Physiology 3: The cardiovascular system

Question 1

What are the functions of the circulatory system?

Answer 1

• Transport – blood and:
  
  • Gases (CO₂ and O₂)
  • Nutrients (e.g. glucose)
  • Fluid and electrolytes
  • Wastes (e.g. urea)
  • Hormones
  • Leukocytes
  • Plasma proteins
• Defence against infection
• Coagulation, healing
• Thermoregulation

Question 2

What is the general purpose of the RH and LH side of the heart?

Answer 2

The heart consists of two pumps in series. The RHS of the heart pumps deoxygenated (from the body) to the lungs and then to the left atrium; the left ventricle pumps oxygenated around the body and to the right atrium.

Question 3

What makes the ‘lub-dub’ sound of the heart?

Answer 3

Blood only flows in one direction because of the valves. It is the closing of these valves that makes the lub-dub sound of the heartbeat.
S1 = closure of the atrioventricular valves
S2 = closure of the semilunar valves

Question 4

What is the generalised structure of the heart (IMAGE)?

Answer 4

Question 5

What is blood flow driven by?

Answer 5

Blood flow is driven by a pressure gradient (ΔP= mean arterial pressure – central venous pressure).

Aortic pressure (MAP) is around 85mmHg, central venous pressure (approximately right atrial pressure) is 0mmHg. This high system pressure is necessary to supply blood to the brain (often against gravity) and extremities.

Pressure in the pulmonary circulation is much lower (15mmHg).

Question 6

What are the different vessels and their functions?

Answer 6

• Arteries – conduit vessels; transport blood under high pressure
• Arterioles – resistance vessels; contract and dilate to control the flow of blood to individual tissue beds; hydraulic filter (convert intermittent flow to relatively steady flow)
• Capillaries – exchange vessels; diffusion of blood gases occurs in this vessels
• Venules/veins – return blood to the heart; capacitance vessels (store blood)

Question 7

What are the key features about the structure of Capilalries and Aorta?

Answer 7

The aorta has a lot of elastic tissue in the wall – very distensible

Capillaries – very thin wall, endothelium only. Easy for nutrients, gases to cross the wall.

Question 8

What is the difference between systole and diastole, and was is its role in continuos blood flow?

Answer 8

• Systole – contraction
• Diastole – relaxation

Even though the heart only beats intermittently, blood flow is continuous because arteries are compliant and store blood during ventricular systole.

During ventricular diastole, the recoil, so blood continues to flow.

Question 9

Do you really understand the change in Blood pressure in a supine pressure? Observe the following image:

Answer 9

• Note that blood pressure changes depending on the vessel.
• Arterioles provide a lot of resistance, lowering pressure and also act as a hydraulic filter, levelling out the flow

Question 10

In the microcirculation, what is the function of the precapillary sphincters?

Answer 10

Microcirculation:

Precapillary sphincters can relax or contract to control blood flow to capillaries in the skin. This assists in thermal regulation. Blood is redirected through non-nutrient vessels

Question 11

What is the blood distribution, and the effect of gravity?

Answer 11

The typical blood volume is approximately 5L. Most blood is resides in the veins and venules (see diagram). Pressure down at the legs is much higher than at the heart (due to the effects of gravity). Vice versa; much lower pressure above the hearts (e.g. negative pressure in the sagittal sinus).

Question 12

How do we move venous blood?

Answer 12

Muscle pump - muscles contracting against the veins helps to push the blood up back to heart. There are also valves in the veins of the legs that stop blood from flowing back down.

Question 13

What is, and how do we measure Cardiac Output?

Answer 13

• Cardiac output is the amount of blood pumped by the heart in a minute.
• Cardiac output = stroke volume x heart rate
• In a 70kg man at rest typical CO is about 5L, SV is 70ml and HR is 70bpm.

Question 14

What are the defintions of Pressure, flow, velocity, conductance and resistance?

Answer 14

• Pressure – the force exerted by blood upon vessel wall (blood pressure refers to arterial pressure)
• Flow – volume of blood movement across a point, determined by force and resistance
• Velocity – the speed an individual particle moves at
• Resistance – the impediment to flow in a vessel. Cannot be directly measured, must be calculated
• Conductance – flow divided by pressure

Question 15

How is the Pressure gradient calculated?

Answer 15

Pressure gradient = flow x resistance

Question 16

Observe the following on the movement of blood, with respect ot resistance, and velocity. Do you understand it?

Answer 16

• Resistance is far higher in system circulation than in pulmonary circulation (about 7x greater).
• Velocity is faster in the aorta than the venae cavae.
• Velocity is very low at the capillaries.
• There is a reciprocal relationship between velocity and the cross sectional area
• Blood velocity is increased as vessel diameter is reduced. Flow remains constant.

Question 17

How is blood flow measured?

Answer 17

Blood pressure is measured using a sphygmomanometer and stethoscope. Inflate cuff above systolic pressure, occluding the brachial artery. As the cuff deflates, Korotkoff sounds are heard at the systolic pressure (where turbulent flow starts) until the diastolic pressure (when laminar flow is reached and sounds stop.)
This allows blood pressure to be measured

Mean pressure = diastolic pressure + 1/3 pulse pressure

Question 18

What are baroreceptors, and what is their function?

Answer 18

Baroreceptors (e.g. aortic baroreceptors or baroreceptors in the carotid sinus) mechanoreceptors located in blood vessels that control blood pressure in the short term. They send signals to the brain stem, in order to adjust heart rate and therefore influence blood pressure.

As BP goes down, HR goes up and vice versa

Question 19

What are the the three basic theory points of the circulatory function?

Answer 19

• Rate of blood flow to each tissue in the body is almost always precisely controlled in relation to tissue need
• Cardiac output is controlled mainly by the sum of all the local tissue flows
• In general, arterial pressure is controlled independently of either local blood flow control or cardiac output control

Question 20

What are the mechanisms of controlled tissue blood flow?

Answer 20

Acute –

• local (e.g. metabolism, active hyperaemia occurs due to metabolic build-up of vasodilators when the rate of metabolisms increases; reactive hyperaemia occurs following a period of ischemia, also due to a build-up of metabolic wastes)
• Neural (autonomic), sympathetic and parasympathetic nervous systems
• Humoral (circulating or hormonal), through vasoconstrictors (adrenaline, noradrenaline, angiotensin 2, Vasopresin, Endothin etc.) or vasodilators (histamine and bradykinin)

Long term – involves alterations in the number and size of vessels

Question 21

Different tissues rely on different control mechanisms more heavily. Elaborate.

Answer 21

Neural regulation predominates in:

• Splanchnic organs (visceral organs)
• Skin

Local factors predominate in:

• Exercising muscle (due to greater nutrient-oxygen needs)
• Vital tissues – brain, heart