Reynolds - CV Histology & Embryology Flashcards
What are the main functions of the CV system?
- Transportation - O2, CO2, other metabolites, hormones, immune cells, waste products
- Regulation (homeostasis) - physiologic pH, hormones
- Protection
When does the heart begin beating in utero?
Day 22
When does the heart begin pumping blood in utero?
Day 24 or 25
From what does the heart begin developing from?
A tube of contractile myocardium lined by endothelium (endocardium)
Heart development steps?
- Cardiogenic field is established cranial to neural plate (brain)
- Heart forms from the visceral (splanchnic) layer of lateral plate mesoderm
- Lateral body folding brings the two sides of the cardiac region to the midline to fuse
- Cardiac looping
How is the cardiogenic field established?
Cranial to the neural plate.
- Blood islands form in the visceral (splanchnic) layer of the lateral plate mesoderm, cranial to the developing brain
- Horseshoe-shaped endothelial tube (primitive heart tube) develops as these blood islands coalesce
What happens with the heart during lateral body folding?
- Brings the two sides of the cardiac region to the midline to fuse.
- Fusion produces a single tube with aortic and venous poles
- The heart tube sprouts aortic arch vessels from the (aortic) outflow region; the venous pole remains paired
- Caudal pole forms venous end of the heart
- Cranial end is the arterial portion of the heart; vessels formed by sprouting are the aortic arches
What is cardiac looping?
The atrial portion grows upward and to the left, while the ventricular region grows downward and to the right. This places the atria superiorly and posteriorly, the ventricles anteriorly and inferiorly, and the heart itself to the L side of the body.
What event establishes the basic morphology of the heart and at what point in time?
Cardiac looping; 28 days
When do external and internal characteristics of the heart develop?
After looping at the end of 4 weeks
What are the 3 main shunts in prenatal circulation?
- Ductus venosus
- Foramen ovale
- Ductus arteriosus
Where is the ductus venosus found in pre-natal circulation and what does it do?
Liver; regulates BP and blood flow from placenta to the heart
Where is the foramen ovale found in pre-natal circulation and what does it do?
Heart; allows for passage of blood, bypassing pulmonary circuit
Where is the ductus arteriosus found in pre-natal circulation and what does it do?
Great vessels; connects pulmonary artery with aortic arch
What are the steps for septum and foramen development in the heart?
- Septum primum forms on Day 28 and extends into the atrium from the cranial-dorsal wall; grows downwards and before fusing at the bottom, there’s an apoptotic event that opens foramen secundum
- Foramen primum exists before septum primum attaches
- Foramen secundum forms by apoptosis of the septum primum
- Septum secundum grows by looping and leaves an open foramen ovale
- At birth, the septum primum and septum secundum are pushed together
What structure in septum development in the heart exists before septum primum attaches?
Foramen primum
What structure in septum development in the heart forms by apoptosis of the septum primum?
Foramen secundum
From what structure does the foramen ovale arise from?
Septum secundum
Name the atrial septal defects
- Excessive resorption of septum primum
- Absence of septum secundum
- Common atrium
What is Atrial Septal Defect (ASD): Excessive resorption of septum primum?
Sometimes the amount of apoptosis is too extensive during formation of the ostium secundum in the upper portion of the septum primum. This results in a hole that’s too big for the septum secundum to cover
What is Atrial Septal Defect (ASD): Absence of the septum secundum?
Occasionally the septum secundum fails to form, leaving a large hole in the atrial septum
What is the Atrial Septal Defect (ASD): Common atrium?
Absence of septum primum and septum secundum
Ventricular Septal Defects (VSD)
20% membranous, 80% muscular
Interventricular septal defects are the most common type of heart defect
Persistent truncus arteriosus
Conotruncal septum never forms
What are the differences between cardiac and skeletal muscle?
- Location
- Shape
- Number and location of nuclei, mitochondria
- Autorhythmicity
- Control
- Presence of intercalated discs
What are intercalated discs?
Junctions between cardiac muscle cells.
- Points of anchorage for myofibrils (mostly actin)
Function of maculae adherens?
Intercalated discs slide
- Maculae adherens prevent the pulling apart of cells during contraction
Function of gap junctions?
Intercalated discs slide
- Allow for the rapid spread of contractile stimuli
How are electrical stimuli propagated in the heart?
Gap junctions
What are the 3 layers of the heart from superficial to deep?
- Epicardium
- Myocardium
- Endocardium
Epicardium
- Visceral pericardium
- Simple squamous epithelium
- Fibrocollagenous tissue with abundant elastic fibers, adipose, nerves, coronary vessels
Myocardium
- Cardiac myocytes
- Collagenous CT skeleton containing adipose, BVs, and nerves
- Difference between atria and ventricle tissues (robustness)
Endocardium
- Inner lining of atria and ventricles
- Layer of simple squamous epithelium
- Layer of fibroelastic tissue containing specialized cardiac muscle fibers (Purkinje fibers), BVs, nerves
What are the factors that control cardiac output (CO)?
CO = SV*HR
- BV reflexes
- Autonomic NS
- Higher centers
What are cardiac conducting cells?
Purkinje system
- Modified, specialized cardiac muscle cells
- Organized into nodes and fibers
- Generate and rapidly transmit the contractile impulse
Basic architecture of BVs: Three layers
Superficial to deep:
- Tunica adventitia
- Tunica media
- Tunica intima
Functions of endothelial cells?
- Permeability barrier: gas and metabolite exchange
- Make their own basement membrane
- Very close to and bathed in blood, so they manipulate blood, release factors that promote good thrombotic events, clotting cascade
- Regulate vessel diameter, release factors that allow dilation and constriction
- Regulate inflammation
*Found in tunica intima
Elastic arteries
- Thick tunica adventitia
- Broad tunica media with lots of elastin
Muscular artery
- Thick tunica media
- Internal/external elastic lamina observed
- Have a lot of muscle and not very elastic; conduct blood to capillaries
Arterioles
- <0.05mm
- Tunica media has little, if any, internal elastic lamina
- 1-3 SM layers
Microcirculation: Capillaries
- Very thin endothelial membrane
- Precapillary sphincters help control blood flow
- Site of gas and nutrient exchange
- Three basic types
What are the the different means of gas and nutrient exchange in capillaries?
- Diffusion across membranes (gases)
- Small molecule transporters
- Transcytosis
- Leaks between cells
What are the three basic types of capillaries?
- Continuous
- Fenestrated
- Sinusoidal (Discontinuous)
Continuous capillary
Found in most tissues, continuous endothelium, scattered tight junctions, transport by diffusion and pinocytosis
Fenestrated capillary
Perforated endothelium enables extensive molecular exchange with the blood, continuous BM, much greater permeability, most notable in small intestine, glomerulus, endocrine glands
Discontinuous (sinusoidal) capillaries
- Greatest permability
- Perforated endothelium and BM
- Liver, lymphoid, endocrine, hematopoietic
How is blood flow in the capillaries controlled?
Diameter, resistance, precapillary sphincters
Venous system
- Always carry blood to the heart
- Categorized by size
- Lower pressure, thinner walls
- Larger veins have the same tunics as the arteries, just not as well-defined
- – Thinner walls allow more flexibility
- – Larger lumen allow more capacity
- Closer to surface of the skin, easier to see
- Valves present in many veins, especially those in extremities
Venules
- Larger than capillaries but smaller than veins
- Carry blood away from capillary beds towards the heart
