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
–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
–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 O , gets rid of CO
2 2
2. Systemic circuit (rest of the body)
O2 is used, CO waste is produced
2
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 CO is given up within pulmonary capillaries
2
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
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
–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
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 O , sphincters open, vasodilation increases
2
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
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
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
