Physiology
Cardiovascular
Which of the following structures prevents a direct electrical pathway from the atria to the ventricles:
Answer:
The atria are separated from the ventricles by a band of fibrous connective tissue called the annulus fibrosus, which provides a skeleton for the attachment of muscle and cardiac valves, and prevents electrical conduction between the atria and ventricles (except at the atrioventricular node).Cardiac Myocyte Microstructure
Physiology / Cardiovascular / Cardiac Cycle
Last Updated: 26th July 2024
The myocardium is composed of cardiac muscle cells called myocytes. The cells are striated due to the arrangement of the thick and thin filaments which make up the bulk of the muscle, although they are less organised than in skeletal muscle. The myocytes are small and branched, with a single nucleus and are rich in mitochondria. The normal pumping action of the heart is dependent on the synchronised contraction of all cardiac cells.
Cardiac myocyte contraction is not dependent on an external nerve supply but instead the heart generates its own rhythm (inherent rhythmicity). The nerves innervating the heart only speed up or slow down the rhythm and can modify the force of contraction.
The synchronicity between myocytes occurs because all the adjacent cells are linked to one another at their ends by specialised gap junctions (formed of connexons), within the intercalated discs, which essentially provide a low-resistance pathway between cells. Gap junctions allow action potentials to spread rapidly from one cell to another and allows the myocardium to act as a functional syncytium. The intercalated discs also provide structural attachments (desmosomes) between myocytes to distribute force.
Although a rise in intracellular [Ca2+] initiates contraction in the same way as in skeletal muscle, the mechanisms leading to the rise in intracellular [Ca2+] are fundamentally different.
Cardiac Myocyte Microstructure. (Image by OpenStax CNX [CC BY 3.0 , via Wikimedia Commons)
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
