed → venules → veins → heart

19. Capillary Exchange

Capillary exchange = movement of substances between:

Blood ↔ interstitial fluid ↔ cells

The capillary wall is thin enough for materials to cross.

Main methods include:

• Diffusion

• Filtration

• Reabsorption

• Transcytosis

20. Diffusion

Diffusion = movement from an area of higher concentration → lower concentration.

This is extremely important for gases.

Oxygen

Oxygen generally moves:

Blood → tissues

because tissues are using oxygen.

Carbon dioxide

Carbon dioxide generally moves:

Tissues → blood

because cells produce carbon dioxide.

21. Filtration

Filtration = fluid moves out of the capillary.

The main force is:

Blood hydrostatic pressure

Hydrostatic pressure is essentially the pressure of fluid pushing against the vessel wall.

Think:

Hydrostatic pressure pushes fluid OUT.

22. Reabsorption

Reabsorption = fluid moves into the capillary.

The main force is:

Blood colloid osmotic pressure

This is primarily created by plasma proteins, especially albumin.

Albumin helps pull water toward the blood.

Think:

Osmotic pressure pulls fluid IN.

23. Filtration vs. Reabsorption

This is very important.

Hydrostatic pressure

Pushes OUT

Colloid osmotic pressure

Pulls IN

Memory trick:

Hydrostatic = pushes
Osmotic = pulls

24. Transcytosis

Transcytosis is the movement of substances through endothelial cells.

The substance is:

1. Taken into the endothelial cell

2. Transported across the cell

3. Released on the other side

This can help transport substances that are too large to easily cross through normal diffusion.

25. Hemodynamics

Hemodynamics = the study of blood flow and the forces that control blood movement through blood vessels.

Three major concepts:

1. Blood pressure

2. Blood flow

3. Resistance

These are closely related.

26. Blood Flow

Blood flow = movement of blood through a vessel, organ, or entire circulation.

Blood naturally moves:

From higher pressure → lower pressure

The heart creates the pressure gradient that drives blood flow.

Simple relationship

Greater pressure difference → greater blood flow

Greater resistance → less blood flow

27. Blood Pressure

Blood pressure = force that blood exerts against the walls of blood vessels.

The greatest pressure occurs in the:

Arteries

because they receive blood directly from the heart.

Blood pressure progressively decreases as blood moves through:

Arteries → arterioles → capillaries → venules → veins

28. Systolic vs. Diastolic Pressure

Systolic pressure

Pressure during:

ventricular contraction

It is the top number.

Example:

120/80 mmHg

120 = systolic

Diastolic pressure

Pressure during:

ventricular relaxation

It is the bottom number.

80 = diastolic

29. Pulse Pressure

Pulse pressure = systolic pressure − diastolic pressure

Example:

120/80

120 − 80 = 40 mmHg

So pulse pressure = 40 mmHg

30. Mean Arterial Pressure

Mean arterial pressure, or MAP, is the average pressure driving blood through systemic circulation.

A common approximation:

MAP ≈ diastolic pressure + ⅓(pulse pressure)

For 120/80:

Pulse pressure = 40

MAP ≈ 80 + ⅓(40)

MAP ≈ 93 mmHg

You should understand the concept even if your instructor does not require calculations.

31. Vascular Resistance

Resistance = opposition to blood flow.

The greater the resistance:

→ the harder it is for blood to move

→ blood flow decreases

The three major factors affecting resistance are:

1. Vessel diameter

2. Blood viscosity

3. Vessel length

32. Vessel Diameter

This is one of the most important factors.

Vasoconstriction

Vessel gets narrower.

↑ Resistance

↓ Blood flow

Vasodilation

Vessel gets wider.

↓ Resistance

↑ Blood flow

Even a small change in vessel diameter can have a large effect on resistance.

This is why arterioles are so important in controlling blood pressure and blood flow.

33. Blood Viscosity

Viscosity = thickness/stickiness of blood.

Thicker blood:

→ more resistance

→ harder to move

Thinner blood:

→ less resistance

Factors affecting viscosity include:

• Number of red blood cells

• Plasma proteins

More RBCs can increase viscosity.

34. Vessel Length

Longer blood vessels create:

More resistance

Shorter vessels create:

Less resistance

This makes sense because blood has to travel through more vessel wall/contact over a longer distance.

Memorize:

Longer vessel → ↑ resistance → ↓ blood flow

35. Relationship Between Blood Pressure, Flow, and Resistance

The basic relationship is:

Blood flow = pressure difference ÷ resistance

So:

If pressure increases:

Blood flow tends to increase.

If resistance increases:

Blood flow tends to decrease.

If resistance decreases:

Blood flow tends to increase.

Think:

Pressure pushes. Resistance opposes.

36. Venous Return

Venous return = amount of blood returning to the heart through the veins.

Because veins have low pressure, the body needs mechanisms to help push blood back toward the heart.

Important mechanisms include:

1. Skeletal muscle pump

When skeletal muscles contract, they squeeze nearby veins.

The valves prevent blood from moving backward.

So blood is pushed:

Toward the heart

This is especially important in the legs.

2. Respiratory pump

During inhalation:

• Pressure in the thoracic cavity decreases

• This helps draw blood toward the heart

Think:

Breathing helps pull venous blood toward the heart.

3. Venoconstriction

Sympathetic stimulation can cause veins to constrict.

This pushes more blood toward the heart.

37. Pulse

A pulse is the pressure wave produced when the heart contracts and pushes blood into the arteries.

You feel the pulse where an artery is close to the surface of the body.

38. Nine Major Pulse Points

Know these:

1. Temporal

2. Facial

3. Common carotid

4. Brachial

5. Radial

6. Femoral

7. Popliteal

8. Posterior tibial

9. Dorsalis pedis

Pulse Point Locations

Temporal

At the side of the head:

Temple

Facial

Along the jaw/face.

Common carotid

Side of the neck.

Important:

Do not press both carotid arteries at the same time.

Brachial

Inside of the upper arm/elbow region.

Also commonly used when measuring blood pressure.

Radial

At the wrist on the thumb side.

This is probably the pulse you use most often.

Femoral

Upper thigh/groin region.

Popliteal

Behind the knee.

Think:

Popliteal = posterior knee

Posterior tibial

Behind the medial ankle.

Dorsalis pedis

Top of the foot.

Think:

Dorsalis pedis = dorsal/top of foot

39. Blood Pressure Measurement

Blood pressure is commonly measured using a:

Sphygmomanometer

This includes an inflatable cuff.

A stethoscope is used to listen to blood flow through an artery.

Usually the brachial artery is used.

40. Korotkoff Sounds

When the cuff is inflated enough, it temporarily compresses the brachial artery.

As the cuff pressure decreases, blood begins moving through the artery again.

The sounds are called:

Korotkoff sounds

First sound

Indicates:

Systolic pressure

Disappearance of sounds

Indicates:

Diastolic pressure

So:

First sound = systolic

Sounds disappear = diastolic

41. Factors That Control Blood Pressure

Blood pressure depends on several major factors.

The most important relationship is:

Blood Pressure = Cardiac Output × Peripheral Resistance

Therefore, blood pressure is affected by:

Cardiac output

How much blood the heart pumps per minute.

Cardiac output depends on:

Heart rate × Stroke volume

Peripheral resistance

How much resistance the blood encounters in the vessels.

This is strongly influenced by:

• Arteriole diameter

• Blood viscosity

• Vessel length

Blood volume

More blood volume generally:

→ increases blood pressure

Less blood volume generally:

→ decreases blood pressure

Blood viscosity

Thicker blood:

→ increases resistance

→ can increase blood pressure

Vessel diameter

Vasoconstriction:

→ ↑ resistance

→ ↑ blood pressure

Vasodilation:

→ ↓ resistance

→ ↓ blood pressure

42. Neural Regulation of Blood Pressure

The nervous system helps maintain blood pressure quickly.

The main control center is in the:

Medulla oblongata

The cardiovascular center receives information from:

Baroreceptors

Baroreceptors detect:

Stretch caused by blood pressure

Important locations:

• Carotid sinus

• Aortic arch

43. Baroreceptor Reflex

If blood pressure becomes too high:

Baroreceptors detect increased stretch

↓

Signal to medulla

↓

Parasympathetic activity increases / sympathetic activity decreases

↓

Heart rate decreases

↓

Blood pressure decreases

If blood pressure becomes too low:

Baroreceptors detect less stretch

↓

Sympathetic activity increases

↓

Heart rate increases

↓

Heart contracts more strongly

↓

Vasoconstriction increases

↓

Blood pressure increases

44. Endocrine Regulation of Blood Pressure

Hormones also help control blood pressure.

Important hormones/systems include:

• Epinephrine

• Norepinephrine

• ADH

• Aldosterone

• Angiotensin II

• ANP

45. Epinephrine and Norepinephrine

Released during sympathetic activation.

They can increase:

• Heart rate

• Contractility

• Vasoconstriction

Overall effect:

Increase blood pressure

46. ADH

ADH = antidiuretic hormone

Released from the posterior pituitary.

Its major effects include:

• Increasing water reabsorption by kidneys

• Helping maintain blood volume

• Promoting vasoconstriction at higher concentrations

More water retained:

→ ↑ blood volume

→ ↑ blood pressure

47. Renin-Angiotensin-Aldosterone System

This is extremely important for blood pressure regulation.

Abbreviated:

RAAS

It becomes especially important when blood pressure or blood volume is low.

Basic sequence

Low blood pressure/low blood volume

↓

Kidneys release renin

↓

Renin helps produce angiotensin II

↓

Angiotensin II causes:

• Vasoconstriction

• Aldosterone release

↓

Aldosterone causes kidneys to retain:

• Sodium

Water follows sodium.

↓

Blood volume increases

↓

Blood pressure increases

48. ANP

ANP = atrial natriuretic peptide

Released by the atria when they are stretched due to increased blood volume.

Its general effect is the opposite of RAAS.

ANP promotes:

• Sodium excretion

• Water excretion

• Reduced blood volume

• Reduced blood pressure

Think:

RAAS raises pressure.
ANP lowers pressure.

49. Erythropoietin

The hormone that stimulates red blood cell production is:

Erythropoietin (EPO)

It is primarily produced by the:

Kidneys

when oxygen levels are low.

EPO stimulates the red bone marrow to produce more:

Erythrocytes = red blood cells

Sequence

Low oxygen

↓

Kidneys release EPO

↓

Red bone marrow increases RBC production

↓

More RBCs

↓

Greater oxygen-carrying capacity

50. Hepatic Portal System

The hepatic portal system is a special blood vessel system that carries blood from the digestive organs to the liver before the blood returns to the heart.

Normally:

Heart → arteries → capillaries → veins → heart

But the hepatic portal system creates an extra capillary bed.

Pathway

Digestive organs

↓

Capillaries

↓

Hepatic portal vein

↓

Liver

↓

Hepatic veins

↓

Inferior vena cava

↓

Heart

51. Why Is the Hepatic Portal System Important?

Blood coming from the digestive tract contains:

• Nutrients

• Toxins

• Drugs

• Other absorbed substances

The blood goes to the liver first.

The liver can:

• Process nutrients

• Store nutrients

• Modify nutrients

• Detoxify substances

• Break down certain drugs

• Process metabolic products

Think:

Digestive tract → liver first → heart

52. Major Vessels Involved in the Hepatic Portal System

Important vessels include:

Hepatic portal vein

Carries blood:

Digestive organs → liver

Hepatic veins

Carry blood:

Liver → inferior vena cava

Splenic vein

Drains blood from the:

• Spleen

• Parts of stomach

• Pancreas

It contributes to the hepatic portal vein.

Superior mesenteric vein

Drains much of the:

• Small intestine

• Large intestine

• Other digestive structures

It also contributes to the hepatic portal vein.

53. Circulation Overview

There are two major circulatory circuits:

Pulmonary circulation

Between:

Heart ↔ lungs

Purpose:

Gas exchange

Pathway

Right ventricle

↓

Pulmonary trunk

↓

Pulmonary arteries

↓

Lungs

↓

Pulmonary veins

↓

Left atrium

Systemic circulation

Between:

Heart ↔ body tissues

Purpose:

Deliver oxygen/nutrients and remove wastes

Pathway

Left ventricle

↓

Aorta

↓

Systemic arteries

↓

Arterioles

↓

Capillaries

↓

Venules

↓

Veins

↓

Venae cavae

↓

Right atrium

54. Pulmonary vs. Systemic: Important Oxygen Pattern

Pulmonary arteries

Deoxygenated

Heart → lungs

Pulmonary veins

Oxygenated

Lungs → heart

Systemic arteries

Usually oxygenated

Heart → body

Systemic veins

Usually deoxygenated

Body → heart

55. Major Blood Vessel Concepts to Memorize

Since your instructor specifically said the Circulatory Pathways section will not need to be memorized for the exam, don't spend huge amounts of time memorizing every artery and vein.

However, you should understand the major pathway.

Know these especially:

Aorta

Largest systemic artery.

Superior vena cava

Returns blood from:

• Head

• Neck

• Upper limbs

• Thorax

to the right atrium.

Inferior vena cava

Returns blood from:

• Abdomen

• Pelvis

• Lower limbs

to the right atrium.

Pulmonary trunk

Carries blood from:

Right ventricle → pulmonary arteries

Pulmonary veins

Carry blood from:

Lungs → left atrium

56. How Blood Pressure Changes Throughout the System

This pattern is very important:

Aorta / large arteries

⬆ Highest pressure

↓

Arteries

↓

Arterioles

⬇ Major pressure drop occurs here

↓

Capillaries

Low pressure

↓

Venules

↓

Veins

Very low pressure

↓

Heart

Why does pressure decrease?

Because resistance in the blood vessels causes pressure to gradually drop as blood travels through the circulation.

57. Why Are Capillaries Low Pressure?

Capillaries need low pressure because their walls are extremely thin.

If pressure were extremely high:

• Capillaries could be damaged

• Excessive fluid could be forced into tissues

The arterioles help regulate how much pressure reaches the capillary beds.

58. Why Are Veins Low Pressure?

By the time blood reaches the veins, much of the pressure generated by the heart has been lost due to resistance.

Therefore veins need additional mechanisms to return blood to the heart:

• Valves

• Skeletal muscle pump

• Respiratory pump

• Sympathetic venoconstriction

59. Arteries vs. Veins

This comparison is extremely testable.

60. Arteries → Capillaries → Veins

Think of the vessels as having different jobs:

Arteries

Transport

Move blood away from heart.

Arterioles

Control

Control how much blood enters tissues.

Capillaries

Exchange

Exchange materials with tissues.

Venules

Collect

Collect blood from capillaries.

Veins

Return + Store

Return blood to heart and store a large volume of blood.

61. High-Yield Memorization Section

If you're short on time, memorize these first.

Vessel direction

Arteries = Away

Veins = toward heart

Vessel order

Artery → arteriole → capillary → venule → vein

Vessel pressure

Arteries = highest

Veins = lowest

Vessel walls

Arteries = thick

Capillaries = extremely thin

Veins = thin

Vein feature

Valves

Capillary function

Exchange

Capillary types

Continuous = least permeable

Fenestrated = windowed

Sinusoidal = most permeable

Hydrostatic vs. osmotic

Hydrostatic = pushes fluid OUT

Osmotic = pulls fluid IN

Resistance

Narrow vessel → ↑ resistance

Wide vessel → ↓ resistance

Long vessel → ↑ resistance

Thicker blood → ↑ resistance

Blood flow

Higher pressure → lower pressure

More resistance → less flow

Blood pressure

BP = Cardiac Output × Peripheral Resistance

Blood pressure measurement

Brachial artery

First Korotkoff sound = systolic

Disappearance = diastolic

Baroreceptors

Located in:

Carotid sinus + aortic arch

Detect:

Blood pressure/stretch

EPO

Kidneys → EPO → bone marrow → RBCs

Hepatic portal system

Digestive organs → hepatic portal vein → liver → hepatic veins → IVC → heart

62. The "Understand It, Don't Just Memorize It" Version

The entire chapter can be understood as one story:

The heart creates pressure.

↓

Arteries carry blood away from the heart.

↓

Arterioles control how much blood enters each tissue.

↓

Capillaries have extremely thin walls so exchange can happen.

↓

Blood loses pressure as it travels through the circulation.

↓

Venules collect the blood.

↓

Veins return the low-pressure blood to the heart.

↓

Valves, skeletal muscles, breathing, and sympathetic stimulation help push venous blood back to the heart.

↓

The nervous and endocrine systems constantly adjust vessel diameter, heart activity, blood volume, and resistance to maintain blood pressure and homeostasis.

That is essentially the big picture of Chapter 20.

⭐ Most Important Exam Connections

Make sure you can explain these without looking at your notes:

1. Why do arteries have thick walls?
Because they receive blood at high pressure directly from the heart.

2. Why are capillaries so thin?
So substances can efficiently move between blood and tissues.

3. Why do veins have valves?
To prevent backflow, especially when blood is moving against gravity.

4. Why do arterioles control blood pressure?
Their smooth muscle changes vessel diameter, which changes resistance and therefore affects blood flow and pressure.

5. Why is blood pressure highest in arteries?
They are closest to the heart's pumping action.

6. Why does blood flow from arteries to capillaries?
Blood moves down a pressure gradient from higher pressure to lower pressure.

7. What pushes fluid out of capillaries?
Hydrostatic pressure.

8. What pulls fluid into capillaries?
Blood colloid osmotic pressure, mainly from plasma proteins.

9. Why is the hepatic portal system special?
Blood from the digestive organs goes to the liver before returning to the heart.

10. How does the body raise blood pressure when it drops?
Sympathetic activity, vasoconstriction, increased heart activity, ADH, and RAAS all help increase blood pressure and/or blood volume.