A+P: Cardiovascular system, blood vessels

Capillaries

Most numerous, smallest diameter (due to branching), and thinnest walls. Site of gas and nutrient exchange into/out of blood vessels. Smallest vessels, but take up the most space in the body. Easy to injure

Blood vessels

Bodies are PACKED with them. Supply nutrients and remove wastes from the body’s cells

Aorta

Main systemic artery; all other systemic arteries branch from it

Vena cava

Superior and inferior. Main systemic veins; all others branch from it

Portal veins

Capillary to capillary connection (diff from usual artery-capillary-vein arrangement)

Basic flow of blood through vessels

Heart← veins ← venules ← capillaries ← arterioles ← arteries ← heart

Hepatic portal vein

Food is digested and nutrients enter the intestinal capillaries, nutrient-rich blood travels via portal vein to the liver capillaries, blood is processed by the liver before traveling to rest of the body

Circulatory anastomosis

Another deviation from the typical artery-capillary-vein arrangement. Artery-vein, artery-artery, or vein-vein connections)

Arteriovenous anastomosis

Allows blood to bypass capillary beds. ‘Circle of willis’ in the brain allows for redundant blood supply in case one of the arteries becomes blocked

Circle of willis

Circle of arteries in the brain that allows redundant blood supply in case one of the arteries becomes blocked

Rate of blood flow is inversely proportional to total cross-sectional area

Blood flow is spread over more “pipes”. Less blood is flowing in each individual blood vessel

Resistance to blood flow

Driven by blood vessel diameter (also blood viscosity and vessel length)

Large diameter = relatively less blood is in contact with vessel wall = less resistance

Turbulent flow = creates more resistance than laminar (straight) flow, more contact between blood and vessel wall

As blood flows through the circulatory system, it loses pressure/flow force due to resistance

The farther blood is from the pumping force generated by the heart:

1. More resistance it has encountered

2. And the less force it contains (lower measured blood pressure)

BP is highest in the aorta, then gradually decreases. Lowest in the veins

Arteries and veins share the same general features

3 tissue layers (tunica intima, tunica media, tunica externa) surrounding a central lumen (center of blood vessel where blood actually flows)

Layers of arteries and veins

Tunica intima, tunica media, tunica externa

Tunica intima

Single layer of epithelial tissue (endothelium). Connective tissue w/ elastic fibers for flexibility and collagen fibers for strength

Sometimes, in larger arteries, an internal elastic membrane that provides both structural support and ability to stretch

Tunica media

In arteries, generally thickest layer. Smooth muscle with supportive elastic fibers

Vasoconstriction

Smaller diameter → more flow resistance → less blood flow

Vasodilation

Larger diameter → less flow resistance → more blood flow

Tunica externa

Connective tissue, mainly collagen. Blends in with surrounding connective tissue to anchor the blood vessel in place. Often thickest layer in veins

Artery structure

• Thicker walls (to withstand high blood pressure)

• More elastic fibers (allows expansion, absorbs large forces generated by heart, helps propel blood away from heart

• More smooth muscle (to shunt blood to different body regions)

Types of arteries

Elastic, muscular, arteriole

As you move away from the heart: diameter, wall thickness, and amount of elastic fibers decreases

Gets smaller

Elastic artery

Conducting artery. Closest to heart. Conducts blood away from the heart

Muscular artery

Distributing artery. Middle to heart. Plays main role in vasoconstriction/dilation. Distributes blood to vast network of arterioles

Arteriole

Resistance vessel. Furthest from heart. Moves into capillaries. Small diameters create a lot of flow resistance; main site of systemic blood pressure regulation; fine-tuned distribution of blood to capillaries.

Atherosclerosis

Plaque build up in the arteries (clots can form around the plaque). Heart must work harder, less blood flow

Angioplasty

Use of balloons to stretch narrowed arteries

Vein structure

• Larger lumens (to reduce resistance to flow)

• Many valves (to ensure movement toward the heart) (because blood pressure is low in veins)

Skeletal muscle pump

Skeletal muscle contraction around a vein pushes blood out of the area. One-way valves ensure movement in the correct direction (muscles relaxed, valves closed)

*Muscles contracted, valve above muscle opens

Respiratory pump

Breathing expands the thorax, drawing blood to the chest/heart

Veins/venules can hold a lot of blood

Large diameter and thin walls (which allow for expansion even at low pressures) means veins/venules can hold a lot of blood

• about 64% of blood is present in the systemic veins/venules

• Blood reservoirs

Blood reservoirs

Contraction of smooth muscle in tunics constrict (and stiffen) the vein/venules, pushing blood out when its needed elsewhere

Varicose veins

Deformed/injured valves allow blood to pool, which enlarges the veins. More common in people with high progesterone or estrogen levels

Potential causes: inactivity, pregnancy, leg injury, etc.

Consequence of having thin walls and blood under low pressure (can easily pool if valves don’t work right)

Diameter and vessel wall thickness is smaller the farther you move from the heart (veins)

Large vein Medium sized vein Venules

Closest to heart ———————————> furthest from heart

Capillaries

• Site of nutrient and gas exchange

• smallest diameter blood vessel, with walls composed of a single layer of endothelial cells

• Gasses, small ions, and lipid-soluble molecules readily pass through the endothelial cell membranes. Other molecules pass through small holes (fenestrations) in the cells, or spaces between the cells

• Capillaries need to be leaky

Precapillary sphincters

Regulate blood flow into capillary beds (can open and close)

Types of capillaries

Continuous Fenestrated Sinusoid

Least leaky ———————→ Least leaky

Hydrostatic pressure

AKA blood pressure. Pushes fluid out of the vessel

Osmotic pressure

Pulls fluid back in (because lots of molecules in the blood)

Blood pressure > osmotic pressure = net movement of fluid out of the capillaries and into the interstitial space

More than a gallon leaves per day

Lymphatic system

Collects the ‘leaked’ fluid that ends up in the interstitial space of the tissues and returns it to the blood vessels. Has important immune functions

Lymphedema

A disruption of the lymphatic that causes a build up of fluid in the tissues

Bulk flow

Movement of fluid into and out of the capillaries (filtration and reabsorption)

Filtration

Movement out of filtration. More net loss of fluid into interstitial space

Reabsorption

Movement into the capillaries

Edema

Build up of the interstitial fluid. Could be caused by: increased BP in capillaries, increased blood hydrostatic pressure, extreme lack of protein in diet (decrease in blood osmotic pressure), damage to/failure of lymph vessels (lymphedema)

Net filtration pressure (NFP)

Related to net flow out of the capillary

NFP = capillary hydrostatic pressure (pushes fluid out of capillary) - blood osmotic pressure (AKA blood colloidal osmotic pressure, pulls fluid back into capillary b/c of plasma proteins that can’t pass through vessel wall)

Start of capillary vessel

• Relatively high hydrostatic pressure

• NFP = 10 mmHg (always more)

• Highest at arterial end

• Filtration

End of capillary vessel

• Lower hydrostatic pressure = -7 mmHg

• Reabsorption

Blood pressure

Force of blood within vessels. Highest in arteries, lowest in veins. Changes during cardiac cycle

• Highest during systole (ventricular contraction)

• Lowest during diastole (ventricular relaxation)

What constrains blood pressure?

Must be high enough to efficiently transport substances

Must be low enough to allow time for adequate diffusion of substances between the blood and tissues

How is blood pressure measured

Usually measured in the brachial artery

1. Inflate cuff until pressure closes artery

2. Lower pressure (release air in cuff) until first sound is heard; this is blood starting to enter the artery and is just below peak systolic pressure

3. Continue lowering pressure; diastolic pressure is when the sound disappears (this is when blood pressure is high enough to keep the artery from being deformed)

Position and artery location affect blood flow

Blood pressure is ideally taken in a standard area/blood vessel (arm/brachial artery) and position (sitting, uncrossed legs and feet)

Korotkoff sounds

Sounds heard because of turbulent flow

Mean arterial blood pressure

Average blood pressure in arteries

MAP = diastolic BP + (systolic - diastolic BP)/3

Not a simple average because heart spends more time in diastolic than in systole

Pulse pressure

Difference between systolic and diastolic pressures (=the force the heart generates when it contracts)

Pulse

Expansion and recoiling of arteries w/ each heart beat. Most readily measured at the radial artery, but can be measured at any of the pulse points

High blood pressure can:

• Damage blood vessels (especially capillaries)

• Over work the heart

• Lead to an aneurysm

• Can lead to:

  • hypertrophic cardiomyopathy

  • Aneurysm

Hypertrophic cardiomyopathy

Enlargement of heart muscle that leads to inefficient pumping of blood

Aneurysm

Weakening/bulging of a blood vessel that can press on surrounding tissues. Extremely dangerous if ruptures

Circulatory shock

Inability of the body to supply adequate blood flow to maintain cellular metabolism

• Vascular shock

  • Anaphylaxis

• Hypovolemic shock

  • Hemorrhage (causes excessive urination)

• Cardiogenic shock

  • Myocardial infarction (heart attack)

Goals of blood flow and blood pressure regulation

1. Maintain overall homeostasis

2. Ensure enough (but not too much!) blood flow (this can change depending on tissue needs)

*Low BP bad! Not enough blood flow to transport oxygen and nutrients

Main mechanisms for blood flow/pressure regulation

• Autoregulatory

  • Chemical

  • Myogenic

• Neural (NS)

• Endocrine system (hormones)

Autoregulatory system

Local mechanism; independent of neural and endocrine control - allows for small changes in a very specific tissue region (chemical and myogenic)

Neural & endocrine

Maintains blood pressure/flow homeostasis on a larger scale (entire body)

Autoregulatory, chemical

Changing levels of local chemical regulators cause the precapillary sphincter to constrict/dilute (allows less/more blood into capillary bed)

Dilation of precapillary sphincter

Occurs in response to decreased O2, increase CO2, increase lactic acid, decrease pH (more acidity), increased histamines, increased body temp (cells - “hungry!!!! Send more!!!!”

Signals trigger endothelial cells to release nitrous oxide (NO), a potent vasodilator

Constriction of precapillary sphincter

Occurs in response to opposite levels (increase O2, decrease CO2, decrease lactic acid, increase pH) (cells - “We’re good. Don’t send more blood!!!”)

Signals trigger endothelial cells to release endothelins, potent vasoconstrictors

Autoregulatory, myogenic

• An automatic response of arteriole smooth muscle

• Stretching the muscle triggers it to contract

• More muscle stretch = triggers constriction = less blood enters the capillary bed

• Less muscle stretch = large diameter = more blood enters capillary bed

• Protects against large fluctuations in blood flow. Ensures a more constant, steady supply to each capillary bed (some change to blood flow is okay when shunting blood to other areas, but all cells need baseline, steady access to blood flow (not too much or too little)

Beta blockers

Block beta adrenergic receptors (receptors that produce different responses) and prevent adrenaline from binding to the receptor. As a result, they reduce the stress on the heart and blood vessels

Neural (NS) as a mechanism

Involves center in medulla oblongata that monitors baroreceptor and chemoreceptor input

Cardiac receptors - cardiac output via changes to sympathetic and parasympathetic nerve activity

Vasomotor center - blood vessel smooth muscle (regulates constriction/dilation)

Only via changes to sympathetic nerve activity (vasomotor has no impact on parasympathetic nerves

Baro and chemoreceptors

Baroreceptors

Monitors blood pressure

Chemoreceptors

Monitors cellular metabolism byproducts (CO2, H+, etc.)

Endocrine mechanisms

Involves hormones that regulate cardiovascular system (heart and blood vessels) and water balance

Blood pressure is highly dependent on volume of blood in blood vessels

More water/plasma = more volume = higher blood pressure

Lower blood pressure/volume causes release of:

• Epinephrine/norepinephrine

• Increased cardiac output

• Constricts some blood vessels and dilates blood vessels to important survival organs (heart, liver, skeletal muscles, etc.)

*Net effect is increased BP

• Erythropoietin (kidneys) - Increased RBC formation = increased blood volume (and pressure)

• Antidiuretic hormone (posterior pituitary) - less urine water loss = increased blood volume (and pressure)

  • Also acts as a vasoconstrictor, this is why it sometimes called ‘vasopressin’ = increased blood pressure