Skeletal and Cardiovascular Systems
Key Takeaways
Bone responds to mechanical stress through remodeling (Wolff's Law)—osteoblasts build bone, osteoclasts resorb bone.
Tendons connect muscle to bone (high tensile strength); ligaments connect bone to bone (provide joint stability).
Cardiac output (Q) = Heart Rate (HR) x Stroke Volume (SV); increases during exercise via both HR and SV.
Blood pressure response: systolic increases with exercise intensity; diastolic remains relatively constant or slightly decreases.
Maximum heart rate can be estimated as 220 - age; however, individual variation is significant.
Quick Answer: Bones adapt to mechanical loading through remodeling (Wolff's Law), with osteoblasts building and osteoclasts resorbing bone tissue. The cardiovascular system increases cardiac output (Q = HR x SV) during exercise, with typical maximal values around 20-25 L/min in trained athletes. Systolic blood pressure rises with exercise intensity while diastolic remains stable.
Bone Anatomy and Physiology
Bone Types
| Bone Type | Characteristics | Location Examples |
|---|---|---|
| Long bones | Longer than wide, shaft (diaphysis) and ends (epiphyses) | Femur, humerus, tibia |
| Short bones | Roughly cube-shaped | Carpals, tarsals |
| Flat bones | Thin, flat, curved | Skull, sternum, scapula |
| Irregular bones | Complex shapes | Vertebrae, pelvis |
| Sesamoid bones | Embedded within tendons | Patella |
Bone Composition
| Component | Percentage | Function |
|---|---|---|
| Mineral (calcium phosphate) | ~65% | Rigidity, compressive strength |
| Organic matrix (collagen) | ~35% | Flexibility, tensile strength |
Bone Cells
| Cell Type | Function | Activity |
|---|---|---|
| Osteoblasts | Build new bone tissue | Bone formation |
| Osteoclasts | Break down bone tissue | Bone resorption |
| Osteocytes | Maintain bone tissue | Mechanosensors |
Wolff's Law and Bone Remodeling
Wolff's Law states that bone adapts to the loads placed upon it:
- Increased loading (weight training) → bone deposition → increased bone density
- Decreased loading (immobilization, bed rest) → bone resorption → decreased bone density
Bone Remodeling Process
- Osteoclasts resorb old or damaged bone tissue
- Osteoblasts deposit new bone matrix
- Mineralization occurs over several weeks
- Osteocytes maintain the mature bone tissue
Exam Tip: Weight-bearing and resistance exercises are particularly effective for increasing bone mineral density, making them important for osteoporosis prevention.
Factors Affecting Bone Health
| Factor | Effect on Bone |
|---|---|
| Mechanical loading | Increases bone density |
| Calcium/Vitamin D intake | Supports mineralization |
| Hormones (estrogen, testosterone) | Promote bone formation |
| Age | Bone loss accelerates after age 30 |
| Inactivity | Promotes bone resorption |
Connective Tissue
Tendons
| Property | Description |
|---|---|
| Function | Connect muscle to bone |
| Composition | Primarily Type I collagen (parallel fibers) |
| Strength | High tensile strength, low elasticity |
| Blood supply | Poor (slow healing) |
| Adaptation | Increases stiffness and cross-sectional area with training |
Ligaments
| Property | Description |
|---|---|
| Function | Connect bone to bone, stabilize joints |
| Composition | Type I collagen (less organized than tendons) |
| Strength | Moderate tensile strength, more elastic than tendons |
| Blood supply | Poor (slow healing) |
| Injury risk | Sprains from excessive joint motion |
Cartilage
| Type | Location | Function |
|---|---|---|
| Hyaline | Joint surfaces, growth plates | Low friction movement |
| Fibrocartilage | Intervertebral discs, menisci | Shock absorption |
| Elastic | Ear, epiglottis | Flexibility |
Key Point: Cartilage is avascular (no blood supply) and has very limited healing capacity when damaged.
Cardiovascular Anatomy
Heart Structure
The heart has four chambers and functions as two pumps:
| Side | Chambers | Function |
|---|---|---|
| Right side | Right atrium, Right ventricle | Pumps deoxygenated blood to lungs |
| Left side | Left atrium, Left ventricle | Pumps oxygenated blood to body |
Heart Valves
| Valve | Location | Function |
|---|---|---|
| Tricuspid | Right atrium → Right ventricle | Prevents backflow |
| Pulmonary | Right ventricle → Pulmonary artery | Prevents backflow |
| Mitral (Bicuspid) | Left atrium → Left ventricle | Prevents backflow |
| Aortic | Left ventricle → Aorta | Prevents backflow |
Blood Flow Pathway
- Deoxygenated blood → Superior/Inferior vena cava → Right atrium
- Right atrium → Tricuspid valve → Right ventricle
- Right ventricle → Pulmonary valve → Pulmonary arteries → Lungs
- Oxygenated blood → Pulmonary veins → Left atrium
- Left atrium → Mitral valve → Left ventricle
- Left ventricle → Aortic valve → Aorta → Body
Cardiac Output
Cardiac Output (Q) is the volume of blood pumped by the heart per minute:
Formula
Q = HR x SV
Where:
- Q = Cardiac output (L/min or mL/min)
- HR = Heart rate (beats/min)
- SV = Stroke volume (mL/beat)
Typical Values
| Population | Resting Q | Maximal Q | Resting SV | Maximal SV |
|---|---|---|---|---|
| Untrained | 5 L/min | 15-20 L/min | 70 mL | 100-120 mL |
| Trained | 5 L/min | 25-35 L/min | 90-100 mL | 150-200 mL |
| Elite endurance | 5 L/min | 35-40+ L/min | 100-120 mL | 180-220 mL |
Exam Tip: Resting cardiac output is similar in trained and untrained individuals (~5 L/min). The difference is that trained individuals achieve this with lower HR and higher SV (cardiac efficiency).
Factors Affecting Stroke Volume
| Factor | Effect |
|---|---|
| Preload | Greater venous return → greater filling → greater SV (Frank-Starling mechanism) |
| Contractility | Increased sympathetic stimulation → stronger contraction → greater SV |
| Afterload | Higher arterial pressure → more resistance → decreased SV |
| Training status | Endurance training increases SV through cardiac hypertrophy |
Blood Pressure Responses to Exercise
Blood pressure is expressed as systolic/diastolic (mmHg):
Normal Values
| Condition | Systolic | Diastolic |
|---|---|---|
| Normal rest | <120 mmHg | <80 mmHg |
| Elevated | 120-129 mmHg | <80 mmHg |
| Hypertension Stage 1 | 130-139 mmHg | 80-89 mmHg |
| Hypertension Stage 2 | >140 mmHg | >90 mmHg |
Exercise Blood Pressure Responses
| Exercise Type | Systolic Response | Diastolic Response |
|---|---|---|
| Aerobic exercise | Increases proportionally with intensity | Stays same or slightly decreases |
| Resistance training | Increases significantly (especially during Valsalva) | Increases moderately |
| Recovery | Gradual return to baseline | May drop below baseline |
Expected systolic values during maximal aerobic exercise:
- Untrained: ~180-200 mmHg
- Trained: ~200-220 mmHg
Warning Sign: Failure of systolic BP to rise with increasing intensity or a drop during exercise may indicate cardiac dysfunction—contraindication for continued exercise.
Heart Rate
Maximum Heart Rate Estimation
Age-predicted HRmax = 220 - age
| Age | Estimated HRmax |
|---|---|
| 20 | 200 bpm |
| 30 | 190 bpm |
| 40 | 180 bpm |
| 50 | 170 bpm |
| 60 | 160 bpm |
Important: This formula has a standard deviation of approximately 10-12 bpm. Individual variation is significant, and actual HRmax should be determined through maximal testing when precision is needed.
Training Heart Rate Zones
| Zone | % HRmax | Purpose |
|---|---|---|
| Recovery | 50-60% | Active recovery, warm-up |
| Aerobic base | 60-70% | Fat oxidation, endurance base |
| Aerobic development | 70-80% | Cardiovascular improvement |
| Threshold | 80-90% | Lactate threshold training |
| Anaerobic | 90-100% | VO2max, speed development |
Oxygen Delivery: The Fick Equation
VO2 = Q x (a-vO2 difference)
Where:
- VO2 = Oxygen consumption (mL/min or L/min)
- Q = Cardiac output (L/min)
- a-vO2 difference = Arterial minus venous oxygen content (mL O2/L blood)
Typical Values
| Population | Resting a-vO2 diff | Maximal a-vO2 diff |
|---|---|---|
| Untrained | 5 mL/dL | 15 mL/dL |
| Trained | 5 mL/dL | 17-18 mL/dL |
Exam Tip: Improvements in VO2max come from increases in both cardiac output (central adaptation) and a-vO2 difference (peripheral adaptation—improved oxygen extraction by muscles).
According to Wolff's Law, what happens to bone when mechanical loading is INCREASED?
Osteoclast activity increases, leading to bone resorption
Osteoblast activity increases, leading to bone deposition and increased density
Bone composition remains unchanged
Bone becomes more flexible due to decreased mineral content
What is the formula for cardiac output?
Q = HR + SV
Q = HR x SV
Q = HR / SV
Q = SV - HR
During steady-state aerobic exercise, what typically happens to diastolic blood pressure?
It increases proportionally with exercise intensity
It remains relatively constant or slightly decreases
It increases more than systolic blood pressure
It drops dramatically below resting values
Which structures connect muscle to bone?
Ligaments
Tendons
Cartilage
Fascia
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