Ch4 Cardiovascular System

Blood vessels

• Arteries: carry blood away from the heart
• Veins: carry blood to the heart

Pulmonary circulation

• Right ventricle → left atrium

Systemic circulation

• Left ventricle → aorta → right atrium via vena cava

Key note:

• The right heart is less oxygenated.

Blood movement

Blood is moved via bulk flow.

Cardiac cycle (process of blood)

1. blood enters atria from veins
2. AV valves open
3. blood enters ventricle (about 80% full)
4. atria contract (remaining 20%)
5. atria relax, ventricle contracts, AV valve closes (lub)
6. pulmonary / aortic valve opens
7. pulmonary / aortic valve closes (dub)

Electrical signaling

Cell types

• Contractile cells

• contract when they receive a signal

• Conducting cells

• do not contract

• controlled by the Cardiac Conduction System (CCS)

Key note:

• The CCS can generate signals and drive pumping on its own.

Cardiac Conduction System (CCS) pathway

1. Sinoatrial (SA) node (right atrium)
2. internodal pathway

3. Atrioventricular (AV) node (right atrium)

4. delay for 100 ms

5. bundle of His
6. left and right ventricles via Purkinje fibers

ECG mapping

• P wave: voltage increase in atrial muscle cells
• QRS complex: ventricular contraction
• T wave: ventricular relaxation

Pacemaker mechanisms (autorhythmicity)

HCN4 (“funny channel”)

Hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4):

• opens when voltage decreases
• contributes to autorhythmicity
• opens when \(V < -40,\text{mV}\) and with cAMP

Pacemaker potential

• pacemaker potential is the slow influx of Na\(^+\)
• cardiac muscle cells do not have leaking Na\(^+\) channels

Interpreting the waveform

• the upward is sodium, which is the start of P wave (atria) or QRS (ventricle)
• the first downward is K\(^+\), which is the end of P wave (atria) or QRS (ventricle)
• the plateau is the gap between P and QRS or QRS and T
• the last downward is K\(^+\), which is QRS (atria) or T (ventricle)

Vessel structure

Layers of a large artery

• lumen
• endothelium
• multiple elastic layers
• many layers of connective tissue and muscle

Arteriole

• lumen
• endothelium
• many layers of muscle

Capillary

• lumen
• endothelium, with intracellular clefts (proteins unable to flow through)

Venule

• lumen
• endothelium

Large vein

• lumen (wide)
• endothelium
• few elastic layers
• few smooth muscle and connective tissue layers
• with valves

Mean Arterial Pressure (MAP)

[ \text{MAP} = \text{CO} \times \text{TPR} ] where:

• CO = cardiac output
• TPR = total peripheral resistance

MAP feedback loop

• Controller: medulla oblongata
• Sensor: mechanoreceptor neurons

• Actuating signals:

• parasympathetic firing rate

• sympathetic firing rate

Effectors

• SA node

• \(\beta\)-adrenergic for SNS

• nAChRs for PNS

• arteriolar smooth muscle

• \(\alpha\)-adrenergic

Autonomic pathways

Sympathetic nervous system (SNS)

NT: noradrenaline

Parasympathetic nervous system (PNS)

NT: ACh

Local change (active hyperemia)

• high level of cellular activity leads to more blood flow to that area (active hyperemia)
• increased arteriole diameter increases blood flow