circulatory system

Humans possess a closed circulatory system. Humans have a double circulatory system, meaning the heart is completely divided into right and left sides

Types of Blood Vessels

There are five primary types of blood vessels,

Arteries: carry blood away from the heart.

• Arterioles: These represent the first branches into smaller vessels.

• Capillary Beds: These are the smallest vessels where the actual exchange of gases ($O_2$, $CO_2$) and nutrients occurs between the blood and underlying tissues.

• Venules: Small vessels that collect blood from the capillary beds to begin the return trip to the heart.

• Veins: Larger vessels that drain blood back into the heart.

Pulmonary and Systemic

The heart is divided by a wall called the septum, which separates it into the right and left sides.

Pulmonary Circulation

• Path: Right side of the heart, Lungs, Left side of the heart.

• Function: A short circuit dedicated to oxygenating deoxygenated blood.

  • Pulmonary Artery: Supplies deoxygenated blood to the lungs.

  • Pulmonary Vein: Returns oxygenated blood to the left side of the heart.

Systemic Circulation

• Path: Left side of the heart —> Everywhere else in the body —> Right side of the heart.

• Key Vessels:

  • Aorta: The largest artery in the body; leaves the left side of the heart to drive systemic circulation.

  • Vena Cava: The large veins returning blood to the right side of the heart.

While most systems have a single capillary bed, certain areas have two capillary beds in sequence, known as portals. Examples include:

• The Digestive Tract and Liver.

• The Kidneys.

• The Brain.

Blood Composition and Production

Blood is made of cellular elements and a fluid component (plasma)

• Albumin and Globulins: Act as transporters for substances that do not dissolve well in water.

• Fibrinogen: Essential for the blood clotting process.

• Platelets: Involved in blood clotting.

Erythropoiesis (Red Blood Cell Production)

• Hematopoiesis: The general term for the production of all blood cells in the bone marrow.

• Erythropoiesis: Specifically the production of RBCs.

• Trigger: Hypoxia (low oxygen levels in the blood) is detected by the kidneys.

• Mechanism: The kidneys secrete a glycoprotein called Erythropoietin (EPO) into the circulation. EPO acts on the bone marrow to trigger RBC maturation.

• Feedback: This is a negative feedback loop; once oxygen-carrying capacity increases, EPO production is switched off.

Cardiac Anatomy and Specialized Cells

Heart Chambers

• Atria: Two chambers at the top (Right and Left). They are described as "flaps of tissue" that rarely fill to capacity as open chambers.

• Ventricles: Two chambers at the bottom (Right and Left). These are the predominant filling and pumping areas.

Unidirectional Flow and Valves

The heart utilizes four valves that open and close in response to pressure changes to ensure one-way flow:

• Atrioventricular (AV) Valves: Separate atria and ventricles.

  • Tricuspid Valve: On the right side; has three valve leaflets.

  • Bicuspid (Mitral) Valve: On the left side; has two valve leaflets.

• Semilunar Valves: Located at the exit points to the large blood vessels.

  • Pulmonary Semilunar Valve: Leads to the pulmonary artery.

  • Aortic Semilunar Valve: Leads to the aorta.

  • Structure: Both semilunar valves consist of three leaflets.

Muscle Thickness and Pressure

• Right Ventricle: Has thin cardiac muscle because it only pumps to the lungs, which are anatomically very close.

• Left Ventricle: Has much thicker muscle because it must generate enough force to drive blood through the entire systemic circulation.

• Clinical Correlation: In cases of chronic high blood pressure, the heart muscle may thicken (hypertrophy). This is detrimental because the internal space (lumen) becomes smaller, reducing the volume of blood the heart can hold.

Electrical Conduction System

Autorhythmic Cells

The heart contains specialized cells that generate their own electrical signal (action potential) without external input from the nervous system. This creates a spontaneous, rhythmic beat.

Electrical Pathway

1. Sinoatrial (SA) Node: Located in the top of the right atrium. Known as the "pacemaker," it fires spontaneously and triggers atrial contraction (depolarization).

2. Atrioventricular (AV) Node: Receives the signal from the SA node and transmits it toward the septum.

3. Bundles of His: Located in the septum; they rapidly transmit the electrical signal to the bottom (apex) of the heart.

4. Purkinje Fibers: Terminal ends of the pathway; they spread the signal from the bottom upward through the ventricles, triggering a bottom-up contraction to eject blood into the vessels at the top.

• Electrocardiogram (ECG): This electrical activity can be measured at the skin using stickers to monitor the health of the electrical pathway.

Mechanical Cycle and Cardiac Performance

The Contractile Cycle

• Diastole: The period of relaxation when the heart fills with blood.

• Systole: The period of contraction when the heart ejects blood.

• Note: The heart spends more time in diastole than systole to allow for adequate filling.

Volumes and Output

• Residual Volume: Approximately $65\,ml$ of blood remains in the ventricles after every beat. This prevents friction/inflammation and provides a reserve to increase output if needed.

• Stroke Volume (SV): The volume of blood ejected per single heartbeat. Standard value: $70\,ml/beat$.

• Heart Rate (HR): The frequency of beats. Average value: $72\,beats/min$.

• Cardiac Output (CO): A measure of the heart's performance over time ($L/min$).

$CO = SV \times HR$

• Regulation: The Autonomic Nervous System alters CO. Sympathetic input (Noradrenaline) increases heart rate; Parasympathetic input (Acetylcholine) slows heart rate.

Principles of Blood Pressure

• Systolic Pressure: The maximum pressure during contraction ($120\,mmHg$).

  • Diastolic Pressure: The minimum pressure during relaxation ($80\,mmHg$)

Hypotension and Venous Return

• Hypotension: Low blood pressure.

• Hypertension: High blood pressure.

• Postural Hypotension: Gravity pulls blood toward lower extremities. Prolonged standing can lead to blood pooling in the feet.

• Skeletal Muscle Pump: Because venous pressure is low, walking helps return blood to the heart. Leg muscles compress the veins, pushing blood upward.

Capillary Transport and The Lymphatic System

Mechanisms of Transport

1. Movement Between Endothelium: Small molecules move through gaps in cell junctions, driven by concentration gradients.

2. Transcytosis: Substances are transported in small vesicles across the cell membrane (active process).

3. Bulk Flow: Mass movement of water and solutes driven by blood pressure. Filtration occurs at the arterial end of the capillary.

The Lymphatic System

• Lost Volume: Significant fluid volume is lost to the interstitial space during bulk flow.

• Function: Lymphatic vessels pick up this excess interstitial fluid and return it to the circulatory system to maintain blood volume and pressure.

• Lymph Nodes: Clusters of immune cells located throughout the system at specific intervals.