Human Respiratory and Circulatory Systems Anatomy

Posted on Apr 20, 2025 in Biology

The Human Respiratory System

Nostrils: Air Intake and Olfaction

Nostrils are lined by mucous membranes containing olfactory receptors for smell detection. These membranes appear red due to blood vessels (which help warm incoming air) or yellowish in the olfactory region.

Functions:

  • Warms the air
  • Moistens the throat
  • Protects against infections with mucus
  • Detects smells

Lungs: Structure and Mechanics

The lungs are housed within the ribcage.

Ribcage, Muscles, and Breathing

The ribcage consists of bones (ribs, sternum, spine) and muscles:

  • Inspiratory muscles: Increase the volume of the rib cage (e.g., diaphragm, external intercostals).
  • Expiratory muscles: Reduce the volume; primarily used for forced expiration (e.g., internal intercostals, abdominal muscles).

Pleura: The Protective Double Membrane

The pleura is a double-layered sac containing pleural fluid. This fluid maintains negative pressure, keeping the pleural layers adhered to each other and, in turn, keeping the lungs attached to the thoracic wall (ribcage).

Bronchial Tree: Airways to Alveoli

The bronchi branch into much narrower bronchioles. All bronchioles have fewer cartilaginous rings than bronchi and are very thin. They lead to the alveolar sacs. Between the bronchioles, there is elastic tissue that allows the lungs to compress and expand.

Essential Respiratory Functions

Pulmonary Ventilation Explained

This is the process of renewing the air within the lungs (breathing).

Gas Exchange: Oxygen and Carbon Dioxide

Gas exchange occurs between the air in the alveoli and the blood in surrounding capillaries. Carbon dioxide (CO2) comes from cellular respiration.

  • Inspired air: Contains oxygen (O2).
  • Exhaled air: Contains less O2 and more CO2 than inspired air.

Understanding Pneumonia

Pneumonia is an infection of the lung tissue. Symptoms include very high fever and difficulty breathing, which can sometimes lead to death. Treatment often involves powerful antibiotics injected into a vein.

The Human Circulatory System

Core Components and Vital Functions

The circulatory system consists of a muscular organ (the heart), whose function is to pump blood, and blood vessels, whose function is to circulate blood throughout the body.

Functions:

  • Supplies oxygen to the body
  • Delivers nutrients
  • Transports toxins for elimination by the kidneys and lungs
  • Transports hormones that regulate body functions
  • Transports leukocytes (white blood cells) for immunity
  • Maintains body temperature

The Heart: Structure and Pumping Action

Layers of the Heart Wall

The heart wall has three layers:

  1. Pericardium: A protective sac surrounding the heart (technically not part of the wall, but closely associated).
  2. Myocardium: The muscular middle layer, responsible for pumping. It is very thick, especially in the ventricles, and thicker in the left ventricle than the right.
  3. Endocardium: The thin inner lining of the heart chambers.

The Heartbeat Cycle: Systole and Diastole

A heartbeat consists of the heart contracting (systole) and relaxing (diastole). The ventricles are much larger and more muscular than the atria.

Heart Rate and Rhythm Regulation

The heartbeat is typically rhythmic, occurring 60-80 times per minute at rest.

  • Tachycardia: An increased heart rate, often defined as over 100-120 beats per minute.
  • Bradycardia: A decreased heart rate, often defined as below 50-60 beats per minute.

The Pacemaker

The heartbeat is initiated by the pacemaker (sinoatrial or SA node), a specialized area in the right atrium that automatically generates electrical pulses, causing the heart to contract.

Systemic Circulation: Major Arterial Pathways

Blood leaves the heart and travels through arteries:

  • Heart -> Aorta (Ascending Aorta, Aortic Arch, Descending Aorta)
  • Ascending Aorta -> Coronary Arteries (supply the heart muscle/myocardium)
  • Aortic Arch -> Carotid Arteries (supply the head) and Subclavian Arteries (supply the arms)
  • Subclavian Artery -> Axillary Artery -> Brachial (Humeral) Artery -> Radial and Ulnar Arteries
  • Descending Aorta (Thoracic) -> Intercostal Arteries (supply respiratory muscles)
  • Descending Aorta (Abdominal) ->
    • Gastric Artery (supplies the stomach)
    • Hepatic Artery (supplies the liver)
    • Superior Mesenteric Artery (supplies the small intestine)
    • Renal Arteries (supply the kidneys)
    • Common Iliac Arteries ->
      • Internal Iliac Artery (supplies the reproductive organs and bladder)
      • External Iliac Artery (supplies the legs) -> Femoral Artery -> Popliteal Artery -> Tibial Arteries

Common Vascular Conditions

Aneurysm: Vessel Wall Dilation

An aneurysm is a dilation or bulging of a blood vessel wall, involving a thinning of the wall. If blood pressure rises significantly, the weakened wall can burst, causing potentially fatal internal bleeding.

Thrombosis: Blood Clot Blockage

Thrombosis occurs when a blood clot (thrombus) blocks a blood vessel. It can be caused by factors like excess cholesterol leading to arteriosclerosis. A thrombus prevents blood supply (including oxygen and glucose) from reaching a body part. This can cause tissue death (gangrene) or be fatal, depending on where the blockage occurs.

Arteriosclerosis: Hardening of Arteries

Arteriosclerosis occurs when substances like cholesterol deposit within blood vessel walls, causing them to harden and narrow. This condition hinders the exchange of substances between the blood and the body’s internal environment and can provoke degenerative organ function. It is often progressive (gets worse over time) and may have no early symptoms, sometimes called a ‘silent disease’.