summary Physiology basic concepts Cardiov & Resp System
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Course
Physiology, Basic Concepts
Institution
Universiteit Leiden (UL)
This is a comprehensive summary of all the content covered in the lectures of the “Cardiovascular and respiratory systems” section of the course Physiology: Basic concepts. On the basis of pictures and text, all material is extensively treated.
,1.1. Cardiac anatomy, electrical aspects
Basic heart anatomy
The heart consists of four rooms. Oxygenated blood from the
lungs enters the Left atrium via the pulmonary vein, after
which it passes the mitral valve into the left ventricle. From
there it is pumped through the aortic valve into the aorta to
the rest of the body. The deoxygenated blood from the body
returns to the heart via the caval veins entering the right
atrium. It passes the tricuspid valve into the right atrium
where it leaves via the pulmonic valve into the pulmonary
artery.
The heart is made up of several tissue layers:
1. Endocardium, a thin internal layer (endothelium and subendothelial connective tissue) or
lining membrane of the heart that also covers its valves.
2. Myocardium, a thick, helical middle layer composed of cardiac muscle.
3. Epicardium, a thin external layer (mesothelium) formed by the visceral layer of serous
pericardium
The cardiac muscle fibers are anchored to the fibrous skeleton of the
heart, a complex framework of collagen forming four rings to which
the valves and septa are attached. It also serves as an electrical
insulator and prevents overdistention of the AV and semilunar valves.
The left and right ventricles can be distinguished by looking at:
Right ventricle Left ventricle
Muscular infundibulum Continuity MV-AoV
Trabecula septomarginalis and moderator band Smooth septal surface
Coarse apical trabecularisation Fine trabecularisation
Ver thick wall! Relatively thin wall
Average cardiac pressures
,Coronary arteries
Coronary arteries are arteries the originate directly from the aorta and supply the cardiac muscle
with its blood. The aorta gives of the Right and left coronary artery.
The left coronary artery consists of a main stem, which splits into Ramus descendens anterior and
the ramus circumflex.
The right coronary artery consists of a main stem as well, which gives of a branch for the SA and AV
node and the Ramus descendens posterior.
Cardiac muscle
the cardiac muscle is composed of several structure levels. The muscle is
composed of muscle fascicles which are divided into muscle fibers. Each
muscle fiber contains many myofibrils which in their turn are composed
of sarcomeres.
Sarcomeres are the contractile units of the muscle, containing actin and
myosin molecules able to create cross bridges and thereby contract. Each
muscle fiber is enclosed in a sarcolemma, which is in a way the cellular
membrane of the muscle cell. This structure contains T-tubules
(1/sarcomere), which penetrate the structure at the level of the Z-line.
The sarcoplasmic reticulum network forms terminals with the T-tubules
(diad). T-tubles are involved in extending the electrical impulse into the
muscle fibers. Depolarization of the T tuble membrane activates voltage
sensor proteins which serve as Ca2+ channels. Ca2+ from the lumen of the
T-tubles is released into the sarcoplasm, causing gated Ca2+-release
channels in the sER. This release initiate steps in the contraction cycle.
Events leading to contractions can be summarized as follows:
, 1. Contraction of a cardiac muscle fiber initiates when the cell membrane depolarization
traveling along Purkinje fibers reaches its destination in cardiac myocytes.
2. General depolarization spreads over the plasma membrane of the muscle cell causing the
opening of voltage-gated Na channels. Na enters the cell.
3. General depolarization continues via membranes of the T tubules.
4. Voltage-sensor proteins (DHSRs) in the plasma membrane of T tubules change their
conformation into functional Ca2 channels.
5. Rise in the cytoplasmic Ca2 concentration opens RyR2- gated Ca2 release channels in the
sarcoplasmic reticulum.
6. Ca2 is rapidly released from the sarcoplasmic reticulum and increases the pool of Ca2 that
entered the sarcoplasm through the calcium channels in the plasma membrane.
7. Accumulated Ca2 diff uses to the myofilaments, where it binds to the TnC portion of the
troponin complex.
8. The actomyosin cross-bridge cycle like that of skeletal muscle is initiated
9. Ca2 is returned to the terminal cisternae of the sarcoplasmic reticulum, where it is
concentrated and captured by calsequestrin, a Ca2 binding protein.
Functional syncytium
Cardiac muscle is in functional syncytium, meaning that
the muscle cells are mechanically, chemically and
electrically connected to each other. This causes the
stimulation of a single muscle cell to result in the
contraction of all muscle cells. The attachment site
between different cardiac muscle cells is called the
Intercalated disk (ID). It consists of the fascia adherens
(transverse boundary between cells), the maculae
adherens/desmosomes (binding sites between the
cells, both in transverse and lateral boundaries), and
gap junctions (provide ions continuity between
adjacent cardiac muscle cells).
Gap junctions consist of 2 connexons (or hemichannels), which are hexamers of transmembrane
protein subunits called connexins. These have an open and closed state and are necessary for the
electrical and metabolic coupling between the cells.
Differences skeletal, smooth and cardiac muscle
Skeletal muscle cell Cardiac muscle cell
Peripheral nucleus Central nucleus
Single multinucleated protoplasmic unit (fibre) Juxtanuclear cytoplasm
End-to-end alignment of cells; individual cells
joined by complex junctions form a functional
unit (1 fiber formed by numerous cells)
Branching pattern of the muscle fibers
Intercalated disks (complex junctions)
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