lOMoARcPSD|255728 27
Heart and Blood Vessels Chapter 8: Lectures 21 and 23
Blood Vessels Transport Blood:
▪
Arteries
– Carry blood away from the heart
– Transport blood under high pressure
– Are thick-walled
▪
Capillaries
– Exchange solutes and water with cells of the body
– Are microscopic
▪
Veins
– Return blood to the heart
– Are thin-walled
Arteries Transport Blood Away from the Heart:
▪
Structure
– Thick-walled, three layers
– Innermost layer: endothelium of squamous epithelial cells
– Middle layer: smooth muscle
– Outer layer: connective tissue
▪
Function
– Arteries carry blood away from heart
– Carry blood under pressure
▪
Aneurism—defect in arterial wall
– Ballooning of the arterial wall
– Some bulge inward, obstructing flow
– Others bulge outward
– Often develop slowly over time
– Often symptomless, until they rupture
– Rupture of aortic aneurism can be rapidly fatal
– May be detected by careful screening and surgically repaired
Arterioles and Precapillary Sphincters Regulate Blood Flow:
▪
Blood flow
– Heart → Arteries → Arterioles → Capillaries
▪
Arterioles: smallest arteries
▪
Precapillary sphincters: control blood flow from arterioles into capillaries
– Vasodilation
– Relaxation of vascular smooth muscle
– Increases blood flow to capillaries
lOMoARcPSD|255728 27
– Vasoconstriction
– Contraction of vascular smooth muscle
– Decreases blood flow to capillaries
Capillaries: Where Blood Exchanges Substances with Tissue:
▪
Structure
– Smallest blood vessels, microscopic
– Thin-walled: one cell layer thick
– Porous
▪
Capillary beds: extensive networks of capillaries
▪
Function: selective exchange of substances with the interstitial fluid
Lymphatic System Helps Maintain Blood Volume:
▪
Function
– Maintains blood volume
– Returns excess interstitial fluid to circulatory system
– Also functions in immune defenses
▪
Structure
– Blind-ended capillaries
– Lymphatic vessels (similar to venous system)
– Lymph—derived from interstitial fluid
Veins Return Blood to the Heart:
▪
Structure
– Three layers, thin-walled
– Larger lumen than arteries
– High distensibility
▪
Functions
– Carry blood toward the heart
– Blood flow
– Capillaries → Venules → Veins → Heart
– Serve as blood volume reservoir
▪
Three mechanisms assisting in blood return
1. Contraction of skeletal muscles
2. One-way valves permit only one-way blood flow
3. Pressure changes associated with breathing push blood toward the heart
The Heart is Mostly Muscle:
▪
Surrounded by fibrous sac—pericardium
– Protects and anchors the heart
▪
Layers of the heart
lOMoARcPSD|255728 27
– Epicardium: thin layer of epithelial and connective tissue
– Myocardium: thick layer of cardiac muscle
– Electrical signals flow directly from cell to cell
– The layer that contracts when the heart beats
– Endocardium: thin layer of endothelial tissue
– Continuous with lining of blood vessels
The Heart Has Four Chambers and Four Valves:
▪
Four chambers
– Two atria: upper chambers
– Two ventricles: lower chambers
– Septum: muscular partition separating right and left sides of the heart
▪
Four valves—prevent backflow
– Two atrioventricular (AV) valves
– Tricuspid valve (right side)
– Bicuspid (mitral) valve (left side)
– Two semilunar valves
– Pulmonary valve
– Aortic valve
The Pattern of Blood Flow Through the Cardiovascular System:
▪
Two circuits—the heart pumps blood through two circuits simultaneously
1. Pulmonary circuit (lungs)
Blood picks up O2, gets rid of CO2
2. Systemic circuit (rest of the body)
O
2
is used, CO
2
waste is produced
Pulmonary Circuit:
1. Deoxygenated blood from the body travels through the vena cava to the right atrium of
the heart
2. Through the right AV valve into the right ventricle
3. Through the pulmonary semilunar valve into the pulmonary trunk, which divides into
the right and left pulmonary arteries and travels to right and left lungs
4. Blood is oxygenated and CO2 is given up within pulmonary capillaries
5. Oxygenated blood travels through the pulmonary veins back to the heart, entering the
left atrium
Systemic Circuit:
1. Oxygenated blood flows from left atrium through left AV valve into left ventricle
2. Oxygenated blood continues from the left ventricle through the aortic semilunar valve
into the aorta
3. Through branching arteries and arterioles to tissues
4. Through the arterioles to capillaries
5. Within capillaries, nutrients and oxygen are delivered and wastes are picked up
6. From capillaries into venules and veins
7. To the vena cava and into the right atrium
lOMoARcPSD|255728 27
The Pattern of Blood Flow Through the Cardiovascular System:
▪
Blood passes through heart twice for every one trip around the body
1. Once as deoxygenated blood, through the right side of the heart
2. Once as oxygenated blood, through the left side of the heart
▪
Deoxygenated blood never mixes with oxygenated blood
Arteries and Veins of the Human Body:
▪
Arteries and veins serving the same region are generally very closely located
▪
Coronary arteries
– Arteries that supply the heart muscle itself
– Supply the myocardium
– Small diameter—may become partially or completely blocked by atherosclerosis
▪
Coronary veins
– Collect blood from myocardial capillaries and channel it back to the right atrium
The Cardiac Cycles: The Heart Contracts and Relaxes
▪
Hearts pumps
▪
Systole: period of contraction
▪
Diastole: period of relaxation
▪
Cardiac cycle: period of relaxation and contraction
▪
Atrial systole
▪
Both atria contract
▪
AV valves open, semilunar valves are closed
▪
Ventricles fill
▪
Ventricular systole
▪
Both ventricles contract
▪
AV valves close, semilunar valves open
▪
Diastole
▪
Both atria and ventricles relax
▪
Semilunar valves close
Heart Sounds Reflect Closing Heart Valves:
▪
Lub-dub heart sound
– Lub: closing of both AV valves during ventricular systole
– Dub: closing of both semilunar valves during ventricular diastole
▪
Heart murmurs
– Caused when blood flow is disturbed
– May be a sign of a defective valve
Cardiac Conduction System Coordinates Contraction:
▪
Sinoatrial (SA) node—small mass of cardiac cells in upper right atrium
lOMoARcPSD|255728 27
– Cardiac pacemaker
– Initiates the heartbeat spontaneously
– Pace can be modified by nervous system
▪
Atrioventricular (AV) node
– Located between atria and ventricles
– Relays impulse
▪
Atrioventricular (AV) bundle and Purkinje fibers
– Located in septum and ventricles
– Carry impulse to ventricles
Electrocardiogram (EKG/ECG) Records the Heart’s Electrical Activity
▪
Tracks the electrical activity of the heart
▪
A healthy heart produces a characteristic pattern
▪
Three formations
– P wave: impulse across atria
– QRS complex: spread of impulse down septum, around ventricles in Purkinje
fibers
– T wave: end of electrical activity in ventricles
▪
EKGs can detect
– Arrhythmias
– Ventricular fibrillation
Blood Exerts Pressure Against Vessel Walls
▪
The blood exerts force on the wall of the blood vessels
– Systolic pressure: highest pressure, as blood is ejected during ventricular systole
– Diastolic pressure: lowest pressure, as blood returns to the heart during
ventricular diastole
▪
Measurement
– Sphygmomanometer: device used to measure blood pressure
– “Normal” readings
– Systolic pressure <120 mmHg
– Diastolic pressure <80 mmHg
Hypertension: High Blood Pressure Can Be Dangerous
▪
Sustained elevation in blood pressure
– Systolic pressure ≥ 140 mmHg
– Diastolic pressure ≥ 90 mmHg
▪
Risk factor for cardiovascular disease
– Higher blood pressure causes greater strain on cardiovascular system
– Blood vessels react by becoming hardened and scarred
– Strain on heart from having to work harder
▪
Silent killer, no symptoms
lOMoARcPSD|255728 27
Hypotension: When Blood Pressure Is Too Low
▪
Low blood pressure
▪
If low enough, may cause dizziness or fainting
▪
May follow abrupt changes in body position
– Standing up suddenly
▪
May result from excessive blood loss or fluid loss from burns
How the Cardiovascular System Is Regulated
▪
Key points
– Homeostatic regulation of the cardiovascular system centers on maintaining a
constant arterial pressure
– Constant arterial pressure is achieved by
1. Regulation of cardiac output
2. Regulation of diameter of arterioles
– Local blood flows are adjusted to meet local requirements
Baroreceptors Maintain Arterial Blood Pressure
▪
One of the most important regulated variables in the body
▪
Regulated within narrow limits by a negative feedback control loop
▪
Baroreceptors: pressure receptors in aorta and carotid arteries
▪
Steps in mechanism
1. Blood pressure rises, vessels are stretched
2. Baroreceptors are activated and signals are sent at an increased rate
3. Signals travel to the cardiovascular center in the brain
4. Signals from cardiovascular center sent to heart to lower heart rate and force of
contraction (decreases the cardiac output)
5. Blood vessels are signaled to reduce vascular resistance, increasing blood flow to
tissues
6. Combined effects lower blood pressure
▪
Mechanism is reversed if blood pressure is too low
Local Requirements Dictate Local Blood Flows
▪
Precapillary sphincters allow fine-tuning of blood flow to local tissues as needed
▪
Metabolically active tissue—needs more O2, sphincters open, vasodilation increases
blood flow
▪
If blood pressure drops precipitously, blood pressure control would cause
vasoconstriction to many organs and shunt blood to brain and heart, where blood supply
and pressure must be maintained
Exercise: Increased Blood Flow and Cardiac Output
lOMoARcPSD|255728 27
▪
Blood flow to active skeletal muscles increases
▪
Cardiac output (CO) is increased to maintain blood pressure
– Non-athletes: up to 20–25 liters/min
– Trained athletes: up to 35 liters/min
Heart Failure: The Heart Becomes Less Efficient:
▪
Heart muscle becomes weaker, less efficient
▪
Congestive heart failure: weakness of heart causes fluid back-up in interstitial
spaces
▪
Out of breath, swollen ankles, legs, neck veins
▪
Why does the heart weaken?
▪
Age, prior heart attacks, leaky heart valves, lung disease
▪
Treatment
▪
Improve cardiac performance, efficiency
▪
Prevent accumulation of interstitial fluid
Embolism: Blockage of a Blood Vessel
▪
Sudden blockage of a blood vessel by material floating in the bloodstream
▪
Often a blood clot breaks away from a larger clot elsewhere
▪
May be cholesterol deposits, tissue fragments, cancer cells, clumps of bacteria,
bubbles of air
▪
Locations
▪
Pulmonary embolism—chest pain, shortness of breath
▪
Cerebral embolism—may cause a stroke
▪
Cardiac embolism—may cause a heart attack
Stroke: Damage to Blood Vessels in the Brain
▪
Damage to part of brain caused by an interruption in blood supply
▪
Two common causes
▪
Embolism blocking a brain blood vessel
▪
Rupture of a cerebral artery
▪
Symptoms: depend on area of brain affected
▪
Immediate medical care is crucial
▪
If embolism, patient receives clot-dissolving drugs
▪
If rupture, surgical repair is sometimes possible
▪
Recovery may require extensive rehabilitation
Replacing a Failing Heart
▪
Heart transplants
– Expensive
– Average post-transplant survival: 15 years
– Problem: shortage of healthy hearts
lOMoARcPSD|255728 27
▪
Temporary solution to shortage of transplant organs:
– Artificial heart
– Xenotransplant (heart from another animal species)
Reducing Your Risk of Cardiovascular Disease
▪
Don’t smoke
– Smokers have twice the risk of heart disease
▪
Watch cholesterol levels
– Risk increases with increasing blood cholesterol
▪
Engage in regular moderate exercise
▪
Monitor blood pressure, treat hypertension
▪
Maintain a healthy weight
▪
Keep diabetes under control
▪
Avoid chronic stress
