7.0 Cardiovascular System 1: Heart and Circulation

Question 1

Identify and describe the function of major structures of the heart

Answer 1

The heart is a dual pump that circulates blood around the body (systemic circulation) and through the lungs (pulmonary circulation).

1. The right atrium receives deoxygenated blood from the vena cavae (blood returning from the body) and empties into the right ventricle.
2. The right ventricle pumps blood into the pulmonary trunk and through the lungs where oxygen is picked up in exchange for carbon dioxide.
3. Oxygenated blood returns from the pulmonary circulation to the left atrium, which empties into the left ventricle.
4. The left ventricle pumps oxygenated blood into the aorta and on to the systemic circulation.

Question 2

Identify the primary coronary vessels

Answer 2

VESSELS ENTERING THE HEART:

Venae cava
Two large veins that carry deoxygenated blood from the upper and lower regions of the body to the right atrium of the heart.

• Superior vena cava – contains blood from the head and the arms.
• Inferior vena cava – the largest vein in the body, returning blood from the legs and lower torso.

Pulmonary veins
These veins transport oxygenated blood from the lungs to the left atrium of the heart.

VESSELS EXITING THE HEART:

Pulmonary arteries
These branch from the pulmonary artery that originates in the right ventricle.
The pulmonary arteries carry deoxygenated blood to the lungs.

Aorta
The largest single blood vessel in the body. It exits the left ventricle and transports oxygenated blood to the body.
The aorta is very elastic and stretches like a balloon as blood enters during ventricular contraction (systole), but during diastole (relaxation of the heart) it recoils so that blood is pushed forwards even though the heart is not contracting.

VESSELS WITHIN THE HEART:

Coronary arteries
A network of blood vessels that carry oxygen and nutrient-rich blood to the cardiac muscle tissue.
The left and right coronary arteries emanate from the base of the ascending aorta. The great vessels originate from the top of the aortic arch.

Question 3

Describe the functional anatomy of the heart valves and how they operate.

Answer 3

Blood flows through the heart in one direction: from the atria to the ventricles and out through the arteries to either the pulmonary or systemic circuit. Four heart valves open and close in response to pressure differences, and ensure that blood flows in only one direction.

The heart has:
- two atrioventricular (AV) valves: tricuspid and mitral valves

Each AV valve contains the following:
- Cusps: flexible resilient fibrous flaps of endocardium reinforced with connective tissue.
- Chordae tendineae: collagen cords that anchor the cusps to papillary muscles. They are known colloquially as the heart strings.
- Papillary muscles: nipplelike structures that protrude from the ventricular walls into the ventricular cavity.

• two semilunar valves:
  pulmonary and aortic valves

Heart valves open and close passively in response to a pressure difference between the two sides of the valve. When pressure is greater behind the valves, the leaflets are blown open, permitting blood to flow through. When pressure is greater in front of the valve, this pressure drives the valves shut, preventing backflow.

Question 4

List the key types of blood vessels and how their structure is related to their function.

Answer 4

Arteries
Pressure reservoirs that deliver blood from the heart to various regions of the body.

Arterioles
High resistance vessels that regulate flow of blood to various tissues. Their diameter is adjusted to the tissues’ needs for oxygen and energy by a combination of nervous innervation and local metabolites.

Capillaries
Small, abundant vessels providing, in total, a large surface area for nutrient and waste exchange between the blood and tissues.

Veins
Large and compliant vessels that deliver blood from the tissues back to the heart.

Venules
Small vessels that drain blood from the capillary beds to the veins.

Question 5

Describe how elastance, compliance and capacitance influence overall blood flow and cardiac output

Answer 5

The elastic properties of the vessel wall (its ability to spring back to its original shape) can be described by the elastance (ΔP/ΔV – the change in pressure divided by the corresponding change in volume).

The distensibility of the wall (its ability to be distended or stretched) is expressed as the compliance (ΔV/ΔP – the inverse of elastance).

– compliance actually decreases at higher volumes and pressures

The vein’s ability to rapidly increase its volume capacity at lower pressures explains why veins are known as capacitance vessels.

Question 6

Summarize the relationships between pressure, area, resistance, velocity, and volume in the major vessel types.

Answer 6

RATE OF FLOW (Q)= CHANGE IN PRESSURE (P) DIVIDED BY RESISTANCE (R)

VISCOSITY
Viscosity arises from cohesive forces between the molecules in the liquid – the stronger the cohesive forces, the greater the resistance to flow.

LENGTH OF TUBE
Friction between liquid and the wall of the tube contributes to resistance. The longer the tube, the greater the resistance to flow. Length and flow are inversely proportional – double the length, halve the flow rate.

RADIUS OF TUBE:
When a liquid is flowing through a tube, the outermost layer moves more slowly than the layer directly inside of it. That layer is then slower than the layer inside of that, and so on, with flow being fastest in the middle. This “streamlined” flow means that flow through a vessel increases very rapidly when the diameter (or radius) is increased.

Resistance is affected much more by changes in radius than it is by changes in length

Question 7

Locate areas where one can palpate a pulse

Answer 7

The pulse can be measured using the radial artery in the wrist or the carotid artery in the neck.

Question 8

Laminar flow

Answer 8

Laminar flow is streamlined and organized. Flow close to the vessel walls is very slow due to friction between the fluid and the wall, while flow in the centre of the vessel is very fast (there is less friction) – this creates a parabolic velocity profile. This type of flow is seen in most blood vessels. A consequence of this is that the blood cells are found towards the center of the flowing stream rather than out in the periphery near the vessel walls.

Question 9

turbulent flow

Answer 9

Turbulent flow is disorganized and occurs in blood vessels when there is some distortion or obstruction (such as when a vessel bifurcates or when it is blocked by a plaque or clot) or when blood velocity exceeds 40 cm/s. There is greater resistance to blood flow when flow is turbulent, and less resistance when flow is laminar. The random motion of particles in turbulent flow causes the velocity profile to flatten.