Chapter 20: Blood Vessels

Arteries

Take blood from the heart to the capillaries.

Capillaries

Microscopic vessels. Exchange substances from blood to tissues and tissues to blood.

Veins

Take blood from capillaries back to the heart.

Lumen

The opening inside the vessel.

Tunica intima

Simple squamous epithelium and areolar. Innermost layer.

Tunica media

Thicker layer, smooth muscle and elastic fibers, middle layer. Able to contract and dilate.

Tunica externa

Areolar connective tissue, anchors the vessel to other structures, has vasa vasorum, which is an arteriovenous system to supply the large vessels with nutrients.

Companion vessels

They lie next to each other. An artery and a vein for the same body region. Maybe even the same name.

Arteries (structure)

Thicker tunica media, thinner lumen, more elastic and collagen fibers. More resilient to changes of blood pressure.

Veins (structure)

Thicker tunica externa, larger lumen, less elastic fibers and collagen fibers.

Elastic arteries

Large arteries that bring blood from the heart to the muscular arteries. They have a lot of elastic fibers, stretch and recoil, and can propel blood during diastole. Examples include the aorta, pulmonary trunk, common carotid, and common iliac.

Muscular arteries

Medium-sized arteries also called distributing arteries. They take blood from the elastic arteries and bring it to specific body regions. They have two layers of elastic tissue: the internal lamina and the external lamina.

Arterioles

The smallest arteries. Larger ones have three tunics, but smaller ones only have a layer of smooth muscle with a thin endothelium. The smooth muscle is constricted, called vasomotor tone, which regulates systemic blood pressure and blood flow.

Atherosclerosis

A progressive disease in elastic and muscular arteries where plaque thickens in the tunica intima, narrowing the lumen. It can be treated by expanding the narrowed region with a balloon or doing a coronary bypass.

Aneurysm

When a part of the wall of the arteriole thins and balloons, making the wall more prone to rupture and bleeding. It is most common in the aorta and the base of the brain.

Capillaries (characteristics)

Small vessels where erythrocytes travel in a single-file line. They connect arterioles to venules and have an endothelial layer on the basement membrane for better gas exchange.

Functional advantage of capillary walls

Thin wall allows for better gas exchange.

Continuous capillaries

Endothelial cells in a continuous lining with tight junctions. Large particles like proteins cannot get through them.

Tight Junctions

They have tight junctions. Large particles cannot get through them like proteins. Small things like glucose can get through.

Fenestrated Capillaries

Fenestrated capillaries have endothelial cells that are continuous, but they have pores. Smaller plasma proteins can get through.

Locations of Fenestrated Capillaries

These are located in the intestines and kidneys.

Sinusoids

Sinusoids have an incomplete lining. Large gaps in the walls. Large things can get through.

Locations of Sinusoids

Located in the bone marrow, spleen, and some endocrine glands.

Capillary Beds

Capillary beds are a group of capillaries that work together. They are in a network. Kind of like a web.

Control of Blood Entry to Capillary Beds

It is controlled by sphincters. Relaxation means blood can enter. Contraction means blood does not enter, and it continues moving past.

Venules

Venules are the smallest veins. Companion vessel to the arteriole. Merge to form veins.

Function of Valves in Veins

These prevent blood from pooling in the limbs. This ensures blood flows to the heart.

Composition of Valves

Valves are made of tunica intima and elastic fibers, and collagen fibers.

Systemic Veins

Systemic veins are blood reservoirs. At rest, they contain 55% of the blood volume.

Blood Movement in Systemic Veins

When they constrict, blood moves into circulation, which is when the body is active. When the body is resting, they dilate and blood pools back in systemic veins.

Blood Pressure

The force of blood against the wall of the vessel.

Blood Pressure Gradient

Change in pressure from one end of the vessel to the other. This propels blood through the vessels. It is higher in arteries and lower in veins.

Arterial Blood Pressure

Blood flow through the arteries pulses with the cardiac cycle.

Systolic Pressure

When the ventricles contract. The highest pressure in the arteries.

Diastolic Pressure

When the ventricles relax. Lowest pressure in the arteries.

Pulse Pressure

Pressure in arteries added by heart contraction. Difference between systolic and diastolic.

Pulse

What vessel type gives you a pulse? Artery.

Factors Affecting Pulse

Higher pressure can make it more forceful. It is absent when blood flow is lacking.

Pulse Points

Where you can compress an artery against a solid structure.

Mean Arterial Pressure

Average arterial pressure across the entire cardiac cycle.

Calculation of Mean Arterial Pressure

Diastolic pressure + ⅓ pulse pressure.

Importance of Mean Arterial Pressure

Provides an index of perfusion. If it is below 60, it can mean insufficient blood flow.

Capillary Blood Pressure

Flow and pressure are smooth. There is no fluctuation between systolic and diastolic. About 40 mmHg at the arterial end and 20 mmHg at the venous end.

Importance of Capillary Blood Pressure Range

It is high enough for the exchange of substances and low enough not to damage vessels.

Venous Blood Pressure

This is the venous return of blood from the heart. It depends on the pressure gradient, skeletal muscle pump, and respiratory pump.

Difference Between Venous and Arterial Blood Pressure

It is low, not pulsatile.

Skeletal Muscle Pump

Helps venous blood return by veins squeezing as muscle contracts, pushing blood, with valves preventing backflow.

Prolonged inactivity

Causes blood to pool in the legs.

Respiratory Pump

Inspiration and expiration cause pressure gradient changes that aid venous return.

Inspiration

Increases abdominal pressure and decreases thoracic pressure, driving blood in abdominal veins towards the thoracic cavity.

Expiration

Increases thoracic pressure and decreases abdominal pressure, driving blood in thoracic veins towards the heart.

Increased breathing rate

Causes blood to move more.

Resistance

Friction that blood encounters when it contacts the vessel wall, opposing the flow of blood.

Viscosity

Resistance of fluid to its flow.

Increasing viscosity

Achieved by greater thickness or more particles in the blood, which increases with higher cell counts.

Decreasing viscosity

Occurs with anemia.

Vessel length

Longer vessels have more resistance, which can change with weight gain or loss.

Vessel radius

Smaller radius leads to more resistance; larger diameters have less resistance, increasing flow.

Blood flow calculation

It is proportional to the pressure gradient divided by resistance.

Pressure gradient effect on blood flow

As the pressure gradient increases, blood flow increases.

Resistance effect on blood flow

Resistance and blood flow oppose each other; if resistance increases, blood flow decreases.

Normal blood pressure range importance

It needs to be high enough to maintain perfusion but not high enough to damage vessels.

Blood pressure regulation

Depends on cardiac output, resistance, and blood volume, regulated by the nervous and endocrine systems.

Autonomic reflexes

Regulate blood pressure on a short-term basis through the cardiovascular center in the medulla oblongata.

Cardiac Center

Influences cardiac output.

Cardio-acceleratory center

Sympathetic nervous system pathway that increases heart rate and force of contraction.

Cardio-inhibitory center

Parasympathetic pathway that decreases heart rate.

Vasomotor Center

Influences blood vessel diameter through sympathetic influence leading to norepinephrine release.

Baroreceptors

Nerve endings that respond to stretch of the vessel wall, located in the aortic arch and carotid sinuses.

Aortic arch baroreceptors

Sense blood pressure and send information to the vagus nerve, then to the cardiovascular center.

Carotid sinus baroreceptors

Use the glossopharyngeal nerve to send information to the cardiovascular center, specific to head and neck pressure.

Baroreceptor reflexes

Activated by a change in blood pressure, useful for quick changes.

Blood pressure decrease response

Vessel stretch declines, baroreceptor firing decreases, activating the cardio-acceleratory center to increase cardiac output.

Cardioinhibitory center

Inhibits cardioinhibitory center, minimizes parasympathetic activity, activates vasomotor center, stimulates sympathetic pathway to increase vasoconstriction, increases cardiac output and resistance, and blood pressure rises.

Cardioaccelatory center

Sends fewer signals when blood pressure increases.

Chemoreceptor Reflexes

Located in arotic and the carotid bodies; negative feedback will return blood chemistry back to normal.

Chemoreceptors

Stimulated by high carbon dioxide, low pH, or very low oxygen, leading to increased BP and shifts blood to the lungs.

Hypothalamus

Can increase cardiac output and resistance due to increased body temperature or fight or flight response.

Limbic system

Can alter blood pressure in response to emotions or memories.

Angiotensinogen

Made in the liver.

Renin

Released by the kidneys when there is low blood pressure.

Angiotensin I

Converted from angiotensinogen by renin.

Angiotensin II

Powerful vasoconstrictor, stimulates the thirst center, kidneys decrease urine formation, and stimulates the release of aldosterone and ADH.

Aldosterone

Released from the adrenal cortex; maintains blood volume and pressure by increasing absorption of sodium ions and water in the kidney.

Anti-diuretic Hormone (ADH)

Released from the posterior pituitary; helps elevate BP or maintain it by increasing water reabsorption and stimulating the thirst center and vasoconstriction.

Atrial Natriuretic Peptide (ANP)

Decreases BP; released from the atria when they're stretched by high blood volume; stimulates vasodilation and increases urine output.

Hypertension

High blood pressure; chronic condition defined as 140/90 or higher.

Hypotension

Chronically decreased blood pressure; systolic under 90, diastolic under 60.

Orthostatic hypotension

Drop in blood pressure due to sudden standing; regulation is not occurring quickly enough.

Systemic arteries

Originate from the aorta.

Ascending aorta

Branches into right and left coronary arteries.

Aortic arch

Branches into the brachiocephalic trunk, left common carotid artery, and left subclavian artery.

Descending aorta

Passes through the diaphragm and becomes the descending abdominal aorta; splits at L4 into right and left common iliac arteries.

Superior vena cava

Drains blood from the head, neck, upper limbs, thoracic, and abdominal walls.

Inferior vena cava

Carries blood from the lower limbs, pelvis, perineum, and abdominal structures.

Coronary sinus

Carries deoxygenated blood from the heart.

Simple Pathways

One major artery brings blood to an organ or a region. Branches into arteries and then arterioles. Arterioles feed into the capillary bed, venule drains the capillary bed, and venules merge into major vein.

Example of Simple Pathway

Splenic artery -> spleen -> Splenic vein

Arterial Anastomosis

An arterial anastomosis is when two or more arteries converge to form arterioles and the capillary bed.

Venous Anastomosis

A venous anastomosis is when multiple veins drain the same region and then merge to head back to the heart.