Controlling the Heart and Blood Pressure

Question 1

Describe blood pressures throughout the systemic circulation

Answer 1

• Blood pressure high in major arteries (oscillatory - due to contractions of heart)
• Blood pressure falls steeply across the arterioles, capillaries and venules (oscillatory nature is reduced)
• Blood pressure is very low in veins
• Large difference in pressure between arterial and venous sides (which creates a driving force for blood flow)

Question 2

Describe how the ejection of blood into the arterial system maintains arterial blood volume and pressure

Answer 2

Blood flow in
- fills arteries
- increases arterial blood volume
- raises arterial pressure

Blood flow out:
- drains arteries
- decreases arterial blood volume
- lowers arterial pressure

Arterial blood volume and pressure are determined by:
- balance between blood flows in and out

Arterial pressure is defined by how much blood is in them.

Question 3

Describe how cardiac output and arterial resistance affect blood pressure

Answer 3

Blood flow in:
- ventricular contraction
- ejection of blood
- CARDIAC OUTPUT

Blood flow out:
- capillary flow
- controlled by resistance of the arteries

Balance flow in and out determines pressure:
- increase cardiac output (increase inflow)
- increases resistance (decrease outflow)
- increase arterial volume and pressure

Question 4

what is the equation for mean arterial blood pressure?

Answer 4

MAP = CO X TRP (total peripheral resistance)

Question 5

What is the equation for cardiac output?

Answer 5

CO = SV (contraction strength) X HR (contraction speed)

SV - stroke volume (how much blood is being pushed out per beat)

• This equation shows that there are two approaches for meeting cardiac needs (increase HR or SV)
• CO is variable due to changes in heart rate and or stroke volume (eg. the values at rest vs. excersise)

Question 6

Describe the homeostasis of arterial blood pressure

Answer 6

Mean arterial pressure is tightly regulated (narrow range)

MAP = CO X TPR
- heart (CO)
- blood vessels (vascular resistance)

Coordinated within the brainstem
- Afferent input from both the CNS and ‘periphery’ (receiving info from the body into the brain)
- efferent output to heart and vessels (info being sent out of the brain to the bod based on what is going on in the brain)

Question 7

Describe the baroreceptors that act as blood pressure sensors

Answer 7

Baroreceptors in the arteries act as sensors of blood pressure. There are two main areas where they are located.
- Carotid artery (brain considers this most critical as it wants to know about any issues in blood pressure leading to the brain)
- Aortic arch (monitors pressure of blood in aorta coming out of the heart)

These receptors are stretch receptors that detect pressure. They send information constantly, and if they stretch any more or less than normal, the brain will be notified.
- If BP goes up: brain will decide to do something soon as high BP is bad for the system long term
- If BP goes down: this is an emergency as it can be fatal so brain makes changes immediately to rectify this.

Question 8

Describe the neural control of cardiac output

Answer 8

The brain is the micromanager of the heart, constantly sending signals to tell it to beat faster or slower. It does this through the sympathetic and parasympathetic systems.

Question 9

How does the brain decrease BP?

Answer 9

“Brake” - Vagus nerve
- If BP is too high brain will activate parasympathetic nervous system, increasing the signalling to the vagus nerve.
- Vegas nerve innervates SA node and VA node
- Vegas nerve tells SA node to slow down signalling and VA node to hold the signal for longer.
- This slows HR down.

Question 10

How does the brain increase BP?

Answer 10

“Accelerator” - Sympathetic trunk ganglion
- If BP is too low brain will activate the sympathetic nervous system, increasing the signalling to the sympathetic trunk ganglion.
- Sympathetic trunk ganglion innervates the SA node, VA node and ventricular walls
- Sympathetic trunk ganglion tells SA node to increase frequency of signalling and VA node to hold the signal for a shorter amount of time. It also tells the contractile cells to release more Ca2+ so the contractions are more powerful.

Question 11

Describe the baroreflex (whole body tilt experiment)

Answer 11

Experiment: lies flat for a while and then is tilted up on a bed. measures changes in CO, SV, HR, etc.

When tilted up:
- SV drops suddenly because now working against gravity
- CO decreases as well but not by much. Why?
- Because heart rate has increased to compensate and keep CO up.

MAP (mean arterial pressure) also doesn’t change, even though CO decreases. Why?
- TPR (total peripheral resistance) increases to compensate.

Remember the equations! They explain this effect.